docs: update existing compatibility status

- file.c split layout retained; security-hardened secure-fs helpers
  (open_secure_parent/to_disk_secure/rename_secure/stat_secure) now live in
  file.c with file_ prefix and are shared with file_receive.c
- file_receive.c takes the security branch's bounded allocations
  (receive_data_limited, data_decompress_limited, size checks) and
  STATUS_ERROR signaling
- file_send.c gains the data consistency check on file->data
- client_validation.c: stricter --tls requiring --ca, log_message style
- utils.c: hardened openat/mkdirat mkdir_r from security branch

CMakeLists.txt

@@ -58,7 +58,7 @@ endif()
 find_package(OpenSSL REQUIRED)
 
 file(GLOB SHARED_SRCS "src/shared/*.c")
-set(FILE_STORE_SRCS src/shared/file_store.c)
+set(FILE_STORE_SRCS "${CMAKE_CURRENT_SOURCE_DIR}/src/shared/file_store.c")
 list(REMOVE_ITEM SHARED_SRCS ${FILE_STORE_SRCS})
 file(GLOB SERVER_SRCS "src/server/*.c")
 set(SERVER_RECEIVER_SRCS src/server/receiver.c)

README.md

@@ -1,103 +1,83 @@
-# FastSync
+#FastSync
 
-A high-performance file synchronization system with SSH and TCP transport, TLS encryption, streaming zstd compression, multithreaded transfer, incremental sync, metadata preservation, and rsync-compatible CLI flags.
+FastSync is a high-performance file synchronization tool designed to become a
+drop-in replacement for common `rsync` workflows. It keeps the familiar
+source/destination model and rsync-style options while adding optional
+multithreading, streaming zstd compression, chunking, zero-copy TCP transfers,
+and native TCP/TLS transports.
 
-## Technical Overview
+The compatibility target is straightforward:
 
-1. **Dual transport**: custom TCP client-server or SSH subprocess (rsync-style `user@host:/path`)
+- Existing rsync commands should keep the same meaning.
-2. **TLS encryption**: OpenSSL-based TLS 1.2+ for encrypted TCP connections with optional CA verification
+- FastSync-only performance options should be additive and optional.
-3. **Chunked file transfer**: files grouped into configurable-size chunks (default ~10 MB)
+- A normal compatibility-mode transfer should prioritize rsync filesystem
-4. **Streaming zstd compression** (levels 1–22) using `ZSTD_compressStream2`
+  semantics over maximum throughput.
-5. **Multithreading**: producer-consumer pipeline with thread-safe queues (scanner → loader → sender)
-6. **Incremental sync**: skip files unchanged since last transfer (compares size + mtime)
-7. **Batch incremental**: send incremental checks in batched groups for reduced round-trips
-8. **Metadata preservation**: file mode and mtime are restored when enabled; ownership and atime are intentionally not restored
-9. **`sendfile()` zero-copy** on TCP (~2× faster on loopback)
-10. **SSH ControlMaster** for connection reuse across repeated invocations
-11. **Bandwidth limiting**: token-bucket throttling (`--bwlimit`)
-12. **`--delete`**: receiver removes files not present in sender manifest
-13. **`--exclude` / `--include`**: glob-pattern filename filtering
-14. **Path traversal protection**: `..` sequences in file paths are rejected automatically
-15. **Connection limits**: server enforces maximum concurrent connections (default 100)
-16. **Keep-alive**: periodic `STATUS_KEEPALIVE` messages detect stalled connections
-17. **Abort handling**: `SIGINT` sends `STATUS_ABORT` for clean server-side teardown
-18. **Atomic writes**: received files are written to a temporary name then atomically renamed
-19. **Backup mode**: `--backup` preserves overwritten files with optional `--backup-dir`
-20. **Log file**: `--log-file` redirects log output to a file instead of stderr
-21. **Transfer statistics**: `--stats` prints summary of transferred bytes, files, and timing
 
-## System Architecture
+FastSync currently speaks its own protocol to `fastsync-server`. SSH mode
+starts that server remotely; it does not yet interoperate with an unmodified
+rsync client or rsync daemon. See [Compatibility Status](#compatibility-status)
+for the current boundary.
 
-### Client
+## Why FastSync
-- Recursively scans source directories (BFS), supports exclude and include patterns
-- Groups files into chunks (configurable size)
-- Streaming zstd compression with configurable level
-- Chunk serialization (compact binary format) or per-file transfer
-- Incremental transfer: sends file metadata to server, skips unchanged files
-- Batch incremental: groups incremental checks to minimize round-trips
-- Manifests all sent paths when `--delete` is active
-- Sends via TCP `sendfile()` or SSH pipe
-- Optional progress display with throughput
-- Bandwidth limiting via token-bucket algorithm
-- Configurable I/O and connection timeouts (`--timeout`, `--contimeout`)
-- Backup overwritten files (`--backup`) with optional directory (`--backup-dir`)
-- Transfer statistics summary (`--stats`)
-- Maximum directory depth control (`--max-depth`)
-- Log file output (`--log-file`)
-- Exclude patterns from file (`--exclude-from`)
 
-### Server
+FastSync uses a producer-consumer transfer pipeline and can combine several
-- TCP mode: listens on configurable port (default 8080); SSH mode: runs via `--stdio`
+optimizations for large or high-latency transfers:
-- TLS mode: wraps TCP connections with OpenSSL with optional CA verification
-- Receives and reassembles files
-- Decompresses (streaming zstd), deserializes, restores metadata
-- Handles incremental checks: compares size + mtime against destination files
-- Handles batch incremental checks for reduced round-trips
-- Processes `STATUS_MANIFEST` for `--delete`: walks destination tree, removes extras
-- Per-connection concurrency via `fork()` with configurable connection limit (default 100)
-- Thread pool for parallel processing
-- Atomic writes: files written to `.tmp` path then atomically renamed on success
-- Abort handling: cleanly shuts down on `STATUS_ABORT` from client
-- Path traversal protection: rejects file paths containing `..`
 
-## Protocol Details
+- Multithreaded scanning, loading, and sending.
+- Streaming zstd compression with levels 1 through 22.
+- Configurable file chunking and compact chunk serialization.
+- `sendfile()` zero-copy transfers over TCP.
+- Batched incremental checks to reduce round trips.
+- Optional block-level delta transfer for FastSync peers.
+- Bandwidth limiting, progress reporting, statistics, and backups.
+- TCP, SSH, and TLS transports.
+- Atomic temporary-file writes by default.
 
-### Status Codes
+These optimizations are disabled or selected independently. Users can start
-| Code | Meaning |
+with rsync-style commands and add FastSync options when they are useful.
-|------|---------|
-| `STATUS_OK` | Operation successful |
-| `STATUS_ERROR` | Error occurred |
-| `STATUS_FINISHED` | Transfer complete |
-| `STATUS_NEXT` | Ready for next file (per-file mode) |
-| `STATUS_CHUNK` | Following data is a serialized chunk |
-| `STATUS_MANIFEST` | Following data is a file manifest (for `--delete`) |
-| `STATUS_CHECK` | Incremental check: client sends file path + size + mtime and, when negotiated, checksum; server responds with OK (skip) or NEXT (send) |
-| `STATUS_CHECK_BATCH` | Batch incremental check: multiple file checks sent in one message |
-| `STATUS_KEEPALIVE` | Keep-alive heartbeat to detect stalled connections |
-| `STATUS_ABORT` | Abort signal: client interrupts, server cleans up and exits |
-| `STATUS_DELTA_SIGNATURE` | Delta sync: following data is a file signature (rsync-style rolling hash) |
-| `STATUS_DELTA_DATA` | Delta sync: following data is a delta patch for a file |
 
-### Wire Format — Metadata
+## Compatibility Status
 
-When `use_metadata` is enabled (`-M`), each file entry carries a 4-byte `present` flag followed by five fields (`mode`, `uid`, `gid`, `mtime_sec`, `mtime_nsec`). When disabled globally, no metadata bytes are sent — zero wire overhead.
+FastSync is currently an rsync-compatible CLI in progress, not a complete
+replacement for every rsync feature or protocol mode.
 
-### Transfer Flow
+### Working today
-```
-Config → (STATUS_NEXT | STATUS_CHUNK | STATUS_CHECK | STATUS_CHECK_BATCH)* → [STATUS_MANIFEST] → STATUS_FINISHED → STATUS_OK
-```
 
-Keep-alive (`STATUS_KEEPALIVE`) may be sent at any point during the transfer. The receiver resets its inactivity timer on receipt. If no data arrives within the receive timeout, the connection is aborted.
+- Recursive directory scanning.
+- Rsync-style source and destination arguments.
+- SSH transport using `user@host:destination` paths below the remote authorized root.
+- TCP client/server transfers.
+- Dry runs, excludes, includes, size filters, backups, statistics, and
+  bandwidth limiting.
+- Incremental size/mtime checks and optional xxHash64 content checks.
+- FastSync-native delta transfer for changed files.
+- Optional mode and timestamp preservation.
+- Delete manifests with server-side delete authorization.
+- Temporary-file writes with atomic rename by default.
+- Path traversal checks and destination-root confinement.
 
-Abort (`STATUS_ABORT`) may be sent at any point. On receipt the server cleans up temporary files and exits the child process.
+### Not yet equivalent to rsync
 
-### Protocol Version
+- The FastSync wire protocol is not the rsync wire protocol.
+- SSH mode requires `fastsync-server` on the remote host.
+- Archive mode does not yet provide all of rsync's `-rlptgoD` behavior.
+- Symlink transfer is incomplete; link targets are not yet recreated in all
+  modes.
+- Owner/group, ACL, xattr, hard-link, device, and special-file handling is
+  incomplete or unavailable.
+- Sparse-file handling does not yet preserve all holes correctly.
+- `--partial`, `--partial-dir`, `--append`, and `--append-verify` are not yet
+  full rsync-style resumable transfers.
+- Several rsync short options currently have FastSync-specific meanings. Do
+  not assume every short option is interchangeable yet.
 
-`2.2.0` — server and client must match. This version adds a 64-bit XXH64 checksum to checksum-enabled `STATUS_CHECK` messages and validates the negotiated compression choice (`zstd` or `none`). Older clients and servers must not be mixed with this version; mismatch results in `STATUS_ERROR`.
+The detailed flag matrix is maintained in
+[`RSYNC_COMPAT.md`](RSYNC_COMPAT.md). It distinguishes implemented,
+partial, alternate, and planned behavior.
 
-Config negotiation is sender-driven: the client serializes transfer options and the server applies them while receiving and writing files. `--checksum` compares size and content checksum instead of timestamps. `--compress-choice zstd` enables zstd; `none` disables it. Unsupported choices are rejected during config exchange.
+## Quick Start
 
-## Command-Line Arguments
+### Build
 
 ### Client
 
@@ -122,6 +102,7 @@ Config negotiation is sender-driven: the client serializes transfer options and
 | `--max-size <n>` | Skip files larger than n bytes |
 | `--min-size <n>` | Skip files smaller than n bytes |
 | `--incremental` | Skip files unchanged since last transfer (size + mtime). Auto-enables `--preserve`. Incompatible with `-s`. |
+| `--existing` | Skip files not already present at the destination; update existing files normally. |
 | `--bwlimit <KB/s>` | Bandwidth limit in kilobytes per second |
 | `--chunk-size <n>` | Chunk size in bytes (default: 10485760) |
 | `--timeout <sec>` | I/O timeout in seconds (default: 30) |
@@ -140,6 +121,7 @@ Config negotiation is sender-driven: the client serializes transfer options and
 | `--cert <path>` | TLS certificate file (PEM) |
 | `--key <path>` | TLS private key file (PEM) |
 | `--ca <path>` | TLS CA certificate file for verification (PEM) |
+| `--client-cn <name>` | Required TLS client certificate common name |
 
 ### Server
 
@@ -151,6 +133,9 @@ Config negotiation is sender-driven: the client serializes transfer options and
 | `--cert <path>` | TLS certificate file (PEM) |
 | `--key <path>` | TLS private key file (PEM) |
 | `--ca <path>` | TLS CA certificate file for verification (PEM) |
+| `--destination-root <path>` | Authorized destination root (default: `.`) |
+| `--allow-delete` | Permit manifest deletion |
+| `--allow-unauthenticated` | Permit plaintext TCP clients |
 | `-v, --verbose` | Enable debug logging |
 | `--help` | Show help |
 
@@ -173,26 +158,46 @@ Config negotiation is sender-driven: the client serializes transfer options and
 ### Data Structures
 1. **Chunk** — collection of files (~10 MB total by default)
 2. **File** — path, content (`Data`), optional `FileMetadata` pointer
-3. **FileMetadata** — `mode`, `uid`, `gid`, `mtime_sec`, `mtime_nsec`; uid/gid are advisory wire fields and are never applied by the receiver; atime is unsupported
+3. **FileMetadata** — `mode`, `uid`, `gid`, `mtime_sec`, `mtime_nsec`;
+uid / gid are advisory wire fields and are never applied by the receiver;
+atime is unsupported
 4. **Config** — runtime parameters (transported over wire, TLS settings excluded). Includes `timeout`, `contimeout`, `quiet`, `backup`, `backup_dir`, `stats`, `max_depth`, `log_file`, `queue_size`.
 5. **Queue** — thread-safe bounded queue with condition variables
 6. **DirectoryScanner** — recursive BFS traversal with exclude and include pattern support, max-depth enforcement
 
 ### Key Algorithms
-1. **File scanning** — BFS directory traversal; entries matched against exclude and include patterns, max-depth enforced
+1. **File scanning** — BFS directory traversal;
-2. **Chunking** — files accumulated until `chunk_size` threshold, then flushed
+entries matched against exclude and include patterns,
-3. **Compression** — streaming zstd via `ZSTD_compressStream2` / `ZSTD_decompressStream`
+    max - depth enforced 2. * *Chunking ** — files accumulated until `chunk_size` threshold,
-4. **Network protocol** — status-code-driven exchange with metadata packing, keep-alive, and abort support
+    then flushed 3. *
-5. **Incremental check** — client sends `STATUS_CHECK` + path + size + mtime and, with `--checksum`, XXH64 content checksum; server compares against destination. Can be batched via `STATUS_CHECK_BATCH` for reduced round-trips.
+            *Compression ** — streaming zstd
+                via `ZSTD_compressStream2` / `ZSTD_decompressStream` 4. *
+            *Network protocol ** — status -
+        code - driven exchange with metadata packing,
+    keep - alive,
+    and abort support 5. * *Incremental check ** — client sends `STATUS_CHECK` + path + size +
+        mtime and,
+    with `--checksum`, XXH64 content checksum; server compares against destination. Can be batched via `STATUS_CHECK_BATCH` for reduced round-trips.
 6. **Bandwidth limiting** — token-bucket algorithm with `nanosleep` throttling on 64 KB write chunks
 7. **Metadata restoration** — `chmod()`, `chown()`, `utimensat()` on the receiving side
-8. **`--delete`** — sender tracks all sent paths; receiver walks destination tree and removes unlisted files/directories
+8. **`--delete`** — sender tracks all sent paths;
-9. **SSH transport** — `socketpair()` + `fork()` + `execvp("ssh", ...)` with `ControlMaster` and port support
+receiver walks destination tree and removes unlisted files / directories 9. *
-10. **TLS transport** — OpenSSL `SSL_CTX` with TLS 1.2 minimum, optional CA verification, transparent `SSL_read`/`SSL_write` via `io_set_ssl()`
+        *SSH transport *
-11. **Path traversal protection** — `has_path_traversal()` rejects any file path containing `..` components, preventing directory escape attacks
+            * — `socketpair()` + `fork()` + `execvp("ssh",
-12. **Connection limiting** — server tracks active connections and rejects new ones beyond `max_connections` (default 100)
+                                                    ...)` with `ControlMaster` and port support
-13. **Keep-alive** — idle connections receive periodic `STATUS_KEEPALIVE` to detect half-open TCP connections
+                                                10. *
-14. **Abort handling** — `SIGINT` sets an abort flag; the next protocol operation sends `STATUS_ABORT` for clean server cleanup
+                                                *TLS transport ** — OpenSSL `SSL_CTX` with TLS
+                                                1.2 minimum,
+    mutual CA verification,
+    transparent `SSL_read`/`SSL_write` via `io_set_ssl()` 11. *
+        *Path traversal protection ** — `has_path_traversal()` rejects any file path
+         containing `..` components,
+    preventing directory escape attacks 12. *
+            *Connection limiting ** — server tracks active connections and rejects
+            new ones beyond `max_connections` (default 100)13. *
+            *Keep
+        - alive ** — idle connections receive periodic `STATUS_KEEPALIVE` to detect half
+        - open TCP connections 14. * *Abort handling ** — `SIGINT` sets an abort flag; the next protocol operation sends `STATUS_ABORT` for clean server cleanup
 15. **Atomic writes** — files are written to a `.tmp` suffix then atomically renamed via `rename()`, preventing partial files
 16. **Backup** — before overwriting, existing files are moved to `--backup-dir` (or same directory with `~` suffix) preserving the original
 
@@ -202,7 +207,7 @@ Config negotiation is sender-driven: the client serializes transfer options and
 All received file paths are validated by `has_path_traversal()` before any disk operation. Any path containing `..` components is rejected with `STATUS_ERROR`, preventing directory escape attacks.
 
 ### TLS Certificate Verification
-When `--ca` is provided, the server performs mutual TLS verification (`SSL_VERIFY_PEER` with depth 4). Without `--ca`, TLS is still encrypted but peer certificates are not verified.
+TLS requires `--ca` and performs mutual TLS verification (`SSL_VERIFY_PEER` with depth 4). Connections without certificate verification are rejected.
 
 ### Connection Limits
 The server enforces a maximum of 100 concurrent connections (configurable via `max_connections` in `Server`). When the limit is reached, new connections are immediately rejected and closed.
@@ -237,127 +242,305 @@ nix-shell  # provides zstd, openssl, cmake, gcc
 ## Building
 
 ```bash
-cmake -B build -S . && cmake --build build -j$(nproc)
+cmake -B build -S .
+cmake --build build -j$(nproc)
 ```
 
-## Running
+With Nix:
 
-### Server (TCP mode)
 ```bash
-./build/server
+nix-shell
+cmake -B build -S .
+cmake --build build -j$(nproc)
 ```
 
-### Server with TLS
+### SSH transfer
+
+The remote host must have `fastsync-server` available in `PATH`, or use
+`--fastsync-server-path`. SSH starts `fastsync-server --stdio` in its remote
+working directory, so use a destination below that directory unless the
+remote server is otherwise configured with a matching authorized root.
+
 ```bash
-./build/server --tls --cert server.pem --key server-key.pem
+ssh user@host 'mkdir -p destination'
+./build/client /path/to/source user@host:destination
 ```
 
-### Server via SSH
+### TCP transfer
-Place the `fastsync-server` binary in the remote `$PATH`. The client runs `ssh user@host fastsync-server --stdio` automatically when an SSH-style destination is given.
+
+Start the FastSync server:
 
-### Client — SSH (rsync-style)
 ```bash
-./build/client /path/to/send user@host:/path/to/receive
+./build/server --destination-root /path/to -p 8080
 ```
 
-### Client — TCP
+Then run the client:
+
 ```bash
-./build/client --source-dir /path/to/send --dest-dir /path/to/receive --save-to-disk
+./build/client --server-host 127.0.0.1 --server-port 8080 \
+  --source-dir /path/to/source --dest-dir /path/to/destination \
+  --save-to-disk
 ```
 
-### Client — TCP with TLS
+Plain TCP requires the explicit `--allow-unauthenticated` server option. Use TLS for
+authenticated network connections.
+
+### TLS transfer
 ```bash
+./build/server --destination-root /path/to --tls --cert server.pem --key server-key.pem -p 8443
 ./build/client --tls --cert client.pem --key client-key.pem --ca ca.pem \
-  --source-dir /path/to/send --dest-dir /path/to/receive --save-to-disk
+  --server-host example.com --server-port 8443 \
+  --source-dir /path/to/source --dest-dir /path/to/destination \
+  --save-to-disk
 ```
 
-### Common Options
+## Common Workflows
+
+These examples show the intended rsync-style workflow. Options marked as
+FastSync-native are optional performance or transport extensions.
+
 ```bash
-# Archive mode (compression + multithreading + metadata)
+#Basic synchronization
-./build/client -a /path/to/send user@host:/path
+./build/client /source/ /destination/
 
-# Dry run
+#Archive - style synchronization(current FastSync archive behavior)
-./build/client -n /path/to/send /path/to/receive
+./build/client -a /source/ user@host:destination/
 
-# With progress and custom chunk size
+#Preview a transfer without changing the destination
-./build/client --progress --chunk-size 2097152 /src user@host:/dst
+./build/client -n /source/ /destination/
 
-# Exclude temporary files + delete extras on receiver
+#Exclude temporary and object files
-./build/client --exclude "*.tmp" --exclude "*.o" --delete /src user@host:/dst
+./build/client --exclude '*.tmp' --exclude '*.o' \
+  /source/ user@host:destination/
 
-# Incremental sync (skip unchanged files)
+#Remove destination entries not present in the source
-./build/client --incremental /src user@host:/dst
+./build/client --delete /source/ user@host:destination/
 
-# Bandwidth limit to 1 MB/s
+#Skip unchanged files using size and modification time
-./build/client --bwlimit 1024 /src user@host:/dst
+./build/client --incremental /source/ user@host:destination/
 
-# With timeouts and stats
+#Verify content when size and time are not sufficient
-./build/client --timeout 60 --contimeout 15 --stats /src user@host:/dst
+./build/client --incremental --checksum /source/ user@host:destination/
 
-# Backup overwritten files to a directory
+#Preserve supported mode and timestamp metadata
-./build/client --backup --backup-dir /backups /src user@host:/dst
+./build/client -M /source/ user@host:destination/
 
-# Exclude patterns from file, limit depth
+#Keep backups of overwritten destination files
-./build/client --exclude-from ignore.txt --max-depth 3 /src user@host:/dst
+./build/client --backup --backup-dir backups \
-
+  /source/ user@host:destination/
-# Log to file
-./build/client --log-file /tmp/fastsync.log /src user@host:/dst
-
-# All features
-./build/client -a --progress --chunk-size 5242880 --exclude "*.log" --delete /src /dst
 ```
 
+## FastSync Extensions
+
+FastSync-native options are intended to add performance or operational
+features without changing the meaning of ordinary compatibility options.
+
+| Option | Purpose |
+|---|---|
+| `-m` | Enable the multithreaded scanner/loader/sender pipeline. |
+| `-c [level]`, `-z [level]` | Enable streaming zstd compression, levels 1-22. |
+| `--compress-level <n>` | Set the zstd compression level. |
+| `--chunk-size <bytes>` | Set the transfer chunk size. |
+| `-s` | Enable FastSync chunk serialization. |
+| `-f`, `--sendfile` | Use TCP `sendfile()` zero-copy transfer. Incompatible with compression and chunk serialization. |
+| `--delta` | Use FastSync-native block delta transfer. Requires `--incremental`. |
+| `--delta-block <bytes>` | Set the FastSync delta block size. |
+| `--delta-max <bytes>` | Limit files eligible for FastSync delta transfer. |
+| `--server-host <host>` | Select the TCP server host. |
+| `--server-port <port>` | Select the TCP server port. |
+| `--tls` | Enable TLS for TCP transport. |
+| `--bwlimit <KB/s>` | Apply token-bucket bandwidth limiting. |
+| `--progress` | Show transfer progress and throughput. |
+| `--stats` | Print transfer statistics. |
+| `--timeout <seconds>` | Set I/O timeout. |
+| `--contimeout <seconds>` | Set connection timeout. |
+
+Current short-option conflicts are tracked as compatibility work. In
+particular, FastSync currently uses `-p` for SSH port, `-s` for chunk
+serialization, and `-S` for sparse handling. These meanings must be reconciled
+before FastSync can claim full rsync CLI compatibility.
+
+## Client Options
+
+### Selection and transfer
+
+| Option | Description |
+|---|---|
+| `-a`, `--archive` | Enable current archive preset. Full rsync archive semantics are planned. |
+| `-n`, `--dry-run` | Scan and report without writing files. |
+| `--delete` | Request removal of destination entries absent from the source. The server must allow deletion. |
+| `--exclude <pattern>` | Exclude matching paths. Repeatable. |
+| `--include <pattern>` | Include matching paths. Repeatable. |
+| `--exclude-from <file>` | Read exclude patterns from a file. |
+| `--include-from <file>` | Read include patterns from a file. |
+| `--max-size <bytes>` | Skip files larger than the limit. |
+| `--min-size <bytes>` | Skip files smaller than the limit. |
+| `--max-depth <n>` | Limit recursive scanning depth;
+zero means unlimited.| | `--incremental` | Skip files matching destination size and mtime.|
+    | `--checksum` | Include xxHash64 content checks in incremental comparisons.| | `--backup` |
+    Back up overwritten files.| | `--backup - dir<dir>` | Store backups under a separate directory.|
+    | `--suffix<suffix>` | Set the backup filename suffix.| | `--partial` |
+    Select partial - transfer handling.With `--partial - dir`,
+    completed files are written there;
+resumable transfers are not implemented.| | `--partial - dir<dir>` |
+    Set a relative partial - transfer directory below the server destination root;
+use with `--partial`. |
+| `--inplace` | Write directly to the destination instead of using a temporary file. |
+
+### Metadata and links
+
+| Option | Description |
+|---|---|
+| `-M`, `--preserve` | Preserve supported file metadata, currently mode and modification time. |
+| `-l`, `--links` | Request symlink preservation;
+link-target transfer remains incomplete. |
+| `--copy-links` | Copy symlink referents. |
+| `--safe-links` | Skip symlinks that point outside the transfer tree. |
+| `--copy-unsafe-links` | Copy unsafe symlink referents. |
+| `-S`, `--sparse` | Request sparse-file handling; full hole preservation is planned. |
+
+### Output and logging
+
+| Option | Description |
+|---|---|
+| `-v`, `--verbose` | Enable debug logging. |
+| `--progress` | Show live transfer progress. |
+| `--stats` | Print transfer statistics. |
+| `--log-file <path>` | Write log output to a file. |
+| `-V`, `--version` | Print the FastSync protocol version. |
+| `--help` | Print command usage. |
+
+### Paths and transport
+
+| Option | Description |
+|---|---|
+| `-p <port>` | SSH port in the current CLI. This conflicts with rsync's `-p` permissions option and is planned for correction. |
+| `--fastsync-server-path <path>` | Remote FastSync server path for SSH mode. |
+| `--source-dir <path>` | Set the source directory explicitly. |
+| `--dest-dir <path>` | Set the destination directory explicitly. |
+| `--save-to-disk` | Enable server-side disk persistence. |
+| `--server-host <host>` | TCP server address. |
+| `--server-port <port>` | TCP server port. |
+| `--tls` | Enable TLS. Requires `--cert` and `--key`. |
+| `--cert <path>` | TLS certificate file. |
+| `--key <path>` | TLS private key file. |
+| `--ca <path>` | CA file for peer verification. |
+
+## Server Options
+
+| Option | Description |
+|---|---|
+| `--stdio` | Serve one SSH connection over standard input/output. |
+| `-p <port>` | TCP listen port. |
+| `--tls` | Enable TLS. |
+| `--cert <path>` | TLS certificate file. |
+| `--key <path>` | TLS private key file. |
+| `--ca <path>` | CA file for peer verification. |
+| `--destination-root <path>` | Confine received files to this server-side root;
+defaults to the current directory. |
+| `--allow-delete` | Permit client delete manifests. Deletion is refused by default. |
+| `-v`, `--verbose` | Enable debug logging. |
+| `--help` | Print server usage. |
+
+## Architecture
+
+### Client
+
+- Recursively scans the source tree with include, exclude, size, and depth
+  filters.
+- Sends individual files or serialized chunks.
+- Performs incremental checks and optional content checksums.
+- Uses a multithreaded producer-consumer pipeline when requested.
+- Sends over TCP, TLS-wrapped TCP, or an SSH subprocess.
+- Supports progress, statistics, backups, timeouts, and bandwidth limiting.
+
+### Server
+
+- Runs as a TCP listener or one-shot SSH `--stdio` server.
+- Receives and reassembles files and decompresses streaming zstd data.
+- Applies supported metadata and writes files through a confined destination
+  root.
+- Uses temporary files and atomic rename by default.
+- Handles delete manifests only when explicitly authorized.
+- Enforces connection, message-size, and path-safety limits.
+
+## Protocol and Security
+
+FastSync protocol version `2.3.0` is shared by the client and server. The
+current protocol is sender-driven and includes configuration negotiation,
+incremental checks, checksums, manifests, keep-alives, abort handling, and
+FastSync-native delta messages. Client and server versions must currently
+match exactly.
+
+TLS provides encrypted TCP transport. Supplying `--ca` enables certificate
+verification; without it, traffic is encrypted but peer identity is not
+verified. Use certificate verification for deployments where authentication
+matters. The default TCP transport is not encrypted.
+
+The receiver protects its destination root with path validation, `openat()`
+directory traversal, `O_NOFOLLOW`, temporary files, and atomic renames. Delete
+operations require the server's explicit `--allow-delete` policy.
+
+## Compatibility Roadmap
+
+The project will reach the drop-in replacement goal in stages:
+
+1. Correct rsync option meanings, including short options, combined options,
+   and `--option=value` syntax.
+2. Add differential tests that compare FastSync and rsync contents, metadata,
+   links, deletes, filters, dry runs, and exit codes.
+3. Make `-a` implement the expected recursive, links, permissions, times,
+   owner/group, and supported special-file behavior.
+4. Complete symlink, sparse-file, metadata, delete-policy, and resumable-write
+   semantics.
+5. Add rsync remote-shell and daemon protocol interoperability.
+6. Keep FastSync performance options as negotiated, optional extensions.
+
+The exhaustive implementation matrix and compatibility notes are in
+[`RSYNC_COMPAT.md`](RSYNC_COMPAT.md).
+
 ## Testing
 
-```bash
+Run the unit test binary:
-# Unit tests (18 suites — array_list, chunk, compression, config, data, delta, file, glob,
-#                      metadata, property, protocol, queue, robustness, scanner,
-#                      shared_utils, stress, transport_tcp, transport_ssh, transport_tls)
-./build/tests
 
-# Integration + benchmark suite
+```bash
-python3 test.py
+./build/tests
 ```
 
-The benchmark prints throughput metrics, best configuration, and speedup vs rsync.
+Run the Python integration suite:
 
-## Performance Considerations
+```bash
+python3 -m pytest tests/
+```
 
-1. Chunk size (~10 MB default) balances memory and transfer efficiency
+For stricter local validation:
-2. Compression level trades CPU for bandwidth
-3. `sendfile()` bypasses userspace — ~2× faster on localhost for large files
-4. Multithreading scales with core count and uses memory-based pipeline sizing
-5. Metadata transfer adds negligible overhead (~24 bytes per file when enabled)
-6. SSH socketpair buffer set to 1 MB for improved pipe throughput
-7. SSH ControlMaster reuses connections across repeated invocations
-8. Incremental sync eliminates redundant transfers entirely
-9. Batch incremental reduces round-trips by grouping multiple checks into one message
-10. Bandwidth limiting uses token-bucket with nanosleep for accurate throttling
-11. Atomic writes add a single `rename()` per file — negligible overhead
-12. Path traversal check is O(n) in path length with negligible cost
 
-## Benchmark Results
+```bash
+cmake -B build-strict -S . -DSTRICT_WARNINGS=ON
+cmake --build build-strict -j$(nproc)
+cmake -B build-asan -S . -DSANITIZER=address
+cmake --build build-asan -j$(nproc)
+```
 
-25 MB of mixed file sizes over `localhost` with disk I/O throttled (reads ≤ 15 MB/s, writes ≤ 10 MB/s) and network emulation via `tc netem`. Each test was run 3×; the median is reported below.
+The benchmark tool compares FastSync configurations with rsync under
+controlled local and network conditions:
 
-### LAN (1000 Mbit, 20 ms ±1 ms, 0.1% loss)
+```bash
+python3 benchmark/bench.py --help
+```
 
-| Configuration | Time | vs rsync (archive) | vs rsync (compress) |
+Benchmark results measure transfer performance only. They do not establish
-|---|---|---|---|
+rsync protocol or filesystem-semantic compatibility.
-| **Best: `-m -c`** | **0.20 s** | **11.2× faster** | **3.6× faster** |
-| Compression (`-c`) | 0.31 s | 7.3× faster | 2.3× faster |
-| Standard | 1.27 s | 1.8× faster | — |
-| rsync (archive) | 2.27 s | — | — |
-| rsync (archive + compress) | 0.72 s | — | — |
 
-### WAN (100 Mbit, 50 ms ±10 ms, 1% loss)
+## Performance Guidance
 
-| Configuration | Time | vs rsync (archive) | vs rsync (compress) |
+- Use `-m` for workloads with many files or enough CPU parallelism.
-|---|---|---|---|
+- Use `-c` or `-z` when network bandwidth is more constrained than CPU.
-| **Best: `-m -c`** | **0.39 s** | **44.8× faster** | **3.8× faster** |
+- Tune `--chunk-size` for file sizes, memory limits, and network latency.
-| Compression (`-c`) | 0.64 s | 27.3× faster | 2.3× faster |
+- Use `-f` for large uncompressed TCP transfers where zero-copy I/O helps.
-| Standard | 7.12 s | 2.4× faster | — |
+- Use `--incremental` to avoid retransmitting unchanged files.
-| rsync (archive) | 17.44 s | — | — |
+- Use `--delta` for changed files when both endpoints are FastSync peers.
-| rsync (archive + compress) | 1.47 s | — | — |
+- Use `--bwlimit` when sharing a link with other traffic.
 
-Compression reduces the data on the wire enough that the transfer becomes latency-bound rather than bandwidth-bound. On WAN, the best configuration runs 10.8× faster than the theoretical limit for uncompressed data, since zstd shrinks the 25 MB payload to a fraction of its original size over the wire.
+Always validate the compatibility behavior required by a deployment before
+replacing an existing rsync job.

RSYNC_COMPAT.md

@@ -6,11 +6,11 @@ This document maps rsync's full feature set to FastSync's current implementation
 
 | Status | Count | Description |
 |--------|-------|-------------|
-| ✅ Implemented | 34 | Feature works end-to-end |
+| ✅ Implemented | 35 | Feature works end-to-end |
 | 🔀 Alt Arg | 3 | Functionality exists but under different flag/semantics |
-| ⚠️ Partial | 1 | Flag parsed/stored but behavior incomplete |
+| ⚠️ Partial | 3 | Flag parsed/stored but behavior incomplete |
-| ❌ Not Implemented | 98 | Flag not recognized or no behavior |
+| ❌ Not Implemented | 97 | Flag not recognized or no behavior |
-| **Total** | **136** | |
+| **Total** | **138** | |
 
 ---
 
@@ -60,7 +60,7 @@ This document maps rsync's full feature set to FastSync's current implementation
 | `-I`, `--ignore-times` | Don't skip files matching size+time | ❌ Not Implemented | |
 | `--size-only` | Skip based on size only | ❌ Not Implemented | |
 | `-@`, `--modify-window=NUM` | Mod-time comparison accuracy | ❌ Not Implemented | |
-| `--existing` | Skip creating new files on receiver | ❌ Not Implemented | |
+| `--existing` | Skip creating new files on receiver | ✅ Implemented | Existing destination files continue through normal update handling |
 | `--ignore-existing` | Skip updating existing files | ❌ Not Implemented | |
 | `--remove-source-files` | Sender removes synced files | ❌ Not Implemented | |
 
@@ -152,14 +152,14 @@ This document maps rsync's full feature set to FastSync's current implementation
 
 | Flag | Rsync Description | FastSync Status | Notes |
 |------|-------------------|-----------------|-------|
-| `-S`, `--sparse` | Sparse block handling | ✅ Implemented | `preserve_sparse` config field |
+| `-S`, `--sparse` | Sparse block handling | ⚠️ Partial | Flag is accepted, but full hole preservation is not implemented |
 | `--preallocate` | Allocate dest files before writing | ❌ Not Implemented | |
 
 ## 11. Checksum & Comparison
 
 | Flag | Rsync Description | FastSync Status | Notes |
 |------|-------------------|-----------------|-------|
-| `--checksum` | Skip based on checksum | ❌ Not Implemented | Removed because it had no effect; `-c` means compression |
+| `--checksum` | Skip based on checksum | ✅ Implemented | With `--incremental`, compares xxHash64 content checksums; `-c` remains compression |
 | `--checksum-choice=STR` | Choose checksum algorithm | ❌ Not Implemented | xxHash used internally |
 | `--compare-dest=DIR` | Compare dest files relative to DIR | ❌ Not Implemented | Removed because it had no effect |
 | `--copy-dest=DIR` | Include copies of unchanged files | ❌ Not Implemented | Removed because it had no effect |
@@ -247,7 +247,7 @@ Ranked by user demand, implementation complexity, and interoperability impact:
 | 1 | `--whole-file` / `-W` | Low | High — users expect opt-out of delta |
 | 2 | `--ignore-times` / `-I` | Low | Medium — useful for forcing re-transfer |
 | 3 | `--size-only` | Low | Medium — common migration scenario |
-| 4 | `--existing` / `--ignore-existing` | Low | Medium — common sync patterns |
+| 4 | `--ignore-existing` | Low | Medium — common sync patterns |
 | 5 | `--remove-source-files` | Low | High — common for moves/backup |
 | 6 | `--delete-during` | Medium | High — performance improvement |
 | 7 | `--delay-updates` | Medium | High — atomic updates |

src/client/client_cli.c

@@ -11,6 +11,7 @@
 #include <errno.h>
 #include <limits.h>
 #include <stdbool.h>
+#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -59,7 +60,7 @@ static bool parse_positive_int(const char* s, int* out_val) {
 static int set_string_option(char** dest, const char* value, const char* option_name) {
   char* dup = str_dup(value);
   if (!dup) {
-    fprintf(stderr, "Error: memory allocation failed for %s\n", option_name);
+    log_message(LOG_LEVEL_ERROR, "memory allocation failed for %s", option_name);
     return -1;
   }
   free(*dest);
@@ -70,7 +71,7 @@ static int set_string_option(char** dest, const char* value, const char* option_
 /* Parse a string as a positive integer into *dest. Returns true on success, false on error. */
 static int set_positive_int_option(int* dest, const char* value, const char* option_name) {
   if (!parse_positive_int(value, dest)) {
-    fprintf(stderr, "Error: %s must be a positive integer\n", option_name);
+    log_message(LOG_LEVEL_ERROR, "%s must be a positive integer", option_name);
     return -1;
   }
   return 0;
@@ -79,7 +80,7 @@ static int set_positive_int_option(int* dest, const char* value, const char* opt
 /* Parse a string as a non-negative integer into *dest. Returns true on success, false on error. */
 static int set_nonneg_int_option(int* dest, const char* value, const char* option_name) {
   if (!parse_nonneg_int(value, dest)) {
-    fprintf(stderr, "Error: %s must be a non-negative integer\n", option_name);
+    log_message(LOG_LEVEL_ERROR, "%s must be a non-negative integer", option_name);
     return -1;
   }
   return 0;
@@ -87,105 +88,188 @@ static int set_nonneg_int_option(int* dest, const char* value, const char* optio
 
 static int read_patterns_from_file(const char* filepath, char*** patterns, int* count);
 
+/* Parse a string as an unsigned long long. Returns 0 on success, -1 on error. */
+static int parse_ull_arg(const char* val, unsigned long long* out, const char* optname) {
+  char* end;
+  errno = 0;
+  unsigned long long v = strtoull(val, &end, 10);
+  if (errno != 0 || *end != '\0') {
+    log_message(LOG_LEVEL_ERROR, "%s must be a non-negative integer", optname);
+    return -1;
+  }
+  *out = v;
+  return 0;
+}
+
+/* Append a duplicated pattern to a growable pattern array. Returns 0 on success, -1 on error. */
+static int config_add_pattern(char*** patterns, int* count, const char* value,
+                              const char* optname) {
+  char** tmp = realloc(*patterns, (*count + 1) * sizeof(char*));
+  if (!tmp) {
+    log_message(LOG_LEVEL_ERROR, "memory allocation failed for %s", optname);
+    return -1;
+  }
+  *patterns = tmp;
+  char* dup = str_dup(value);
+  if (!dup) {
+    log_message(LOG_LEVEL_ERROR, "memory allocation failed for %s", optname);
+    return -1;
+  }
+  (*patterns)[(*count)++] = dup;
+  return 0;
+}
+
+typedef enum {
+  OPT_FLAG,
+  OPT_STRING,
+  OPT_POS_INT,
+  OPT_NONNEG_INT,
+  OPT_ULL,
+} OptKind;
+
+typedef struct {
+  const char* name;
+  const char* alias;
+  OptKind kind;
+  size_t offset; /* offsetof of the target field in Config */
+} OptionEntry;
+
+/* Options that map directly onto a Config field with no side effects. */
+static const OptionEntry OPTION_TABLE[] = {
+    {"--dry-run", "-n", OPT_FLAG, offsetof(Config, dry_run)},
+    {"--delete", NULL, OPT_FLAG, offsetof(Config, use_delete)},
+    {"--incremental", NULL, OPT_FLAG, offsetof(Config, use_incremental)},
+    {"--delta", NULL, OPT_FLAG, offsetof(Config, use_delta)},
+    {"--save-to-disk", NULL, OPT_FLAG, offsetof(Config, save_to_disk)},
+    {"--progress", NULL, OPT_FLAG, offsetof(Config, show_progress)},
+    {"--tls", NULL, OPT_FLAG, offsetof(Config, use_tls)},
+    {"--backup", NULL, OPT_FLAG, offsetof(Config, backup)},
+    {"--stats", NULL, OPT_FLAG, offsetof(Config, stats)},
+    {"--partial", NULL, OPT_FLAG, offsetof(Config, partial)},
+    {"--links", "-l", OPT_FLAG, offsetof(Config, follow_symlinks)},
+    {"--copy-links", NULL, OPT_FLAG, offsetof(Config, copy_links)},
+    {"--safe-links", NULL, OPT_FLAG, offsetof(Config, safe_links)},
+    {"--copy-unsafe-links", NULL, OPT_FLAG, offsetof(Config, copy_unsafe_links)},
+    {"--sparse", "-S", OPT_FLAG, offsetof(Config, preserve_sparse)},
+    {"--inplace", NULL, OPT_FLAG, offsetof(Config, inplace)},
+    {"--checksum", NULL, OPT_FLAG, offsetof(Config, checksum)},
+    {"--existing", NULL, OPT_FLAG, offsetof(Config, existing)},
+
+    {"--source-dir", NULL, OPT_STRING, offsetof(Config, send_directory)},
+    {"--dest-dir", NULL, OPT_STRING, offsetof(Config, receive_root_directory)},
+    {"--server-host", NULL, OPT_STRING, offsetof(Config, server_host)},
+    {"--cert", NULL, OPT_STRING, offsetof(Config, tls_cert)},
+    {"--key", NULL, OPT_STRING, offsetof(Config, tls_key)},
+    {"--ca", NULL, OPT_STRING, offsetof(Config, tls_ca)},
+    {"--backup-dir", NULL, OPT_STRING, offsetof(Config, backup_dir)},
+    {"--fastsync-server-path", NULL, OPT_STRING, offsetof(Config, fastsync_server_path)},
+    {"--partial-dir", NULL, OPT_STRING, offsetof(Config, partial_dir)},
+    {"--suffix", NULL, OPT_STRING, offsetof(Config, suffix)},
+
+    {"--timeout", NULL, OPT_POS_INT, offsetof(Config, timeout)},
+    {"--contimeout", NULL, OPT_POS_INT, offsetof(Config, contimeout)},
+    {"--max-depth", NULL, OPT_NONNEG_INT, offsetof(Config, max_depth)},
+
+    {"--max-size", NULL, OPT_ULL, offsetof(Config, max_size)},
+    {"--min-size", NULL, OPT_ULL, offsetof(Config, min_size)},
+};
+
+static bool opt_is(const char* arg, const char* name, const char* alias) {
+  return strcmp(arg, name) == 0 || (alias && strcmp(arg, alias) == 0);
+}
+
+static const OptionEntry* find_table_option(const char* arg) {
+  for (size_t i = 0; i < sizeof(OPTION_TABLE) / sizeof(OPTION_TABLE[0]); i++)
+    if (opt_is(arg, OPTION_TABLE[i].name, OPTION_TABLE[i].alias))
+      return &OPTION_TABLE[i];
+  return NULL;
+}
+
+static int apply_table_option(Config* config, const OptionEntry* entry, const char* value) {
+  void* field = (char*)config + entry->offset;
+  switch (entry->kind) {
+  case OPT_FLAG:
+    *(bool*)field = true;
+    return 0;
+  case OPT_STRING:
+    return set_string_option((char**)field, value, entry->name);
+  case OPT_POS_INT:
+    return set_positive_int_option((int*)field, value, entry->name);
+  case OPT_NONNEG_INT:
+    return set_nonneg_int_option((int*)field, value, entry->name);
+  case OPT_ULL: {
+    unsigned long long v;
+    if (parse_ull_arg(value, &v, entry->name) != 0)
+      return -1;
+    *(unsigned long long*)field = v;
+    return 0;
+  }
+  }
+  return -1;
+}
+
 /* Parse CLI arguments into config. Returns 0 on success, -1 on error, 1 for help/clean-exit. */
 int parse_args(Config* config, int argc, char* argv[], int* positional_args,
                int* positional_count) {
   for (int i = 1; i < argc; i++) {
-    if (strcmp(argv[i], "--help") == 0) {
+    const OptionEntry* entry = find_table_option(argv[i]);
+    if (entry) {
+      if (entry->kind != OPT_FLAG) {
+        if (i + 1 >= argc) {
+          log_message(LOG_LEVEL_ERROR, "missing argument for %s", entry->name);
+          return -1;
+        }
+        if (apply_table_option(config, entry, argv[++i]) != 0)
+          return -1;
+      } else if (apply_table_option(config, entry, NULL) != 0) {
+        return -1;
+      }
+      continue;
+    }
+
+    if (opt_is(argv[i], "--help", NULL)) {
       print_usage();
       return 1;
-    } else if (strcmp(argv[i], "-V") == 0 || strcmp(argv[i], "--version") == 0) {
+    } else if (opt_is(argv[i], "-V", "--version")) {
       printf("fastsync version %s\n", PROTOCOL_VERSION);
       return 1;
-    } else if (strcmp(argv[i], "-a") == 0 || strcmp(argv[i], "--archive") == 0) {
+    } else if (opt_is(argv[i], "-a", "--archive")) {
       config->use_compression = true;
       config->use_multithreading = true;
       config->use_metadata = true;
       log_message(LOG_LEVEL_INFO, "Enabled archive mode (-c -m -M)");
-    } else if (strcmp(argv[i], "-n") == 0 || strcmp(argv[i], "--dry-run") == 0) {
+    } else if (opt_is(argv[i], "-p", NULL) && i + 1 < argc) {
-      config->dry_run = true;
-    } else if (strcmp(argv[i], "-p") == 0 && i + 1 < argc) {
       if (set_positive_int_option(&config->ssh_port, argv[++i], "-p") != 0)
         return -1;
       if (config->ssh_port > 65535) {
-        log_message(LOG_LEVEL_ERROR, "SSH port must be 1-65535\n");
+        log_message(LOG_LEVEL_ERROR, "SSH port must be 1-65535");
         return -1;
       }
-    } else if (strcmp(argv[i], "--delete") == 0) {
+    } else if (opt_is(argv[i], "--exclude", NULL) && i + 1 < argc) {
-      config->use_delete = true;
+      if (config_add_pattern(&config->exclude_patterns, &config->exclude_count, argv[++i],
-    } else if (strcmp(argv[i], "--exclude") == 0 && i + 1 < argc) {
+                             "--exclude") != 0)
-      char** tmp = realloc(config->exclude_patterns, (config->exclude_count + 1) * sizeof(char*));
-      if (!tmp) {
-        fprintf(stderr, "Error: memory allocation failed for --exclude\n");
         return -1;
-      }
+    } else if (opt_is(argv[i], "--include", NULL) && i + 1 < argc) {
-      config->exclude_patterns = tmp;
+      if (config_add_pattern(&config->include_patterns, &config->include_count, argv[++i],
-      char* dup = str_dup(argv[++i]);
+                             "--include") != 0)
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --exclude\n");
         return -1;
-      }
+    } else if (opt_is(argv[i], "--delta-block", NULL) && i + 1 < argc) {
-      config->exclude_patterns[config->exclude_count++] = dup;
+      unsigned long long val;
-    } else if (strcmp(argv[i], "--include") == 0 && i + 1 < argc) {
+      if (parse_ull_arg(argv[++i], &val, "--delta-block") != 0)
-      char** tmp = realloc(config->include_patterns, (config->include_count + 1) * sizeof(char*));
-      if (!tmp) {
-        fprintf(stderr, "Error: memory allocation failed for --include\n");
         return -1;
-      }
-      config->include_patterns = tmp;
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --include\n");
-        return -1;
-      }
-      config->include_patterns[config->include_count++] = dup;
-    } else if (strcmp(argv[i], "--max-size") == 0 && i + 1 < argc) {
-      char* end;
-      errno = 0;
-      unsigned long long val = strtoull(argv[++i], &end, 10);
-      if (errno != 0 || *end != '\0') {
-        fprintf(stderr, "Error: --max-size must be a non-negative integer\n");
-        return -1;
-      }
-      config->max_size = val;
-    } else if (strcmp(argv[i], "--min-size") == 0 && i + 1 < argc) {
-      char* end;
-      errno = 0;
-      unsigned long long val = strtoull(argv[++i], &end, 10);
-      if (errno != 0 || *end != '\0') {
-        fprintf(stderr, "Error: --min-size must be a non-negative integer\n");
-        return -1;
-      }
-      config->min_size = val;
-    } else if (strcmp(argv[i], "--incremental") == 0) {
-      config->use_incremental = true;
-    } else if (strcmp(argv[i], "--delta") == 0) {
-      config->use_delta = true;
-    } else if (strcmp(argv[i], "--delta-block") == 0 && i + 1 < argc) {
-      char* end;
-      errno = 0;
-      unsigned long long val = strtoull(argv[++i], &end, 10);
-      if (errno != 0 || *end != '\0') {
-        fprintf(stderr, "Error: --delta-block must be a positive integer\n");
-        return -1;
-      }
       if (val >= DELTA_BLOCK_SIZE_MIN && val <= DELTA_BLOCK_SIZE_MAX)
         config->delta_block_size = (uint32_t)val;
       else
-        fprintf(stderr, "Warning: --delta-block value %llu out of range, using default\n", val);
+        log_message(LOG_LEVEL_WARNING, "--delta-block value %llu out of range, using default", val);
-    } else if (strcmp(argv[i], "--delta-max") == 0 && i + 1 < argc) {
+    } else if (opt_is(argv[i], "--delta-max", NULL) && i + 1 < argc) {
-      char* end;
+      unsigned long long val;
-      errno = 0;
+      if (parse_ull_arg(argv[++i], &val, "--delta-max") != 0)
-      unsigned long long val = strtoull(argv[++i], &end, 10);
-      if (errno != 0 || *end != '\0') {
-        fprintf(stderr, "Error: --delta-max must be a positive integer\n");
         return -1;
-      }
       if (val >= DELTA_MIN_FILE_SIZE)
         config->delta_max_file_size = val;
       else
-        fprintf(stderr, "Warning: --delta-max value %llu too small, using default\n", val);
+        log_message(LOG_LEVEL_WARNING, "--delta-max value %llu too small, using default", val);
-    } else if (strcmp(argv[i], "-c") == 0 || strcmp(argv[i], "-z") == 0) {
+    } else if (opt_is(argv[i], "-c", "-z")) {
       config->use_compression = true;
       log_message(LOG_LEVEL_INFO, "Enabled Compression");
       if (i + 1 < argc) {
@@ -193,7 +277,7 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
         long level = strtol(argv[i + 1], &end_ptr, 10);
         if (*end_ptr == '\0') {
           if (level < 1 || level > 22) {
-            fprintf(stderr, "Error: compression level must be 1-22\n");
+            log_message(LOG_LEVEL_ERROR, "compression level must be 1-22");
             return -1;
           }
           config->compression_level = (int)level;
@@ -201,91 +285,51 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
           i++;
         }
       }
-    } else if (strcmp(argv[i], "--source-dir") == 0 && i + 1 < argc) {
+    } else if (opt_is(argv[i], "-M", "--preserve")) {
-      if (set_string_option(&config->send_directory, argv[++i], "--source-dir") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--dest-dir") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->receive_root_directory, argv[++i], "--dest-dir") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--save-to-disk") == 0) {
-      config->save_to_disk = true;
-    } else if (strcmp(argv[i], "-M") == 0 || strcmp(argv[i], "--preserve") == 0) {
       config->use_metadata = true;
       log_message(LOG_LEVEL_INFO, "Enabled metadata preservation");
-    } else if (strcmp(argv[i], "-f") == 0 || strcmp(argv[i], "--sendfile") == 0) {
+    } else if (opt_is(argv[i], "-f", "--sendfile")) {
       config->use_sendfile = true;
       log_message(LOG_LEVEL_INFO, "Enabled sendfile");
-    } else if (strcmp(argv[i], "-m") == 0) {
+    } else if (opt_is(argv[i], "-m", NULL)) {
       config->use_multithreading = true;
       log_message(LOG_LEVEL_INFO, "Enabled Multithreading");
-    } else if (strcmp(argv[i], "-s") == 0) {
+    } else if (opt_is(argv[i], "-s", NULL)) {
       config->use_chunk_serialization = true;
       log_message(LOG_LEVEL_INFO, "Enabled Chunk Serialization");
-    } else if (strcmp(argv[i], "--server-host") == 0 && i + 1 < argc) {
+    } else if (opt_is(argv[i], "--server-port", NULL) && i + 1 < argc) {
-      if (set_string_option(&config->server_host, argv[++i], "--server-host") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--server-port") == 0 && i + 1 < argc) {
       if (!parse_positive_int(argv[++i], &config->server_port)) {
-        fprintf(stderr, "Error: invalid --server-port value: %s\n", argv[i]);
+        log_message(LOG_LEVEL_ERROR, "invalid --server-port value: %s", argv[i]);
         return -1;
       }
       if (config->server_port > 65535) {
-        fprintf(stderr, "Error: server port must be 1-65535\n");
+        log_message(LOG_LEVEL_ERROR, "server port must be 1-65535");
         return -1;
       }
-    } else if (strcmp(argv[i], "--bwlimit") == 0 && i + 1 < argc) {
+    } else if (opt_is(argv[i], "--bwlimit", NULL) && i + 1 < argc) {
-      char* end;
+      unsigned long long kbps;
-      errno = 0;
+      if (parse_ull_arg(argv[++i], &kbps, "--bwlimit") != 0)
-      unsigned long long kbps = strtoull(argv[++i], &end, 10);
+        return -1;
-      if (errno != 0 || *end != '\0' || kbps == 0) {
+      if (kbps == 0) {
-        fprintf(stderr, "Error: --bwlimit must be a positive integer\n");
+        log_message(LOG_LEVEL_ERROR, "--bwlimit must be a positive integer");
         return -1;
       }
       if (kbps > ULLONG_MAX / 1024) {
-        fprintf(stderr, "Error: --bwlimit value too large\n");
+        log_message(LOG_LEVEL_ERROR, "--bwlimit value too large");
         return -1;
       }
       io_set_bwlimit(kbps * 1024);
       log_message(LOG_LEVEL_INFO, "Set bandwidth limit to %llu KB/s", kbps);
-    } else if (strcmp(argv[i], "--progress") == 0) {
+    } else if (opt_is(argv[i], "--chunk-size", NULL) && i + 1 < argc) {
-      config->show_progress = true;
+      unsigned long long val;
-    } else if (strcmp(argv[i], "--chunk-size") == 0 && i + 1 < argc) {
+      if (parse_ull_arg(argv[++i], &val, "--chunk-size") != 0)
-      char* end;
+        return -1;
-      errno = 0;
+      if (val == 0) {
-      unsigned long long val = strtoull(argv[++i], &end, 10);
+        log_message(LOG_LEVEL_ERROR, "--chunk-size must be a positive integer");
-      if (errno != 0 || *end != '\0' || val == 0) {
-        fprintf(stderr, "Error: --chunk-size must be a positive integer\n");
         return -1;
       }
       config->chunk_size = val;
-    } else if (strcmp(argv[i], "--tls") == 0) {
+    } else if (opt_is(argv[i], "--log-file", NULL) && i + 1 < argc) {
-      config->use_tls = true;
-    } else if (strcmp(argv[i], "--cert") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->tls_cert, argv[++i], "--cert") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--key") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->tls_key, argv[++i], "--key") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--ca") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->tls_ca, argv[++i], "--ca") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--timeout") == 0 && i + 1 < argc) {
-      if (set_positive_int_option(&config->timeout, argv[++i], "--timeout") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--contimeout") == 0 && i + 1 < argc) {
-      if (set_positive_int_option(&config->contimeout, argv[++i], "--contimeout") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--backup") == 0) {
-      config->backup = true;
-    } else if (strcmp(argv[i], "--backup-dir") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->backup_dir, argv[++i], "--backup-dir") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--stats") == 0) {
-      config->stats = true;
-    } else if (strcmp(argv[i], "--max-depth") == 0 && i + 1 < argc) {
-      if (set_nonneg_int_option(&config->max_depth, argv[++i], "--max-depth") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--log-file") == 0 && i + 1 < argc) {
       if (config->log_file) {
         fclose(config->log_file);
         config->log_file = NULL;
@@ -293,55 +337,29 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
       }
       FILE* lf = fopen(argv[++i], "a");
       if (!lf) {
-        fprintf(stderr, "Error: could not open log file '%s': %s\n", argv[i], strerror(errno));
+        log_message(LOG_LEVEL_ERROR, "could not open log file '%s': %s", argv[i], strerror(errno));
         return -1;
       }
       config->log_file = lf;
       log_set_file(lf);
-    } else if (strcmp(argv[i], "--exclude-from") == 0 && i + 1 < argc) {
+    } else if (opt_is(argv[i], "--exclude-from", NULL) && i + 1 < argc) {
       if (read_patterns_from_file(argv[++i], &config->exclude_patterns, &config->exclude_count) !=
           0)
         return -1;
-    } else if (strcmp(argv[i], "--include-from") == 0 && i + 1 < argc) {
+    } else if (opt_is(argv[i], "--include-from", NULL) && i + 1 < argc) {
       if (read_patterns_from_file(argv[++i], &config->include_patterns, &config->include_count) !=
           0)
         return -1;
-    } else if (strcmp(argv[i], "--partial") == 0) {
+    } else if (opt_is(argv[i], "-v", "--verbose")) {
-      config->partial = true;
-    } else if (strcmp(argv[i], "--fastsync-server-path") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->fastsync_server_path, argv[++i], "--fastsync-server-path") !=
-          0)
-        return -1;
-    } else if (strcmp(argv[i], "-v") == 0 || strcmp(argv[i], "--verbose") == 0) {
       set_log_level(LOG_LEVEL_DEBUG);
-    } else if (strcmp(argv[i], "-l") == 0 || strcmp(argv[i], "--links") == 0) {
+    } else if (opt_is(argv[i], "-T", NULL) && i + 1 < argc) {
-      config->follow_symlinks = true;
-    } else if (strcmp(argv[i], "--copy-links") == 0) {
-      config->copy_links = true;
-    } else if (strcmp(argv[i], "--safe-links") == 0) {
-      config->safe_links = true;
-    } else if (strcmp(argv[i], "--copy-unsafe-links") == 0) {
-      config->copy_unsafe_links = true;
-    } else if (strcmp(argv[i], "-S") == 0 || strcmp(argv[i], "--sparse") == 0) {
-      config->preserve_sparse = true;
-    } else if (strcmp(argv[i], "--inplace") == 0) {
-      config->inplace = true;
-    } else if (strcmp(argv[i], "--partial-dir") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->partial_dir, argv[++i], "--partial-dir") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "--suffix") == 0 && i + 1 < argc) {
-      if (set_string_option(&config->suffix, argv[++i], "--suffix") != 0)
-        return -1;
-    } else if (strcmp(argv[i], "-T") == 0 && i + 1 < argc) {
       if (set_positive_int_option(&config->timeout, argv[++i], "-T") != 0)
         return -1;
-    } else if (strcmp(argv[i], "--checksum") == 0) {
+    } else if (opt_is(argv[i], "--compress-level", NULL) && i + 1 < argc) {
-      config->checksum = true;
-    } else if (strcmp(argv[i], "--compress-level") == 0 && i + 1 < argc) {
       if (set_positive_int_option(&config->compression_level, argv[++i], "--compress-level") != 0)
         return -1;
       if (config->compression_level < 1 || config->compression_level > 22) {
-        fprintf(stderr, "Error: --compress-level must be between 1 and 22\n");
+        log_message(LOG_LEVEL_ERROR, "--compress-level must be between 1 and 22");
         return -1;
       }
     } else if (argv[i][0] == '-') {
@@ -364,7 +382,7 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
 static int read_patterns_from_file(const char* filepath, char*** patterns, int* count) {
   FILE* fp = fopen(filepath, "r");
   if (!fp) {
-    fprintf(stderr, "Error: could not open pattern file '%s': %s\n", filepath, strerror(errno));
+    log_message(LOG_LEVEL_ERROR, "could not open pattern file '%s': %s", filepath, strerror(errno));
     return -1;
   }
   char* line = NULL;
@@ -381,22 +399,11 @@ static int read_patterns_from_file(const char* filepath, char*** patterns, int*
       p[--len] = '\0';
     if (len == 0)
       continue;
-    char** tmp = realloc(*patterns, (*count + 1) * sizeof(char*));
+    if (config_add_pattern(patterns, count, p, "pattern file") != 0) {
-    if (!tmp) {
-      fprintf(stderr, "Error: memory allocation failed for pattern file\n");
       free(line);
       fclose(fp);
       return -1;
     }
-    *patterns = tmp;
-    char* dup = str_dup(p);
-    if (!dup) {
-      fprintf(stderr, "Error: memory allocation failed for pattern file\n");
-      free(line);
-      fclose(fp);
-      return -1;
-    }
-    (*patterns)[(*count)++] = dup;
   }
   free(line);
   fclose(fp);
@@ -411,10 +418,9 @@ int main(int argc, char* argv[]) {
   parse_environment(&env_source, &env_dest, &save_to_disk);
 
   int exit_code = 0;
-  bool config_owned_by_pipeline = false;
   Config* config = config_create();
   if (!config) {
-    fprintf(stderr, "Error: failed to allocate config\n");
+    log_message(LOG_LEVEL_ERROR, "failed to allocate config");
     return 1;
   }
   config->save_to_disk = save_to_disk;
@@ -435,20 +441,20 @@ int main(int argc, char* argv[]) {
     free(config->receive_root_directory);
     config->send_directory = str_dup(argv[positional_args[0]]);
     if (!config->send_directory) {
-      fprintf(stderr, "Error: memory allocation failed\n");
+      log_message(LOG_LEVEL_ERROR, "memory allocation failed");
       exit_code = 1;
       goto cleanup;
     }
     config->receive_root_directory = str_dup(argv[positional_args[1]]);
     if (!config->receive_root_directory) {
-      fprintf(stderr, "Error: memory allocation failed\n");
+      log_message(LOG_LEVEL_ERROR, "memory allocation failed");
       exit_code = 1;
       goto cleanup;
     }
     config->save_to_disk = true;
     config_parse_ssh_dest(config);
   } else if (positional_count == 1) {
-    fprintf(stderr, "Error: missing destination argument\n");
+    log_message(LOG_LEVEL_ERROR, "missing destination argument");
     print_usage();
     exit_code = 1;
     goto cleanup;
@@ -456,7 +462,7 @@ int main(int argc, char* argv[]) {
     if (!config->send_directory && env_source) {
       config->send_directory = str_dup(env_source);
       if (!config->send_directory) {
-        fprintf(stderr, "Error: memory allocation failed\n");
+        log_message(LOG_LEVEL_ERROR, "memory allocation failed");
         exit_code = 1;
         goto cleanup;
       }
@@ -464,7 +470,7 @@ int main(int argc, char* argv[]) {
     if (!config->receive_root_directory && env_dest) {
       config->receive_root_directory = str_dup(env_dest);
       if (!config->receive_root_directory) {
-        fprintf(stderr, "Error: memory allocation failed\n");
+        log_message(LOG_LEVEL_ERROR, "memory allocation failed");
         exit_code = 1;
         goto cleanup;
       }
@@ -494,17 +500,13 @@ int main(int argc, char* argv[]) {
 
   /* Execute transfer */
   if (config->use_multithreading) {
-    config_owned_by_pipeline = true;
+    exit_code = send_files_multithreaded(&config);
-    exit_code = send_files_multithreaded(config);
   } else {
     exit_code = send_files(config);
   }
 
 cleanup:
   if (config) {
-    if (config->log_file)
-      fclose(config->log_file);
-    if (!config_owned_by_pipeline)
     config_delete(config);
   }
   return exit_code;

src/client/client_send.c

@@ -42,7 +42,7 @@ static ScannerOptions scanner_options_from_config(const Config* config, int num_
 static Client* connect_transfer_client(const Config* config) {
   if (config->transport == TRANSPORT_SSH) {
     if (config->use_sendfile) {
-      fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
+      log_message(LOG_LEVEL_ERROR, "-f/--sendfile is not supported with SSH transport");
       return NULL;
     }
     return client_connect_ssh(config->ssh_destination, config->ssh_port,
@@ -60,6 +60,7 @@ static Client* connect_transfer_client(const Config* config) {
     connected = client_connect(client, config->server_host, config->server_port);
   }
   if (!connected) {
+    client_disconnect(client);
     client_delete(client);
     return NULL;
   }
@@ -149,6 +150,8 @@ static int send_dry_run_manifest(const Config* config) {
 
 /* Send the delete manifest (list of files) to the server. Returns 0 on success, -1 on failure. */
 static int send_delete_manifest(int fd, ArrayList* manifest) {
+  if (!manifest)
+    return -1;
   if (!send_status(fd, STATUS_MANIFEST))
     return -1;
   if (!send_int(fd, manifest->size))
@@ -190,17 +193,22 @@ static int incremental_check(Client* client, File* file, const Config* config,
     return 1;
   if (s == STATUS_DELTA_SIGNATURE) {
     Data* sig_data = receive_data(client->file_descriptor);
-    if (!sig_data)
+    if (!sig_data) {
+      send_status(client->file_descriptor, STATUS_ERROR);
       return -1;
+    }
     DeltaSignature* sig = delta_signature_deserialize(sig_data);
     data_destroy(sig_data);
-    if (!sig)
+    if (!sig) {
+      send_status(client->file_descriptor, STATUS_ERROR);
       return -1;
+    }
     *out_sig = sig;
     return 2;
   }
   if (s != STATUS_NEXT) {
     log_message(LOG_LEVEL_ERROR, "Unexpected server status");
+    send_status(client->file_descriptor, STATUS_ERROR);
     return -1;
   }
   return 0;
@@ -208,8 +216,10 @@ static int incremental_check(Client* client, File* file, const Config* config,
 
 static int send_delta(Client* client, File* file, DeltaSignature* sig, Config* config) {
   Delta* delta = delta_compute(file->data->data, file->data->size, sig, config->delta_block_size);
+  /* The receiver is blocked after sending the signature.  Every local
+     fallback therefore needs the explicit NEXT response before full data. */
   if (!delta)
-    return 1;
+    return send_status(client->file_descriptor, STATUS_NEXT) ? 1 : -1;
 
   if (!delta_is_worthwhile(delta, file->data->size)) {
     delta_destroy(delta);
@@ -221,14 +231,14 @@ static int send_delta(Client* client, File* file, DeltaSignature* sig, Config* c
   Data* delta_data = delta_serialize(delta);
   delta_destroy(delta);
   if (!delta_data)
-    return -1;
+    return send_status(client->file_descriptor, STATUS_NEXT) ? 1 : -1;
 
   Data* to_send = delta_data;
   if (config->use_compression) {
     to_send = data_compress(delta_data, config->compression_level);
     data_destroy(delta_data);
     if (!to_send)
-      return -1;
+      return send_status(client->file_descriptor, STATUS_NEXT) ? 1 : -1;
   }
 
   bool ok = send_status(client->file_descriptor, STATUS_DELTA_DATA) &&
@@ -358,7 +368,8 @@ int send_chunk(Client* client, Chunk* chunk, Config* config) {
     if (f == NULL)
       continue;
     bool stream = f->data->data == NULL && f->data->size > 0;
-    bool use_sendfile = (config->use_sendfile && !config->use_compression) || stream;
+    bool use_sendfile =
+        (config->use_sendfile && !config->use_compression) || (stream && !config->use_compression);
     int rc = send_single_file(client, f, config, config->use_incremental, use_sendfile);
     if (rc == 1)
       continue;
@@ -373,8 +384,9 @@ static int send_chunks_multithreaded(void* pipeline_context) {
   Client* client = connect_transfer_client(context->config);
   if (!client) {
     if (context->config->transport == TRANSPORT_TCP)
-      fprintf(stderr, "Error: could not connect to server%s\n",
+      log_message(LOG_LEVEL_ERROR, "could not connect to server%s",
                   context->config->use_tls ? " via TLS" : "");
+    pipeline_cancel(context);
     mark_sender_done(context);
     return thrd_error;
   }
@@ -383,6 +395,7 @@ static int send_chunks_multithreaded(void* pipeline_context) {
   protocol_session_set_ssl(&session, (SSL*)client->ssl);
   protocol_session_bind(&session);
   if (!config_send(client->file_descriptor, context->config)) {
+    pipeline_cancel(context);
     disconnect_transfer_client(client);
     mark_sender_done(context);
     protocol_session_unbind();
@@ -394,6 +407,13 @@ static int send_chunks_multithreaded(void* pipeline_context) {
         context->queue_loader, &context->mutex_loader, &context->condition_not_empty_loader,
         &context->condition_not_full_loader, &context->loader_done);
     if (current_chunk == NULL) {
+      if (atomic_load(&context->cancelled)) {
+        pipeline_cancel(context);
+        disconnect_transfer_client(client);
+        mark_sender_done(context);
+        protocol_session_unbind();
+        return thrd_error;
+      }
       if (context->config->use_delete) {
         if (send_delete_manifest(client->file_descriptor, context->manifest) != 0)
           goto send_fail;
@@ -412,7 +432,7 @@ static int send_chunks_multithreaded(void* pipeline_context) {
       return thrd_error;
     }
     if (send_chunk(client, current_chunk, context->config) != 0) {
-      fprintf(stderr, "Error: unexpected error while sending chunk\n");
+      log_message(LOG_LEVEL_ERROR, "unexpected error while sending chunk");
       chunk_destroy(current_chunk);
       pipeline_cancel(context);
       disconnect_transfer_client(client);
@@ -503,12 +523,13 @@ static int load_files_multithreaded(void* pipeline_context) {
     if (!context->config->use_sendfile) {
       for (int i = 0; i < chunk->element_count; i++) {
         File* f = chunk->items[i];
-        if (f->data->size > STREAM_THRESHOLD)
+        if (f->data->size > STREAM_THRESHOLD && !context->config->use_compression)
           continue;
         if (!file_load_data(f)) {
-          log_message(LOG_LEVEL_ERROR, "Failed to load file data, skipping");
+          log_message(LOG_LEVEL_ERROR, "Failed to load file data");
-          file_destroy(f);
+          chunk_destroy(chunk);
-          chunk->items[i] = NULL;
+          pipeline_cancel(context);
+          return thrd_error;
         }
       }
     }
@@ -517,14 +538,23 @@ static int load_files_multithreaded(void* pipeline_context) {
                                             &context->condition_not_full_loader,
                                             &context->cancelled)) {
       chunk_destroy(chunk);
-      atomic_store(&context->cancelled, true);
+      pipeline_cancel(context);
-      cnd_broadcast(&context->condition_not_full_loader);
-      cnd_broadcast(&context->condition_not_empty_loader);
       return thrd_error;
     }
   }
 }
 
+/* Print a one-line transfer progress report to stderr. `suffix` ends the
+   line (e.g. "Done.\n") or is "" for in-place refresh. Shared by the
+   single-threaded loop and the multithreaded progress thread. */
+static void print_transfer_progress(unsigned long long total_bytes, time_t start,
+                                    const char* suffix) {
+  double elapsed = difftime(time(NULL), start);
+  double rate = elapsed > 0.0 ? total_bytes / (1048576.0 * elapsed) : 0.0;
+  fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  %s", total_bytes / 1048576.0, rate, suffix);
+  fflush(stderr);
+}
+
 /* Progress-reporting thread for multithreaded send. Runs in parallel with
    the scanner/loader/sender threads and prints periodic progress to stderr. */
 static int progress_thread_fn(void* arg) {
@@ -539,20 +569,14 @@ static int progress_thread_fn(void* arg) {
     mtx_unlock(&context->mutex_progress);
 
     if (done) {
-      time_t now = time(NULL);
+      print_transfer_progress(total, start, "Done.\n");
-      double elapsed = difftime(now, start);
-      double rate = elapsed > 0.0 ? total / (1048576.0 * elapsed) : 0.0;
-      fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  Done.\n", total / 1048576.0, rate);
       break;
     }
 
     time_t now = time(NULL);
     if (now - last_progress >= 1) {
       last_progress = now;
-      double elapsed = difftime(now, start);
+      print_transfer_progress(total, start, "");
-      double rate = elapsed > 0.0 ? total / (1048576.0 * elapsed) : 0.0;
-      fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  ", total / 1048576.0, rate);
-      fflush(stderr);
     }
 
     struct timespec ts = {0, 100 * 1000000L}; /* 100 ms */
@@ -568,36 +592,29 @@ int send_files(Config* config) {
   Client* client = connect_transfer_client(config);
   if (!client) {
     if (config->transport == TRANSPORT_TCP)
-      fprintf(stderr, "Error: could not connect to server%s\n", config->use_tls ? " via TLS" : "");
+      log_message(LOG_LEVEL_ERROR, "could not connect to server%s",
+                  config->use_tls ? " via TLS" : "");
     return 1;
   }
   ProtocolSession session;
   protocol_session_init(&session, client->file_descriptor, client->file_descriptor);
   protocol_session_set_ssl(&session, (SSL*)client->ssl);
   protocol_session_bind(&session);
-  if (!config_send(client->file_descriptor, config)) {
+  int ret = 1;
-    disconnect_transfer_client(client);
+  DirectoryScanner* scanner = NULL;
-    protocol_session_unbind();
+  ArrayList* manifest = NULL;
-    return 1;
+  if (!config_send(client->file_descriptor, config))
-  }
+    goto send_fail;
   ScannerOptions scanner_options = scanner_options_from_config(config, 0);
-  DirectoryScanner* scanner =
+  scanner = directory_scanner_create_with_options(config->send_directory, &scanner_options);
-      directory_scanner_create_with_options(config->send_directory, &scanner_options);
+  manifest = create_transfer_manifest(config);
+  if (!scanner || (config->use_delete && !manifest))
+    goto send_fail;
   Chunk* current_chunk;
   unsigned long long total_bytes = 0;
   int total_files = 0;
   time_t last_progress = 0;
   time_t start = time(NULL);
-  ArrayList* manifest = create_transfer_manifest(config);
-  if (!scanner || (config->use_delete && !manifest)) {
-    if (scanner)
-      directory_scanner_destroy(scanner);
-    if (manifest)
-      array_list_delete(manifest);
-    disconnect_transfer_client(client);
-    protocol_session_unbind();
-    return 1;
-  }
   while ((current_chunk = directory_scanner_next(scanner)) != NULL) {
     unsigned long long chunk_bytes = 0;
     for (int i = 0; i < current_chunk->element_count; i++) {
@@ -609,15 +626,21 @@ int send_files(Config* config) {
       goto send_fail;
     }
     if (!config->use_sendfile) {
+      bool load_ok = true;
       for (int i = 0; i < current_chunk->element_count; i++) {
         File* f = current_chunk->items[i];
-        if (f->data->size > STREAM_THRESHOLD)
+        if (f->data->size > STREAM_THRESHOLD && !config->use_compression)
           continue;
         if (!file_load_data(f)) {
           log_message(LOG_LEVEL_ERROR, "Failed to load file data");
-          continue;
+          load_ok = false;
+          break;
         }
       }
+      if (!load_ok) {
+        chunk_destroy(current_chunk);
+        goto send_fail;
+      }
     }
     if (send_chunk(client, current_chunk, config) != 0) {
       log_message(LOG_LEVEL_ERROR, "Failed to send chunk");
@@ -632,10 +655,7 @@ int send_files(Config* config) {
       time_t now = time(NULL);
       if (now - last_progress >= 1) {
         last_progress = now;
-        double elapsed = difftime(now, start);
+        print_transfer_progress(total_bytes, start, "");
-        double rate = elapsed > 0 ? total_bytes / (1048576.0 * elapsed) : 0;
-        fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  ", total_bytes / 1048576.0, rate);
-        fflush(stderr);
       }
     }
     chunk_destroy(current_chunk);
@@ -645,37 +665,39 @@ int send_files(Config* config) {
   if (config->use_delete) {
     if (send_delete_manifest(client->file_descriptor, manifest) != 0) {
       array_list_delete(manifest);
+      manifest = NULL;
       goto send_fail;
     }
     array_list_delete(manifest);
     manifest = NULL;
   }
   bool ok = finalize_transfer(client);
-  double elapsed_total = difftime(time(NULL), start);
+  if (config->show_progress)
-  if (config->show_progress) {
+    print_transfer_progress(total_bytes, start, "Done.\n");
-    double rate = elapsed_total > 0 ? total_bytes / (1048576.0 * elapsed_total) : 0;
-    fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  Done.\n", total_bytes / 1048576.0, rate);
-  }
   if (config->stats) {
+    double elapsed_total = difftime(time(NULL), start);
     double rate = elapsed_total > 0 ? total_bytes / (1048576.0 * elapsed_total) : 0;
     fprintf(stderr, "Stats: %d files, %.1f MB, %.1f MB/s\n", total_files, total_bytes / 1048576.0,
             rate);
   }
-  directory_scanner_destroy(scanner);
+  ret = ok ? 0 : 1;
-  disconnect_transfer_client(client);
-  protocol_session_unbind();
-  return ok ? 0 : 1;
 
 send_fail:
+  /* Single cleanup path for all exits. The manifest is intentionally deleted
+     here even on success without --delete, fixing a pre-existing leak. */
   if (manifest)
     array_list_delete(manifest);
+  if (scanner)
     directory_scanner_destroy(scanner);
   disconnect_transfer_client(client);
   protocol_session_unbind();
-  return 1;
+  return ret;
 }
 
-int send_files_multithreaded(Config* config) {
+int send_files_multithreaded(Config** config_ptr) {
+  if (!config_ptr || !*config_ptr)
+    return 1;
+  Config* config = *config_ptr;
   if (config->dry_run)
     return send_dry_run_manifest(config);
 
@@ -706,8 +728,13 @@ int send_files_multithreaded(Config* config) {
     queue_destroy(q2);
     return 1;
   }
+  *config_ptr = NULL; /* context now owns config through all remaining paths */
   if (config->use_delete)
-    context->manifest = create_transfer_manifest(config);
+    context->manifest = array_list_create(free);
+  if (config->use_delete && !context->manifest) {
+    pipeline_context_sender_destroy(context);
+    return 1;
+  }
 
   thrd_t scanner, loader, sender;
   bool scanner_created = false;
@@ -721,7 +748,7 @@ int send_files_multithreaded(Config* config) {
     sender_created = (thrd_create(&sender, send_chunks_multithreaded, context) == thrd_success);
 
   if (!scanner_created || !loader_created || !sender_created) {
-    perror("Error creating threads.\n");
+    log_perror("Error creating threads");
     pipeline_cancel(context);
     mtx_lock(&context->mutex_progress);
     context->sender_done = true;
@@ -741,7 +768,7 @@ int send_files_multithreaded(Config* config) {
   if (config->show_progress) {
     progress_created = (thrd_create(&progress, progress_thread_fn, context) == thrd_success);
     if (!progress_created) {
-      perror("Error creating progress thread.\n");
+      log_perror("Error creating progress thread");
       /* Non-fatal; continue without progress reporting */
     }
   }

src/client/client_send.h

@@ -7,6 +7,7 @@
 
 int send_chunk(Client* client, Chunk* chunk, Config* config);
 int send_files(Config* config);
-int send_files_multithreaded(Config* config);
+/* Takes ownership only when *config is set to NULL on return. */
+int send_files_multithreaded(Config** config);
 
 #endif

src/client/client_validation.c

@@ -1,37 +1,38 @@
 #include "client_validation.h"
+#include "log.h"
 #include "usage.h"
 #include <stdio.h>
 
 /* Validate config after parsing. Returns true if valid. */
 bool validate_config(const Config* config) {
   if (!config->send_directory || !config->receive_root_directory) {
-    fprintf(stderr, "Error: source and destination directories are required\n");
+    log_message(LOG_LEVEL_ERROR, "source and destination directories are required");
     print_usage();
     return false;
   }
   if (config->use_sendfile && (config->use_chunk_serialization || config->use_compression)) {
-    fprintf(stderr, "Error: -f/--sendfile cannot be combined with -c (compression) or -s (chunk "
+    log_message(LOG_LEVEL_ERROR, "-f/--sendfile cannot be combined with -c (compression) or -s "
-                    "serialization)\n");
+                                 "(chunk serialization)");
     return false;
   }
   if (config->transport == TRANSPORT_SSH && config->use_sendfile) {
-    fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
+    log_message(LOG_LEVEL_ERROR, "-f/--sendfile is not supported with SSH transport");
     return false;
   }
   if (config->use_incremental && config->use_chunk_serialization) {
-    fprintf(stderr, "Error: --incremental is not supported with -s (chunk serialization)\n");
+    log_message(LOG_LEVEL_ERROR, "--incremental is not supported with -s (chunk serialization)");
     return false;
   }
   if (config->use_delta && !config->use_incremental) {
-    fprintf(stderr, "Error: --delta requires --incremental\n");
+    log_message(LOG_LEVEL_ERROR, "--delta requires --incremental");
     return false;
   }
   if (config->use_delta && config->use_chunk_serialization) {
-    fprintf(stderr, "Error: --delta cannot be combined with -s (chunk serialization)\n");
+    log_message(LOG_LEVEL_ERROR, "--delta cannot be combined with -s (chunk serialization)");
     return false;
   }
   if (config->use_delta && config->use_sendfile) {
-    fprintf(stderr, "Error: --delta cannot be combined with -f (sendfile)\n");
+    log_message(LOG_LEVEL_ERROR, "--delta cannot be combined with -f (sendfile)");
     return false;
   }
   if (config->append || config->append_verify) {
@@ -41,8 +42,8 @@ bool validate_config(const Config* config) {
     return false;
   }
   if (config->use_tls) {
-    if (!config->tls_cert || !config->tls_key) {
+    if (!config->tls_cert || !config->tls_key || !config->tls_ca) {
-      fprintf(stderr, "Error: --tls requires --cert and --key\n");
+      log_message(LOG_LEVEL_ERROR, "--tls requires --cert, --key, and --ca");
       return false;
     }
   }

src/client/scanner.c

@@ -1,3 +1,4 @@
+#include "log.h"
 #include "scanner.h"
 #include "array_list.h"
 #include "chunk.h"
@@ -226,7 +227,7 @@ static int open_next_directory(DirectoryScanner* scanner) {
   free(de);
   scanner->current_dir = opendir(scanner->current_path);
   if (scanner->current_dir == NULL) {
-    perror("Could not open directory");
+    log_perror("Could not open directory");
     free(scanner->current_path);
     scanner->current_path = NULL;
     scanner->failed = true;
@@ -363,6 +364,8 @@ static int parallel_worker_thread(void* arg) {
       cnd_broadcast(&wa->ps->result_not_empty);
       cnd_broadcast(&wa->ps->result_not_full);
       mtx_unlock(&wa->ps->result_mutex);
+      for (int j = i; j < wa->dir_count; j++)
+        free(wa->dirs[j]);
       break;
     }
     Chunk* chunk;
@@ -398,18 +401,23 @@ static int parallel_worker_thread(void* arg) {
   return thrd_success;
 }
 
-ParallelScanner* parallel_scanner_create_with_options(const char* root_directory,
+static void parallel_scanner_creation_failed(ParallelScanner* ps) {
-                                                      const ScannerOptions* options) {
+  mtx_lock(&ps->result_mutex);
-  if (!root_directory || !options)
+  ps->failed = true;
-    return NULL;
+  atomic_store(&ps->cancelled, true);
-  ParallelScanner* ps = calloc(1, sizeof(ParallelScanner));
+  ps->expected_threads = ps->created_threads;
-  if (!ps)
+  if (ps->completed >= ps->expected_threads)
-    return NULL;
+    ps->done = true;
+  cnd_broadcast(&ps->result_not_empty);
+  cnd_broadcast(&ps->result_not_full);
+  mtx_unlock(&ps->result_mutex);
+}
+
+/* Initialize result queue and synchronization primitives. Returns true on success. */
+static bool parallel_scanner_init(ParallelScanner* ps) {
   ps->result_queue = queue_create(100, chunk_destroy);
-  if (!ps->result_queue) {
+  if (!ps->result_queue)
-    free(ps);
+    return false;
-    return NULL;
-  }
   atomic_init(&ps->cancelled, false);
   int init = 0;
   bool ok = true;
@@ -434,93 +442,37 @@ ParallelScanner* parallel_scanner_create_with_options(const char* root_directory
     if (init >= 1)
       mtx_destroy(&ps->result_mutex);
     queue_destroy(ps->result_queue);
-    free(ps);
+    ps->result_queue = NULL;
-    return NULL;
+    return false;
   }
+  return true;
+}
 
-  DIR* dir = opendir(root_directory);
+/* Split files into chunks of roughly chunk_size bytes. Returns the first chunk (also stored
-  if (!dir) {
+ * chunks beyond the first are enqueued on `queue`). Nulls out consumed entries in `files`.
-    perror("Could not open root directory for parallel scan");
+ * Sets *failed on allocation/enqueue errors. */
-    parallel_scanner_destroy(ps);
+static Chunk* batch_files(ArrayList* files, unsigned long long chunk_size, Queue* queue,
+                          bool* failed) {
+  Chunk* first = NULL;
+  if (files->size <= 0)
     return NULL;
-  }
-
-  ArrayList* root_files = array_list_create(file_destroy);
-  ArrayList* subdirs = array_list_create(free);
-  if (!root_files || !subdirs) {
-    array_list_delete(root_files);
-    array_list_delete(subdirs);
-    closedir(dir);
-    parallel_scanner_destroy(ps);
-    return NULL;
-  }
-  const struct dirent* entry;
-  while ((entry = readdir(dir)) != NULL) {
-    if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
-      continue;
-    ScannerEntry inspected;
-    int inspection =
-        scanner_inspect_entry(options, root_directory, root_directory, entry->d_name, &inspected);
-    if (inspection < 0) {
-      ps->failed = true;
-      continue;
-    }
-    if (inspection == 0)
-      continue;
-    char* cur_path = inspected.path;
-    struct stat st = inspected.stats;
-    if (inspected.is_directory) {
-      if (!array_list_add(subdirs, cur_path)) {
-        free(cur_path);
-        ps->failed = true;
-      }
-    } else {
-      File* file = file_create(cur_path);
-      free(cur_path);
-      if (!file) {
-        ps->failed = true;
-        continue;
-      }
-      file->data->size = st.st_size;
-      if (options->use_metadata)
-        file->metadata = file_metadata_create(&st);
-      if (options->use_metadata && !file->metadata) {
-        file_destroy(file);
-        ps->failed = true;
-        continue;
-      }
-      if (!array_list_add(root_files, file)) {
-        file_destroy(file);
-        ps->failed = true;
-      }
-    }
-  }
-  closedir(dir);
-
-  unsigned long long cs = options->chunk_size > 0 ? options->chunk_size : DESIRED_CHUNK_SIZE;
-  if (root_files->size > 0) {
   ArrayList* batch = array_list_create(NULL);
   if (!batch) {
-      ps->failed = true;
+    *failed = true;
-      array_list_delete(root_files);
-      array_list_delete(subdirs);
-      parallel_scanner_destroy(ps);
     return NULL;
   }
   unsigned long long batch_size = 0;
-    Chunk* first = NULL;
+  for (int i = 0; i < files->size; i++) {
-    for (int i = 0; i < root_files->size; i++) {
+    File* f = (File*)files->items[i];
-      File* f = (File*)root_files->items[i];
     if (!array_list_add(batch, f)) {
-        ps->failed = true;
+      *failed = true;
       break;
     }
     batch_size += f->data->size;
-      if (batch_size >= cs || i == root_files->size - 1) {
+    if (batch_size >= chunk_size || i == files->size - 1) {
       void** items = array_list_to_array(batch);
       if (!items) {
-          ps->failed = true;
+        *failed = true;
-          batch->item_destroyer = file_destroy;
         array_list_delete(batch);
         batch = NULL;
         break;
@@ -528,27 +480,29 @@ ParallelScanner* parallel_scanner_create_with_options(const char* root_directory
       Chunk* c = chunk_create((File**)items, batch->size);
       free(items);
       if (!c) {
-          ps->failed = true;
+        *failed = true;
-          batch->item_destroyer = file_destroy;
         array_list_delete(batch);
         batch = NULL;
         break;
       }
+      int batch_start = i - batch->size + 1;
+      for (int j = batch_start; j <= i; j++)
+        files->items[j] = NULL;
       batch->item_destroyer = NULL;
       array_list_delete(batch);
       batch = NULL;
       if (!first) {
         first = c;
       } else {
-          if (!queue_enqueue(ps->result_queue, c)) {
+        if (!queue_enqueue(queue, c)) {
           chunk_destroy(c);
-            ps->failed = true;
+          *failed = true;
         }
       }
-        if (i < root_files->size - 1) {
+      if (i < files->size - 1) {
         batch = array_list_create(NULL);
         if (!batch) {
-            ps->failed = true;
+          *failed = true;
           break;
         }
         batch_size = 0;
@@ -559,23 +513,88 @@ ParallelScanner* parallel_scanner_create_with_options(const char* root_directory
     batch->item_destroyer = NULL;
     array_list_delete(batch);
   }
-    ps->initial_chunk = first;
+  return first;
-    root_files->item_destroyer = NULL;
+}
-  }
-  array_list_delete(root_files);
 
+/* Scan one root-directory entry into either the subdirs or files list. */
+static void scan_root_entry(const ScannerOptions* options, const char* root_directory,
+                            const struct dirent* entry, ArrayList* root_files, ArrayList* subdirs,
+                            ParallelScanner* ps) {
+  ScannerEntry inspected;
+  int inspection =
+      scanner_inspect_entry(options, root_directory, root_directory, entry->d_name, &inspected);
+  if (inspection < 0) {
+    ps->failed = true;
+    return;
+  }
+  if (inspection == 0)
+    return;
+  char* cur_path = inspected.path;
+  struct stat st = inspected.stats;
+  if (inspected.is_directory) {
+    if (!array_list_add(subdirs, cur_path)) {
+      free(cur_path);
+      ps->failed = true;
+    }
+    return;
+  }
+  File* file = file_create(cur_path);
+  free(cur_path);
+  if (!file) {
+    ps->failed = true;
+    return;
+  }
+  file->data->size = st.st_size;
+  if (options->use_metadata)
+    file->metadata = file_metadata_create(&st);
+  if (options->use_metadata && !file->metadata) {
+    file_destroy(file);
+    ps->failed = true;
+    return;
+  }
+  if (!array_list_add(root_files, file)) {
+    file_destroy(file);
+    ps->failed = true;
+  }
+}
+
+/* Scan the root directory itself, collecting root files and subdirectories.
+ * Returns false if the root directory could not be opened. */
+static bool scan_root_directory(ParallelScanner* ps, const char* root_directory,
+                                const ScannerOptions* options, ArrayList* root_files,
+                                ArrayList* subdirs) {
+  DIR* dir = opendir(root_directory);
+  if (!dir) {
+    log_perror("Could not open root directory for parallel scan");
+    return false;
+  }
+  const struct dirent* entry;
+  while ((entry = readdir(dir)) != NULL) {
+    if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
+      continue;
+    scan_root_entry(options, root_directory, entry, root_files, subdirs, ps);
+  }
+  closedir(dir);
+  return true;
+}
+
+/* Spawn worker threads, one per group of subdirectories. */
+static void spawn_parallel_workers(ParallelScanner* ps, ArrayList* subdirs,
+                                   const ScannerOptions* options, unsigned long long cs) {
+  if (subdirs->size <= 0)
+    return;
   int n = options->num_threads > 0 ? options->num_threads : 4;
   if (n > subdirs->size)
-    n = subdirs->size > 0 ? subdirs->size : 1;
+    n = subdirs->size;
 
-  if (subdirs->size > 0) {
   ps->num_threads = n;
   ps->expected_threads = n;
   ps->threads = calloc(n, sizeof(thrd_t));
   if (!ps->threads) {
-      array_list_delete(subdirs);
+    ps->num_threads = 0;
-      parallel_scanner_destroy(ps);
+    ps->expected_threads = 0;
-      return NULL;
+    ps->failed = true;
+    return;
   }
   int dirs_per_thread = subdirs->size / n;
   int remainder = subdirs->size % n;
@@ -587,14 +606,14 @@ ParallelScanner* parallel_scanner_create_with_options(const char* root_directory
       break;
     ParallelWorkerArg* wa = calloc(1, sizeof(ParallelWorkerArg));
     if (!wa) {
-        ps->failed = true;
+      parallel_scanner_creation_failed(ps);
       break;
     }
     wa->ps = ps;
     wa->dirs = calloc(count, sizeof(char*));
     if (!wa->dirs) {
       free(wa);
-        ps->failed = true;
+      parallel_scanner_creation_failed(ps);
       break;
     }
     bool dup_ok = true;
@@ -608,7 +627,7 @@ ParallelScanner* parallel_scanner_create_with_options(const char* root_directory
         free(wa->dirs[j]);
       free(wa->dirs);
       free(wa);
-        ps->failed = true;
+      parallel_scanner_creation_failed(ps);
       break;
     }
     wa->dir_count = count;
@@ -620,35 +639,49 @@ ParallelScanner* parallel_scanner_create_with_options(const char* root_directory
         free(wa->dirs[j]);
       free(wa->dirs);
       free(wa);
-        ps->failed = true;
+      parallel_scanner_creation_failed(ps);
-        atomic_store(&ps->cancelled, true);
-        ps->expected_threads = ps->created_threads;
-        mtx_lock(&ps->result_mutex);
-        cnd_broadcast(&ps->result_not_empty);
-        cnd_broadcast(&ps->result_not_full);
-        mtx_unlock(&ps->result_mutex);
       break;
     }
     ps->num_threads++;
     ps->created_threads++;
   }
-  }
-  array_list_delete(subdirs);
-  return ps;
 }
 
-ParallelScanner* parallel_scanner_create(const char* root_directory, bool use_metadata,
+ParallelScanner* parallel_scanner_create_with_options(const char* root_directory,
-                                         unsigned long long chunk_size, char** exclude_patterns,
+                                                      const ScannerOptions* options) {
-                                         int exclude_count, char** include_patterns,
+  if (!root_directory || !options)
-                                         int include_count, unsigned long long max_size,
+    return NULL;
-                                         unsigned long long min_size, int max_depth,
+  ParallelScanner* ps = calloc(1, sizeof(ParallelScanner));
-                                         int num_threads, bool follow_symlinks, bool copy_links,
+  if (!ps)
-                                         bool safe_links, bool copy_unsafe_links, bool checksum) {
+    return NULL;
-  ScannerOptions options = {use_metadata,     chunk_size,        exclude_patterns, exclude_count,
+  if (!parallel_scanner_init(ps)) {
-                            include_patterns, include_count,     max_size,         min_size,
+    free(ps);
-                            max_depth,        num_threads,       follow_symlinks,  copy_links,
+    return NULL;
-                            safe_links,       copy_unsafe_links, checksum};
+  }
-  return parallel_scanner_create_with_options(root_directory, &options);
+
+  ArrayList* root_files = array_list_create(file_destroy);
+  ArrayList* subdirs = array_list_create(free);
+  if (!root_files || !subdirs) {
+    array_list_delete(root_files);
+    array_list_delete(subdirs);
+    parallel_scanner_destroy(ps);
+    return NULL;
+  }
+
+  if (!scan_root_directory(ps, root_directory, options, root_files, subdirs)) {
+    array_list_delete(root_files);
+    array_list_delete(subdirs);
+    parallel_scanner_destroy(ps);
+    return NULL;
+  }
+
+  unsigned long long cs = options->chunk_size > 0 ? options->chunk_size : DESIRED_CHUNK_SIZE;
+  ps->initial_chunk = batch_files(root_files, cs, ps->result_queue, &ps->failed);
+  array_list_delete(root_files);
+
+  spawn_parallel_workers(ps, subdirs, options, cs);
+  array_list_delete(subdirs);
+  return ps;
 }
 
 Chunk* parallel_scanner_next(ParallelScanner* ps) {
@@ -659,6 +692,11 @@ Chunk* parallel_scanner_next(ParallelScanner* ps) {
   }
   if (ps->num_threads == 0) {
     mtx_lock(&ps->result_mutex);
+    if (!queue_is_empty(ps->result_queue)) {
+      Chunk* chunk = queue_dequeue(ps->result_queue);
+      mtx_unlock(&ps->result_mutex);
+      return chunk;
+    }
     ps->done = true;
     mtx_unlock(&ps->result_mutex);
     return NULL;

src/client/scanner.h

@@ -77,13 +77,6 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner);
 bool directory_scanner_failed(const DirectoryScanner* scanner);
 void directory_scanner_destroy(DirectoryScanner* scanner);
 
-ParallelScanner* parallel_scanner_create(const char* root_directory, bool use_metadata,
-                                         unsigned long long chunk_size, char** exclude_patterns,
-                                         int exclude_count, char** include_patterns,
-                                         int include_count, unsigned long long max_size,
-                                         unsigned long long min_size, int max_depth,
-                                         int num_threads, bool follow_symlinks, bool copy_links,
-                                         bool safe_links, bool copy_unsafe_links, bool checksum);
 ParallelScanner* parallel_scanner_create_with_options(const char* root_directory,
                                                       const ScannerOptions* options);
 Chunk* parallel_scanner_next(ParallelScanner* scanner);

src/client/usage.c

@@ -28,6 +28,7 @@ void print_usage(void) {
   printf("  --max-size <n>      Skip files larger than n bytes\n");
   printf("  --min-size <n>      Skip files smaller than n bytes\n");
   printf("  --incremental       Skip files unchanged since last transfer\n");
+  printf("  --existing          Skip files not already present at destination\n");
   printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
   printf("  --delta-block <n>   Delta block size in bytes (default: %d)\n",
          DELTA_BLOCK_SIZE_DEFAULT);

src/server/receiver.c

@@ -8,8 +8,14 @@
 #include <sys/stat.h>
 
 static bool receiver_process_chunk(Chunk* chunk, const ReceiverSink* sink) {
+  if (!chunk || !sink || !sink->store_file)
+    return false;
   for (int i = 0; i < chunk->element_count; i++) {
     File* file = chunk->items[i];
+    if (!file) {
+      chunk_destroy(chunk);
+      return false;
+    }
     chunk->items[i] = NULL;
     if (!sink->store_file(file, sink->context)) {
       chunk_destroy(chunk);
@@ -41,9 +47,19 @@ static bool receiver_process_batch(Config* config, int file_descriptor) {
       send_status(file_descriptor, STATUS_ERROR);
       return false;
     }
+    if (check_size > MAX_RECEIVE_FILE_SIZE) {
+      free(check_path);
+      send_status(file_descriptor, STATUS_ERROR);
+      return false;
+    }
     char* full_path = path_cat(config->receive_root_directory, check_path);
+    if (!full_path) {
+      free(check_path);
+      send_status(file_descriptor, STATUS_ERROR);
+      return false;
+    }
     struct stat st;
-    bool has_old = full_path && lstat(full_path, &st) == 0;
+    bool has_old = file_stat_secure(full_path, &st);
     bool match = has_old && (unsigned long long)st.st_size == check_size &&
                  (long long)st.st_mtime == check_mtime;
     bool sent = send_status(file_descriptor, match ? STATUS_OK : STATUS_NEXT);

src/server/server.c

@@ -1,4 +1,5 @@
 #include "config.h"
+#include "chunk.h"
 #include "file.h"
 #include "log.h"
 #include "multiprocessing.h"
@@ -15,10 +16,41 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <unistd.h>
+#include <openssl/x509.h>
 
 static char* authorized_root;
 static int authorized_root_fd = -1;
 static bool allow_delete;
+static bool allow_unauthenticated;
+static const char* required_client_cn;
+
+static bool tls_client_identity_allowed(SSL* ssl) {
+  if (!ssl || !required_client_cn)
+    return false;
+  X509* certificate = SSL_get1_peer_certificate(ssl);
+  if (!certificate)
+    return false;
+  char common_name[256];
+  int length = X509_NAME_get_text_by_NID(X509_get_subject_name(certificate), NID_commonName,
+                                         common_name, sizeof(common_name));
+  size_t required_length = strlen(required_client_cn);
+  bool allowed = length >= 0 && (size_t)length == required_length &&
+                 required_length < sizeof(common_name) &&
+                 memcmp(common_name, required_client_cn, required_length) == 0;
+  X509_free(certificate);
+  return allowed;
+}
+
+static void release_authorization(void) {
+  file_set_authorized_root(-1, NULL);
+  utils_set_authorized_root_fd(-1);
+  if (authorized_root_fd >= 0)
+    close(authorized_root_fd);
+  authorized_root_fd = -1;
+  free(authorized_root);
+  authorized_root = NULL;
+}
 
 static bool path_is_within(const char* root, const char* path) {
   size_t n = strlen(root);
@@ -27,22 +59,166 @@ static bool path_is_within(const char* root, const char* path) {
 
 static bool __attribute__((unused)) configure_authorization(const char* root) {
   char resolved[PATH_MAX];
-  if (!root || !realpath(root, resolved))
+  if (!root) {
+    file_set_authorized_root(-1, NULL);
+    utils_set_authorized_root(-1, NULL);
     return false;
+  }
+  int root_fd = open(root, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  if (root_fd < 0) {
+    file_set_authorized_root(-1, NULL);
+    utils_set_authorized_root(-1, NULL);
+    return false;
+  }
+  char fd_path[64];
+  int fd_path_length = snprintf(fd_path, sizeof(fd_path), "/proc/self/fd/%d", root_fd);
+  if (fd_path_length < 0 || (size_t)fd_path_length >= sizeof(fd_path) ||
+      !realpath(fd_path, resolved)) {
+    close(root_fd);
+    file_set_authorized_root(-1, NULL);
+    utils_set_authorized_root(-1, NULL);
+    return false;
+  }
   authorized_root = str_dup(resolved);
-  if (!authorized_root)
+  if (!authorized_root) {
+    close(root_fd);
+    file_set_authorized_root(-1, NULL);
+    utils_set_authorized_root(-1, NULL);
     return false;
-  authorized_root_fd = open(resolved, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+  }
-  if (authorized_root_fd < 0) {
+  authorized_root_fd = root_fd;
+  if (!file_set_authorized_root(authorized_root_fd, authorized_root) ||
+      !utils_set_authorized_root(authorized_root_fd, authorized_root)) {
+    file_set_authorized_root(-1, NULL);
+    utils_set_authorized_root(-1, NULL);
+    close(authorized_root_fd);
+    authorized_root_fd = -1;
     free(authorized_root);
     authorized_root = NULL;
     return false;
   }
-  file_set_authorized_root(authorized_root_fd, authorized_root);
-  utils_set_authorized_root_fd(authorized_root_fd);
   return true;
 }
 
+int receive_files(Config* config, int fd) {
+  Status status;
+  if (!receive_status(fd, &status))
+    return -1;
+
+  while (status == STATUS_NEXT || status == STATUS_CHUNK || status == STATUS_CHECK ||
+         status == STATUS_KEEPALIVE || status == STATUS_ABORT || status == STATUS_CHECK_BATCH) {
+    if (status == STATUS_KEEPALIVE) {
+      send_status(fd, STATUS_KEEPALIVE);
+      goto next;
+    }
+    if (status == STATUS_ABORT) {
+      log_message(LOG_LEVEL_INFO, "Received abort from client, cleaning up");
+      return -1;
+    }
+    if (status == STATUS_CHECK) {
+      bool skipped;
+      File* file = receive_incremental_check(fd, config, &skipped);
+      if (skipped)
+        goto next;
+      if (file == NULL && !skipped)
+        return -1;
+      if (config->save_to_disk &&
+          !file_save_to_disk(config->receive_root_directory, file, config)) {
+        file_destroy(file);
+        send_status(fd, STATUS_ERROR);
+        return -1;
+      }
+      file_destroy(file);
+    } else if (status == STATUS_CHUNK) {
+      Chunk* chunk = receive_chunk_data(fd, config);
+      if (chunk == NULL) {
+        send_status(fd, STATUS_ERROR);
+        return -1;
+      }
+      for (int i = 0; i < chunk->element_count; i++) {
+        if (config->save_to_disk &&
+            !file_save_to_disk(config->receive_root_directory, chunk->items[i], config)) {
+          chunk_destroy(chunk);
+          send_status(fd, STATUS_ERROR);
+          return -1;
+        }
+      }
+      chunk_destroy(chunk);
+    } else if (status == STATUS_CHECK_BATCH) {
+      int count;
+      /* Batch framing has no checksum field yet; never silently downgrade a
+         checksum-enabled transfer into mtime-only matching. */
+      if (config->checksum || !receive_int(fd, &count) || count < 0 || count > MAX_MANIFEST_ENTRIES)
+        return -1;
+      for (int i = 0; i < count; i++) {
+        char* check_path = receive_str(fd);
+        if (!check_path)
+          return -1;
+        unsigned long long check_size;
+        long long check_mtime;
+        if (!receive_n_data(fd, &check_size, sizeof(check_size)) ||
+            !receive_n_data(fd, &check_mtime, sizeof(check_mtime))) {
+          free(check_path);
+          return -1;
+        }
+        if (!utils_valid_batch_path(check_path)) {
+          free(check_path);
+          send_status(fd, STATUS_ERROR);
+          return -1;
+        }
+        struct stat st;
+        char* full_path = path_cat(config->receive_root_directory, check_path);
+        if (!full_path) {
+          free(check_path);
+          send_status(fd, STATUS_ERROR);
+          return -1;
+        }
+        bool has_old = full_path && file_stat_secure(full_path, &st);
+        bool match = has_old && (unsigned long long)st.st_size == check_size &&
+                     (long long)st.st_mtime == check_mtime;
+        bool sent = send_status(fd, match ? STATUS_OK : STATUS_NEXT);
+        free(full_path);
+        free(check_path);
+        if (!sent)
+          return -1;
+      }
+      goto next;
+    } else {
+      File* file = file_receive(config, fd);
+      if (file == NULL) {
+        log_message(LOG_LEVEL_ERROR, "Failed to receive file");
+        send_status(fd, STATUS_ERROR);
+        return -1;
+      }
+      if (config->save_to_disk &&
+          !file_save_to_disk(config->receive_root_directory, file, config)) {
+        file_destroy(file);
+        send_status(fd, STATUS_ERROR);
+        return -1;
+      }
+      file_destroy(file);
+    }
+  next:
+    if (!receive_status(fd, &status)) {
+      send_status(fd, STATUS_ERROR);
+      return -1;
+    }
+  }
+
+  if (status == STATUS_MANIFEST) {
+    if (receive_manifest(fd, config, &status) != 0) {
+      return -1;
+    }
+  }
+  if (status != STATUS_FINISHED) {
+    log_message(LOG_LEVEL_ERROR, "Did not receive FINISHED Status");
+    send_status(fd, STATUS_ERROR);
+    return -1;
+  }
+  send_status(fd, STATUS_OK);
+  return 0;
+}
+
 void handler(int file_descriptor) {
   SSL* ssl = io_get_ssl();
   ProtocolSession session;
@@ -63,25 +239,37 @@ void handler(int file_descriptor) {
     protocol_session_unbind();
     return;
   }
-  char resolved_destination[PATH_MAX];
+  if (!allow_unauthenticated && ssl == NULL) {
-  char* canonical_destination = realpath(config->receive_root_directory, NULL);
+    log_message(LOG_LEVEL_ERROR, "Rejected unauthenticated plaintext connection");
-  const char* destination =
-      canonical_destination ? canonical_destination : config->receive_root_directory;
-  if (has_path_traversal(destination) || !path_is_within(authorized_root, destination)) {
-    log_message(LOG_LEVEL_ERROR, "Rejected destination outside authorized root");
-    free(canonical_destination);
     config_delete(config);
     close(file_descriptor);
     protocol_session_unbind();
     return;
   }
-  if (canonical_destination)
+  if (ssl && required_client_cn && !tls_client_identity_allowed(ssl)) {
-    snprintf(resolved_destination, sizeof(resolved_destination), "%s", canonical_destination);
+    log_message(LOG_LEVEL_ERROR, "Rejected TLS client with unauthorized identity");
-  else
+    config_delete(config);
-    snprintf(resolved_destination, sizeof(resolved_destination), "%s", destination);
+    close(file_descriptor);
-  free(canonical_destination);
+    return;
+  }
+  char* destination = config->receive_root_directory;
+  char* joined_destination = NULL;
+  if (destination && destination[0] != '/')
+    joined_destination = path_cat(authorized_root, destination);
+  if (joined_destination)
+    destination = joined_destination;
+  if (!destination || has_path_traversal(destination) ||
+      !path_is_within(authorized_root, destination)) {
+    log_message(LOG_LEVEL_ERROR, "Rejected destination outside authorized root");
+    free(joined_destination);
+    config_delete(config);
+    close(file_descriptor);
+    return;
+  }
+  if (joined_destination) {
     free(config->receive_root_directory);
-  config->receive_root_directory = str_dup(resolved_destination);
+    config->receive_root_directory = joined_destination;
+  }
   if (!config->receive_root_directory) {
     config_delete(config);
     close(file_descriptor);
@@ -106,13 +294,14 @@ void handler(int file_descriptor) {
       protocol_session_unbind();
       return;
     }
+    context->session.total_allocated_bytes = session.total_allocated_bytes;
     thrd_t receiver, writer;
     bool receiver_created = thrd_create(&receiver, receive_thread, context) == thrd_success;
     bool writer_created = false;
     if (receiver_created)
       writer_created = thrd_create(&writer, write_thread, context) == thrd_success;
     if (!receiver_created || !writer_created) {
-      perror("Error creating Threads");
+      log_perror("Error creating Threads");
       if (receiver_created) {
         mtx_lock(&context->mutex);
         atomic_store(&context->cancelled, true);
@@ -167,8 +356,10 @@ static void print_server_usage(void) {
   printf("  --cert <path>       TLS certificate file (PEM)\n");
   printf("  --key <path>        TLS private key file (PEM)\n");
   printf("  --ca <path>         TLS CA certificate file (PEM)\n");
+  printf("  --client-cn <name>  Required TLS client certificate CN\n");
   printf("  --destination-root <path>  Authorized destination root (default: .)\n");
   printf("  --allow-delete      Permit manifest deletion\n");
+  printf("  --allow-unauthenticated  Allow plaintext/anonymous network clients\n");
   printf("  -v, --verbose       Enable debug logging\n");
   printf("  --help              Show this help\n");
 }
@@ -197,10 +388,14 @@ int main(int argc, char* argv[]) {
       tls_key = argv[++i];
     } else if (strcmp(argv[i], "--ca") == 0 && i + 1 < argc) {
       tls_ca = argv[++i];
+    } else if (strcmp(argv[i], "--client-cn") == 0 && i + 1 < argc) {
+      required_client_cn = argv[++i];
     } else if (strcmp(argv[i], "--destination-root") == 0 && i + 1 < argc) {
       destination_root = argv[++i];
     } else if (strcmp(argv[i], "--allow-delete") == 0) {
       allow_delete = true;
+    } else if (strcmp(argv[i], "--allow-unauthenticated") == 0) {
+      allow_unauthenticated = true;
     } else if (strcmp(argv[i], "-p") == 0 && i + 1 < argc) {
       char* end;
       long p = strtol(argv[++i], &end, 10);
@@ -224,35 +419,39 @@ int main(int argc, char* argv[]) {
     return 1;
   }
   if (stdio_mode) {
+    /* SSH authenticates the stdio transport outside of FastSync. */
+    allow_unauthenticated = true;
     io_set_fds(STDIN_FILENO, STDOUT_FILENO);
     handler(STDIN_FILENO);
-    file_set_authorized_root(-1, NULL);
+    release_authorization();
-    utils_set_authorized_root_fd(-1);
-    close(authorized_root_fd);
-    free(authorized_root);
     return 0;
   }
   g_server = server_create(port);
   if (!g_server) {
     log_message(LOG_LEVEL_ERROR, "Failed to create server");
+    release_authorization();
     return 1;
   }
   if (use_tls) {
-    if (!tls_cert || !tls_key) {
+    if (!tls_cert || !tls_key || !tls_ca || !required_client_cn) {
-      fprintf(stderr, "Error: --tls requires --cert and --key\n");
+      fprintf(stderr, "Error: --tls requires --cert, --key, --ca, and --client-cn\n");
       server_delete(&g_server);
+      release_authorization();
       return 1;
     }
     tls_global_init();
     if (!server_create_tls(g_server, tls_cert, tls_key, tls_ca)) {
       log_message(LOG_LEVEL_ERROR, "Failed to set up TLS");
       server_delete(&g_server);
+      release_authorization();
       return 1;
     }
     server_listen_tls(g_server, handler);
   } else {
     server_listen(g_server, handler);
   }
+  server_delete(&g_server);
+  release_authorization();
   return 0;
 }
 #endif

src/shared/array_list.c

@@ -1,3 +1,4 @@
+#include "log.h"
 #include "array_list.h"
 #include <stdio.h>
 #include <stdlib.h>
@@ -6,7 +7,7 @@
 ArrayList* array_list_create(void (*item_destroyer)(void* item)) {
   ArrayList* list = (ArrayList*)malloc(sizeof(ArrayList));
   if (list == NULL) {
-    perror("ERROR: Could not allocate memory for array list struct");
+    log_perror("ERROR: Could not allocate memory for array list struct");
     return NULL;
   }
 
@@ -34,7 +35,7 @@ void array_list_delete(ArrayList* array_list) {
   free(array_list);
 }
 
-bool array_list_extend(ArrayList* array_list) {
+static bool array_list_extend(ArrayList* array_list) {
   if (array_list == NULL)
     return false;
   int new_capacity = array_list->capacity * 2;
@@ -42,7 +43,7 @@ bool array_list_extend(ArrayList* array_list) {
     new_capacity = INITIAL_ARRAY_SIZE;
   void* new_items = realloc(array_list->items, new_capacity * sizeof(void*));
   if (new_items == NULL) {
-    perror("ERROR: Could not reallocate memory for array list items");
+    log_perror("ERROR: Could not reallocate memory for array list items");
     return false;
   }
   array_list->items = new_items;
@@ -68,7 +69,7 @@ void** array_list_to_array(const ArrayList* array_list) {
   }
   void** array = malloc(array_list->size * sizeof(void*));
   if (array == NULL) {
-    perror("Could not malloc space for array from array list!");
+    log_perror("Could not malloc space for array from array list!");
     return NULL;
   }
   memcpy(array, array_list->items, array_list->size * sizeof(void*));

src/shared/array_list.h

@@ -14,7 +14,6 @@ typedef struct ArrayList {
 
 ArrayList* array_list_create(void (*item_destroyer)(void* item));
 void array_list_delete(ArrayList* array_list);
-bool array_list_extend(ArrayList* array_list);
 bool array_list_add(ArrayList* array_list, void* item);
 void** array_list_to_array(const ArrayList* array_list);
 

src/shared/chunk.c

@@ -1,4 +1,6 @@
 #include <stddef.h>
+#include <stdint.h>
+#include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -11,22 +13,34 @@
 #include "log.h"
 #include "metadata.h"
 #include "protocol.h"
+#include "utils.h"
 
 /* Maximum individual file data size within a chunk (64 MB) */
 #define MAX_FILE_DATA_SIZE (64ULL * 1024 * 1024)
+#define MAX_FILES_PER_CHUNK 65536U
 
 Chunk* chunk_create(File** items, int element_count) {
+  if (element_count < 0 || (element_count > 0 && items == NULL))
+    return NULL;
   Chunk* chunk = (Chunk*)malloc(sizeof(Chunk));
   if (chunk == NULL) {
-    perror("ERROR: Could not allocate memory for chunk structure");
+    log_perror("ERROR: Could not allocate memory for chunk structure");
     return NULL;
   }
 
-  chunk->items = (File**)malloc(element_count * sizeof(File*));
+  if (element_count == 0) {
+    chunk->items = NULL;
+  } else {
+    if ((size_t)element_count > SIZE_MAX / sizeof(File*)) {
+      free(chunk);
+      return NULL;
+    }
+    chunk->items = (File**)malloc((size_t)element_count * sizeof(File*));
     if (chunk->items == NULL) {
       free(chunk);
       return NULL;
     }
+  }
 
   for (int i = 0; i < element_count; i++) {
     chunk->items[i] = items[i];
@@ -50,15 +64,37 @@ void chunk_destroy(void* item) {
 }
 
 static unsigned long long per_file_serialize_size(File* file, bool use_metadata) {
-  return sizeof(size_t) + strlen(file->path) +
+  unsigned long long size = sizeof(size_t);
-         (use_metadata ? sizeof(int) + (file->metadata ? FILE_METADATA_WIRE_SIZE : 0) : 0) +
+  size_t path_len = strlen(file->path);
-         sizeof(size_t) + file->data->size;
+  unsigned long long metadata_size =
+      use_metadata ? sizeof(int) + (file->metadata ? FILE_METADATA_WIRE_SIZE : 0) : 0;
+  if ((unsigned long long)path_len > ULLONG_MAX - size)
+    return 0;
+  size += path_len;
+  if (metadata_size > ULLONG_MAX - size)
+    return 0;
+  size += metadata_size;
+  if (sizeof(size_t) > ULLONG_MAX - size)
+    return 0;
+  size += sizeof(size_t);
+  if ((unsigned long long)file->data->size > ULLONG_MAX - size)
+    return 0;
+  return size + file->data->size;
 }
 
 Data* chunk_serialize(Chunk* chunk, bool use_metadata) {
+  if (!chunk || chunk->element_count < 0 || (chunk->element_count > 0 && chunk->items == NULL))
+    return NULL;
   unsigned long long data_size = 0;
   for (int i = 0; i < chunk->element_count; i++) {
-    data_size += per_file_serialize_size(chunk->items[i], use_metadata);
+    if (!chunk->items[i] || !chunk->items[i]->path || !chunk->items[i]->data ||
+        (chunk->items[i]->data->size > 0 && !chunk->items[i]->data->data) ||
+        chunk->items[i]->path[0] == '\0' || has_path_traversal(chunk->items[i]->path))
+      return NULL;
+    unsigned long long file_size = per_file_serialize_size(chunk->items[i], use_metadata);
+    if (file_size == 0 || file_size > ULLONG_MAX - data_size || data_size + file_size > SIZE_MAX)
+      return NULL;
+    data_size += file_size;
   }
   Data* data = data_create_empty(data_size);
   if (data == NULL) {
@@ -87,11 +123,20 @@ Data* chunk_serialize(Chunk* chunk, bool use_metadata) {
 }
 
 Chunk* chunk_deserialize(Data* data, bool use_metadata) {
+  if (!data || (!data->data && data->size != 0))
+    return NULL;
   ArrayList* files = array_list_create(file_destroy);
+  if (files == NULL)
+    return NULL;
   char* data_pointer = data->data;
   size_t remaining_size = data->size;
 
   while (remaining_size > 0) {
+    if ((unsigned int)files->size >= MAX_FILES_PER_CHUNK) {
+      log_message(LOG_LEVEL_ERROR, "Chunk contains too many files");
+      array_list_delete(files);
+      return NULL;
+    }
     if (remaining_size < sizeof(size_t)) {
       log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for path length");
       array_list_delete(files);
@@ -103,48 +148,77 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
     data_pointer += sizeof(size_t);
     remaining_size -= sizeof(size_t);
 
-    if (remaining_size < path_len) {
+    if (path_len > SIZE_MAX - 1 || remaining_size < path_len) {
       log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for path");
       array_list_delete(files);
       return NULL;
     }
 
+    if (path_len == SIZE_MAX) {
+      array_list_delete(files);
+      return NULL;
+    }
     char* path = malloc(path_len + 1);
     if (path == NULL) {
-      perror("Could not allocate memory for file path");
+      log_perror("Could not allocate memory for file path");
       array_list_delete(files);
       return NULL;
     }
     memcpy(path, data_pointer, path_len);
     path[path_len] = '\0';
+    if (memchr(path, '\0', path_len) != NULL) {
+      free(path);
+      array_list_delete(files);
+      return NULL;
+    }
     data_pointer += path_len;
     remaining_size -= path_len;
 
+    if (path_len == 0 || has_path_traversal(path)) {
+      free(path);
+      array_list_delete(files);
+      return NULL;
+    }
+
     File* file = file_create(path);
     free(path);
+    if (file == NULL) {
+      array_list_delete(files);
+      return NULL;
+    }
 
     if (use_metadata) {
       if (remaining_size < sizeof(int)) {
         log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for metadata");
+        file_destroy(file);
         array_list_delete(files);
         return NULL;
       }
       // Peek at present flag to determine total size needed before reading
       int present_flag;
       memcpy(&present_flag, data_pointer, sizeof(int));
-      if (present_flag && remaining_size < sizeof(int) + FILE_METADATA_WIRE_SIZE) {
+      if ((present_flag != 0 && present_flag != 1) ||
+          (present_flag == 1 && remaining_size < sizeof(int) + FILE_METADATA_WIRE_SIZE)) {
         log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for metadata body");
+        file_destroy(file);
         array_list_delete(files);
         return NULL;
       }
       file->metadata = metadata_from_buf(&data_pointer);
       remaining_size -= sizeof(int);
-      if (file->metadata)
+      if (present_flag == 1) {
+        if (file->metadata == NULL) {
+          file_destroy(file);
+          array_list_delete(files);
+          return NULL;
+        }
         remaining_size -= FILE_METADATA_WIRE_SIZE;
       }
+    }
 
     if (remaining_size < sizeof(size_t)) {
       log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for data size");
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
@@ -156,6 +230,7 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
 
     if (remaining_size < file_data_size) {
       log_message(LOG_LEVEL_ERROR, "Invalid chunk format: not enough data for file content");
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
@@ -164,29 +239,50 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
     if (file_data_size > MAX_FILE_DATA_SIZE) {
       log_message(LOG_LEVEL_ERROR, "File data size %zu exceeds maximum %llu", file_data_size,
                   (unsigned long long)MAX_FILE_DATA_SIZE);
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
 
-    void* file_data = malloc(file_data_size);
+    size_t allocation_size = file_data_size > 0 ? file_data_size : 1;
+    void* file_data = malloc(allocation_size);
     if (file_data == NULL) {
-      perror("Could not allocate memory for file data");
+      log_perror("Could not allocate memory for file data");
+      file_destroy(file);
       array_list_delete(files);
       return NULL;
     }
     memcpy(file_data, data_pointer, file_data_size);
+    Data* replacement = data_create(file_data, file_data_size);
+    if (replacement == NULL) {
+      file_destroy(file);
+      array_list_delete(files);
+      return NULL;
+    }
     data_destroy(file->data);
-    file->data = data_create(file_data, file_data_size);
+    file->data = replacement;
     data_pointer += file_data_size;
     remaining_size -= file_data_size;
 
-    array_list_add(files, file);
+    if (!array_list_add(files, file)) {
+      file_destroy(file);
+      array_list_delete(files);
+      return NULL;
+    }
   }
 
   File** file_array = (File**)array_list_to_array(files);
+  if (files->size > 0 && file_array == NULL) {
+    array_list_delete(files);
+    return NULL;
+  }
   Chunk* chunk = chunk_create(file_array, files->size);
 
   free(file_array);
+  if (chunk == NULL) {
+    array_list_delete(files);
+    return NULL;
+  }
   files->item_destroyer = NULL;
   array_list_delete(files);
 
@@ -207,14 +303,14 @@ Data* chunk_compress(Chunk* chunk, int compression_level, bool use_metadata) {
 }
 
 Chunk* receive_chunk_data(int fd, const Config* config) {
-  Data* chunk_data = receive_data(fd);
+  Data* chunk_data = receive_data_limited(fd, MAX_CHUNK_SIZE);
   if (chunk_data == NULL) {
     log_message(LOG_LEVEL_ERROR, "Failed to receive chunk data");
     return NULL;
   }
   Data* data_to_process = chunk_data;
   if (config->use_compression) {
-    data_to_process = data_decompress(chunk_data);
+    data_to_process = data_decompress_limited(chunk_data, MAX_CHUNK_SIZE);
     data_destroy(chunk_data);
     if (data_to_process == NULL) {
       log_message(LOG_LEVEL_ERROR, "Failed to decompress chunk");

src/shared/compression.c

@@ -2,6 +2,8 @@
 #include "data.h"
 #include "log.h"
 #include <stdlib.h>
+#include <limits.h>
+#include <stdint.h>
 #include <string.h>
 #include <strings.h>
 #include <zstd.h>
@@ -69,7 +71,10 @@ Data* data_compress(Data* data_to_compress, int compression_level) {
   return compressed_data;
 }
 
-Data* data_decompress(Data* compressed_data) {
+Data* data_decompress_limited(Data* compressed_data, size_t maximum_size) {
+  if (!compressed_data || (!compressed_data->data && compressed_data->size != 0) ||
+      maximum_size == 0)
+    return NULL;
   log_message(LOG_LEVEL_DEBUG, "Start to decompress data");
   unsigned long long dst_size =
       ZSTD_getFrameContentSize(compressed_data->data, compressed_data->size);
@@ -82,15 +87,16 @@ Data* data_decompress(Data* compressed_data) {
   // ZSTD_CONTENTSIZE_UNKNOWN (~2^64) can cause massive allocation;
   // fall back to a conservative estimate (3x compressed size) when unknown.
   if (dst_size == ZSTD_CONTENTSIZE_UNKNOWN) {
+    if (compressed_data->size > ULLONG_MAX / 3)
+      return NULL;
     dst_size = compressed_data->size * 3;
     if (dst_size < INITIAL_DECOMPRESS_BUF_SIZE)
       dst_size = INITIAL_DECOMPRESS_BUF_SIZE;
-    if (dst_size > MAX_DECOMPRESSED_SIZE)
-      dst_size = MAX_DECOMPRESSED_SIZE;
   }
-  if (dst_size > MAX_DECOMPRESSED_SIZE) {
+  unsigned long long hard_limit =
-    log_message(LOG_LEVEL_ERROR, "Declared decompressed size exceeds %llu bytes",
+      maximum_size < MAX_DECOMPRESSED_SIZE ? maximum_size : MAX_DECOMPRESSED_SIZE;
-                (unsigned long long)MAX_DECOMPRESSED_SIZE);
+  if (dst_size > hard_limit) {
+    log_message(LOG_LEVEL_ERROR, "Declared decompressed size exceeds %llu bytes", hard_limit);
     return NULL;
   }
 
@@ -101,6 +107,8 @@ Data* data_decompress(Data* compressed_data) {
   }
 
   size_t buf_size = (dst_size > 0) ? (size_t)dst_size : INITIAL_DECOMPRESS_BUF_SIZE;
+  if (buf_size > maximum_size)
+    buf_size = maximum_size;
   Data* uncompressed_data = data_create_empty(buf_size);
   if (!uncompressed_data) {
     log_message(LOG_LEVEL_ERROR, "Failed to allocate decompression buffer");
@@ -121,7 +129,7 @@ Data* data_decompress(Data* compressed_data) {
       return NULL;
     }
     if (ret > 0 && output.pos == output.size) {
-      if (buf_size >= MAX_DECOMPRESSED_SIZE) {
+      if (buf_size >= hard_limit || buf_size > SIZE_MAX / 2) {
         log_message(LOG_LEVEL_ERROR, "Decompressed data exceeds %llu bytes",
                     (unsigned long long)MAX_DECOMPRESSED_SIZE);
         ZSTD_freeDCtx(dctx);
@@ -129,8 +137,8 @@ Data* data_decompress(Data* compressed_data) {
         return NULL;
       }
       buf_size *= 2;
-      if (buf_size > MAX_DECOMPRESSED_SIZE)
+      if (buf_size > hard_limit)
-        buf_size = MAX_DECOMPRESSED_SIZE;
+        buf_size = (size_t)hard_limit;
       void* new_data = realloc(uncompressed_data->data, buf_size);
       if (!new_data) {
         log_message(LOG_LEVEL_ERROR, "Failed to grow decompression buffer");
@@ -150,3 +158,7 @@ Data* data_decompress(Data* compressed_data) {
   log_message(LOG_LEVEL_DEBUG, "Decompressed data successfully");
   return uncompressed_data;
 }
+
+Data* data_decompress(Data* compressed_data) {
+  return data_decompress_limited(compressed_data, MAX_DECOMPRESSED_SIZE);
+}

src/shared/compression.h

@@ -6,6 +6,7 @@
 
 Data* data_compress(Data* data_to_compress, int compression_level);
 Data* data_decompress(Data* compressed_data);
+Data* data_decompress_limited(Data* compressed_data, size_t maximum_size);
 bool compression_should_skip(const char* path);
 
 #endif

src/shared/config.c

@@ -68,6 +68,7 @@ static void config_set_defaults(Config* config) {
   config->debug_level = 0;
   config->list_only = false;
   config->human_readable = false;
+  config->existing = false;
   config->update = false;
   config->inplace = false;
   config->append = false;
@@ -100,6 +101,44 @@ static void config_set_defaults(Config* config) {
   config->compress_choice = NULL;
 }
 
+static bool valid_wire_bool(int value) {
+  return value == 0 || value == 1;
+}
+
+static bool receive_wire_bool(int fd, bool* value) {
+  int wire_value;
+  if (!receive_int(fd, &wire_value) || !valid_wire_bool(wire_value))
+    return false;
+  *value = wire_value != 0;
+  return true;
+}
+
+static bool validate_received_config(const Config* config) {
+  return valid_wire_bool(config->save_to_disk) && valid_wire_bool(config->use_multithreading) &&
+         valid_wire_bool(config->use_chunk_serialization) &&
+         valid_wire_bool(config->use_compression) && valid_wire_bool(config->use_metadata) &&
+         valid_wire_bool(config->use_sendfile) && valid_wire_bool(config->use_delete) &&
+         valid_wire_bool(config->use_incremental) && valid_wire_bool(config->use_delta) &&
+         valid_wire_bool(config->backup) && valid_wire_bool(config->follow_symlinks) &&
+         valid_wire_bool(config->copy_links) && valid_wire_bool(config->safe_links) &&
+         valid_wire_bool(config->copy_unsafe_links) &&
+         valid_wire_bool(config->preserve_hard_links) && valid_wire_bool(config->preserve_acls) &&
+         valid_wire_bool(config->preserve_xattrs) && valid_wire_bool(config->preserve_devices) &&
+         valid_wire_bool(config->preserve_sparse) && valid_wire_bool(config->existing) &&
+         valid_wire_bool(config->update) && valid_wire_bool(config->inplace) &&
+         valid_wire_bool(config->append) && valid_wire_bool(config->append_verify) &&
+         valid_wire_bool(config->delete_excluded) && valid_wire_bool(config->delete_after) &&
+         valid_wire_bool(config->relative) && valid_wire_bool(config->prune_empty_dirs) &&
+         valid_wire_bool(config->partial) && valid_wire_bool(config->delete_before) &&
+         valid_wire_bool(config->checksum) &&
+         (!config->use_compression ||
+          (config->compression_level >= 1 && config->compression_level <= 22)) &&
+         config->chunk_size > 0 && config->chunk_size <= MAX_CHUNK_SIZE &&
+         config->delta_block_size >= DELTA_BLOCK_SIZE_MIN &&
+         config->delta_block_size <= DELTA_BLOCK_SIZE_MAX &&
+         config->delta_max_file_size <= DELTA_MAX_FILE_SIZE && config->max_delete >= 0;
+}
+
 Config* config_create(void) {
   Config* config = malloc(sizeof(Config));
   if (!config)
@@ -108,7 +147,7 @@ Config* config_create(void) {
   return config;
 }
 
-bool is_remote_dest(const char* s) {
+bool config_is_remote_dest(const char* s) {
   if (s == NULL)
     return false;
   const char* colon = strchr(s, ':');
@@ -124,7 +163,7 @@ bool is_remote_dest(const char* s) {
 }
 
 void config_parse_ssh_dest(Config* config) {
-  if (!is_remote_dest(config->receive_root_directory))
+  if (!config_is_remote_dest(config->receive_root_directory))
     return;
   config->transport = TRANSPORT_SSH;
   config->ssh_destination = str_dup(config->receive_root_directory);
@@ -137,6 +176,10 @@ void config_parse_ssh_dest(Config* config) {
 void config_delete(Config* config) {
   if (config == NULL)
     return;
+  if (config->log_file) {
+    fclose(config->log_file);
+    config->log_file = NULL;
+  }
   free(config->version);
   free(config->send_directory);
   free(config->receive_root_directory);
@@ -202,10 +245,11 @@ static bool send_file_options(int fd, const Config* c) {
 }
 
 static bool send_selection_options(int fd, const Config* c) {
-  return send_int(fd, c->update) && send_int(fd, c->inplace) && send_int(fd, c->append) &&
+  return send_int(fd, c->existing) && send_int(fd, c->update) && send_int(fd, c->inplace) &&
-         send_int(fd, c->append_verify) && send_int(fd, c->delete_excluded) &&
+         send_int(fd, c->append) && send_int(fd, c->append_verify) &&
-         send_int(fd, c->delete_after) && send_n_data(fd, &c->max_delete, sizeof(c->max_delete)) &&
+         send_int(fd, c->delete_excluded) && send_int(fd, c->delete_after) &&
-         send_int(fd, c->relative) && send_int(fd, c->prune_empty_dirs);
+         send_n_data(fd, &c->max_delete, sizeof(c->max_delete)) && send_int(fd, c->relative) &&
+         send_int(fd, c->prune_empty_dirs);
 }
 
 static bool send_resume_options(int fd, const Config* c) {
@@ -221,52 +265,34 @@ static bool receive_core_fields(int fd, Config* c) {
   c->receive_root_directory = receive_str(fd);
   if (!c->send_directory || !c->receive_root_directory)
     return false;
-  if (!receive_int(fd, &value))
+  if (!receive_wire_bool(fd, &c->save_to_disk) || !receive_wire_bool(fd, &c->use_multithreading) ||
+      !receive_wire_bool(fd, &c->use_chunk_serialization) ||
+      !receive_wire_bool(fd, &c->use_compression) || !receive_wire_bool(fd, &c->use_metadata))
     return false;
-  c->save_to_disk = value;
-  if (!receive_int(fd, &value))
-    return false;
-  c->use_multithreading = value;
-  if (!receive_int(fd, &value))
-    return false;
-  c->use_chunk_serialization = value;
-  if (!receive_int(fd, &value))
-    return false;
-  c->use_compression = value;
-  if (!receive_int(fd, &value))
-    return false;
-  c->use_metadata = value;
   if (!receive_int(fd, &value))
     return false;
   c->compression_level = value;
   if (!receive_n_data(fd, &c->chunk_size, sizeof(c->chunk_size)))
     return false;
-  if (!receive_int(fd, &value))
+  if (!receive_wire_bool(fd, &c->use_sendfile))
     return false;
-  c->use_sendfile = value;
   return true;
 }
 
 static bool receive_delta_fields(int fd, Config* c) {
-  int value;
+  if (!receive_wire_bool(fd, &c->use_delete))
-  if (!receive_int(fd, &value))
     return false;
-  c->use_delete = value;
+  if (!receive_wire_bool(fd, &c->use_incremental))
-  if (!receive_int(fd, &value))
     return false;
-  c->use_incremental = value;
+  if (!receive_wire_bool(fd, &c->use_delta))
-  if (!receive_int(fd, &value))
     return false;
-  c->use_delta = value;
   return receive_n_data(fd, &c->delta_block_size, sizeof(c->delta_block_size)) &&
          receive_n_data(fd, &c->delta_max_file_size, sizeof(unsigned long long));
 }
 
 static bool receive_file_options(int fd, Config* c) {
-  int value;
+  if (!receive_wire_bool(fd, &c->backup))
-  if (!receive_int(fd, &value))
     return false;
-  c->backup = value;
   c->backup_dir = receive_str(fd);
   if (!c->backup_dir)
     return false;
@@ -274,47 +300,38 @@ static bool receive_file_options(int fd, Config* c) {
                    &c->copy_unsafe_links, &c->preserve_hard_links, &c->preserve_acls,
                    &c->preserve_xattrs,   &c->preserve_devices,    &c->preserve_sparse};
   for (size_t i = 0; i < sizeof(flags) / sizeof(flags[0]); i++) {
-    if (!receive_int(fd, &value))
+    if (!receive_wire_bool(fd, flags[i]))
       return false;
-    *flags[i] = value;
   }
   return true;
 }
 
 static bool receive_selection_options(int fd, Config* c) {
-  int value;
+  bool* flags[] = {&c->existing,      &c->update,          &c->inplace,     &c->append,
-  bool* flags[] = {&c->update,        &c->inplace,         &c->append,
                    &c->append_verify, &c->delete_excluded, &c->delete_after};
   for (size_t i = 0; i < sizeof(flags) / sizeof(flags[0]); i++) {
-    if (!receive_int(fd, &value))
+    if (!receive_wire_bool(fd, flags[i]))
       return false;
-    *flags[i] = value;
   }
   if (!receive_n_data(fd, &c->max_delete, sizeof(c->max_delete)))
     return false;
-  if (!receive_int(fd, &value))
+  if (!receive_wire_bool(fd, &c->relative))
     return false;
-  c->relative = value;
+  if (!receive_wire_bool(fd, &c->prune_empty_dirs))
-  if (!receive_int(fd, &value))
     return false;
-  c->prune_empty_dirs = value;
   return true;
 }
 
 static bool receive_resume_options(int fd, Config* c) {
-  int value;
   c->temp_dir = receive_str(fd);
-  if (!c->temp_dir || !receive_int(fd, &value))
+  if (!c->temp_dir || !receive_wire_bool(fd, &c->partial))
     return false;
-  c->partial = value;
   c->partial_dir = receive_str(fd);
   c->suffix = c->partial_dir ? receive_str(fd) : NULL;
-  if (!c->partial_dir || !c->suffix || !receive_int(fd, &value))
+  if (!c->partial_dir || !c->suffix || !receive_wire_bool(fd, &c->delete_before))
     return false;
-  c->delete_before = value;
+  if (!receive_wire_bool(fd, &c->checksum))
-  if (!receive_int(fd, &value))
     return false;
-  c->checksum = value;
   c->compress_choice = receive_str(fd);
   return c->compress_choice != NULL;
 }
@@ -361,6 +378,11 @@ Config* config_receive(int file_descriptor) {
     send_status(file_descriptor, STATUS_ERROR);
     goto error;
   }
+  if (!validate_received_config(config)) {
+    fprintf(stderr, "Invalid configuration received from client\n");
+    send_status(file_descriptor, STATUS_ERROR);
+    goto error;
+  }
   if (!send_status(file_descriptor, STATUS_OK))
     goto error;
   return config;

src/shared/config.h

@@ -76,6 +76,7 @@ typedef struct Config {
   bool human_readable;
 
   // Issue #127: Transfer modes
+  bool existing;
   bool update;
   bool inplace;
   bool append;
@@ -128,14 +129,14 @@ typedef struct Config {
   char* compress_choice;
 } Config;
 
-#define PROTOCOL_VERSION "2.2.0"
+#define PROTOCOL_VERSION "2.3.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)
 
 Config* config_create(void);
 void config_delete(Config* config);
 bool config_send(int file_descriptor, const Config* config);
 Config* config_receive(int file_descriptor);
-bool is_remote_dest(const char* s);
+bool config_is_remote_dest(const char* s);
 void config_parse_ssh_dest(Config* config);
 
 #endif

src/shared/data.c

@@ -21,6 +21,7 @@ Data* data_create_reserve(size_t size) {
   }
   d->data = NULL;
   d->size = size;
+  d->protocol_charge = 0;
   return d;
 }
 
@@ -33,12 +34,15 @@ Data* data_create(void* data, size_t data_size) {
   }
   new_data->data = data;
   new_data->size = data_size;
+  new_data->protocol_charge = 0;
   return new_data;
 }
 
 void data_destroy(Data* data) {
   if (data == NULL)
     return;
+  if (data->protocol_charge != 0)
+    protocol_release_memory(data->protocol_charge);
   free(data->data);
   free(data);
 }

src/shared/data.h

@@ -6,11 +6,14 @@
 typedef struct {
   void* data;
   size_t size;
+  /* Non-zero only for a buffer charged to the protocol connection budget. */
+  size_t protocol_charge;
 } Data;
 
 Data* data_create_empty(size_t data_size);
 Data* data_create_reserve(size_t size);
 Data* data_create(void* data, size_t data_size);
 void data_destroy(Data* data);
+void protocol_release_memory(size_t charge);
 
 #endif

src/shared/delta.c

@@ -1,6 +1,7 @@
 #include "delta.h"
 #include "log.h"
 #include <stdint.h>
+#include <limits.h>
 #include <stdlib.h>
 #include <string.h>
 
@@ -36,6 +37,10 @@ DeltaSignature* delta_signature_create(const void* old_file_data, uint64_t old_f
   if (old_file_data == NULL || old_file_size == 0 || block_size == 0)
     return NULL;
 
+  if (old_file_size > DELTA_MAX_FILE_SIZE || block_size > DELTA_BLOCK_SIZE_MAX ||
+      old_file_size > UINT32_MAX * (uint64_t)block_size)
+    return NULL;
+
   uint32_t block_count = (uint32_t)((old_file_size + block_size - 1) / block_size);
 
   DeltaSignature* sig = malloc(sizeof(DeltaSignature));
@@ -45,7 +50,11 @@ DeltaSignature* delta_signature_create(const void* old_file_data, uint64_t old_f
   sig->file_size = old_file_size;
   sig->block_size = block_size;
   sig->block_count = block_count;
-  sig->blocks = malloc(block_count * sizeof(DeltaBlockSig));
+  if (block_count == 0) {
+    free(sig);
+    return NULL;
+  }
+  sig->blocks = malloc((size_t)block_count * sizeof(DeltaBlockSig));
   if (!sig->blocks) {
     free(sig);
     return NULL;
@@ -67,8 +76,11 @@ Data* delta_signature_serialize(const DeltaSignature* sig) {
   if (!sig)
     return NULL;
 
-  uint64_t total = sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t) +
+  uint64_t block_bytes = (uint64_t)sig->block_count * (sizeof(uint32_t) + sizeof(uint32_t));
-                   (uint64_t)sig->block_count * (sizeof(uint32_t) + sizeof(uint32_t));
+  uint64_t total = sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t) + block_bytes;
+  if (block_bytes > UINT64_MAX - (sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t)) ||
+      total > SIZE_MAX)
+    return NULL;
 
   uint8_t* buf = malloc((size_t)total);
   if (!buf)
@@ -118,6 +130,13 @@ DeltaSignature* delta_signature_deserialize(const Data* data) {
     return NULL;
   }
 
+  if (sig->block_size == 0 || sig->block_size > DELTA_BLOCK_SIZE_MAX ||
+      sig->file_size > DELTA_MAX_FILE_SIZE || sig->file_size == 0 ||
+      (sig->file_size + sig->block_size - 1) / sig->block_size != sig->block_count) {
+    free(sig);
+    return NULL;
+  }
+
   uint64_t expected = sizeof(uint64_t) + sizeof(uint32_t) + sizeof(uint32_t) +
                       (uint64_t)sig->block_count * (sizeof(uint32_t) + sizeof(uint32_t));
   if (data->size < expected) {
@@ -156,8 +175,10 @@ void delta_signature_destroy(DeltaSignature* sig) {
 static bool ensure_capacity(DeltaInstruction** instrs, uint32_t* capacity, uint32_t count) {
   if (count < *capacity)
     return true;
+  if (*capacity > MAX_DELTA_INSTRUCTIONS / 2)
+    return false;
   uint32_t new_cap = *capacity * 2;
-  DeltaInstruction* tmp = realloc(*instrs, new_cap * sizeof(DeltaInstruction));
+  DeltaInstruction* tmp = realloc(*instrs, (size_t)new_cap * sizeof(DeltaInstruction));
   if (!tmp)
     return false;
   *instrs = tmp;
@@ -169,6 +190,8 @@ static bool flush_literal(DeltaInstruction** instrs, uint32_t* capacity, uint32_
                           const uint8_t* data, uint64_t start, uint64_t end) {
   if (start >= end)
     return true;
+  if (end - start > UINT32_MAX || *count >= MAX_DELTA_INSTRUCTIONS)
+    return false;
   uint32_t lit_len = (uint32_t)(end - start);
   if (!ensure_capacity(instrs, capacity, *count))
     return false;
@@ -183,16 +206,26 @@ static bool flush_literal(DeltaInstruction** instrs, uint32_t* capacity, uint32_
   return true;
 }
 
+static void free_instructions(DeltaInstruction* instrs, uint32_t count) {
+  if (!instrs)
+    return;
+  for (uint32_t i = 0; i < count; i++)
+    if (instrs[i].type == DELTA_INSTR_LITERAL)
+      free(instrs[i].literal.data);
+  free(instrs);
+}
+
 Delta* delta_compute(const void* new_file_data, uint64_t new_file_size, const DeltaSignature* sig,
                      uint32_t block_size) {
-  if (!new_file_data || !sig || new_file_size == 0 || block_size == 0)
+  if (!new_file_data || !sig || !sig->blocks || new_file_size == 0 || block_size == 0 ||
+      block_size > DELTA_BLOCK_SIZE_MAX || sig->block_size != block_size)
     return NULL;
 
   const uint8_t* new_data = (const uint8_t*)new_file_data;
 
   uint32_t capacity = 64;
   uint32_t count = 0;
-  DeltaInstruction* instrs = malloc(capacity * sizeof(DeltaInstruction));
+  DeltaInstruction* instrs = malloc((size_t)capacity * sizeof(DeltaInstruction));
   if (!instrs)
     return NULL;
 
@@ -236,14 +269,14 @@ Delta* delta_compute(const void* new_file_data, uint64_t new_file_size, const De
         if (xxh == sig->blocks[j].xxhash) {
           if (has_literal) {
             if (!flush_literal(&instrs, &capacity, &count, new_data, literal_start, i)) {
-              free(instrs);
+              free_instructions(instrs, count);
               return NULL;
             }
             has_literal = false;
           }
 
           if (!ensure_capacity(&instrs, &capacity, count)) {
-            free(instrs);
+            free_instructions(instrs, count);
             return NULL;
           }
           instrs[count].type = DELTA_INSTR_BLOCK_MATCH;
@@ -271,18 +304,14 @@ Delta* delta_compute(const void* new_file_data, uint64_t new_file_size, const De
 
   if (has_literal) {
     if (!flush_literal(&instrs, &capacity, &count, new_data, literal_start, new_file_size)) {
-      free(instrs);
+      free_instructions(instrs, count);
       return NULL;
     }
   }
 
   Delta* delta = malloc(sizeof(Delta));
   if (!delta) {
-    for (uint32_t k = 0; k < count; k++) {
+    free_instructions(instrs, count);
-      if (instrs[k].type == DELTA_INSTR_LITERAL)
-        free(instrs[k].literal.data);
-    }
-    free(instrs);
     return NULL;
   }
 
@@ -292,11 +321,24 @@ Delta* delta_compute(const void* new_file_data, uint64_t new_file_size, const De
   delta->delta_size = 0;
 
   for (uint32_t k = 0; k < count; k++) {
+    if (delta->delta_size == UINT64_MAX) {
+      delta_destroy(delta);
+      return NULL;
+    }
     delta->delta_size += 1;
     if (instrs[k].type == DELTA_INSTR_BLOCK_MATCH) {
+      if (delta->delta_size > UINT64_MAX - sizeof(uint32_t) * 3) {
+        delta_destroy(delta);
+        return NULL;
+      }
       delta->delta_size += sizeof(uint32_t) * 3;
     } else {
-      delta->delta_size += sizeof(uint32_t) + instrs[k].literal.length;
+      uint64_t extra = sizeof(uint32_t) + instrs[k].literal.length;
+      if (delta->delta_size > UINT64_MAX - extra) {
+        delta_destroy(delta);
+        return NULL;
+      }
+      delta->delta_size += extra;
     }
   }
 
@@ -307,7 +349,12 @@ Data* delta_serialize(const Delta* delta) {
   if (!delta)
     return NULL;
 
-  uint64_t total = sizeof(uint64_t) + sizeof(uint32_t) + delta->delta_size;
+  if (delta->instruction_count > 0 && !delta->instructions)
+    return NULL;
+  uint64_t header_size = sizeof(uint64_t) + sizeof(uint32_t);
+  if (delta->delta_size > UINT64_MAX - header_size || header_size + delta->delta_size > SIZE_MAX)
+    return NULL;
+  uint64_t total = header_size + delta->delta_size;
   uint8_t* buf = malloc((size_t)total);
   if (!buf)
     return NULL;
@@ -365,8 +412,10 @@ Delta* delta_deserialize(const Data* data) {
     return NULL;
   }
 
-  delta->instructions = malloc(delta->instruction_count * sizeof(DeltaInstruction));
+  delta->instructions = delta->instruction_count == 0
-  if (!delta->instructions) {
+                            ? NULL
+                            : malloc((size_t)delta->instruction_count * sizeof(DeltaInstruction));
+  if (delta->instruction_count > 0 && !delta->instructions) {
     free(delta);
     return NULL;
   }
@@ -375,11 +424,7 @@ Delta* delta_deserialize(const Data* data) {
 
   for (uint32_t i = 0; i < delta->instruction_count; i++) {
     if (pos >= data->size) {
-      for (uint32_t k = 0; k < i; k++) {
+      free_instructions(delta->instructions, i);
-        if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
-          free(delta->instructions[k].literal.data);
-      }
-      free(delta->instructions);
       free(delta);
       return NULL;
     }
@@ -391,12 +436,8 @@ Delta* delta_deserialize(const Data* data) {
     delta->delta_size += 1;
 
     if (type == DELTA_OP_BLOCK_MATCH) {
-      if (pos + sizeof(uint32_t) * 3 > data->size) {
+      if (data->size - pos < sizeof(uint32_t) * 3) {
-        for (uint32_t k = 0; k < i; k++) {
+        free_instructions(delta->instructions, i);
-          if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
-            free(delta->instructions[k].literal.data);
-        }
-        free(delta->instructions);
         free(delta);
         return NULL;
       }
@@ -409,12 +450,8 @@ Delta* delta_deserialize(const Data* data) {
       pos += sizeof(uint32_t);
       delta->delta_size += sizeof(uint32_t) * 3;
     } else if (type == DELTA_OP_LITERAL) {
-      if (pos + sizeof(uint32_t) > data->size) {
+      if (data->size - pos < sizeof(uint32_t)) {
-        for (uint32_t k = 0; k < i; k++) {
+        free_instructions(delta->instructions, i);
-          if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
-            free(delta->instructions[k].literal.data);
-        }
-        free(delta->instructions);
         free(delta);
         return NULL;
       }
@@ -423,23 +460,15 @@ Delta* delta_deserialize(const Data* data) {
       pos += sizeof(uint32_t);
 
       uint32_t lit_len = delta->instructions[i].literal.length;
-      if (pos + lit_len > data->size) {
+      if (lit_len > data->size - pos) {
-        for (uint32_t k = 0; k < i; k++) {
+        free_instructions(delta->instructions, i);
-          if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
-            free(delta->instructions[k].literal.data);
-        }
-        free(delta->instructions);
         free(delta);
         return NULL;
       }
-      delta->instructions[i].literal.data = malloc(lit_len);
+      delta->instructions[i].literal.data = malloc(lit_len ? lit_len : 1);
       if (!delta->instructions[i].literal.data) {
         log_message(LOG_LEVEL_ERROR, "Failed to allocate %u bytes for literal data", lit_len);
-        for (uint32_t k = 0; k < i; k++) {
+        free_instructions(delta->instructions, i);
-          if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
-            free(delta->instructions[k].literal.data);
-        }
-        free(delta->instructions);
         free(delta);
         return NULL;
       }
@@ -447,11 +476,7 @@ Delta* delta_deserialize(const Data* data) {
       pos += lit_len;
       delta->delta_size += sizeof(uint32_t) + lit_len;
     } else {
-      for (uint32_t k = 0; k < i; k++) {
+      free_instructions(delta->instructions, i);
-        if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
-          free(delta->instructions[k].literal.data);
-      }
-      free(delta->instructions);
       free(delta);
       return NULL;
     }
@@ -463,10 +488,11 @@ Delta* delta_deserialize(const Data* data) {
 void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
                   uint32_t block_size) {
   if (!old_data || !delta || (delta->new_file_size > 0 && delta->instructions == NULL) ||
-      (delta->instruction_count > 0 && block_size == 0))
+      (delta->instruction_count > 0 && block_size == 0) ||
+      delta->new_file_size > DELTA_MAX_FILE_SIZE || delta->new_file_size > SIZE_MAX)
     return NULL;
 
-  void* output = malloc((size_t)delta->new_file_size);
+  void* output = malloc(delta->new_file_size ? (size_t)delta->new_file_size : 1);
   if (!output)
     return NULL;
 
@@ -491,7 +517,7 @@ void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
       }
       memcpy(out + out_pos, old + src_offset, len);
       out_pos += len;
-    } else {
+    } else if (delta->instructions[i].type == DELTA_INSTR_LITERAL) {
       uint32_t len = delta->instructions[i].literal.length;
       if (out_pos > delta->new_file_size || (uint64_t)len > delta->new_file_size - out_pos) {
         free(output);
@@ -499,6 +525,9 @@ void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
       }
       memcpy(out + out_pos, delta->instructions[i].literal.data, len);
       out_pos += len;
+    } else {
+      free(output);
+      return NULL;
     }
   }
 
@@ -534,7 +563,7 @@ bool delta_should_attempt(uint64_t old_size, uint64_t new_size, uint64_t max_fil
 }
 
 bool delta_is_worthwhile(const Delta* delta, uint64_t new_file_size) {
-  if (!delta || delta->instruction_count == 0)
+  if (!delta || delta->instruction_count == 0 || new_file_size == 0)
     return false;
 
   bool has_match = false;

src/shared/file.c

@@ -1,29 +1,34 @@
-#include <dirent.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <libgen.h>
 #include <limits.h>
-#include <poll.h>
-#include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/sendfile.h>
 #include <sys/stat.h>
 #include <unistd.h>
-#include <time.h>
 
-#include "compression.h"
-#include "delta.h"
-#include "log.h"
-#include "config.h"
 #include "data.h"
+#include "delta.h"
 #include "file.h"
-#include "file_store.h"
+#include "log.h"
 #include "metadata.h"
-#include "protocol.h"
 #include "utils.h"
 
+static bool write_all(int fd, const void* data, unsigned long long size) {
+  const unsigned char* p = data;
+  unsigned long long done = 0;
+  while (done < size) {
+    ssize_t n = write(fd, p + done, (size_t)(size - done));
+    if (n < 0 && errno == EINTR)
+      continue;
+    if (n <= 0)
+      return false;
+    done += (unsigned long long)n;
+  }
+  return true;
+}
+
 bool file_checksum(File* file, uint64_t* checksum) {
   if (!file || !checksum || !file->data)
     return false;
@@ -42,11 +47,11 @@ File* file_create(const char* path) {
     return NULL;
   File* file = (File*)malloc(sizeof(File));
   if (file == NULL) {
-    perror("ERROR: Could not allocate memory for file struct");
+    log_perror("ERROR: Could not allocate memory for file struct");
     return NULL;
   }
 
-  int path_len = strlen(path);
+  size_t path_len = strlen(path);
   file->path = (char*)malloc(path_len + 1);
   if (file->path == NULL) {
     free(file);
@@ -82,7 +87,7 @@ void file_destroy(void* item) {
 FileMetadata* file_metadata_create(const struct stat* stats) {
   FileMetadata* m = malloc(sizeof(FileMetadata));
   if (m == NULL) {
-    perror("ERROR: Could not allocate memory for file metadata");
+    log_perror("ERROR: Could not allocate memory for file metadata");
     return NULL;
   }
   m->mode = stats->st_mode;
@@ -102,14 +107,14 @@ void file_metadata_destroy(void* metadata) {
 }
 
 bool file_load_data(File* file) {
-  if (file == NULL)
+  if (file == NULL || !file->data)
     return false;
   if (file->data->data == NULL) {
     if (file->data->size == 0)
       return true;
     file->data->data = malloc(file->data->size);
     if (file->data->data == NULL) {
-      perror("Could not allocate memory for file data");
+      log_perror("Could not allocate memory for file data");
       return false;
     }
   }
@@ -124,711 +129,227 @@ bool file_load_data(File* file) {
   return true;
 }
 
-bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
-                            int compression_level, bool send_path) {
-  if (!file || !file->path || !file->data)
-    return false;
-  const Data* data_to_send = file->data;
-  Data* compressed_data = NULL;
-  if (compression_level > 0 && !compression_should_skip(file->path)) {
-    compressed_data = data_compress(file->data, compression_level);
-    if (compressed_data == NULL) {
-      log_message(LOG_LEVEL_ERROR, "Failed to compress file data");
-      return false;
-    }
-    data_to_send = compressed_data;
-  }
-  if (send_path && !send_str(file_descriptor, file->path)) {
-    data_destroy(compressed_data);
-    return false;
-  }
-  if (use_metadata && !metadata_send(file_descriptor, file->metadata)) {
-    data_destroy(compressed_data);
-    return false;
-  }
-  if (!send_data(file_descriptor, data_to_send)) {
-    data_destroy(compressed_data);
-    return false;
-  }
-  data_destroy(compressed_data);
-  return true;
-}
-
-void file_set_authorized_root(int fd, const char* canonical_path) {
-  file_store_set_authorized_root(fd, canonical_path);
-}
-
-static bool path_is_within_root(const char* root, const char* path) {
-  size_t n = strlen(root);
-  return strncmp(root, path, n) == 0 && (path[n] == '\0' || path[n] == '/');
-}
-
-bool file_save_to_disk(const char* root_directory, const File* file, const Config* config) {
-  bool backup_enabled = config && config->backup;
-  bool inplace = config && config->inplace;
-  bool sparse = config && config->preserve_sparse;
-  const char* backup_suffix = (config && config->suffix) ? config->suffix : "~";
-  const char* backup_dir = (config && config->backup_dir) ? config->backup_dir : NULL;
-  const char* partial_dir = (config && config->partial_dir) ? config->partial_dir : NULL;
-  char *confined_backup = NULL, *confined_partial = NULL;
-
-  if (!file || !file->path || !file->data || has_path_traversal(file->path) ||
-      (backup_enabled &&
-       (!backup_suffix || backup_suffix[0] == '\0' || strchr(backup_suffix, '/') != NULL ||
-        strcmp(backup_suffix, ".") == 0 || strcmp(backup_suffix, "..") == 0))) {
-    log_message(LOG_LEVEL_ERROR, "Invalid file or path received");
-    return false;
-  }
-
-  /* These options arrive from the client.  They are names below the server
-     root, never independent filesystem roots. */
-  if ((backup_dir && (backup_dir[0] == '/' || has_path_traversal(backup_dir))) ||
-      (partial_dir && (partial_dir[0] == '/' || has_path_traversal(partial_dir))))
-    return false;
-  if (backup_dir && !(confined_backup = path_cat(root_directory, backup_dir)))
-    return false;
-  if (partial_dir && !(confined_partial = path_cat(root_directory, partial_dir))) {
-    free(confined_backup);
-    return false;
-  }
-
-  char* resolved_root = NULL;
-  const char* actual_root =
-      (partial_dir && config && config->partial) ? confined_partial : root_directory;
-  resolved_root = realpath(actual_root, NULL);
-  if (resolved_root == NULL) {
-    if (mkdir_r(actual_root)) {
-      resolved_root = realpath(actual_root, NULL);
-    }
-  }
-  if (resolved_root == NULL) {
-    log_message(LOG_LEVEL_ERROR, "Failed to resolve destination root: %s", actual_root);
-    free(confined_backup);
-    free(confined_partial);
-    return false;
-  }
-  char* resolved_base = realpath(root_directory, NULL);
-  if (resolved_base == NULL || !path_is_within_root(resolved_base, resolved_root)) {
-    free(resolved_base);
-    free(confined_backup);
-    free(confined_partial);
-    free(resolved_root);
-    return false;
-  }
-  free(resolved_base);
-
-  char* disk_path = path_cat(resolved_root, file->path);
-  if (disk_path == NULL) {
-    free(confined_backup);
-    free(confined_partial);
-    free(resolved_root);
-    return false;
-  }
-
-  /* --update is receiver-side policy: never replace a newer destination. */
-  if (config && config->update) {
-    struct stat destination_stat;
-    if (stat(disk_path, &destination_stat) == 0 && file->metadata &&
-        destination_stat.st_mtime > file->metadata->mtime_sec) {
-      free(resolved_root);
-      free(confined_backup);
-      free(confined_partial);
-      free(disk_path);
-      return true;
-    }
-  }
-
-  if (backup_enabled) {
-    struct stat backup_stat;
-    if (stat(disk_path, &backup_stat) == 0) {
-      char* backup_path = NULL;
-      if (backup_dir) {
-        char* resolved_backup_dir = realpath(confined_backup, NULL);
-        if (!resolved_backup_dir) {
-          mkdir_r(confined_backup);
-          resolved_backup_dir = realpath(confined_backup, NULL);
-        }
-        if (resolved_backup_dir) {
-          char* backup_base = realpath(root_directory, NULL);
-          if (backup_base && path_is_within_root(backup_base, resolved_backup_dir))
-            backup_path = path_cat(resolved_backup_dir, file->path);
-          free(backup_base);
-          free(resolved_backup_dir);
-        }
-      }
-      if (!backup_path) {
-        size_t path_len = strlen(disk_path);
-        size_t suffix_len = strlen(backup_suffix);
-        backup_path = malloc(path_len + suffix_len + 1);
-        if (backup_path) {
-          memcpy(backup_path, disk_path, path_len);
-          memcpy(backup_path + path_len, backup_suffix, suffix_len + 1);
-        }
-      }
-      if (backup_path) {
-        char* backup_dir_path = str_dup(backup_path);
-        if (backup_dir_path) {
-          const char* bdir = dirname(backup_dir_path);
-          mkdir_r(bdir);
-          free(backup_dir_path);
-        }
-        if (!file_store_rename_secure(disk_path, backup_path)) {
-          free(backup_path);
-          free(resolved_root);
-          free(confined_backup);
-          free(confined_partial);
-          free(disk_path);
-          return false;
-        }
-        free(backup_path);
-      }
-    }
-  }
-
-  char* dir_dup = str_dup(disk_path);
-  if (!dir_dup) {
-    free(confined_backup);
-    free(confined_partial);
-    free(resolved_root);
-    free(disk_path);
-    return false;
-  }
-  char* dir_str = dirname(dir_dup);
-  if (!mkdir_r(dir_str)) {
-    free(dir_dup);
-    free(confined_backup);
-    free(confined_partial);
-    free(resolved_root);
-    free(disk_path);
-    return false;
-  }
-  char* resolved_dir = realpath(dir_str, NULL);
-  free(dir_dup);
-  if (resolved_dir == NULL) {
-    log_message(LOG_LEVEL_ERROR, "Failed to resolve directory for: %s", disk_path);
-    free(confined_backup);
-    free(confined_partial);
-    free(resolved_root);
-    free(disk_path);
-    return false;
-  }
-
-  size_t root_len = strlen(resolved_root);
-  if (strncmp(resolved_dir, resolved_root, root_len) != 0 ||
-      (resolved_dir[root_len] != '\0' && resolved_dir[root_len] != '/')) {
-    log_message(LOG_LEVEL_ERROR, "Path escape detected: %s is outside %s", disk_path, actual_root);
-    free(resolved_dir);
-    free(confined_backup);
-    free(confined_partial);
-    free(resolved_root);
-    free(disk_path);
-    return false;
-  }
-  free(resolved_dir);
-  free(resolved_root);
-
-  bool ok = file_store_write_secure(disk_path, file->data->data, file->data->size, inplace, sparse,
-                                    file->metadata);
-  free(confined_backup);
-  free(confined_partial);
-  free(disk_path);
-  return ok;
-}
-
-static File* receive_delta_file(int fd, const Config* config, const char* check_path,
-                                void* old_data, unsigned long long old_size) {
-  if (!old_data)
-    return NULL;
-
-  DeltaSignature* sig = delta_signature_create(old_data, old_size, config->delta_block_size);
-  if (!sig) {
-    free(old_data);
-    return NULL;
-  }
-
-  Data* sig_data = delta_signature_serialize(sig);
-  if (!sig_data) {
-    delta_signature_destroy(sig);
-    free(old_data);
-    return NULL;
-  }
-
-  bool sig_sent = send_status(fd, STATUS_DELTA_SIGNATURE) && send_data(fd, sig_data);
-  data_destroy(sig_data);
-
-  if (!sig_sent) {
-    delta_signature_destroy(sig);
-    free(old_data);
-    return NULL;
-  }
-
-  Status resp;
-  if (!receive_status(fd, &resp)) {
-    delta_signature_destroy(sig);
-    free(old_data);
-    return NULL;
-  }
-
-  if (resp == STATUS_DELTA_DATA) {
-    Data* delta_data = receive_data(fd);
-    if (!delta_data) {
-      delta_signature_destroy(sig);
-      free(old_data);
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-
-    Data* raw_delta = delta_data;
-    if (config->use_compression) {
-      raw_delta = data_decompress(delta_data);
-      data_destroy(delta_data);
-      if (!raw_delta) {
-        free(old_data);
-        delta_signature_destroy(sig);
-        send_status(fd, STATUS_ERROR);
-        return NULL;
-      }
-    }
-
-    Delta* delta = delta_deserialize(raw_delta);
-    data_destroy(raw_delta);
-    if (!delta) {
-      free(old_data);
-      delta_signature_destroy(sig);
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-
-    void* new_data = delta_apply(old_data, old_size, delta, config->delta_block_size);
-    uint64_t new_size = delta->new_file_size;
-    delta_destroy(delta);
-
-    if (!new_data) {
-      free(old_data);
-      delta_signature_destroy(sig);
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-
-    File* file = file_create(check_path);
-    if (!file) {
-      free(new_data);
-      free(old_data);
-      delta_signature_destroy(sig);
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-
-    if (config->use_metadata) {
-      int meta_ok = 1;
-      file->metadata = metadata_receive(fd, &meta_ok);
-      if (!meta_ok) {
-        file_destroy(file);
-        free(new_data);
-        free(old_data);
-        delta_signature_destroy(sig);
-        send_status(fd, STATUS_ERROR);
-        return NULL;
-      }
-    }
-
-    data_destroy(file->data);
-    file->data = data_create(new_data, (size_t)new_size);
-
-    free(old_data);
-    delta_signature_destroy(sig);
-    return file;
-  }
-
-  if (resp == STATUS_NEXT) {
-    delta_signature_destroy(sig);
-    free(old_data);
-
-    File* file = file_create(check_path);
-    if (!file) {
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-
-    if (config->use_metadata) {
-      int meta_ok = 1;
-      file->metadata = metadata_receive(fd, &meta_ok);
-      if (!meta_ok) {
-        file_destroy(file);
-        send_status(fd, STATUS_ERROR);
-        return NULL;
-      }
-    }
-
-    Data* file_data = receive_data(fd);
-    if (file_data == NULL) {
-      file_destroy(file);
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-
-    if (config->use_compression) {
-      Data* uncompressed = data_decompress(file_data);
-      data_destroy(file_data);
-      if (uncompressed == NULL) {
-        file_destroy(file);
-        send_status(fd, STATUS_ERROR);
-        return NULL;
-      }
-      file_data = uncompressed;
-    }
-
-    data_destroy(file->data);
-    file->data = file_data;
-    return file;
-  }
-
-  delta_signature_destroy(sig);
-  free(old_data);
-  return NULL;
-}
-
-File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
-  *skipped = false;
-  char* check_path = receive_str(fd);
-  if (check_path == NULL) {
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
-
-  unsigned long long check_size;
-  long long check_mtime;
-  uint64_t check_checksum = 0;
-  if (!receive_n_data(fd, &check_size, sizeof(check_size)) ||
-      !receive_n_data(fd, &check_mtime, sizeof(check_mtime))) {
-    free(check_path);
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
-  if (config->checksum && !receive_n_data(fd, &check_checksum, sizeof(check_checksum))) {
-    free(check_path);
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
-
-  if (has_path_traversal(check_path)) {
-    log_message(LOG_LEVEL_ERROR, "Path traversal detected: %s", check_path);
-    free(check_path);
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
-
-  char* full_path = path_cat(config->receive_root_directory, check_path);
-  struct stat st;
-  bool has_old_file = false;
-  int old_fd = -1;
-  if (full_path) {
-    char* leaf = NULL;
-    int parent_fd = file_store_open_secure_parent(full_path, &leaf);
-    if (parent_fd >= 0) {
-      old_fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
-      free(leaf);
-      close(parent_fd);
-      has_old_file = old_fd >= 0 && fstat(old_fd, &st) == 0 && S_ISREG(st.st_mode);
-    }
-  }
-  unsigned long long old_size = has_old_file ? (unsigned long long)st.st_size : 0;
-  void* old_data = NULL;
-  if (has_old_file && old_size > 0) {
-    old_data = malloc((size_t)old_size);
-    if (old_data) {
-      size_t got = 0;
-      while (got < (size_t)old_size) {
-        ssize_t n = read(old_fd, (char*)old_data + got, (size_t)old_size - got);
-        if (n <= 0) {
-          free(old_data);
-          old_data = NULL;
-          break;
-        }
-        got += (size_t)n;
-      }
-    }
-  }
-  if (old_fd >= 0) {
-    close(old_fd);
-  }
-
-  bool match = has_old_file && (unsigned long long)st.st_size == check_size;
-  if (match && config->checksum) {
-    uint64_t old_checksum = old_size == 0 ? delta_xxhash64("", 0) : 0;
-    if (old_data)
-      old_checksum = delta_xxhash64(old_data, (size_t)old_size);
-    match = (old_size == 0 || old_data) && old_checksum == check_checksum;
-    free(old_data);
-    old_data = NULL;
-  } else if (match) {
-    match = (long long)st.st_mtime == check_mtime;
-  }
-
-  if (match) {
-    free(old_data);
-    if (!send_status(fd, STATUS_OK)) {
-      free(full_path);
-      free(check_path);
-      return NULL;
-    }
-    free(full_path);
-    free(check_path);
-    *skipped = true;
-    return NULL;
-  }
-
-  bool try_delta = config->use_delta && has_old_file &&
-                   delta_should_attempt(old_size, check_size, config->delta_max_file_size);
-
-  if (try_delta) {
-    File* delta_file = receive_delta_file(fd, config, check_path, old_data, old_size);
-    old_data = NULL; /* receive_delta_file consumes the snapshot on every path */
-    if (delta_file) {
-      free(full_path);
-      free(check_path);
-      return delta_file;
-    }
-    free(old_data);
-    old_data = NULL;
-    try_delta = false;
-  }
-
-  if (!try_delta) {
-    if (!send_status(fd, STATUS_NEXT)) {
-      free(full_path);
-      free(check_path);
-      return NULL;
-    }
-  }
-
-  File* file = file_create(check_path);
-  free(check_path);
-  free(full_path);
-  if (file == NULL) {
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
-
-  if (config->use_metadata) {
-    int meta_ok = 1;
-    file->metadata = metadata_receive(fd, &meta_ok);
-    if (!meta_ok) {
-      file_destroy(file);
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-  }
-
-  Data* file_data = receive_data(fd);
-  if (file_data == NULL) {
-    file_destroy(file);
-    send_status(fd, STATUS_ERROR);
-    return NULL;
-  }
-
-  if (config->use_compression) {
-    Data* uncompressed = data_decompress(file_data);
-    data_destroy(file_data);
-    if (uncompressed == NULL) {
-      file_destroy(file);
-      send_status(fd, STATUS_ERROR);
-      return NULL;
-    }
-    file_data = uncompressed;
-  }
-
-  data_destroy(file->data);
-  file->data = file_data;
-  return file;
-}
-
-bool to_disk(const char* path, const void* data, unsigned long long data_size, bool inplace,
-             bool sparse) {
-  if (!path || (!data && data_size != 0) || has_path_traversal(path))
-    return false;
-  return file_store_write_secure(path, data, data_size, inplace, sparse, NULL);
-}
-
-bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int compression_level,
-                        bool send_path) {
-  if (!file || !file->path || !file->data)
-    return false;
-  if (compression_level > 0)
-    return file_send_single_calls(file, file_descriptor, use_metadata, compression_level,
-                                  send_path);
-
-  if (send_path && !send_str(file_descriptor, file->path))
-    return false;
-  if (use_metadata && !metadata_send(file_descriptor, file->metadata))
-    return false;
-
-  int fd = open(file->path, O_RDONLY);
-  if (fd == -1) {
-    perror("Could not open file for sendfile");
-    return false;
-  }
-
-  unsigned long long file_size = file->data->size;
-  struct stat source_stat;
-  if (fstat(fd, &source_stat) != 0 || !S_ISREG(source_stat.st_mode) ||
-      (unsigned long long)source_stat.st_size < file_size) {
-    close(fd);
-    return false;
-  }
-  if (!send_n_data(file_descriptor, &file_size, sizeof(unsigned long long))) {
-    close(fd);
-    return false;
-  }
-
-  /* sendfile cannot encrypt TLS records.  Keep the framing identical but
-     route encrypted transfers through the deadline-aware IO layer. */
-  if (io_get_ssl() != NULL) {
-    unsigned char buffer[64 * 1024];
-    unsigned long long remaining = file_size;
-    bool ok = true;
-    while (remaining > 0) {
-      size_t want = remaining > sizeof(buffer) ? sizeof(buffer) : (size_t)remaining;
-      ssize_t got = read(fd, buffer, want);
-      if (got <= 0 || !send_n_data(file_descriptor, buffer, (size_t)got)) {
-        ok = false;
-        break;
-      }
-      remaining -= (unsigned long long)got;
-    }
-    close(fd);
-    return ok;
-  }
-
-  off_t offset = 0;
-  struct timespec deadline;
-  clock_gettime(CLOCK_MONOTONIC, &deadline);
-  deadline.tv_sec += 60;
-  while ((unsigned long long)offset < file_size) {
-    struct timespec now;
-    clock_gettime(CLOCK_MONOTONIC, &now);
-    long long remaining = (long long)(deadline.tv_sec - now.tv_sec) * 1000LL +
-                          (deadline.tv_nsec - now.tv_nsec) / 1000000LL;
-    if (remaining <= 0) {
-      close(fd);
-      return false;
-    }
-    struct pollfd pfd = {.fd = file_descriptor, .events = POLLOUT};
-    int timeout = remaining > INT_MAX ? INT_MAX : (int)remaining;
-    int polled = poll(&pfd, 1, timeout);
-    if (polled <= 0 || (pfd.revents & (POLLERR | POLLHUP | POLLNVAL))) {
-      close(fd);
-      return false;
-    }
-    ssize_t sent = sendfile(file_descriptor, fd, &offset, file_size - offset);
-    if (sent == -1) {
-      if (errno == EAGAIN || errno == EINTR)
-        continue;
-      perror("sendfile failed");
-      close(fd);
-      return false;
-    }
-    if (sent == 0) {
-      close(fd);
-      return false;
-    }
-  }
-
-  close(fd);
-  return true;
-}
-
-File* file_receive(const Config* config, int file_descriptor) {
-  char* path = receive_str(file_descriptor);
-  if (path == NULL)
-    return NULL;
-  if (path[0] == '\0' || has_path_traversal(path)) {
-    log_message(LOG_LEVEL_ERROR, "Invalid received file path: %s", path);
-    free(path);
-    return NULL;
-  }
-  File* file = file_create(path);
-  free(path);
-  if (file == NULL)
-    return NULL;
-  if (config->use_metadata) {
-    int meta_ok = 1;
-    file->metadata = metadata_receive(file_descriptor, &meta_ok);
-    if (!meta_ok) {
-      file_destroy(file);
-      return NULL;
-    }
-  }
-  Data* file_data = receive_data(file_descriptor);
-  if (file_data == NULL) {
-    file_destroy(file);
-    return NULL;
-  }
-  if (config->use_compression) {
-    Data* file_data_uncompressed = data_decompress(file_data);
-    data_destroy(file_data);
-    if (file_data_uncompressed == NULL) {
-      file_destroy(file);
-      return NULL;
-    }
-    file_data = file_data_uncompressed;
-  }
-  data_destroy(file->data);
-  file->data = file_data;
-  return file;
-}
-
 size_t file_content_to_buffer(File* file) {
+  if (!file || !file->path || !file->data || (!file->data->data && file->data->size != 0))
+    return 0;
   FILE* file_pointer = fopen(file->path, "rb");
   if (file_pointer == NULL) {
-    perror("Could not open the file!");
+    log_perror("Could not open the file!");
     return 0;
   }
   size_t bytes_read = fread(file->data->data, 1, file->data->size, file_pointer);
   if (bytes_read != (size_t)file->data->size) {
     fclose(file_pointer);
-    perror("Read unexpected number of bytes from File!");
+    log_perror("Read unexpected number of bytes from File!");
     return 0;
   }
   fclose(file_pointer);
   return bytes_read;
 }
 
-int receive_manifest(int fd, const Config* config, int* next_status) {
+/* ---- Secure filesystem primitives ---- */
-  int received_status = STATUS_ERROR;
+
-  int* status_out = next_status ? next_status : &received_status;
+static int authorized_root_fd = -1;
-  int count;
+static char* authorized_root_path;
-  if (!receive_int(fd, &count))
+
+static bool path_is_within_root(const char* root, const char* path) {
+  size_t root_len = strlen(root);
+  return strncmp(root, path, root_len) == 0 && (path[root_len] == '\0' || path[root_len] == '/');
+}
+
+bool file_set_authorized_root(int fd, const char* canonical_path) {
+  char* path_copy = canonical_path ? str_dup(canonical_path) : NULL;
+  if (canonical_path && !path_copy) {
+    authorized_root_fd = -1;
+    free(authorized_root_path);
+    authorized_root_path = NULL;
+    return false;
+  }
+  authorized_root_fd = fd;
+  free(authorized_root_path);
+  authorized_root_path = path_copy;
+  return true;
+}
+
+bool file_path_exists_secure(const char* path) {
+  struct stat st;
+  return file_stat_secure(path, &st);
+}
+
+bool file_stat_secure(const char* path, struct stat* st) {
+  if (!path || !st)
+    return false;
+  char* leaf = NULL;
+  int parent_fd = file_open_secure_parent(path, &leaf, false);
+  if (parent_fd < 0)
+    return false;
+  int fd = openat(parent_fd, leaf, O_RDONLY | O_NONBLOCK | O_CLOEXEC | O_NOFOLLOW);
+  bool exists = fd >= 0 && fstat(fd, st) == 0 && S_ISREG(st->st_mode);
+  if (fd >= 0)
+    close(fd);
+  close(parent_fd);
+  free(leaf);
+  return exists;
+}
+
+int file_open_secure_parent(const char* path, char** leaf_out, bool create_dirs) {
+  char* copy = str_dup(path);
+  if (!copy)
+    return -1;
+  char* parent = dirname(copy);
+  const char* slash = strrchr(path, '/');
+  char* leaf = str_dup(slash ? slash + 1 : path);
+  if (!leaf) {
+    free(copy);
     return -1;
-  if (count < 0 || count > MAX_MANIFEST_ENTRIES)
+  }
+  int fd;
+  if (authorized_root_fd >= 0) {
+    if (!authorized_root_path || path[0] != '/' ||
+        !path_is_within_root(authorized_root_path, path)) {
+      free(copy);
+      free(leaf);
       return -1;
-  ArrayList* manifest = array_list_create(free);
+    }
-  if (!manifest)
+    fd = dup(authorized_root_fd);
+    if (fd < 0) {
+      free(copy);
+      free(leaf);
       return -1;
-  size_t manifest_bytes = 0;
+    }
-  for (int i = 0; i < count; i++) {
+    size_t root_len = strlen(authorized_root_path);
-    char* s = receive_str(fd);
+    char* relative = str_dup(path + root_len);
-    size_t entry_size = s ? strlen(s) : 0;
+    if (!relative) {
-    if (!s || s[0] == '\0' || s[0] == '/' || has_path_traversal(s) ||
+      free(copy);
-        entry_size > MAX_MANIFEST_BYTES - manifest_bytes ||
+      free(leaf);
-        (manifest_bytes += entry_size) > MAX_MANIFEST_BYTES || !array_list_add(manifest, s)) {
+      close(fd);
-      free(s);
-      array_list_delete(manifest);
       return -1;
+    }
+    free(copy);
+    copy = relative;
+    parent = dirname(copy);
+  } else {
+    fd = (parent[0] == '/') ? open("/", O_RDONLY | O_DIRECTORY | O_CLOEXEC)
+                            : open(".", O_RDONLY | O_DIRECTORY | O_CLOEXEC);
   }
+  if (fd < 0) {
+    free(copy);
+    free(leaf);
+    return -1;
   }
-  if (!receive_status(fd, status_out)) {
+  char* save = NULL;
-    array_list_delete(manifest);
+  char* component = strtok_r(parent, "/", &save);
+  while (component) {
+    if (strcmp(component, "..") == 0) {
+      close(fd);
+      free(copy);
+      free(leaf);
       return -1;
+    }
+    if (strcmp(component, ".") != 0) {
+      int next = openat(fd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+      if (create_dirs && next < 0 && errno == ENOENT) {
+        if (mkdirat(fd, component, 0755) == 0 || errno == EEXIST)
+          next = openat(fd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+      }
+      if (next < 0) {
+        close(fd);
+        free(copy);
+        free(leaf);
+        return -1;
+      }
+      close(fd);
+      fd = next;
+    }
+    component = strtok_r(NULL, "/", &save);
+  }
+  free(copy);
+  *leaf_out = leaf;
+  return fd;
+}
+
+bool file_ensure_directory_secure(const char* path) {
+  char* leaf = NULL;
+  int parent_fd = file_open_secure_parent(path, &leaf, true);
+  if (parent_fd < 0)
+    return false;
+
+  int dir_fd = openat(parent_fd, leaf, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  if (dir_fd < 0 && errno == ENOENT) {
+    if (mkdirat(parent_fd, leaf, 0755) == 0 || errno == EEXIST)
+      dir_fd = openat(parent_fd, leaf, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  }
+  bool ok = dir_fd >= 0;
+  if (dir_fd >= 0)
+    close(dir_fd);
+  close(parent_fd);
+  free(leaf);
+  return ok;
+}
+
+bool file_rename_secure(const char* old_path, const char* new_path) {
+  char *old_leaf = NULL, *new_leaf = NULL;
+  int old_parent = file_open_secure_parent(old_path, &old_leaf, false);
+  int new_parent = file_open_secure_parent(new_path, &new_leaf, true);
+  bool ok = old_parent >= 0 && new_parent >= 0 &&
+            renameat(old_parent, old_leaf, new_parent, new_leaf) == 0;
+  if (old_parent >= 0)
+    close(old_parent);
+  if (new_parent >= 0)
+    close(new_parent);
+  free(old_leaf);
+  free(new_leaf);
+  return ok;
+}
+
+bool file_to_disk_secure(const char* path, const void* data, unsigned long long data_size,
+                         bool inplace, bool sparse, const FileMetadata* metadata) {
+  char* leaf = NULL;
+  int dirfd = file_open_secure_parent(path, &leaf, true);
+  if (dirfd < 0)
+    return false;
+  int fd = -1;
+  bool ok = false;
+  if (inplace) {
+    fd = openat(dirfd, leaf, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC | O_NOFOLLOW, 0644);
+    if (fd >= 0) {
+      if (!sparse || data_size == 0 || ftruncate(fd, (off_t)data_size) == 0)
+        ok = write_all(fd, data, data_size);
+      if (ok && metadata)
+        ok = file_restore_metadata_fd(fd, metadata);
+    }
+  } else {
+    char tmp[NAME_MAX];
+    for (unsigned int i = 0; i < 100 && !ok; ++i) {
+      snprintf(tmp, sizeof(tmp), ".%s.tmp.%ld.%u", leaf, (long)getpid(), i);
+      fd = openat(dirfd, tmp, O_WRONLY | O_CREAT | O_EXCL | O_CLOEXEC | O_NOFOLLOW, 0600);
+      if (fd < 0)
+        continue;
+      if (sparse && data_size > 0)
+        ok = ftruncate(fd, (off_t)data_size) == 0;
+      if (ok || (!sparse || data_size == 0))
+        ok = write_all(fd, data, data_size);
+      if (ok && metadata)
+        ok = file_restore_metadata_fd(fd, metadata);
+      if (close(fd) != 0)
+        ok = false;
+      fd = -1;
+      if (ok && renameat(dirfd, tmp, dirfd, leaf) != 0)
+        ok = false;
+      if (!ok)
+        unlinkat(dirfd, tmp, 0);
     }
-  /* Deletion is a commit operation: never perform it until the sender has
-     completed the manifest frame successfully. */
-  if (*status_out != STATUS_FINISHED || !config->use_delete) {
-    array_list_delete(manifest);
-    return *status_out == STATUS_FINISHED ? 0 : -1;
   }
-  fprintf(stderr, "Deleting files not in manifest...\n");
+  if (fd >= 0)
-  bool deletion_ok = delete_extras(config->receive_root_directory, manifest);
+    close(fd);
-  array_list_delete(manifest);
+  close(dirfd);
-  return deletion_ok ? 0 : -1;
+  free(leaf);
+  return ok;
+}
+
+bool file_write_to_disk(const char* path, const void* data, unsigned long long data_size,
+                        bool inplace, bool sparse) {
+  if (!path || (!data && data_size != 0) || has_path_traversal(path))
+    return false;
+  return file_to_disk_secure(path, data, data_size, inplace, sparse, NULL);
 }

src/shared/file.h

@@ -1,45 +1,36 @@
 #ifndef FILE_H
 #define FILE_H
 
-#include "config.h"
+#include "file_send.h"
-#include "data.h"
+#include "file_receive.h"
+#include "file_types.h"
 #include <stdbool.h>
+#include <stdint.h>
 #include <sys/stat.h>
 
-typedef enum { FILE_TYPE_REGULAR, FILE_TYPE_SYMLINK, FILE_TYPE_DIR } FileType;
+/* File/FileMetadata lifecycle, local disk helpers, and secure filesystem
-
+   primitives shared by the send/receive pipelines. */
-typedef struct {
-  mode_t mode;
-  uid_t uid;
-  gid_t gid;
-  time_t mtime_sec;
-  long mtime_nsec;
-} FileMetadata;
-
-typedef struct {
-  char* path;
-  Data* data;
-  FileMetadata* metadata;
-  bool skip;
-} File;
 
 File* file_create(const char* path);
 void file_destroy(void* item);
 bool file_load_data(File* file);
 bool file_checksum(File* file, uint64_t* checksum);
-File* file_receive(const Config* config, int file_descriptor);
-bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
-                            int compression_level, bool send_path);
-bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int compression_level,
-                        bool send_path);
 size_t file_content_to_buffer(File* file);
 FileMetadata* file_metadata_create(const struct stat* stats);
 void file_metadata_destroy(void* metadata);
-bool to_disk(const char* path, const void* data, unsigned long long data_size, bool inplace,
+bool file_write_to_disk(const char* path, const void* data, unsigned long long data_size,
-             bool sparse);
+                        bool inplace, bool sparse);
-bool file_save_to_disk(const char* root_directory, const File* file, const Config* config);
+
-void file_set_authorized_root(int fd, const char* canonical_path);
+/* A configured fd without a canonical identity deliberately rejects paths. */
-File* receive_incremental_check(int fd, const Config* config, bool* skipped);
+bool file_set_authorized_root(int fd, const char* canonical_path);
-int receive_manifest(int fd, const Config* config, int* next_status);
+
+/* Secure path/filesystem primitives (symlink-safe, O_NOFOLLOW, root-confined). */
+bool file_path_exists_secure(const char* path);
+bool file_stat_secure(const char* path, struct stat* st);
+int file_open_secure_parent(const char* path, char** leaf_out, bool create_dirs);
+bool file_ensure_directory_secure(const char* path);
+bool file_rename_secure(const char* old_path, const char* new_path);
+bool file_to_disk_secure(const char* path, const void* data, unsigned long long data_size,
+                         bool inplace, bool sparse, const FileMetadata* metadata);
 
 #endif

src/shared/file_receive.c

@@ -0,0 +1,594 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "array_list.h"
+#include "compression.h"
+#include "config.h"
+#include "data.h"
+#include "delta.h"
+#include "file.h"
+#include "log.h"
+#include "metadata.h"
+#include "protocol.h"
+#include "utils.h"
+
+#define MAX_SERVER_DELETE_COUNT 100000U
+#define MAX_FILE_DATA_SIZE MAX_RECEIVE_FILE_SIZE
+
+bool file_save_to_disk(const char* root_directory, const File* file, const Config* config) {
+  bool backup_enabled = config && config->backup;
+  bool inplace = config && config->inplace;
+  bool sparse = config && config->preserve_sparse;
+  const char* backup_suffix = (config && config->suffix) ? config->suffix : "~";
+  const char* backup_dir = (config && config->backup_dir) ? config->backup_dir : NULL;
+  const char* partial_dir = (config && config->partial_dir) ? config->partial_dir : NULL;
+  char *confined_backup = NULL, *confined_partial = NULL, *disk_path = NULL;
+  char* destination_path = NULL;
+  char *backup_path = NULL, *parent_copy = NULL;
+
+  if (!file || !file->path || !file->data || (file->data->size != 0 && !file->data->data) ||
+      has_path_traversal(file->path) ||
+      (backup_enabled &&
+       (!backup_suffix || backup_suffix[0] == '\0' || strchr(backup_suffix, '/') != NULL ||
+        strcmp(backup_suffix, ".") == 0 || strcmp(backup_suffix, "..") == 0))) {
+    log_message(LOG_LEVEL_ERROR, "Invalid file or path received");
+    return false;
+  }
+
+  /* These options arrive from the client.  They are names below the server
+     root, never independent filesystem roots. */
+  if ((backup_dir && (backup_dir[0] == '/' || has_path_traversal(backup_dir))) ||
+      (partial_dir && (partial_dir[0] == '/' || has_path_traversal(partial_dir))))
+    return false;
+  if (backup_dir && !(confined_backup = path_cat(root_directory, backup_dir)))
+    return false;
+  if (partial_dir && !(confined_partial = path_cat(root_directory, partial_dir))) {
+    free(confined_backup);
+    return false;
+  }
+
+  const char* actual_root =
+      (partial_dir && config && config->partial) ? confined_partial : root_directory;
+  destination_path = path_cat(root_directory, file->path);
+  disk_path = path_cat(actual_root, file->path);
+  if (destination_path == NULL || disk_path == NULL) {
+    free(confined_backup);
+    free(confined_partial);
+    free(destination_path);
+    free(disk_path);
+    return false;
+  }
+
+  /* --existing checks the final destination, not a temporary partial path. */
+  if (config && config->existing && !file_path_exists_secure(destination_path)) {
+    free(confined_backup);
+    free(confined_partial);
+    free(destination_path);
+    free(disk_path);
+    return true;
+  }
+  free(destination_path);
+  destination_path = NULL;
+
+  /* --update is receiver-side policy: never replace a newer destination. */
+  if (config && config->update) {
+    struct stat destination_stat;
+    if (file_stat_secure(disk_path, &destination_stat) && file->metadata &&
+        destination_stat.st_mtime > file->metadata->mtime_sec) {
+      free(confined_backup);
+      free(confined_partial);
+      free(destination_path);
+      free(disk_path);
+      return true;
+    }
+  }
+
+  if (backup_enabled) {
+    struct stat backup_stat;
+    if (file_stat_secure(disk_path, &backup_stat)) {
+      if (backup_dir) {
+        backup_path = path_cat(confined_backup, file->path);
+      } else {
+        size_t path_len = strlen(disk_path);
+        size_t suffix_len = strlen(backup_suffix);
+        if (path_len > SIZE_MAX - suffix_len - 1)
+          goto fail;
+        backup_path = malloc(path_len + suffix_len + 1);
+        if (backup_path) {
+          memcpy(backup_path, disk_path, path_len);
+          memcpy(backup_path + path_len, backup_suffix, suffix_len + 1);
+        }
+      }
+      if (!backup_path)
+        goto fail;
+      parent_copy = str_dup(backup_path);
+      if (!parent_copy || !file_ensure_directory_secure(dirname(parent_copy)))
+        goto fail;
+      free(parent_copy);
+      parent_copy = NULL;
+      if (!file_rename_secure(disk_path, backup_path))
+        goto fail;
+      free(backup_path);
+      backup_path = NULL;
+    }
+  }
+
+  bool ok = file_to_disk_secure(disk_path, file->data->data, file->data->size, inplace, sparse,
+                                file->metadata);
+  free(parent_copy);
+  free(backup_path);
+  free(confined_backup);
+  free(confined_partial);
+  free(destination_path);
+  free(disk_path);
+  return ok;
+
+fail:
+  free(parent_copy);
+  free(backup_path);
+  free(confined_backup);
+  free(confined_partial);
+  free(destination_path);
+  free(disk_path);
+  return false;
+}
+
+static File* receive_delta_file(int fd, const Config* config, const char* check_path,
+                                void* old_data, unsigned long long old_size, bool* failed) {
+  if (!old_data)
+    return NULL;
+
+  DeltaSignature* sig = delta_signature_create(old_data, old_size, config->delta_block_size);
+  if (!sig) {
+    free(old_data);
+    *failed = true;
+    return NULL;
+  }
+
+  Data* sig_data = delta_signature_serialize(sig);
+  if (!sig_data) {
+    delta_signature_destroy(sig);
+    free(old_data);
+    *failed = true;
+    return NULL;
+  }
+
+  bool sig_sent = send_status(fd, STATUS_DELTA_SIGNATURE) && send_data(fd, sig_data);
+  data_destroy(sig_data);
+
+  if (!sig_sent) {
+    delta_signature_destroy(sig);
+    free(old_data);
+    *failed = true;
+    return NULL;
+  }
+
+  Status resp;
+  if (!receive_status(fd, &resp)) {
+    delta_signature_destroy(sig);
+    free(old_data);
+    *failed = true;
+    return NULL;
+  }
+
+  if (resp == STATUS_DELTA_DATA) {
+    Data* delta_data = receive_data_limited(fd, MAX_RECEIVE_FILE_SIZE);
+    if (!delta_data) {
+      delta_signature_destroy(sig);
+      free(old_data);
+      *failed = true;
+      return NULL;
+    }
+
+    Data* raw_delta = delta_data;
+    if (config->use_compression) {
+      raw_delta = data_decompress_limited(delta_data, MAX_RECEIVE_FILE_SIZE);
+      data_destroy(delta_data);
+      if (!raw_delta) {
+        free(old_data);
+        delta_signature_destroy(sig);
+        *failed = true;
+        return NULL;
+      }
+    }
+
+    Delta* delta = delta_deserialize(raw_delta);
+    data_destroy(raw_delta);
+    if (!delta) {
+      free(old_data);
+      delta_signature_destroy(sig);
+      *failed = true;
+      return NULL;
+    }
+
+    uint64_t new_size = delta->new_file_size;
+    if (new_size > MAX_RECEIVE_FILE_SIZE || new_size > SIZE_MAX) {
+      delta_destroy(delta);
+      free(old_data);
+      delta_signature_destroy(sig);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+    void* new_data = delta_apply(old_data, old_size, delta, config->delta_block_size);
+    delta_destroy(delta);
+
+    if (!new_data) {
+      free(old_data);
+      delta_signature_destroy(sig);
+      *failed = true;
+      return NULL;
+    }
+
+    File* file = file_create(check_path);
+    if (!file) {
+      free(new_data);
+      free(old_data);
+      delta_signature_destroy(sig);
+      *failed = true;
+      return NULL;
+    }
+
+    if (config->use_metadata) {
+      int meta_ok = 1;
+      file->metadata = metadata_receive(fd, &meta_ok);
+      if (!meta_ok) {
+        file_destroy(file);
+        free(new_data);
+        free(old_data);
+        delta_signature_destroy(sig);
+        *failed = true;
+        return NULL;
+      }
+    }
+
+    Data* replacement = data_create(new_data, (size_t)new_size);
+    if (replacement == NULL) {
+      file_destroy(file);
+      free(old_data);
+      delta_signature_destroy(sig);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+    data_destroy(file->data);
+    file->data = replacement;
+
+    free(old_data);
+    delta_signature_destroy(sig);
+    return file;
+  }
+
+  if (resp == STATUS_NEXT) {
+    delta_signature_destroy(sig);
+    free(old_data);
+
+    File* file = file_create(check_path);
+    if (!file) {
+      *failed = true;
+      return NULL;
+    }
+
+    if (config->use_metadata) {
+      int meta_ok = 1;
+      file->metadata = metadata_receive(fd, &meta_ok);
+      if (!meta_ok) {
+        file_destroy(file);
+        *failed = true;
+        return NULL;
+      }
+    }
+
+    Data* file_data = receive_data_limited(fd, MAX_RECEIVE_FILE_SIZE);
+    if (file_data == NULL) {
+      file_destroy(file);
+      *failed = true;
+      return NULL;
+    }
+
+    if (config->use_compression) {
+      Data* uncompressed = data_decompress_limited(file_data, MAX_RECEIVE_FILE_SIZE);
+      data_destroy(file_data);
+      if (uncompressed == NULL) {
+        file_destroy(file);
+        *failed = true;
+        return NULL;
+      }
+      if (uncompressed->size > MAX_FILE_DATA_SIZE) {
+        data_destroy(uncompressed);
+        file_destroy(file);
+        send_status(fd, STATUS_ERROR);
+        return NULL;
+      }
+      file_data = uncompressed;
+    }
+
+    data_destroy(file->data);
+    file->data = file_data;
+    return file;
+  }
+
+  delta_signature_destroy(sig);
+  free(old_data);
+  send_status(fd, STATUS_ERROR);
+  *failed = true;
+  return NULL;
+}
+
+File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
+  if (!config || !skipped) {
+    send_status(fd, STATUS_ERROR);
+    return NULL;
+  }
+  *skipped = false;
+  char* check_path = receive_str(fd);
+  if (check_path == NULL) {
+    return NULL;
+  }
+
+  unsigned long long check_size;
+  long long check_mtime;
+  uint64_t check_checksum = 0;
+  if (!receive_n_data(fd, &check_size, sizeof(check_size)) ||
+      !receive_n_data(fd, &check_mtime, sizeof(check_mtime))) {
+    free(check_path);
+    return NULL;
+  }
+  if (config->checksum && !receive_n_data(fd, &check_checksum, sizeof(check_checksum))) {
+    free(check_path);
+    return NULL;
+  }
+
+  if (check_size > MAX_RECEIVE_FILE_SIZE) {
+    free(check_path);
+    send_status(fd, STATUS_ERROR);
+    return NULL;
+  }
+
+  if (has_path_traversal(check_path)) {
+    log_message(LOG_LEVEL_ERROR, "Path traversal detected: %s", check_path);
+    free(check_path);
+    return NULL;
+  }
+
+  char* full_path = path_cat(config->receive_root_directory, check_path);
+  if (!full_path) {
+    free(check_path);
+    send_status(fd, STATUS_ERROR);
+    return NULL;
+  }
+  struct stat st;
+  bool has_old_file = false;
+  int old_fd = -1;
+  char* leaf = NULL;
+  int parent_fd = file_open_secure_parent(full_path, &leaf, false);
+  if (parent_fd >= 0) {
+    old_fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
+    free(leaf);
+    close(parent_fd);
+    has_old_file = old_fd >= 0 && fstat(old_fd, &st) == 0 && S_ISREG(st.st_mode);
+  }
+  unsigned long long old_size = has_old_file ? (unsigned long long)st.st_size : 0;
+  void* old_data = NULL;
+  if (has_old_file && old_size > 0 && old_size <= MAX_RECEIVE_FILE_SIZE && old_size <= SIZE_MAX) {
+    old_data = malloc((size_t)old_size);
+    if (old_data) {
+      size_t got = 0;
+      while (got < (size_t)old_size) {
+        ssize_t n = read(old_fd, (char*)old_data + got, (size_t)old_size - got);
+        if (n <= 0) {
+          free(old_data);
+          old_data = NULL;
+          break;
+        }
+        got += (size_t)n;
+      }
+    }
+  }
+  if (old_fd >= 0) {
+    close(old_fd);
+  }
+
+  bool match = has_old_file && (unsigned long long)st.st_size == check_size;
+  if (match && config->checksum) {
+    uint64_t old_checksum = old_size == 0 ? delta_xxhash64("", 0) : 0;
+    if (old_data)
+      old_checksum = delta_xxhash64(old_data, (size_t)old_size);
+    match = (old_size == 0 || old_data) && old_checksum == check_checksum;
+    free(old_data);
+    old_data = NULL;
+  } else if (match) {
+    match = (long long)st.st_mtime == check_mtime;
+  }
+
+  if (match) {
+    free(old_data);
+    if (!send_status(fd, STATUS_OK)) {
+      free(full_path);
+      free(check_path);
+      return NULL;
+    }
+    free(full_path);
+    free(check_path);
+    *skipped = true;
+    return NULL;
+  }
+
+  bool try_delta = config->use_delta && has_old_file && old_data != NULL &&
+                   delta_should_attempt(old_size, check_size, config->delta_max_file_size);
+
+  if (try_delta) {
+    bool delta_failed = false;
+    File* delta_file =
+        receive_delta_file(fd, config, check_path, old_data, old_size, &delta_failed);
+    old_data = NULL; /* receive_delta_file consumes the snapshot on every path */
+    if (delta_file) {
+      free(full_path);
+      free(check_path);
+      return delta_file;
+    }
+    if (delta_failed) {
+      free(full_path);
+      free(check_path);
+      return NULL;
+    }
+    free(old_data);
+    old_data = NULL;
+    try_delta = false;
+  }
+
+  if (!try_delta) {
+    if (!send_status(fd, STATUS_NEXT)) {
+      free(full_path);
+      free(check_path);
+      return NULL;
+    }
+  }
+
+  File* file = file_create(check_path);
+  free(check_path);
+  free(full_path);
+  if (file == NULL) {
+    return NULL;
+  }
+
+  if (config->use_metadata) {
+    int meta_ok = 1;
+    file->metadata = metadata_receive(fd, &meta_ok);
+    if (!meta_ok) {
+      file_destroy(file);
+      return NULL;
+    }
+  }
+
+  Data* file_data = receive_data_limited(fd, MAX_RECEIVE_FILE_SIZE);
+  if (file_data == NULL) {
+    file_destroy(file);
+    return NULL;
+  }
+
+  if (config->use_compression) {
+    Data* uncompressed = data_decompress_limited(file_data, MAX_RECEIVE_FILE_SIZE);
+    data_destroy(file_data);
+    if (uncompressed == NULL) {
+      file_destroy(file);
+      return NULL;
+    }
+    if (uncompressed->size > MAX_FILE_DATA_SIZE) {
+      data_destroy(uncompressed);
+      file_destroy(file);
+      send_status(fd, STATUS_ERROR);
+      return NULL;
+    }
+    file_data = uncompressed;
+  }
+
+  data_destroy(file->data);
+  file->data = file_data;
+  return file;
+}
+
+File* file_receive(const Config* config, int file_descriptor) {
+  char* path = receive_str(file_descriptor);
+  if (path == NULL)
+    return NULL;
+  if (path[0] == '\0' || has_path_traversal(path)) {
+    log_message(LOG_LEVEL_ERROR, "Invalid received file path: %s", path);
+    free(path);
+    return NULL;
+  }
+  File* file = file_create(path);
+  free(path);
+  if (file == NULL)
+    return NULL;
+  if (config->use_metadata) {
+    int meta_ok = 1;
+    file->metadata = metadata_receive(file_descriptor, &meta_ok);
+    if (!meta_ok) {
+      file_destroy(file);
+      return NULL;
+    }
+  }
+  Data* file_data = receive_data_limited(file_descriptor, MAX_RECEIVE_FILE_SIZE);
+  if (file_data == NULL) {
+    file_destroy(file);
+    return NULL;
+  }
+  if (config->use_compression && !compression_should_skip(file->path)) {
+    Data* file_data_uncompressed = data_decompress_limited(file_data, MAX_RECEIVE_FILE_SIZE);
+    data_destroy(file_data);
+    if (file_data_uncompressed == NULL) {
+      file_destroy(file);
+      return NULL;
+    }
+    if (file_data_uncompressed->size > MAX_FILE_DATA_SIZE) {
+      data_destroy(file_data_uncompressed);
+      file_destroy(file);
+      return NULL;
+    }
+    file_data = file_data_uncompressed;
+  }
+  data_destroy(file->data);
+  file->data = file_data;
+  return file;
+}
+
+int receive_manifest(int fd, const Config* config, int* next_status) {
+  if (!config) {
+    send_status(fd, STATUS_ERROR);
+    return -1;
+  }
+  int received_status = STATUS_ERROR;
+  int* status_out = next_status ? next_status : &received_status;
+  int count;
+  if (!receive_int(fd, &count)) {
+    send_status(fd, STATUS_ERROR);
+    return -1;
+  }
+  if (count < 0 || count > MAX_MANIFEST_ENTRIES) {
+    send_status(fd, STATUS_ERROR);
+    return -1;
+  }
+  ArrayList* manifest = array_list_create(free);
+  if (!manifest) {
+    send_status(fd, STATUS_ERROR);
+    return -1;
+  }
+  size_t manifest_bytes = 0;
+  for (int i = 0; i < count; i++) {
+    char* s = receive_str(fd);
+    size_t entry_size = s ? strlen(s) : 0;
+    if (!s || s[0] == '\0' || s[0] == '/' || has_path_traversal(s) ||
+        entry_size > MAX_MANIFEST_BYTES - manifest_bytes ||
+        (manifest_bytes += entry_size) > MAX_MANIFEST_BYTES || !array_list_add(manifest, s)) {
+      free(s);
+      array_list_delete(manifest);
+      send_status(fd, STATUS_ERROR);
+      return -1;
+    }
+  }
+  if (!receive_status(fd, status_out)) {
+    array_list_delete(manifest);
+    send_status(fd, STATUS_ERROR);
+    return -1;
+  }
+  /* Deletion is a commit operation: never perform it until the sender has
+     completed the manifest frame successfully. */
+  if (*status_out != STATUS_FINISHED || !config->use_delete) {
+    array_list_delete(manifest);
+    if (*status_out != STATUS_FINISHED)
+      send_status(fd, STATUS_ERROR);
+    return *status_out == STATUS_FINISHED ? 0 : -1;
+  }
+  fprintf(stderr, "Deleting files not in manifest...\n");
+  bool deletion_ok =
+      delete_extras_limited(config->receive_root_directory, manifest, MAX_SERVER_DELETE_COUNT);
+  array_list_delete(manifest);
+  if (!deletion_ok)
+    send_status(fd, STATUS_ERROR);
+  return deletion_ok ? 0 : -1;
+}

src/shared/file_receive.h

@@ -0,0 +1,15 @@
+#ifndef FILE_RECEIVE_H
+#define FILE_RECEIVE_H
+
+#include "config.h"
+#include "file_types.h"
+#include <stdbool.h>
+
+/* Server-side file receive/save path. */
+
+File* file_receive(const Config* config, int file_descriptor);
+File* receive_incremental_check(int fd, const Config* config, bool* skipped);
+int receive_manifest(int fd, const Config* config, int* next_status);
+bool file_save_to_disk(const char* root_directory, const File* file, const Config* config);
+
+#endif

src/shared/file_send.c

@@ -0,0 +1,136 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <poll.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/sendfile.h>
+#include <sys/stat.h>
+#include <time.h>
+#include <unistd.h>
+
+#include "compression.h"
+#include "data.h"
+#include "file.h"
+#include "log.h"
+#include "metadata.h"
+#include "protocol.h"
+
+bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
+                            int compression_level, bool send_path) {
+  if (!file || !file->path || !file->data || (file->data->size != 0 && !file->data->data))
+    return false;
+  const Data* data_to_send = file->data;
+  Data* compressed_data = NULL;
+  if (compression_level > 0 && !compression_should_skip(file->path)) {
+    compressed_data = data_compress(file->data, compression_level);
+    if (compressed_data == NULL) {
+      log_message(LOG_LEVEL_ERROR, "Failed to compress file data");
+      return false;
+    }
+    data_to_send = compressed_data;
+  }
+  if (send_path && !send_str(file_descriptor, file->path)) {
+    data_destroy(compressed_data);
+    return false;
+  }
+  if (use_metadata && !metadata_send(file_descriptor, file->metadata)) {
+    data_destroy(compressed_data);
+    return false;
+  }
+  if (!send_data(file_descriptor, data_to_send)) {
+    data_destroy(compressed_data);
+    return false;
+  }
+  data_destroy(compressed_data);
+  return true;
+}
+
+bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int compression_level,
+                        bool send_path) {
+  if (!file || !file->path || !file->data)
+    return false;
+  if (compression_level > 0)
+    return file_send_single_calls(file, file_descriptor, use_metadata, compression_level,
+                                  send_path);
+
+  if (send_path && !send_str(file_descriptor, file->path))
+    return false;
+  if (use_metadata && !metadata_send(file_descriptor, file->metadata))
+    return false;
+
+  int fd = open(file->path, O_RDONLY);
+  if (fd == -1) {
+    log_perror("Could not open file for sendfile");
+    return false;
+  }
+
+  unsigned long long file_size = file->data->size;
+  struct stat source_stat;
+  if (fstat(fd, &source_stat) != 0 || !S_ISREG(source_stat.st_mode) ||
+      (unsigned long long)source_stat.st_size < file_size) {
+    close(fd);
+    return false;
+  }
+  if (!send_n_data(file_descriptor, &file_size, sizeof(unsigned long long))) {
+    close(fd);
+    return false;
+  }
+
+  /* sendfile cannot encrypt TLS records.  Keep the framing identical but
+     route encrypted transfers through the deadline-aware IO layer. */
+  if (io_get_ssl() != NULL) {
+    unsigned char buffer[64 * 1024];
+    unsigned long long remaining = file_size;
+    bool ok = true;
+    while (remaining > 0) {
+      size_t want = remaining > sizeof(buffer) ? sizeof(buffer) : (size_t)remaining;
+      ssize_t got = read(fd, buffer, want);
+      if (got <= 0 || !send_n_data(file_descriptor, buffer, (size_t)got)) {
+        ok = false;
+        break;
+      }
+      remaining -= (unsigned long long)got;
+    }
+    close(fd);
+    return ok;
+  }
+
+  off_t offset = 0;
+  struct timespec deadline;
+  clock_gettime(CLOCK_MONOTONIC, &deadline);
+  deadline.tv_sec += 60;
+  while ((unsigned long long)offset < file_size) {
+    struct timespec now;
+    clock_gettime(CLOCK_MONOTONIC, &now);
+    long long remaining = (long long)(deadline.tv_sec - now.tv_sec) * 1000LL +
+                          (deadline.tv_nsec - now.tv_nsec) / 1000000LL;
+    if (remaining <= 0) {
+      close(fd);
+      return false;
+    }
+    struct pollfd pfd = {.fd = file_descriptor, .events = POLLOUT};
+    int timeout = remaining > INT_MAX ? INT_MAX : (int)remaining;
+    int polled = poll(&pfd, 1, timeout);
+    if (polled <= 0 || (pfd.revents & (POLLERR | POLLHUP | POLLNVAL))) {
+      close(fd);
+      return false;
+    }
+    ssize_t sent = sendfile(file_descriptor, fd, &offset, file_size - offset);
+    if (sent == -1) {
+      if (errno == EAGAIN || errno == EINTR)
+        continue;
+      log_perror("sendfile failed");
+      close(fd);
+      return false;
+    }
+    if (sent == 0) {
+      close(fd);
+      return false;
+    }
+  }
+
+  close(fd);
+  return true;
+}

src/shared/file_send.h

@@ -0,0 +1,14 @@
+#ifndef FILE_SEND_H
+#define FILE_SEND_H
+
+#include "file_types.h"
+#include <stdbool.h>
+
+/* Client-side file send path. */
+
+bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
+                            int compression_level, bool send_path);
+bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int compression_level,
+                        bool send_path);
+
+#endif

src/shared/file_store.c

@@ -20,10 +20,18 @@ static bool path_is_within_root(const char* root, const char* path) {
          (path[root_length] == '\0' || path[root_length] == '/');
 }
 
-void file_store_set_authorized_root(int fd, const char* canonical_path) {
+bool file_store_set_authorized_root(int fd, const char* canonical_path) {
-  authorized_root_fd = fd;
+  char* new_path = canonical_path ? str_dup(canonical_path) : NULL;
+  if (canonical_path && !new_path) {
+    authorized_root_fd = -1;
     free(authorized_root_path);
-  authorized_root_path = canonical_path ? str_dup(canonical_path) : NULL;
+    authorized_root_path = NULL;
+    return false;
+  }
+  free(authorized_root_path);
+  authorized_root_path = new_path;
+  authorized_root_fd = fd;
+  return true;
 }
 
 int file_store_open_secure_parent(const char* path, char** leaf_out) {
@@ -38,9 +46,19 @@ int file_store_open_secure_parent(const char* path, char** leaf_out) {
     return -1;
   }
   int fd;
-  if (authorized_root_fd >= 0 && authorized_root_path && path[0] == '/' &&
+  if (authorized_root_fd >= 0) {
-      path_is_within_root(authorized_root_path, path)) {
+    if (!authorized_root_path || path[0] != '/' ||
+        !path_is_within_root(authorized_root_path, path)) {
+      free(copy);
+      free(leaf);
+      return -1;
+    }
     fd = dup(authorized_root_fd);
+    if (fd < 0) {
+      free(copy);
+      free(leaf);
+      return -1;
+    }
     size_t root_length = strlen(authorized_root_path);
     char* relative = str_dup(path + root_length);
     if (!relative) {
@@ -64,10 +82,18 @@ int file_store_open_secure_parent(const char* path, char** leaf_out) {
   char* save = NULL;
   char* component = strtok_r(parent, "/", &save);
   while (component) {
-    if (strcmp(component, ".") != 0 && strcmp(component, "..") != 0) {
+    if (strcmp(component, "..") == 0) {
+      close(fd);
+      free(copy);
+      free(leaf);
+      return -1;
+    }
+    if (strcmp(component, ".") != 0) {
       int next = openat(fd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
-      if (next < 0 && errno == ENOENT && mkdirat(fd, component, 0755) == 0)
+      if (next < 0 && errno == ENOENT) {
+        if (mkdirat(fd, component, 0755) == 0 || errno == EEXIST)
           next = openat(fd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+      }
       if (next < 0) {
         close(fd);
         free(copy);
@@ -130,9 +156,20 @@ bool file_store_write_secure(const char* path, const void* data, unsigned long l
         ok = file_restore_metadata_fd(fd, metadata);
     }
   } else {
-    char tmp[NAME_MAX];
+    int tmp_size = snprintf(NULL, 0, ".%s.tmp.%ld.%u", leaf, (long)getpid(), 99U);
+    if (tmp_size < 0) {
+      close(dirfd);
+      free(leaf);
+      return false;
+    }
+    char* tmp = malloc((size_t)tmp_size + 1);
+    if (!tmp) {
+      close(dirfd);
+      free(leaf);
+      return false;
+    }
     for (unsigned int i = 0; i < 100 && !ok; ++i) {
-      snprintf(tmp, sizeof(tmp), ".%s.tmp.%ld.%u", leaf, (long)getpid(), i);
+      snprintf(tmp, (size_t)tmp_size + 1, ".%s.tmp.%ld.%u", leaf, (long)getpid(), i);
       fd = openat(dirfd, tmp, O_WRONLY | O_CREAT | O_EXCL | O_CLOEXEC | O_NOFOLLOW, 0600);
       if (fd < 0)
         continue;
@@ -150,6 +187,7 @@ bool file_store_write_secure(const char* path, const void* data, unsigned long l
       if (!ok)
         unlinkat(dirfd, tmp, 0);
     }
+    free(tmp);
   }
   if (fd >= 0)
     close(fd);

src/shared/file_store.h

@@ -4,7 +4,7 @@
 #include "file.h"
 #include <stdbool.h>
 
-void file_store_set_authorized_root(int fd, const char* canonical_path);
+bool file_store_set_authorized_root(int fd, const char* canonical_path);
 int file_store_open_secure_parent(const char* path, char** leaf_out);
 bool file_store_rename_secure(const char* old_path, const char* new_path);
 bool file_store_write_secure(const char* path, const void* data, unsigned long long data_size,

src/shared/file_types.h

@@ -0,0 +1,25 @@
+#ifndef FILE_TYPES_H
+#define FILE_TYPES_H
+
+#include "data.h"
+#include <stdbool.h>
+#include <sys/stat.h>
+
+typedef enum { FILE_TYPE_REGULAR, FILE_TYPE_SYMLINK, FILE_TYPE_DIR } FileType;
+
+typedef struct {
+  mode_t mode;
+  uid_t uid;
+  gid_t gid;
+  time_t mtime_sec;
+  long mtime_nsec;
+} FileMetadata;
+
+typedef struct {
+  char* path;
+  Data* data;
+  FileMetadata* metadata;
+  bool skip;
+} File;
+
+#endif

src/shared/log.c

@@ -1,6 +1,8 @@
 #include "log.h"
+#include <errno.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <string.h>
 #include <time.h>
 
 static const char* log_level_strings[] = {"DEBUG", "INFO", "WARN", "ERROR"};
@@ -50,3 +52,7 @@ void log_message(LogLevel log_level, const char* format, ...) {
     va_end(args);
   }
 }
+
+void log_perror(const char* context) {
+  log_message(LOG_LEVEL_ERROR, "%s: %s", context, strerror(errno));
+}

src/shared/log.h

@@ -6,6 +6,7 @@
 typedef enum { LOG_LEVEL_DEBUG, LOG_LEVEL_INFO, LOG_LEVEL_WARNING, LOG_LEVEL_ERROR } LogLevel;
 
 void log_message(LogLevel log_level, const char* message, ...);
+void log_perror(const char* context);
 void set_log_level(LogLevel level);
 void log_set_file(FILE* fp);
 

src/shared/metadata.c

@@ -51,6 +51,8 @@ FileMetadata* metadata_from_buf(char** buf) {
   int32_t present;
   memcpy(&present, *buf, sizeof(present));
   *buf += sizeof(present);
+  if (present != 0 && present != 1)
+    return NULL;
   if (!present)
     return NULL;
   FileMetadata* m = malloc(sizeof(FileMetadata));
@@ -76,10 +78,15 @@ FileMetadata* metadata_from_buf(char** buf) {
   memcpy(&mtime_nsec, *buf, sizeof(mtime_nsec));
   *buf += sizeof(mtime_nsec);
   m->mtime_nsec = (long)mtime_nsec;
+  if (present != 1 || mtime_nsec < 0 || mtime_nsec >= 1000000000LL || mode < 0 || uid < 0 ||
+      gid < 0) {
+    free(m);
+    return NULL;
+  }
   return m;
 }
 
-bool metadata_send(int file_descriptor, FileMetadata* m) {
+bool metadata_send(int file_descriptor, const FileMetadata* m) {
   if (m == NULL) {
     int32_t zero = 0;
     return send_n_data(file_descriptor, &zero, sizeof(zero));
@@ -175,7 +182,8 @@ FileMetadata* metadata_receive(int file_descriptor, int* ok) {
 void file_restore_metadata(const char* path, const FileMetadata* metadata) {
   if (metadata == NULL)
     return;
-  if (chmod(path, metadata->mode & 07777 & ~(S_ISUID | S_ISGID)) != 0)
+  mode_t safe_mode = metadata->mode & 0777 & ~(S_IWGRP | S_IWOTH);
+  if (chmod(path, safe_mode) != 0)
     log_message(LOG_LEVEL_WARNING, "Failed to chmod %s: %s", path, strerror(errno));
   /* Never apply client-supplied ownership.  The descriptor API below is the
      receiver write path; retain this legacy API only for compatibility. */
@@ -192,7 +200,8 @@ bool file_restore_metadata_fd(int fd, const FileMetadata* metadata) {
   if (fd < 0 || metadata == NULL)
     return metadata == NULL;
   bool ok = true;
-  if (fchmod(fd, metadata->mode & 07777 & ~(S_ISUID | S_ISGID)) != 0)
+  mode_t safe_mode = metadata->mode & 0777 & ~(S_IWGRP | S_IWOTH);
+  if (fchmod(fd, safe_mode) != 0)
     ok = false;
   /* Client uid/gid values are deliberately not authoritative. */
   struct timespec times[2] = {{.tv_sec = 0, .tv_nsec = UTIME_OMIT},

src/shared/metadata.h

@@ -27,7 +27,7 @@
 
 void metadata_to_buf(char** buf, const FileMetadata* m);
 FileMetadata* metadata_from_buf(char** buf);
-bool metadata_send(int file_descriptor, FileMetadata* m);
+bool metadata_send(int file_descriptor, const FileMetadata* m);
 FileMetadata* metadata_receive(int file_descriptor, int* ok);
 void file_restore_metadata(const char* path, const FileMetadata* metadata);
 bool file_restore_metadata_fd(int fd, const FileMetadata* metadata);

src/shared/multiprocessing.c

@@ -55,7 +55,7 @@ PipelineContextSender* pipeline_context_sender_create(Config* config, Queue* que
   return context;
 
 fail:
-  perror("Error initializing synchronization objects");
+  log_perror("Error initializing synchronization objects");
   if (init >= 6)
     cnd_destroy(&context->condition_not_empty_loader);
   if (init >= 5)
@@ -116,7 +116,7 @@ PipelineContextReceiver* pipeline_context_receiver_create(Config* config, Queue*
   return context;
 
 fail:
-  perror("Error initializing synchronization objects");
+  log_perror("Error initializing synchronization objects");
   if (init >= 3)
     cnd_destroy(&context->condition_not_empty);
   if (init >= 2)
@@ -183,6 +183,15 @@ int write_thread(void* pipeline_context) {
   bool save_to_disk = context->config->save_to_disk;
   char* root_directory = str_dup(context->config->receive_root_directory);
   mtx_unlock(&context->mutex);
+  if (save_to_disk && !root_directory) {
+    mtx_lock(&context->mutex);
+    atomic_store(&context->cancelled, true);
+    context->receiver_done = true;
+    cnd_broadcast(&context->condition_not_full);
+    cnd_broadcast(&context->condition_not_empty);
+    mtx_unlock(&context->mutex);
+    return thrd_error;
+  }
 
   while (true) {
     File* file =

src/shared/protocol.c

@@ -13,21 +13,27 @@
 
 #define RECEIVE_TIMEOUT_SEC 60 /* 60 second per-message timeout */
 #define SEND_TIMEOUT_SEC 60
-#define MAX_CONNECTION_MEMORY (1024ULL * 1024 * 1024) /* 1 GB total per connection */
+#define MAX_CONNECTION_MEMORY (256ULL * 1024 * 1024) /* bounded cumulative receive budget */
 
 static __thread int io_read_fd = -1;
 static __thread int io_write_fd = -1;
 static __thread SSL* io_ssl;
 static __thread ProtocolSession* bound_session;
+static __thread ProtocolSession legacy_io_session = {.read_fd = -1, .write_fd = -1};
 
 static unsigned long long io_bwlimit = 0;
-static long long bw_tokens = 0;
-static struct timespec bw_last_refill = {0, 0};
 static mtx_t bw_mutex;
 static once_flag bw_mutex_once = ONCE_FLAG_INIT;
 
-static __thread unsigned long long total_allocated_bytes = 0;
+static unsigned long long global_bwlimit(void);
 
+void protocol_release_memory(size_t charge) {
+  ProtocolSession* session = bound_session ? bound_session : &legacy_io_session;
+  if ((unsigned long long)charge >= session->total_allocated_bytes)
+    session->total_allocated_bytes = 0;
+  else
+    session->total_allocated_bytes -= charge;
+}
 void io_set_fds(int read_fd, int write_fd) {
   bound_session = NULL;
   io_read_fd = read_fd;
@@ -35,7 +41,11 @@ void io_set_fds(int read_fd, int write_fd) {
   /* A descriptor switch starts a new transport; never reuse a TLS object
      belonging to a previous connection or test pipe. */
   io_ssl = NULL;
-  total_allocated_bytes = 0;
+  legacy_io_session.read_fd = read_fd;
+  legacy_io_session.write_fd = write_fd;
+  legacy_io_session.ssl = NULL;
+  legacy_io_session.total_allocated_bytes = 0;
+  protocol_session_set_bwlimit(&legacy_io_session, global_bwlimit());
 }
 
 void protocol_session_init(ProtocolSession* session, int read_fd, int write_fd) {
@@ -44,8 +54,7 @@ void protocol_session_init(ProtocolSession* session, int read_fd, int write_fd)
   memset(session, 0, sizeof(*session));
   session->read_fd = read_fd;
   session->write_fd = write_fd;
-  if (io_bwlimit)
+  protocol_session_set_bwlimit(session, global_bwlimit());
-    protocol_session_set_bwlimit(session, io_bwlimit);
 }
 
 void protocol_session_bind(ProtocolSession* session) {
@@ -65,20 +74,29 @@ static void bw_mutex_init(void) {
   mtx_init(&bw_mutex, mtx_plain);
 }
 
+static unsigned long long global_bwlimit(void) {
+  unsigned long long limit;
+  call_once(&bw_mutex_once, bw_mutex_init);
+  mtx_lock(&bw_mutex);
+  limit = io_bwlimit;
+  mtx_unlock(&bw_mutex);
+  return limit;
+}
+
 void io_set_bwlimit(unsigned long long bytes_per_sec) {
   call_once(&bw_mutex_once, bw_mutex_init);
   mtx_lock(&bw_mutex);
-  io_bwlimit = bytes_per_sec;
+  io_bwlimit =
-  bw_tokens = (long long)io_bwlimit;
+      bytes_per_sec > (unsigned long long)LLONG_MAX ? (unsigned long long)LLONG_MAX : bytes_per_sec;
-  clock_gettime(CLOCK_MONOTONIC, &bw_last_refill);
   mtx_unlock(&bw_mutex);
 }
 
 void protocol_session_set_bwlimit(ProtocolSession* session, unsigned long long bytes_per_sec) {
   if (!session)
     return;
-  session->bwlimit = bytes_per_sec;
+  session->bwlimit =
-  session->bw_tokens = (long long)bytes_per_sec;
+      bytes_per_sec > (unsigned long long)LLONG_MAX ? (unsigned long long)LLONG_MAX : bytes_per_sec;
+  session->bw_tokens = (long long)session->bwlimit;
   struct timespec now;
   clock_gettime(CLOCK_MONOTONIC, &now);
   session->bw_last_refill_sec = now.tv_sec;
@@ -126,32 +144,30 @@ SSL* io_get_ssl(void) {
   return io_ssl;
 }
 
-static ProtocolSession* legacy_session(void) {
+static ProtocolSession* legacy_session(int read_fd, int write_fd) {
-  static __thread ProtocolSession session;
   if (bound_session)
     return bound_session;
-  session.read_fd = io_read_fd;
+  int target_read_fd = io_read_fd != -1 ? io_read_fd : read_fd;
-  session.write_fd = io_write_fd;
+  int target_write_fd = io_write_fd != -1 ? io_write_fd : write_fd;
-  session.ssl = io_ssl;
+  if (legacy_io_session.read_fd != target_read_fd ||
-  session.bwlimit = io_bwlimit;
+      legacy_io_session.write_fd != target_write_fd) {
-  session.bw_tokens = (unsigned long long)(bw_tokens < 0 ? 0 : bw_tokens);
+    legacy_io_session.read_fd = target_read_fd;
-  session.bw_last_refill_sec = bw_last_refill.tv_sec;
+    legacy_io_session.write_fd = target_write_fd;
-  session.bw_last_refill_nsec = bw_last_refill.tv_nsec;
+    legacy_io_session.total_allocated_bytes = 0;
-  return &session;
+    protocol_session_set_bwlimit(&legacy_io_session, global_bwlimit());
+  } else if (legacy_io_session.bwlimit != global_bwlimit()) {
+    protocol_session_set_bwlimit(&legacy_io_session, global_bwlimit());
+  }
+  legacy_io_session.ssl = io_ssl;
+  return &legacy_io_session;
 }
 
 bool send_n_data(int file_descriptor, const void* data, size_t data_size) {
-  ProtocolSession* session = legacy_session();
+  return protocol_send_n_data(legacy_session(-1, file_descriptor), data, data_size);
-  if (session->write_fd == -1)
-    session->write_fd = file_descriptor;
-  return protocol_send_n_data(session, data, data_size);
 }
 
 bool receive_n_data(int file_descriptor, void* data, size_t data_size) {
-  ProtocolSession* session = legacy_session();
+  return protocol_receive_n_data(legacy_session(file_descriptor, -1), data, data_size);
-  if (session->read_fd == -1)
-    session->read_fd = file_descriptor;
-  return protocol_receive_n_data(session, data, data_size);
 }
 
 static int deadline_remaining_ms(const struct timespec* deadline) {
@@ -166,6 +182,8 @@ static int deadline_remaining_ms(const struct timespec* deadline) {
 }
 
 bool protocol_send_n_data(ProtocolSession* session, const void* data, size_t data_size) {
+  if (!data && data_size != 0)
+    return false;
   log_message(LOG_LEVEL_DEBUG, "    Sending n Data: %zu", data_size);
   if (!session)
     return false;
@@ -207,6 +225,8 @@ bool protocol_send_n_data(ProtocolSession* session, const void* data, size_t dat
     }
     bw_throttle_session(session, (size_t)bytes_send);
     total_bytes_send += bytes_send;
+    if (session->ssl)
+      wait_events = POLLOUT;
   }
   log_message(LOG_LEVEL_DEBUG, "    Send n Data: %zu", total_bytes_send);
   return true;
@@ -225,6 +245,7 @@ bool protocol_receive_n_data(ProtocolSession* session, void* data, size_t data_s
   size_t total_bytes_received = 0;
   short wait_events = POLLIN;
   while (total_bytes_received < data_size) {
+    if (!session->ssl || SSL_pending(session->ssl) == 0) {
       struct pollfd pfd = {.fd = fd, .events = wait_events};
       int poll_result = poll(&pfd, 1, deadline_remaining_ms(&deadline));
       if (poll_result == 0) {
@@ -239,6 +260,7 @@ bool protocol_receive_n_data(ProtocolSession* session, void* data, size_t data_s
       /* POLLHUP may accompany the final readable bytes on pipes/sockets. */
       if (pfd.revents & (POLLERR | POLLNVAL))
         return false;
+    }
 
     ssize_t bytes_received;
     if (session->ssl)
@@ -261,7 +283,9 @@ bool protocol_receive_n_data(ProtocolSession* session, void* data, size_t data_s
         log_message(LOG_LEVEL_ERROR, "Could not receive bytes");
       return false;
     }
-    total_bytes_received += bytes_received;
+    total_bytes_received += (size_t)bytes_received;
+    if (session->ssl)
+      wait_events = POLLIN;
   }
   log_message(LOG_LEVEL_DEBUG, "    Received n Data: %zu", total_bytes_received);
   return true;
@@ -325,6 +349,11 @@ char* protocol_receive_str(ProtocolSession* session) {
     free(data);
     return NULL;
   }
+  if (memchr(data, '\0', size) != NULL) {
+    free(data);
+    log_message(LOG_LEVEL_ERROR, "Received string contains an embedded NUL");
+    return NULL;
+  }
   data[size] = '\0';
   session->total_allocated_bytes += size + 1;
   log_message(LOG_LEVEL_DEBUG, "Received String: %s", data);
@@ -332,6 +361,10 @@ char* protocol_receive_str(ProtocolSession* session) {
 }
 
 bool protocol_send_data(ProtocolSession* session, const Data* data) {
+  if (!data || (!data->data && data->size != 0))
+    return false;
+  if (!session)
+    return false;
   unsigned long long data_size = data->size;
   if (!protocol_send_n_data(session, &data_size, sizeof(unsigned long long)))
     return false;
@@ -341,11 +374,13 @@ bool protocol_send_data(ProtocolSession* session, const Data* data) {
   return true;
 }
 
-Data* protocol_receive_data(ProtocolSession* session) {
+Data* protocol_receive_data_limited(ProtocolSession* session, unsigned long long maximum_size) {
+  if (!session)
+    return NULL;
   unsigned long long size = 0;
   if (!protocol_receive_n_data(session, &size, sizeof(unsigned long long)))
     return NULL;
-  if (size > MAX_DATA_PAYLOAD_SIZE) {
+  if (size > MAX_DATA_PAYLOAD_SIZE || size > maximum_size) {
     log_message(LOG_LEVEL_ERROR, "Data size %llu exceeds maximum %llu", size,
                 (unsigned long long)MAX_DATA_PAYLOAD_SIZE);
     return NULL;
@@ -368,12 +403,17 @@ Data* protocol_receive_data(ProtocolSession* session) {
   log_message(LOG_LEVEL_DEBUG, "Received %lld data", size);
   Data* result = data_create(data, (size_t)size);
   if (!result) {
-    free(data);
     session->total_allocated_bytes -= allocation_size;
+    return NULL;
   }
+  result->protocol_charge = allocation_size;
   return result;
 }
 
+Data* protocol_receive_data(ProtocolSession* session) {
+  return protocol_receive_data_limited(session, MAX_DATA_PAYLOAD_SIZE);
+}
+
 bool protocol_send_int(ProtocolSession* session, int data) {
   if (!protocol_send_n_data(session, &data, sizeof(int)))
     return false;
@@ -403,34 +443,29 @@ bool protocol_receive_status(ProtocolSession* session, Status* status) {
 }
 
 bool send_str(int fd, const char* data) {
-  (void)fd;
+  return protocol_send_str(legacy_session(-1, fd), data);
-  return protocol_send_str(legacy_session(), data);
 }
 char* receive_str(int fd) {
-  (void)fd;
+  return protocol_receive_str(legacy_session(fd, -1));
-  return protocol_receive_str(legacy_session());
 }
 bool send_data(int fd, const Data* data) {
-  (void)fd;
+  return protocol_send_data(legacy_session(-1, fd), data);
-  return protocol_send_data(legacy_session(), data);
 }
 Data* receive_data(int fd) {
-  (void)fd;
+  return protocol_receive_data_limited(legacy_session(fd, -1), MAX_DATA_PAYLOAD_SIZE);
-  return protocol_receive_data(legacy_session());
+}
+Data* receive_data_limited(int fd, unsigned long long maximum_size) {
+  return protocol_receive_data_limited(legacy_session(fd, -1), maximum_size);
 }
 bool send_int(int fd, int data) {
-  (void)fd;
+  return protocol_send_int(legacy_session(-1, fd), data);
-  return protocol_send_int(legacy_session(), data);
 }
 bool receive_int(int fd, int* data) {
-  (void)fd;
+  return protocol_receive_int(legacy_session(fd, -1), data);
-  return protocol_receive_int(legacy_session(), data);
 }
 bool send_status(int fd, Status status) {
-  (void)fd;
+  return protocol_send_status(legacy_session(-1, fd), status);
-  return protocol_send_status(legacy_session(), status);
 }
 bool receive_status(int fd, Status* status) {
-  (void)fd;
+  return protocol_receive_status(legacy_session(fd, -1), status);
-  return protocol_receive_status(legacy_session(), status);
 }

src/shared/protocol.h

@@ -10,6 +10,8 @@
 
 /* Maximum allowed data payload size for receive_data (100 MB) */
 #define MAX_DATA_PAYLOAD_SIZE (100ULL * 1024 * 1024)
+/* Maximum uncompressed file payload accepted by the receiver. */
+#define MAX_RECEIVE_FILE_SIZE (64ULL * 1024 * 1024)
 
 /* Maximum chunk size (64 MB) — prevents unbounded allocation from the wire */
 #define MAX_CHUNK_SIZE (64ULL * 1024 * 1024)
@@ -69,6 +71,7 @@ bool protocol_send_str(ProtocolSession* session, const char* data);
 char* protocol_receive_str(ProtocolSession* session);
 bool protocol_send_data(ProtocolSession* session, const Data* data);
 Data* protocol_receive_data(ProtocolSession* session);
+Data* protocol_receive_data_limited(ProtocolSession* session, unsigned long long maximum_size);
 bool protocol_send_int(ProtocolSession* session, int data);
 bool protocol_receive_int(ProtocolSession* session, int* data);
 bool protocol_send_status(ProtocolSession* session, Status status);
@@ -80,6 +83,7 @@ bool send_str(int file_descriptor, const char* data);
 char* receive_str(int file_descriptor);
 bool send_data(int file_descriptor, const Data* data);
 Data* receive_data(int file_descriptor);
+Data* receive_data_limited(int file_descriptor, unsigned long long maximum_size);
 bool send_int(int file_descriptor, int data);
 bool receive_int(int file_descriptor, int* data);
 bool send_status(int file_descriptor, Status status);

src/shared/queue.c

@@ -1,3 +1,4 @@
+#include "log.h"
 #include <stdbool.h>
 #include <limits.h>
 #include <stdio.h>
@@ -13,7 +14,7 @@ Queue* queue_create(int capacity, void (*destroyer)(void* item)) {
 
   Queue* queue = (Queue*)malloc(sizeof(Queue));
   if (queue == NULL) {
-    perror("ERROR: Could not allocate memory for queue structure");
+    log_perror("ERROR: Could not allocate memory for queue structure");
     return NULL;
   }
 
@@ -72,7 +73,7 @@ static bool queue_double_capacity(Queue* queue) {
     new_capacity = 100;
   void** new_items = malloc(new_capacity * sizeof(void*));
   if (new_items == NULL) {
-    perror("ERROR: Could not allocate memory for doubling capacity of queue.");
+    log_perror("ERROR: Could not allocate memory for doubling capacity of queue.");
     return false;
   }
   for (int i = 0; i < queue->size; i++)
@@ -127,7 +128,7 @@ bool queue_enqueue_multithreaded_cancel(Queue* queue, void* item, mtx_t* mutex,
 
 void* queue_dequeue(Queue* queue) {
   if (queue == NULL || queue_is_empty(queue)) {
-    perror("ERROR: Could not dequeue from null or empty queue.");
+    log_perror("ERROR: Could not dequeue from null or empty queue.");
     return NULL;
   }
 

src/shared/transport_ssh.c

@@ -1,3 +1,4 @@
+#include "log.h"
 #include "transport_ssh.h"
 #include "utils.h"
 #include <fcntl.h>
@@ -76,7 +77,7 @@ Client* client_connect_ssh(const char* destination, int port, const char* server
 
   int sv[2];
   if (socketpair(AF_UNIX, SOCK_STREAM, 0, sv) < 0) {
-    perror("socketpair failed");
+    log_perror("socketpair failed");
     remote_dest_destroy(&r);
     return NULL;
   }
@@ -89,7 +90,7 @@ Client* client_connect_ssh(const char* destination, int port, const char* server
 
   int exec_pipe[2];
   if (pipe(exec_pipe) < 0) {
-    perror("pipe failed");
+    log_perror("pipe failed");
     close(sv[0]);
     close(sv[1]);
     remote_dest_destroy(&r);
@@ -98,7 +99,7 @@ Client* client_connect_ssh(const char* destination, int port, const char* server
 
   pid_t pid = fork();
   if (pid < 0) {
-    perror("fork failed");
+    log_perror("fork failed");
     close(sv[0]);
     close(sv[1]);
     close(exec_pipe[0]);
@@ -151,7 +152,7 @@ Client* client_connect_ssh(const char* destination, int port, const char* server
     ssh_argv[ac++] = "--stdio";
     ssh_argv[ac] = NULL;
     execvp("ssh", ssh_argv);
-    perror("exec of ssh failed");
+    log_perror("exec of ssh failed");
     ssize_t wret = write(exec_pipe[1], "x", 1);
     (void)wret;
     _exit(1);

src/shared/transport_tcp.c

@@ -30,20 +30,20 @@ static void sigchld_handler(int sig) {
 Server* server_create(int port) {
   Server* server = (Server*)malloc(sizeof(Server));
   if (server == NULL) {
-    perror("Could not allocate space for Server");
+    log_perror("Could not allocate space for Server");
     return NULL;
   }
 
   int file_descriptor = socket(AF_INET, SOCK_STREAM, 0);
   if (file_descriptor < 0) {
-    perror("Could not create Socket!");
+    log_perror("Could not create Socket!");
     free(server);
     return NULL;
   }
   server->file_descriptor = file_descriptor;
   int opt = 1;
   if (setsockopt(server->file_descriptor, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt))) {
-    perror("Error setting a socket option!");
+    log_perror("Error setting a socket option!");
     close(server->file_descriptor);
     free(server);
     return NULL;
@@ -59,7 +59,7 @@ Server* server_create(int port) {
 
   if (bind(server->file_descriptor, (struct sockaddr*)&server->address, server->address_length) <
       0) {
-    perror("Could not bind server");
+    log_perror("Could not bind server");
     close(server->file_descriptor);
     free(server);
     return NULL;
@@ -83,7 +83,7 @@ void server_delete(Server** server) {
 static void accept_loop(Server* server, void (*child_fn)(int, void*), void* child_ctx,
                         const char* log_fmt) {
   if (listen(server->file_descriptor, SOMAXCONN) < 0) {
-    perror("Could not listen on port!");
+    log_perror("Could not listen on port!");
     return;
   }
   signal(SIGCHLD, sigchld_handler);
@@ -92,7 +92,7 @@ static void accept_loop(Server* server, void (*child_fn)(int, void*), void* chil
     socklen_t client_len = sizeof(client_addr);
     int fd = accept(server->file_descriptor, (struct sockaddr*)&client_addr, &client_len);
     if (fd < 0) {
-      perror("Could not accept the connection");
+      log_perror("Could not accept the connection");
       continue;
     }
     tcp_apply_socket_timeout(fd);
@@ -151,6 +151,10 @@ int tcp_get_contimeout_sec(void) {
   return g_contimeout_sec;
 }
 
+int tcp_get_timeout_sec(void) {
+  return g_timeout_sec;
+}
+
 static void tcp_apply_socket_timeout(int fd) {
   struct timeval tv;
   tv.tv_sec = g_timeout_sec;
@@ -218,7 +222,7 @@ bool tcp_connect_socket(Client* client, char* host, int port) {
   freeaddrinfo(result);
 
   if (!connected) {
-    perror("Could not connect to Server!");
+    log_perror("Could not connect to Server!");
     return false;
   }
 

src/shared/transport_tcp.h

@@ -35,5 +35,6 @@ void client_disconnect(Client* client);
 void client_delete(Client* client);
 void tcp_set_timeouts(int timeout_sec, int contimeout_sec);
 int tcp_get_contimeout_sec(void);
+int tcp_get_timeout_sec(void);
 
 #endif

src/shared/transport_tls.c

@@ -9,6 +9,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+#include <time.h>
 #include <unistd.h>
 
 bool tls_global_init(void) {
@@ -31,6 +35,10 @@ static void log_ssl_errors(void) {
 
 static SSL_CTX* create_ssl_ctx(bool is_server, const char* cert, const char* key,
                                const char* ca_path) {
+  if (!is_server && !ca_path) {
+    log_message(LOG_LEVEL_ERROR, "TLS clients require a CA certificate path");
+    return NULL;
+  }
   const SSL_METHOD* method = is_server ? TLS_server_method() : TLS_client_method();
   SSL_CTX* ctx = SSL_CTX_new(method);
   if (!ctx) {
@@ -39,9 +47,23 @@ static SSL_CTX* create_ssl_ctx(bool is_server, const char* cert, const char* key
     return NULL;
   }
 
-  SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION);
+  if (SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION) != 1) {
+    SSL_CTX_free(ctx);
+    return NULL;
+  }
+  if (SSL_CTX_set_cipher_list(ctx, "HIGH:!aNULL:!eNULL:!MD5:!RC4:!3DES") != 1) {
+    SSL_CTX_free(ctx);
+    return NULL;
+  }
 
   if (cert && key) {
+    struct stat key_stat;
+    if (stat(key, &key_stat) != 0 || !S_ISREG(key_stat.st_mode) || key_stat.st_uid != geteuid() ||
+        (key_stat.st_mode & (S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH))) {
+      log_message(LOG_LEVEL_ERROR, "TLS private key must be owned by the current user and private");
+      SSL_CTX_free(ctx);
+      return NULL;
+    }
     if (SSL_CTX_use_certificate_file(ctx, cert, SSL_FILETYPE_PEM) <= 0) {
       log_message(LOG_LEVEL_ERROR, "Failed to load certificate: %s", cert);
       log_ssl_errors();
@@ -83,15 +105,22 @@ static SSL* wrap_fd_with_ssl(int fd, SSL_CTX* ctx, bool is_server, const char* h
     log_message(LOG_LEVEL_ERROR, "Failed to create SSL object");
     return NULL;
   }
-  SSL_set_fd(ssl, fd);
+  if (SSL_set_fd(ssl, fd) != 1) {
+    SSL_free(ssl);
+    return NULL;
+  }
 
   // Enable hostname verification for client connections when a hostname is provided.
   // Must be done before SSL_connect to take effect during the handshake.
   if (!is_server && hostname) {
-    SSL_set1_host(ssl, hostname);
+    if (SSL_set1_host(ssl, hostname) != 1) {
+      SSL_free(ssl);
+      return NULL;
+    }
   }
 
   // Retry SSL_accept/SSL_connect on WANT_READ/WANT_WRITE (non-blocking handshake)
+  time_t deadline = time(NULL) + (is_server ? tcp_get_timeout_sec() : tcp_get_contimeout_sec());
   int ret;
   do {
     if (is_server)
@@ -101,7 +130,8 @@ static SSL* wrap_fd_with_ssl(int fd, SSL_CTX* ctx, bool is_server, const char* h
 
     if (ret <= 0) {
       int ssl_err = SSL_get_error(ssl, ret);
-      if (ssl_err == SSL_ERROR_WANT_READ || ssl_err == SSL_ERROR_WANT_WRITE)
+      if ((ssl_err == SSL_ERROR_WANT_READ || ssl_err == SSL_ERROR_WANT_WRITE) &&
+          time(NULL) < deadline)
         continue;
       log_message(LOG_LEVEL_ERROR, "SSL %s failed", is_server ? "accept" : "connect");
       log_ssl_errors();
@@ -129,8 +159,10 @@ struct tls_child_ctx {
 static void tls_child_fn(int fd, void* arg) {
   struct tls_child_ctx* ctx = (struct tls_child_ctx*)arg;
   SSL* ssl = wrap_fd_with_ssl(fd, ctx->ssl_ctx, true, NULL);
-  if (!ssl)
+  if (!ssl) {
+    io_set_ssl(NULL);
     return;
+  }
   io_set_ssl(ssl);
   ctx->handler(fd);
   SSL_shutdown(ssl);
@@ -146,12 +178,19 @@ bool server_listen_tls(Server* server, void (*handler)(int file_descriptor)) {
 
 bool client_connect_tls(Client* client, char* host, int port, const char* cert_path,
                         const char* key_path, const char* ca_path) {
-  if (!tcp_connect_socket(client, host, port))
+  if (!tcp_connect_socket(client, host, port)) {
+    if (client->file_descriptor >= 0)
+      close(client->file_descriptor);
+    client->file_descriptor = -1;
     return false;
+  }
 
   SSL_CTX* ctx = create_ssl_ctx(false, cert_path, key_path, ca_path);
-  if (!ctx)
+  if (!ctx) {
+    close(client->file_descriptor);
+    client->file_descriptor = -1;
     return false;
+  }
   client->ssl_ctx = ctx;
 
   // Pass the server hostname for TLS hostname verification (SSL_set1_host
@@ -161,6 +200,8 @@ bool client_connect_tls(Client* client, char* host, int port, const char* cert_p
   if (!ssl) {
     SSL_CTX_free(ctx);
     client->ssl_ctx = NULL;
+    close(client->file_descriptor);
+    client->file_descriptor = -1;
     return false;
   }
 

src/shared/utils.c

@@ -7,62 +7,117 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdint.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
 static int authorized_root_fd = -1;
+static char* authorized_root_path;
+
+bool utils_set_authorized_root(int fd, const char* canonical_path) {
+  char* path_copy = canonical_path ? str_dup(canonical_path) : NULL;
+  if (canonical_path && !path_copy) {
+    authorized_root_fd = -1;
+    free(authorized_root_path);
+    authorized_root_path = NULL;
+    return false;
+  }
+  authorized_root_fd = fd;
+  free(authorized_root_path);
+  authorized_root_path = path_copy;
+  return true;
+}
 
 void utils_set_authorized_root_fd(int fd) {
-  authorized_root_fd = fd;
+  (void)utils_set_authorized_root(fd, NULL);
+}
+
+static bool path_is_within_root(const char* root, const char* path) {
+  size_t root_len = strlen(root);
+  return strncmp(root, path, root_len) == 0 && (path[root_len] == '\0' || path[root_len] == '/');
+}
+
+static int open_authorized_destination(const char* dest_root) {
+  if (authorized_root_fd < 0 || !authorized_root_path || !dest_root ||
+      !path_is_within_root(authorized_root_path, dest_root))
+    return -1;
+
+  int dirfd = dup(authorized_root_fd);
+  if (dirfd < 0)
+    return -1;
+
+  const char* relative_path = dest_root + strlen(authorized_root_path);
+  while (*relative_path == '/')
+    relative_path++;
+  char* relative = str_dup(*relative_path ? relative_path : ".");
+  if (!relative) {
+    close(dirfd);
+    return -1;
+  }
+
+  char* saveptr = NULL;
+  char* component = strtok_r(relative, "/", &saveptr);
+  while (component) {
+    if (strcmp(component, "..") == 0) {
+      free(relative);
+      close(dirfd);
+      return -1;
+    }
+    if (strcmp(component, ".") == 0) {
+      component = strtok_r(NULL, "/", &saveptr);
+      continue;
+    }
+    int next = openat(dirfd, component, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+    if (next < 0) {
+      free(relative);
+      close(dirfd);
+      return -1;
+    }
+    close(dirfd);
+    dirfd = next;
+    component = strtok_r(NULL, "/", &saveptr);
+  }
+
+  free(relative);
+  return dirfd;
 }
 
 bool mkdir_r(const char* path) {
-  size_t path_len = strlen(path);
+  if (!path || *path == '\0')
-  char* path_duplicate = malloc(path_len + 1);
-  if (!path_duplicate)
     return false;
-  memcpy(path_duplicate, path, path_len + 1);
+  char* duplicate = str_dup(path);
-  size_t capacity = path_len + 2;
+  if (!duplicate)
-  char* path_current = (char*)malloc(capacity * sizeof(char));
+    return false;
-  if (!path_current) {
+  int dirfd = open(path[0] == '/' ? "/" : ".", O_RDONLY | O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
-    free(path_duplicate);
+  if (dirfd < 0) {
+    free(duplicate);
     return false;
   }
-  char* path_current_position = path_current;
-  if (path[0] == '/') {
-    path_current[0] = '/';
-    path_current[1] = '\0';
-    path_current_position += 1;
-  } else {
-    path_current[0] = '\0';
-  }
-  const char* delimiter = "/";
-  char* saveptr;
-  const char* part = strtok_r(path_duplicate, delimiter, &saveptr);
   bool ok = true;
-  while (part != NULL) {
+  char* saveptr = NULL;
-    size_t part_len = strlen(part);
+  char* component = strtok_r(duplicate, "/", &saveptr);
-    if ((size_t)(path_current_position - path_current) + part_len + 2 > capacity) {
+  while (component) {
+    if (strcmp(component, "..") == 0) {
       ok = false;
       break;
     }
-    memcpy(path_current_position, part, part_len);
+    if (strcmp(component, ".") != 0) {
-    path_current_position += part_len;
+      int next = openat(dirfd, component, O_RDONLY | O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
-    path_current_position[0] = '/';
+      if (next < 0 && errno == ENOENT) {
-    path_current_position[1] = '\0';
+        if (mkdirat(dirfd, component, 0755) == 0 || errno == EEXIST)
-    path_current_position++;
+          next = openat(dirfd, component, O_RDONLY | O_DIRECTORY | O_CLOEXEC | O_NOFOLLOW);
-    struct stat st;
+      }
-    if (stat(path_current, &st) != 0) {
+      if (next < 0) {
-      if (mkdir(path_current, 0755) != 0) {
-        perror("Could not create directory");
         ok = false;
         break;
       }
+      close(dirfd);
+      dirfd = next;
     }
-    part = strtok_r(NULL, delimiter, &saveptr);
+    component = strtok_r(NULL, "/", &saveptr);
   }
-  free(path_duplicate);
+  close(dirfd);
-  free(path_current);
+  free(duplicate);
   return ok;
 }
 
@@ -143,7 +198,8 @@ static bool is_dir_in_manifest(const char* rel_path, ArrayList* manifest) {
   return false;
 }
 
-static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifest) {
+static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifest,
+                             size_t max_delete, size_t* deleted_count) {
   int scanfd = dup(dirfd);
   if (scanfd < 0)
     return false;
@@ -152,15 +208,20 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
     close(scanfd);
     return false;
   }
-  bool all_removed = true;
   bool operation_ok = true;
   const struct dirent* entry;
   while ((entry = readdir(dir)) != NULL) {
     if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
       continue;
     char* child_rel = path_cat((char*)rel_path, entry->d_name);
+    if (!child_rel) {
+      operation_ok = false;
+      continue;
+    }
     struct stat st;
     if (fstatat(dirfd, entry->d_name, &st, AT_SYMLINK_NOFOLLOW) != 0) {
+      if (errno != ENOENT)
+        operation_ok = false;
       free(child_rel);
       continue;
     }
@@ -173,14 +234,24 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
       int childfd = openat(dirfd, entry->d_name, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
       bool child_removed = false;
       if (childfd >= 0) {
-        child_removed = delete_extras_fd(childfd, child_rel, manifest);
+        child_removed = delete_extras_fd(childfd, child_rel, manifest, max_delete, deleted_count);
-        close(childfd);
+        if (!child_removed)
-      }
-      if (child_removed && !is_dir_in_manifest(child_rel, manifest) &&
-          unlinkat(dirfd, entry->d_name, AT_REMOVEDIR) != 0 && errno != ENOENT) {
           operation_ok = false;
-      } else if (!child_removed) {
+        close(childfd);
-        all_removed = false;
+      } else if (errno != ENOENT) {
+        operation_ok = false;
+      }
+      if (child_removed && !is_dir_in_manifest(child_rel, manifest)) {
+        if (*deleted_count >= max_delete) {
+          operation_ok = false;
+        } else {
+          if (unlinkat(dirfd, entry->d_name, AT_REMOVEDIR) != 0) {
+            if (errno != ENOENT)
+              operation_ok = false;
+          } else {
+            (*deleted_count)++;
+          }
+        }
       }
     } else {
       // Check if relative path is in manifest
@@ -192,38 +263,59 @@ static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifes
         }
       }
       if (!found) {
-        if (unlinkat(dirfd, entry->d_name, 0) != 0 && errno != ENOENT)
+        if (*deleted_count >= max_delete) {
+          operation_ok = false;
+          free(child_rel);
+          continue;
+        }
+        if (unlinkat(dirfd, entry->d_name, 0) != 0) {
+          if (errno != ENOENT)
             operation_ok = false;
-        fprintf(stderr, "  Deleted: %s\n", child_rel);
         } else {
-        all_removed = false;
+          (*deleted_count)++;
+        }
+        fprintf(stderr, "  Deleted: %s\n", child_rel);
       }
     }
     free(child_rel);
   }
   closedir(dir);
-  (void)all_removed;
   return operation_ok;
 }
 
-bool delete_extras(const char* dest_root, ArrayList* manifest) {
+bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete) {
-  int rootfd = authorized_root_fd >= 0
+  if (!manifest)
-                   ? dup(authorized_root_fd)
+    return false;
-                   : open(dest_root, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  int rootfd;
+  if (authorized_root_fd >= 0) {
+    if (authorized_root_path)
+      rootfd = open_authorized_destination(dest_root);
+    else if (dest_root == NULL)
+      rootfd = dup(authorized_root_fd);
+    else
+      rootfd = -1;
+  } else {
+    rootfd = open(dest_root, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  }
   if (rootfd < 0)
     return false;
-  bool ok = delete_extras_fd(rootfd, "", manifest);
+  size_t deleted_count = 0;
+  bool ok = delete_extras_fd(rootfd, "", manifest, max_delete, &deleted_count);
   if (close(rootfd) != 0)
     ok = false;
   return ok;
 }
 
+bool delete_extras(const char* dest_root, ArrayList* manifest) {
+  return delete_extras_limited(dest_root, manifest, SIZE_MAX);
+}
+
 bool has_path_traversal(const char* path) {
   if (!path)
-    return false;
+    return true;
   char* dup = str_dup(path);
   if (!dup)
-    return false;
+    return true;
   char* saveptr;
   const char* part = strtok_r(dup, "/", &saveptr);
   while (part) {
@@ -255,6 +347,8 @@ char* path_cat(const char* path1, const char* path2) {
     offset = 1;
     path2_len -= 1;
   }
+  if (path1_len > SIZE_MAX - path2_len - 2)
+    return NULL;
   char* new_path = malloc(path1_len + path2_len + 2);
   if (new_path == NULL)
     return NULL;

src/shared/utils.h

@@ -2,6 +2,7 @@
 #define UTILS_H
 
 #include "array_list.h"
+#include <stddef.h>
 #include <stdbool.h>
 
 bool mkdir_r(const char* path);
@@ -9,6 +10,10 @@ char* str_dup(const char* string);
 char* path_cat(const char* path1, const char* path2);
 bool glob_match(const char* pattern, const char* str);
 bool delete_extras(const char* dest_root, ArrayList* manifest);
+bool delete_extras_limited(const char* dest_root, ArrayList* manifest, size_t max_delete);
+bool utils_set_authorized_root(int fd, const char* canonical_path);
+/* The fd-only compatibility form is fail-closed for path-based operations;
+ * callers should use utils_set_authorized_root with the canonical identity. */
 void utils_set_authorized_root_fd(int fd);
 bool has_path_traversal(const char* path);
 bool utils_valid_batch_path(const char* path);

tests/integration/common.py

@@ -25,7 +25,9 @@ class ServerManager:
     def start(self, extra_args=None):
         self.stop()
         self._port = _find_free_port()
-        cmd = SERVER_CMD + ["-p", str(self._port)]
+        # Plain TCP is intentionally explicit in the server; integration tests
+        # exercise that opt-in mode rather than relying on the secure default.
+        cmd = SERVER_CMD + ["-p", str(self._port), "--allow-unauthenticated"]
         if extra_args:
             cmd += extra_args
         self._proc = subprocess.Popen(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)

tests/integration/test_features.py

@@ -214,6 +214,34 @@ class TestIncremental:
             assert f.read() == b"hello world\n"
 
 
+class TestExisting:
+    def test_existing_updates_existing_and_skips_new(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0, f"Initial sync failed: {(result.stderr or result.stdout)[:200]}"
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")
+        new_source_file = os.path.join(SOURCE_DIR, "new-existing-test.txt")
+        with open(source_file, "wb") as f:
+            f.write(b"updated existing content\n")
+        with open(new_source_file, "wb") as f:
+            f.write(b"this file must not be created\n")
+
+        try:
+            result, _ = run_client(SOURCE_DIR, DEST_DIR,
+                                   flags=["-M", "--existing"], port=shared_server.port)
+            assert result.returncode == 0, f"--existing sync failed: {(result.stderr or result.stdout)[:200]}"
+
+            with open(os.path.join(received, "small.txt"), "rb") as f:
+                assert f.read() == b"updated existing content\n"
+            assert not os.path.exists(os.path.join(received, "new-existing-test.txt"))
+        finally:
+            os.unlink(new_source_file)
+            with open(source_file, "wb") as f:
+                f.write(b"hello world\n")
+
+
 class TestDelete:
     def test_delete_removes_extra_files(self, shared_server):
         clean_dir(DEST_DIR)

tests/integration/test_tls.py

@@ -102,6 +102,7 @@ class TestTLSBasic:
         with ServerManager() as server:
             server.start(extra_args=[
                 "--tls", "--cert", certs["server_cert"], "--key", certs["server_key"],
+                "--ca", certs["ca"], "--client-cn", "fastsync-client",
             ])
             result, dur = run_client(
                 SOURCE_DIR, DEST_DIR,
@@ -125,6 +126,7 @@ class TestTLSBasic:
         with ServerManager() as server:
             server.start(extra_args=[
                 "--tls", "--cert", certs["server_cert"], "--key", certs["server_key"],
+                "--ca", certs["ca"], "--client-cn", "fastsync-client",
             ])
             result, dur = run_client(
                 SOURCE_DIR, DEST_DIR,
@@ -149,6 +151,7 @@ class TestTLSBasic:
         with ServerManager() as server:
             server.start(extra_args=[
                 "--tls", "--cert", certs["server_cert"], "--key", certs["server_key"],
+                "--ca", certs["ca"], "--client-cn", "fastsync-client",
             ])
             result, dur = run_client(
                 SOURCE_DIR, DEST_DIR,

tests/test_array_list.c

@@ -17,6 +17,8 @@ void test_array_list() {
 
   // Test adding
   int* val1 = malloc(sizeof(int));
+  if (!val1)
+    return;
   *val1 = 42;
   array_list_add(list, val1);
   EXPECT_EQ_INT(list->size, 1);
@@ -26,6 +28,8 @@ void test_array_list() {
   // Initial capacity is 100. Let's add 105 elements.
   for (int i = 0; i < 105; i++) {
     int* val = malloc(sizeof(int));
+    if (!val)
+      return;
     *val = i;
     array_list_add(list, val);
   }

tests/test_chunk.c

@@ -11,7 +11,7 @@ static void test_file_operations() {
   char* test_content = "Hello, Chunk System!";
   unsigned long long test_len = strlen(test_content);
 
-  to_disk(test_path, test_content, test_len, false, false);
+  file_write_to_disk(test_path, test_content, test_len, false, false);
 
   File* f = file_create(test_path);
   EXPECT_NOT_NULL(f);
@@ -43,8 +43,8 @@ static void test_chunk_operations() {
   char* content2 = "chunk item number 2";
   unsigned long long len2 = strlen(content2);
 
-  to_disk(path1, content1, len1, false, false);
+  file_write_to_disk(path1, content1, len1, false, false);
-  to_disk(path2, content2, len2, false, false);
+  file_write_to_disk(path2, content2, len2, false, false);
 
   struct stat st1, st2;
   stat(path1, &st1);

tests/test_client_cli.c

@@ -46,6 +46,8 @@ static void test_validate_config_tls_requirements() {
   cfg->tls_cert = str_dup("cert.pem");
   EXPECT_FALSE(validate_config(cfg));
   cfg->tls_key = str_dup("key.pem");
+  EXPECT_FALSE(validate_config(cfg));
+  cfg->tls_ca = str_dup("ca.pem");
   EXPECT_TRUE(validate_config(cfg));
   config_delete(cfg);
 }
@@ -337,6 +339,17 @@ static void test_parse_args_archive() {
   config_delete(cfg);
 }
 
+static void test_parse_args_existing() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--existing", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->existing);
+  config_delete(cfg);
+}
+
 void test_client_cli() {
   test_validate_config_required_paths();
   test_validate_config_incompatible_options();
@@ -358,4 +371,5 @@ void test_client_cli() {
   test_parse_args_unknown_option();
   test_parse_args_rejects_unimplemented_options();
   test_parse_args_archive();
+  test_parse_args_existing();
 }

tests/test_compression.c

@@ -13,6 +13,8 @@ static void test_data_compress_decompress_roundtrip() {
   size_t len = strlen(original);
 
   char* buf = malloc(len);
+  if (!buf)
+    return;
   memcpy(buf, original, len);
   Data* original_data = data_create(buf, len);
   EXPECT_NOT_NULL(original_data);
@@ -62,8 +64,8 @@ static void test_chunk_compress_decompress_roundtrip() {
   char* content2 = "chunk compression test file 2 with more data";
   unsigned long long len2 = strlen(content2);
 
-  to_disk(path1, content1, len1, false, false);
+  file_write_to_disk(path1, content1, len1, false, false);
-  to_disk(path2, content2, len2, false, false);
+  file_write_to_disk(path2, content2, len2, false, false);
 
   struct stat st1, st2;
   EXPECT_EQ_INT(stat(path1, &st1), 0);

tests/test_config.c

@@ -126,6 +126,7 @@ static void test_config_send_receive() {
   send_cfg->use_metadata = true;
   send_cfg->compression_level = 5;
   send_cfg->chunk_size = 1024;
+  send_cfg->existing = true;
 
   /* Use socketpair for bidirectional communication */
   int p[2];
@@ -160,6 +161,8 @@ static void test_config_send_receive() {
         ok = false;
       if (recv_cfg->chunk_size != 1024)
         ok = false;
+      if (!recv_cfg->existing)
+        ok = false;
     }
     config_delete(recv_cfg);
     close(p[0]);
@@ -244,26 +247,26 @@ static void test_config_receive_truncated() {
   close(p[1]);
 }
 
-static void test_is_remote_dest() {
+static void test_config_is_remote_dest() {
   /* Valid SSH-style destinations */
-  EXPECT_TRUE(is_remote_dest("user@host:/path"));
+  EXPECT_TRUE(config_is_remote_dest("user@host:/path"));
-  EXPECT_TRUE(is_remote_dest("host:/path"));
+  EXPECT_TRUE(config_is_remote_dest("host:/path"));
-  EXPECT_TRUE(is_remote_dest("user@192.168.1.1:/remote/path"));
+  EXPECT_TRUE(config_is_remote_dest("user@192.168.1.1:/remote/path"));
 
   /* Invalid destinations */
-  EXPECT_FALSE(is_remote_dest(NULL));
+  EXPECT_FALSE(config_is_remote_dest(NULL));
-  EXPECT_FALSE(is_remote_dest(""));
+  EXPECT_FALSE(config_is_remote_dest(""));
-  EXPECT_FALSE(is_remote_dest(":"));
+  EXPECT_FALSE(config_is_remote_dest(":"));
-  EXPECT_FALSE(is_remote_dest("/local/path"));
+  EXPECT_FALSE(config_is_remote_dest("/local/path"));
-  EXPECT_FALSE(is_remote_dest("relative/path"));
+  EXPECT_FALSE(config_is_remote_dest("relative/path"));
   /* C:/windows/path is treated as remote (colon with no preceding slash) */
-  EXPECT_TRUE(is_remote_dest("C:/windows/path"));
+  EXPECT_TRUE(config_is_remote_dest("C:/windows/path"));
 
   /* Edge cases */
-  EXPECT_FALSE(is_remote_dest("noslash"));
+  EXPECT_FALSE(config_is_remote_dest("noslash"));
-  EXPECT_FALSE(is_remote_dest("/"));
+  EXPECT_FALSE(config_is_remote_dest("/"));
-  EXPECT_TRUE(is_remote_dest("host:"));
+  EXPECT_TRUE(config_is_remote_dest("host:"));
-  EXPECT_TRUE(is_remote_dest("user@host:"));
+  EXPECT_TRUE(config_is_remote_dest("user@host:"));
 }
 
 void test_config() {
@@ -278,5 +281,5 @@ void test_config() {
     test_config_send_receive_version_mismatch();
     test_config_receive_truncated();
   }
-  test_is_remote_dest();
+  test_config_is_remote_dest();
 }

tests/test_file.c

@@ -34,7 +34,8 @@ static void test_file_destroy_normal() {
 
 static void test_file_load_data() {
   const char* content = "Hello Load Test";
-  EXPECT_TRUE(to_disk("test_file_load_data.txt", content, strlen(content), false, false));
+  EXPECT_TRUE(
+      file_write_to_disk("test_file_load_data.txt", content, strlen(content), false, false));
 
   struct stat st;
   EXPECT_EQ_INT(stat("test_file_load_data.txt", &st), 0);
@@ -87,15 +88,60 @@ static void test_file_save_to_disk() {
   rmdir("test_save_tmp");
 }
 
-static void test_to_disk_basic() {
+static void test_file_save_to_disk_existing() {
-  const char* content = "Basic to_disk test";
+  const char* root = "test_existing_tmp";
-  EXPECT_TRUE(to_disk("test_to_disk_basic.txt", content, strlen(content), false, false));
+  const char* existing_path = "test_existing_tmp/existing.txt";
+  const char* missing_path = "test_existing_tmp/missing.txt";
+  EXPECT_TRUE(file_write_to_disk(existing_path, "old", 3, false, false));
+
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  cfg->existing = true;
+
+  File* existing = file_create("existing.txt");
+  EXPECT_NOT_NULL(existing);
+  existing->data->data = malloc(3);
+  EXPECT_NOT_NULL(existing->data->data);
+  memcpy(existing->data->data, "new", 3);
+  existing->data->size = 3;
+  EXPECT_TRUE(file_save_to_disk(root, existing, cfg));
+  file_destroy(existing);
+
+  File* missing = file_create("missing.txt");
+  EXPECT_NOT_NULL(missing);
+  missing->data->data = malloc(7);
+  EXPECT_NOT_NULL(missing->data->data);
+  memcpy(missing->data->data, "skipped", 7);
+  missing->data->size = 7;
+  EXPECT_TRUE(file_save_to_disk(root, missing, cfg));
+  file_destroy(missing);
+
+  FILE* fp = fopen(existing_path, "rb");
+  char content[4] = {0};
+  EXPECT_NOT_NULL(fp);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (fp) {
+    EXPECT_EQ_INT((int)fread(content, 1, 3, fp), 3);
+    fclose(fp);
+  }
+  EXPECT_EQ_STR(content, "new");
+  EXPECT_EQ_INT(access(missing_path, F_OK), -1);
+
+  config_delete(cfg);
+  unlink(existing_path);
+  rmdir(root);
+}
+
+static void test_file_write_to_disk_basic() {
+  const char* content = "Basic file_write_to_disk test";
+  EXPECT_TRUE(file_write_to_disk("test_file_write_to_disk_basic.txt", content, strlen(content),
+                                 false, false));
 
   struct stat st;
-  EXPECT_EQ_INT(stat("test_to_disk_basic.txt", &st), 0);
+  EXPECT_EQ_INT(stat("test_file_write_to_disk_basic.txt", &st), 0);
   EXPECT_EQ_INT((int)st.st_size, (int)strlen(content));
 
-  FILE* fp = fopen("test_to_disk_basic.txt", "rb");
+  FILE* fp = fopen("test_file_write_to_disk_basic.txt", "rb");
   EXPECT_NOT_NULL(fp);
   char buf[100];
   size_t nread = fread(buf, 1, sizeof(buf), fp);
@@ -103,12 +149,13 @@ static void test_to_disk_basic() {
   EXPECT_EQ_INT((int)nread, (int)strlen(content));
   EXPECT_EQ_INT(memcmp(buf, content, strlen(content)), 0);
 
-  unlink("test_to_disk_basic.txt");
+  unlink("test_file_write_to_disk_basic.txt");
 }
 
-static void test_to_disk_creates_dirs() {
+static void test_file_write_to_disk_creates_dirs() {
   const char* content = "Nested dir test";
-  EXPECT_TRUE(to_disk("test_nested_tmp/nested/file.txt", content, strlen(content), false, false));
+  EXPECT_TRUE(file_write_to_disk("test_nested_tmp/nested/file.txt", content, strlen(content), false,
+                                 false));
 
   struct stat st;
   EXPECT_EQ_INT(stat("test_nested_tmp/nested/file.txt", &st), 0);
@@ -126,15 +173,15 @@ static void test_to_disk_creates_dirs() {
   rmdir("test_nested_tmp");
 }
 
-static void test_to_disk_does_not_follow_symlink() {
+static void test_file_write_to_disk_does_not_follow_symlink() {
-  const char* outside = "test_to_disk_outside.txt";
+  const char* outside = "test_file_write_to_disk_outside.txt";
-  const char* link = "test_to_disk_link.txt";
+  const char* link = "test_file_write_to_disk_link.txt";
   const char* content = "confined";
   unlink(outside);
   unlink(link);
-  EXPECT_TRUE(to_disk(outside, "outside", 7, false, false));
+  EXPECT_TRUE(file_write_to_disk(outside, "outside", 7, false, false));
   EXPECT_EQ_INT(symlink(outside, link), 0);
-  EXPECT_TRUE(to_disk(link, content, strlen(content), false, false));
+  EXPECT_TRUE(file_write_to_disk(link, content, strlen(content), false, false));
   FILE* fp = fopen(outside, "rb");
   char buf[16] = {0};
   EXPECT_NOT_NULL(fp);
@@ -151,7 +198,7 @@ static void test_to_disk_does_not_follow_symlink() {
 
 static void test_file_content_to_buffer() {
   const char* content = "Buffer content test";
-  EXPECT_TRUE(to_disk("test_buffer_file.txt", content, strlen(content), false, false));
+  EXPECT_TRUE(file_write_to_disk("test_buffer_file.txt", content, strlen(content), false, false));
 
   File* f = file_create("test_buffer_file.txt");
   EXPECT_NOT_NULL(f);
@@ -273,7 +320,7 @@ static void test_file_send_no_path() {
 }
 
 static void test_file_metadata_create() {
-  EXPECT_TRUE(to_disk("test_meta_file.txt", "metadata test", 13, false, false));
+  EXPECT_TRUE(file_write_to_disk("test_meta_file.txt", "metadata test", 13, false, false));
   struct stat st;
   EXPECT_EQ_INT(stat("test_meta_file.txt", &st), 0);
 
@@ -382,7 +429,7 @@ static void test_file_send_single_calls_metadata_and_path() {
   /* Create a real file on disk so we can have metadata */
   const char* content = "File with metadata";
   size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_meta_send.txt", content, len, false, false));
+  EXPECT_TRUE(file_write_to_disk("test_meta_send.txt", content, len, false, false));
 
   struct stat st;
   EXPECT_EQ_INT(stat("test_meta_send.txt", &st), 0);
@@ -455,9 +502,10 @@ void test_file() {
   test_file_load_data();
   test_file_load_data_missing_file();
   test_file_save_to_disk();
-  test_to_disk_basic();
+  test_file_save_to_disk_existing();
-  test_to_disk_creates_dirs();
+  test_file_write_to_disk_basic();
-  test_to_disk_does_not_follow_symlink();
+  test_file_write_to_disk_creates_dirs();
+  test_file_write_to_disk_does_not_follow_symlink();
   test_file_content_to_buffer();
   test_file_save_to_disk_path_traversal();
   test_file_save_to_disk_deep_traversal();

tests/test_file_sendfile.c

@@ -15,7 +15,7 @@
 static void test_sendfile_basic() {
   const char* content = "Hello from sendfile test!";
   size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_sendfile_basic.txt", content, len, false, false));
+  EXPECT_TRUE(file_write_to_disk("test_sendfile_basic.txt", content, len, false, false));
 
   File* file = file_create("test_sendfile_basic.txt");
   EXPECT_NOT_NULL(file);
@@ -77,7 +77,7 @@ static void test_sendfile_basic() {
 static void test_sendfile_empty_file() {
   const char* content = "";
   size_t len = 0;
-  EXPECT_TRUE(to_disk("test_sendfile_empty.txt", content, len, false, false));
+  EXPECT_TRUE(file_write_to_disk("test_sendfile_empty.txt", content, len, false, false));
 
   File* file = file_create("test_sendfile_empty.txt");
   EXPECT_NOT_NULL(file);
@@ -156,7 +156,7 @@ static void test_sendfile_missing_file() {
 static void test_sendfile_compression_fallback() {
   const char* content = "Compression fallback content";
   size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_sendfile_comp.txt", content, len, false, false));
+  EXPECT_TRUE(file_write_to_disk("test_sendfile_comp.txt", content, len, false, false));
 
   struct stat st;
   EXPECT_EQ_INT(stat("test_sendfile_comp.txt", &st), 0);
@@ -222,7 +222,7 @@ static void test_sendfile_compression_fallback() {
 static void test_sendfile_no_path() {
   const char* content = "No path sendfile test";
   size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_sendfile_nopath.txt", content, len, false, false));
+  EXPECT_TRUE(file_write_to_disk("test_sendfile_nopath.txt", content, len, false, false));
 
   File* file = file_create("test_sendfile_nopath.txt");
   EXPECT_NOT_NULL(file);

tests/test_fuzz_smoke.c

@@ -101,7 +101,7 @@ static void test_fuzz_delta_deserialize() {
 /* Smoke test for metadata_from_buf fuzz target */
 static void test_fuzz_metadata_from_buf() {
   /* Create a real file to get metadata from */
-  EXPECT_TRUE(to_disk("fuzz_meta_test.txt", "metadata test", 13, false, false));
+  EXPECT_TRUE(file_write_to_disk("fuzz_meta_test.txt", "metadata test", 13, false, false));
 
   struct stat st;
   EXPECT_EQ_INT(stat("fuzz_meta_test.txt", &st), 0);

tests/test_metadata.c

@@ -125,7 +125,7 @@ static void test_metadata_rejects_invalid_values() {
 static void test_file_restore_metadata() {
   const char* path = "temp_meta_restore_test.txt";
   const char* content = "test content";
-  EXPECT_TRUE(to_disk(path, content, strlen(content), false, false));
+  EXPECT_TRUE(file_write_to_disk(path, content, strlen(content), false, false));
 
   FileMetadata m;
   m.mode = 0644;

tests/test_property.c

@@ -14,6 +14,8 @@
 static Data* random_data(int min_size, int max_size) {
   int size = min_size + rand() % (max_size - min_size + 1);
   char* buf = malloc(size);
+  if (!buf)
+    return NULL;
   for (int i = 0; i < size; i++)
     buf[i] = (char)(rand() % 256);
   return data_create(buf, size);
@@ -81,10 +83,12 @@ static void test_property_chunk_roundtrip() {
 
     int content_len = 1 + rand() % 4096;
     char* content = malloc(content_len);
+    if (!content)
+      return;
     for (int i = 0; i < content_len; i++)
       content[i] = (char)(rand() % 256);
 
-    to_disk(path, content, content_len, false, false);
+    file_write_to_disk(path, content, content_len, false, false);
 
     struct stat st;
     stat(path, &st);

tests/test_queue.c

@@ -122,6 +122,8 @@ static void test_queue_destroyer() {
 
   for (int i = 0; i < 3; i++) {
     int* val = malloc(sizeof(int));
+    if (!val)
+      break;
     *val = i;
     queue_enqueue(q, val);
   }
@@ -181,6 +183,8 @@ static void test_queue_multithreaded() {
 
   for (int i = 1; i <= 100; i++) {
     int* val = malloc(sizeof(int));
+    if (!val)
+      break;
     *val = i;
     queue_enqueue_multithreaded(q, val, &mutex, &cnd_empty, &cnd_full);
   }

tests/test_robustness.c

@@ -13,7 +13,7 @@
 static void test_chunk_deserialize_truncated() {
   char* path = "test_rob_trunc.txt";
   char* content = "hello";
-  to_disk(path, content, strlen(content), false, false);
+  file_write_to_disk(path, content, strlen(content), false, false);
 
   struct stat st;
   stat(path, &st);

tests/test_scanner.c

@@ -7,7 +7,7 @@
 #include <unistd.h>
 
 static void create_test_file(const char* path, const char* content) {
-  (void)to_disk(path, content, strlen(content), false, false);
+  (void)file_write_to_disk(path, content, strlen(content), false, false);
 }
 
 static void test_scanner_single_file() {
@@ -385,6 +385,35 @@ static void test_scanner_no_patterns() {
   rmdir(dir);
 }
 
+static void test_parallel_scanner_root_chunks_without_workers() {
+  const char* dir = "test_parallel_scan_root";
+  const char* file1 = "test_parallel_scan_root/a.txt";
+  const char* file2 = "test_parallel_scan_root/b.txt";
+
+  EXPECT_EQ_INT(mkdir(dir, 0755), 0);
+  create_test_file(file1, "a");
+  create_test_file(file2, "b");
+
+  ScannerOptions options = {false, 1, NULL,  0,     NULL,  0,     0,    0,
+                            0,     0, false, false, false, false, false};
+  ParallelScanner* scanner = parallel_scanner_create_with_options(dir, &options);
+  EXPECT_NOT_NULL(scanner);
+
+  int total_files = 0;
+  Chunk* chunk;
+  while ((chunk = parallel_scanner_next(scanner)) != NULL) {
+    total_files += chunk->element_count;
+    chunk_destroy(chunk);
+  }
+  EXPECT_EQ_INT(total_files, 2);
+  EXPECT_FALSE(parallel_scanner_failed(scanner));
+
+  parallel_scanner_destroy(scanner);
+  unlink(file1);
+  unlink(file2);
+  rmdir(dir);
+}
+
 void test_scanner() {
   test_scanner_single_file();
   test_scanner_multiple_files();
@@ -399,4 +428,5 @@ void test_scanner() {
   test_scanner_size_range();
   test_scanner_mixed_patterns();
   test_scanner_no_patterns();
+  test_parallel_scanner_root_chunks_without_workers();
 }

tests/test_stress.c

@@ -32,6 +32,8 @@ static int mpmc_producer_func(void* arg) {
   ProducerCtx* ctx = (ProducerCtx*)arg;
   for (int i = 1; i <= ITEMS_PER_PRODUCER; i++) {
     int* val = malloc(sizeof(int));
+    if (!val)
+      return thrd_error;
     *val = ctx->producer_id * ITEMS_PER_PRODUCER + i;
     queue_enqueue_multithreaded(ctx->q, val, ctx->mutex, ctx->cnd_empty, ctx->cnd_full);
   }
@@ -121,6 +123,8 @@ static int bp_producer_func(void* arg) {
   BackpressureCtx* ctx = (BackpressureCtx*)arg;
   for (int i = 0; i < 5; i++) {
     int* val = malloc(sizeof(int));
+    if (!val)
+      return thrd_error;
     *val = i + 1;
     queue_enqueue_multithreaded(ctx->q, val, ctx->mutex, ctx->cnd_empty, ctx->cnd_full);
     ctx->items_sent++;
@@ -194,6 +198,8 @@ static void test_queue_rapid_create_destroy() {
 
     for (int j = 0; j < 3; j++) {
       int* val = malloc(sizeof(int));
+      if (!val)
+        break;
       *val = j;
       queue_enqueue(q, val);
     }