docs: clarify rsync compatibility roadmap

CMakeLists.txt

@@ -58,15 +58,18 @@ endif()
 find_package(OpenSSL REQUIRED)
 
 file(GLOB SHARED_SRCS "src/shared/*.c")
+set(FILE_STORE_SRCS 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)
 file(GLOB CLIENT_SRCS "src/client/*.c")
 
 # --- Main executables ---
-add_executable(server ${SERVER_SRCS} ${SHARED_SRCS})
+add_executable(server ${SERVER_SRCS} ${SHARED_SRCS} ${FILE_STORE_SRCS})
 target_include_directories(server PRIVATE src/shared src/server src/client)
 target_link_libraries(server PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 
-add_executable(client ${CLIENT_SRCS} ${SHARED_SRCS})
+add_executable(client ${CLIENT_SRCS} ${SHARED_SRCS} ${FILE_STORE_SRCS} ${SERVER_RECEIVER_SRCS})
 target_include_directories(client PRIVATE src/shared src/server src/client)
 target_link_libraries(client PRIVATE Threads::Threads ${ZSTD_LIBRARY} OpenSSL::SSL OpenSSL::Crypto xxhash)
 
@@ -79,7 +82,7 @@ set(TEST_INCLUDES tests src/shared src/server src/client)
 
 # Monolithic test binary (backward compatible)
 file(GLOB TEST_SRCS "tests/test_*.c" "tests/runner.c")
-add_executable(tests ${TEST_SRCS} ${SHARED_SRCS} src/client/scanner.c src/client/client_cli.c)
+add_executable(tests ${TEST_SRCS} ${SHARED_SRCS} ${FILE_STORE_SRCS} ${SERVER_RECEIVER_SRCS} src/client/scanner.c src/client/client_cli.c src/client/client_validation.c src/client/usage.c)
 target_include_directories(tests PRIVATE ${TEST_INCLUDES})
 target_compile_definitions(tests PRIVATE FASTSYNC_TEST_BUILD)
 target_link_libraries(tests PRIVATE ${TEST_LIBS})
@@ -94,7 +97,7 @@ if(ENABLE_FUZZ)
   file(GLOB FUZZ_SRCS "tests/fuzz/*.c")
 	foreach(FUZZ_SRC ${FUZZ_SRCS})
 		get_filename_component(FUZZ_NAME ${FUZZ_SRC} NAME_WE)
-		add_executable(${FUZZ_NAME} ${FUZZ_SRC} ${SHARED_SRCS})
+    add_executable(${FUZZ_NAME} ${FUZZ_SRC} ${SHARED_SRCS} ${FILE_STORE_SRCS} ${SERVER_RECEIVER_SRCS})
 		target_include_directories(${FUZZ_NAME} PRIVATE ${TEST_INCLUDES})
 		target_compile_options(${FUZZ_NAME} PRIVATE -fsanitize=fuzzer,address,undefined -fno-omit-frame-pointer)
 		target_link_options(${FUZZ_NAME} PRIVATE -fsanitize=fuzzer,address,undefined)

README.md

@@ -1,363 +1,377 @@
 # 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`)
-2. **TLS encryption**: OpenSSL-based TLS 1.2+ for encrypted TCP connections with optional CA verification
-3. **Chunked file transfer**: files grouped into configurable-size chunks (default ~10 MB)
-4. **Streaming zstd compression** (levels 1–22) using `ZSTD_compressStream2`
-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
+- Existing rsync commands should keep the same meaning.
+- FastSync-only performance options should be additive and optional.
+- A normal compatibility-mode transfer should prioritize rsync filesystem
+  semantics over maximum throughput.
 
-## 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
-- 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`)
+## Why FastSync
 
-### Server
-- TCP mode: listens on configurable port (default 8080); SSH mode: runs via `--stdio`
-- 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 `..`
+FastSync uses a producer-consumer transfer pipeline and can combine several
+optimizations for large or high-latency transfers:
 
-## 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
-| Code | Meaning |
-|------|---------|
-| `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 |
+These optimizations are disabled or selected independently. Users can start
+with rsync-style commands and add FastSync options when they are useful.
 
-### 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
-```
-Config → (STATUS_NEXT | STATUS_CHUNK | STATUS_CHECK | STATUS_CHECK_BATCH)* → [STATUS_MANIFEST] → STATUS_FINISHED → STATUS_OK
-```
+### Working today
 
-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:/path` destinations.
+- 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
-
-| Argument | Description |
-|----------|-------------|
-| Positional | `<source> <dest>` — automatic SSH detection if dest contains `:` |
-| `-c [level]` | Compression with optional level (1–22, default 5) |
-| `-z [level]` | Alias for `-c` |
-| `-a, --archive` | Archive mode: enables `-c -m -M` (no `-s`) |
-| `-m` | Multithreading mode |
-| `-s` | Chunk serialization (batch all files per chunk) |
-| `-f, --sendfile` | Sendfile zero-copy. Incompatible with `-c` / `-s`. TCP only. |
-| `-M, --preserve` | Preserve supported file metadata (mode and mtime; ownership and atime are unsupported) |
-| `-n, --dry-run` | Scan and print what would be transferred |
-| `-p <port>` | SSH port (default: 22) |
-| `-v, --verbose` | Enable debug logging |
-| `--progress` | Show real-time transfer speed |
-| `--delete` | Delete files on receiver not present in source |
-| `--exclude <pattern>` | Exclude files matching glob pattern (repeatable) |
-| `--exclude-from <file>` | Read exclude patterns from a file (one per line) |
-| `--include <pattern>` | Only transfer files matching glob pattern (repeatable, whitelist) |
-| `--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`. |
-| `--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) |
-| `--contimeout <sec>` | Connection timeout in seconds (default: 10) |
-| `--backup` | Backup existing destination files before overwriting |
-| `--backup-dir <dir>` | Target directory for backups (requires `--backup`) |
-| `--stats` | Print transfer statistics at end (bytes, files, timing) |
-| `--max-depth <n>` | Maximum directory depth to recurse (0 = unlimited, default: 0) |
-| `--log-file <path>` | Write log messages to file instead of stderr |
-| `--source-dir <path>` | Source directory (overrides `FASTSYNC_SOURCE_DIR`) |
-| `--dest-dir <path>` | Server destination directory (overrides `FASTSYNC_DEST_DIR`) |
-| `--save-to-disk` | Write received files to disk |
-| `--server-host <ip>` | Server IP address (default: `127.0.0.1`) |
-| `--server-port <n>` | Server port (default: `8080`) |
-| `--tls` | Enable TLS encryption |
-| `--cert <path>` | TLS certificate file (PEM) |
-| `--key <path>` | TLS private key file (PEM) |
-| `--ca <path>` | TLS CA certificate file for verification (PEM) |
-
-### Server
-
-| Argument | Description |
-|----------|-------------|
-| `--stdio` | Run in stdio mode (for SSH transport; single connection then exits) |
-| `-p <port>` | TCP listen port (default: 8080, range: 1–65535) |
-| `--tls` | Enable TLS encryption |
-| `--cert <path>` | TLS certificate file (PEM) |
-| `--key <path>` | TLS private key file (PEM) |
-| `--ca <path>` | TLS CA certificate file for verification (PEM) |
-| `-v, --verbose` | Enable debug logging |
-| `--help` | Show help |
-
-## Environment Variables
-
-| Variable | Default | Description |
-|----------|---------|-------------|
-| `FASTSYNC_SOURCE_DIR` | — | Source directory fallback |
-| `FASTSYNC_DEST_DIR` | — | Destination directory fallback |
-| `FASTSYNC_SAVE_TO_DISK` | `false` | Disk persistence fallback |
-| `FASTSYNC_SSH_PORT` | `22` | Default SSH port |
-| `FASTSYNC_SERVER_HOST` | `127.0.0.1` | Default server host |
-| `FASTSYNC_SERVER_PORT` | `8080` | Default server port |
-| `FASTSYNC_TLS_CERT` | — | Default TLS certificate path |
-| `FASTSYNC_TLS_KEY` | — | Default TLS private key path |
-| `FASTSYNC_TLS_CA` | — | Default TLS CA certificate path |
-
-## Implementation Details
-
-### 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
-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
-2. **Chunking** — files accumulated until `chunk_size` threshold, then flushed
-3. **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
-9. **SSH transport** — `socketpair()` + `fork()` + `execvp("ssh", ...)` with `ControlMaster` and port support
-10. **TLS transport** — OpenSSL `SSL_CTX` with TLS 1.2 minimum, optional 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
-
-## Security Features
-
-### Path Traversal Protection
-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.
-
-### 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.
-
-### Abort Handling
-If the client receives `SIGINT` (Ctrl+C) during a transfer, it sends `STATUS_ABORT` to the server. The server then cleans up temporary files and exits the child process, preventing incomplete files from remaining on disk.
-
-### Atomic Writes
-Received files are written to a temporary path (suffixed with `.tmp`) and then atomically renamed to the final filename via `rename()`. This prevents partial or corrupted files from appearing at the destination if the transfer is interrupted.
-
-## Build Requirements
-
-- C11 compiler
-- CMake >= 3.22
-- zstd library
-- OpenSSL (development headers and libraries)
-- pthreads
-- SSH client (for SSH transport mode only)
-
-### Installing Dependencies
-
-**Ubuntu/Debian:**
-```bash
-sudo apt install cmake build-essential libzstd-dev libssl-dev openssh-client
-```
-
-**Nix:**
-```bash
-nix-shell  # provides zstd, openssl, cmake, gcc
-```
-
-## Building
+Requirements: C11 compiler, CMake 3.22 or newer, zstd, OpenSSL, pthreads,
+and an SSH client for SSH transport.
 
 ```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`.
+
 ```bash
-./build/server --tls --cert server.pem --key server-key.pem
+./build/client /path/to/source user@host:/path/to/destination
 ```
 
-### Server via SSH
-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.
+### TCP transfer
+
+Start the FastSync server:
 
-### Client — SSH (rsync-style)
 ```bash
-./build/client /path/to/send user@host:/path/to/receive
+./build/server -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
+### TLS transfer
+
 ```bash
+./build/server --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)
-./build/client -a /path/to/send user@host:/path
+# Basic synchronization
+./build/client /source/ /destination/
 
-# Dry run
-./build/client -n /path/to/send /path/to/receive
+# Archive-style synchronization (current FastSync archive behavior)
+./build/client -a /source/ user@host:/destination/
 
-# With progress and custom chunk size
-./build/client --progress --chunk-size 2097152 /src user@host:/dst
+# Preview a transfer without changing the destination
+./build/client -n /source/ /destination/
 
-# Exclude temporary files + delete extras on receiver
-./build/client --exclude "*.tmp" --exclude "*.o" --delete /src user@host:/dst
+# Exclude temporary and object files
+./build/client --exclude '*.tmp' --exclude '*.o' \
+  /source/ user@host:/destination/
 
-# Incremental sync (skip unchanged files)
-./build/client --incremental /src user@host:/dst
+# Remove destination entries not present in the source
+./build/client --delete /source/ user@host:/destination/
 
-# Bandwidth limit to 1 MB/s
-./build/client --bwlimit 1024 /src user@host:/dst
+# Skip unchanged files using size and modification time
+./build/client --incremental /source/ user@host:/destination/
 
-# With timeouts and stats
-./build/client --timeout 60 --contimeout 15 --stats /src user@host:/dst
+# Verify content when size and time are not sufficient
+./build/client --incremental --checksum /source/ user@host:/destination/
 
-# Backup overwritten files to a directory
-./build/client --backup --backup-dir /backups /src user@host:/dst
+# Preserve supported mode and timestamp metadata
+./build/client -M /source/ user@host:/destination/
 
-# Exclude patterns from file, limit depth
-./build/client --exclude-from ignore.txt --max-depth 3 /src user@host:/dst
-
-# 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
+# Keep backups of overwritten destination files
+./build/client --backup --backup-dir /backups \
+  /source/ user@host:/destination/
 ```
 
+## 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` | Keep received data under the configured partial location. Full resume semantics are planned. |
+| `--partial-dir <dir>` | Set the partial transfer directory. |
+| `--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. |
+| `--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 `2.2.0` 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
-# 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
+Run the unit test binary:
 
-# Integration + benchmark suite
-python3 test.py
+```bash
+./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
-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
+For stricter local validation:
 
-## 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) |
-|---|---|---|---|
-| **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 | — | — |
+Benchmark results measure transfer performance only. They do not establish
+rsync protocol or filesystem-semantic compatibility.
 
-### WAN (100 Mbit, 50 ms ±10 ms, 1% loss)
+## Performance Guidance
 
-| Configuration | Time | vs rsync (archive) | vs rsync (compress) |
-|---|---|---|---|
-| **Best: `-m -c`** | **0.39 s** | **44.8× faster** | **3.8× faster** |
-| Compression (`-c`) | 0.64 s | 27.3× faster | 2.3× faster |
-| Standard | 7.12 s | 2.4× faster | — |
-| rsync (archive) | 17.44 s | — | — |
-| rsync (archive + compress) | 1.47 s | — | — |
+- Use `-m` for workloads with many files or enough CPU parallelism.
+- Use `-c` or `-z` when network bandwidth is more constrained than CPU.
+- Tune `--chunk-size` for file sizes, memory limits, and network latency.
+- Use `-f` for large uncompressed TCP transfers where zero-copy I/O helps.
+- Use `--incremental` to avoid retransmitting unchanged files.
+- Use `--delta` for changed files when both endpoints are FastSync peers.
+- 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.

src/client/client_cli.c

@@ -1,10 +1,12 @@
 #include "client_send.h"
+#include "client_validation.h"
 #include "config.h"
 #include "delta.h"
 #include "log.h"
 #include "protocol.h"
 #include "transport_tcp.h"
 #include "transport_tls.h"
+#include "usage.h"
 #include "utils.h"
 #include <errno.h>
 #include <limits.h>
@@ -12,7 +14,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include "usage.c"
 
 #ifndef FASTSYNC_TEST_BUILD
 /* Parse environment variables for source/destination directories and save-to-disk flag. */
@@ -360,55 +361,6 @@ int parse_args(Config* config, int argc, char* argv[], int* positional_args,
   return 0;
 }
 
-#ifndef FASTSYNC_TEST_BUILD
-/* Validate config after parsing. Returns true if valid. */
-static bool validate_config(const Config* config) {
-  if (!config->send_directory || !config->receive_root_directory) {
-    fprintf(stderr, "Error: source and destination directories are required\n");
-    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 "
-                    "serialization)\n");
-    return false;
-  }
-  if (config->transport == TRANSPORT_SSH && config->use_sendfile) {
-    fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
-    return false;
-  }
-  if (config->use_incremental && config->use_chunk_serialization) {
-    fprintf(stderr, "Error: --incremental is not supported with -s (chunk serialization)\n");
-    return false;
-  }
-  if (config->use_delta && !config->use_incremental) {
-    fprintf(stderr, "Error: --delta requires --incremental\n");
-    return false;
-  }
-  if (config->use_delta && config->use_chunk_serialization) {
-    fprintf(stderr, "Error: --delta cannot be combined with -s (chunk serialization)\n");
-    return false;
-  }
-  if (config->use_delta && config->use_sendfile) {
-    fprintf(stderr, "Error: --delta cannot be combined with -f (sendfile)\n");
-    return false;
-  }
-  if (config->append || config->append_verify) {
-    fprintf(
-        stderr,
-        "Error: --append and --append-verify are not supported yet; refusing to ignore option\n");
-    return false;
-  }
-  if (config->use_tls) {
-    if (!config->tls_cert || !config->tls_key) {
-      fprintf(stderr, "Error: --tls requires --cert and --key\n");
-      return false;
-    }
-  }
-  return true;
-}
-#endif /* FASTSYNC_TEST_BUILD */
-
 static int read_patterns_from_file(const char* filepath, char*** patterns, int* count) {
   FILE* fp = fopen(filepath, "r");
   if (!fp) {

src/client/client_send.c

@@ -28,6 +28,87 @@
 /* Forward declaration for progress-reporting thread used in multithreaded send. */
 static int progress_thread_fn(void* arg);
 
+static ScannerOptions scanner_options_from_config(const Config* config, int num_threads) {
+  ScannerOptions options = {
+      config->use_metadata,  config->chunk_size,        config->exclude_patterns,
+      config->exclude_count, config->include_patterns,  config->include_count,
+      config->max_size,      config->min_size,          config->max_depth,
+      num_threads,           config->follow_symlinks,   config->copy_links,
+      config->safe_links,    config->copy_unsafe_links, config->checksum};
+  return options;
+}
+
+/* Select the configured transport for both transfer execution paths. */
+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");
+      return NULL;
+    }
+    return client_connect_ssh(config->ssh_destination, config->ssh_port,
+                              config->fastsync_server_path);
+  }
+
+  Client* client = client_create();
+  if (!client)
+    return NULL;
+  bool connected;
+  if (config->use_tls) {
+    connected = client_connect_tls(client, config->server_host, config->server_port,
+                                   config->tls_cert, config->tls_key, config->tls_ca);
+  } else {
+    connected = client_connect(client, config->server_host, config->server_port);
+  }
+  if (!connected) {
+    client_delete(client);
+    return NULL;
+  }
+  return client;
+}
+
+static void disconnect_transfer_client(Client* client) {
+  if (!client)
+    return;
+  client_disconnect(client);
+  client_delete(client);
+}
+
+static ArrayList* create_transfer_manifest(const Config* config) {
+  return config->use_delete ? array_list_create(free) : NULL;
+}
+
+static bool add_chunk_to_manifest(ArrayList* manifest, const Chunk* chunk) {
+  if (!manifest)
+    return true;
+  for (int i = 0; i < chunk->element_count; i++) {
+    const char* path = chunk->items[i]->path;
+    if (*path == '/')
+      path++;
+    char* entry = str_dup(path);
+    if (!entry) {
+      log_message(LOG_LEVEL_ERROR, "Failed to allocate manifest entry");
+      return false;
+    }
+    if (!array_list_add(manifest, entry)) {
+      free(entry);
+      return false;
+    }
+  }
+  return true;
+}
+
+static bool finalize_transfer(Client* client) {
+  Status status;
+  return send_status(client->file_descriptor, STATUS_FINISHED) &&
+         receive_status(client->file_descriptor, &status) && status == STATUS_OK;
+}
+
+static void mark_sender_done(PipelineContextSender* context) {
+  mtx_lock(&context->mutex_progress);
+  context->sender_done = true;
+  mtx_unlock(&context->mutex_progress);
+}
+
 static void pipeline_cancel(PipelineContextSender* context) {
   mtx_lock(&context->mutex_scanner);
   mtx_lock(&context->mutex_loader);
@@ -43,12 +124,10 @@ static void pipeline_cancel(PipelineContextSender* context) {
 }
 
 /* Print dry-run manifest showing files that would be transferred. Returns 0 on success. */
-static int send_dry_run_manifest(Config* config) {
-  DirectoryScanner* scanner = directory_scanner_create(
-      config->send_directory, config->use_metadata, config->chunk_size, config->exclude_patterns,
-      config->exclude_count, config->include_patterns, config->include_count, config->max_size,
-      config->min_size, config->max_depth, config->follow_symlinks, config->copy_links,
-      config->safe_links, config->copy_unsafe_links, config->checksum);
+static int send_dry_run_manifest(const Config* config) {
+  ScannerOptions options = scanner_options_from_config(config, 0);
+  DirectoryScanner* scanner =
+      directory_scanner_create_with_options(config->send_directory, &options);
   if (!scanner)
     return -1;
   Chunk* chunk;
@@ -291,49 +370,22 @@ int send_chunk(Client* client, Chunk* chunk, Config* config) {
 
 static int send_chunks_multithreaded(void* pipeline_context) {
   PipelineContextSender* context = (PipelineContextSender*)pipeline_context;
-  Client* client;
-  if (context->config->transport == TRANSPORT_SSH) {
-    if (context->config->use_sendfile) {
-      fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
-      mtx_lock(&context->mutex_progress);
-      context->sender_done = true;
-      mtx_unlock(&context->mutex_progress);
-      return 1;
-    }
-    client = client_connect_ssh(context->config->ssh_destination, context->config->ssh_port,
-                                context->config->fastsync_server_path);
-  } else if (context->config->use_tls) {
-    client = client_create();
-    if (!client || !client_connect_tls(client, context->config->server_host,
-                                       context->config->server_port, context->config->tls_cert,
-                                       context->config->tls_key, context->config->tls_ca)) {
-      if (client)
-        client_delete(client);
-      fprintf(stderr, "Error: could not connect to server via TLS\n");
-      mtx_lock(&context->mutex_progress);
-      context->sender_done = true;
-      mtx_unlock(&context->mutex_progress);
+  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",
+              context->config->use_tls ? " via TLS" : "");
+    mark_sender_done(context);
     return thrd_error;
   }
-  } else {
-    client = client_create();
-    if (!client ||
-        !client_connect(client, context->config->server_host, context->config->server_port)) {
-      if (client)
-        client_delete(client);
-      fprintf(stderr, "Error: could not connect to server\n");
-      mtx_lock(&context->mutex_progress);
-      context->sender_done = true;
-      mtx_unlock(&context->mutex_progress);
-      return thrd_error;
-    }
-  }
+  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, context->config)) {
-    client_disconnect(client);
-    client_delete(client);
-    mtx_lock(&context->mutex_progress);
-    context->sender_done = true;
-    mtx_unlock(&context->mutex_progress);
+    disconnect_transfer_client(client);
+    mark_sender_done(context);
+    protocol_session_unbind();
     return thrd_error;
   }
 
@@ -346,35 +398,26 @@ static int send_chunks_multithreaded(void* pipeline_context) {
         if (send_delete_manifest(client->file_descriptor, context->manifest) != 0)
           goto send_fail;
       }
-      if (!send_status(client->file_descriptor, STATUS_FINISHED))
-        goto send_fail;
-      Status s;
-      int ok = receive_status(client->file_descriptor, &s) && s == STATUS_OK;
-      client_disconnect(client);
-      client_delete(client);
-      mtx_lock(&context->mutex_progress);
-      context->sender_done = true;
-      mtx_unlock(&context->mutex_progress);
+      bool ok = finalize_transfer(client);
+      disconnect_transfer_client(client);
+      mark_sender_done(context);
+      protocol_session_unbind();
       return ok ? thrd_success : thrd_error;
 
     send_fail:
       pipeline_cancel(context);
-      client_disconnect(client);
-      client_delete(client);
-      mtx_lock(&context->mutex_progress);
-      context->sender_done = true;
-      mtx_unlock(&context->mutex_progress);
+      disconnect_transfer_client(client);
+      mark_sender_done(context);
+      protocol_session_unbind();
       return thrd_error;
     }
     if (send_chunk(client, current_chunk, context->config) != 0) {
       fprintf(stderr, "Error: unexpected error while sending chunk\n");
       chunk_destroy(current_chunk);
       pipeline_cancel(context);
-      client_disconnect(client);
-      client_delete(client);
-      mtx_lock(&context->mutex_progress);
-      context->sender_done = true;
-      mtx_unlock(&context->mutex_progress);
+      disconnect_transfer_client(client);
+      mark_sender_done(context);
+      protocol_session_unbind();
       return thrd_error;
     }
     if (context->config->show_progress) {
@@ -393,13 +436,9 @@ static int send_chunks_multithreaded(void* pipeline_context) {
 
 static int scan_directory_multithreaded(void* pipeline_context) {
   PipelineContextSender* context = (PipelineContextSender*)pipeline_context;
-  ParallelScanner* scanner = parallel_scanner_create(
-      context->config->send_directory, context->config->use_metadata, context->config->chunk_size,
-      context->config->exclude_patterns, context->config->exclude_count,
-      context->config->include_patterns, context->config->include_count, context->config->max_size,
-      context->config->min_size, context->config->max_depth, 4, context->config->follow_symlinks,
-      context->config->copy_links, context->config->safe_links, context->config->copy_unsafe_links,
-      context->config->checksum);
+  ScannerOptions options = scanner_options_from_config(context->config, 4);
+  ParallelScanner* scanner =
+      parallel_scanner_create_with_options(context->config->send_directory, &options);
 
   Chunk* current_chunk;
   if (scanner == NULL) {
@@ -410,29 +449,15 @@ static int scan_directory_multithreaded(void* pipeline_context) {
   while ((current_chunk = parallel_scanner_next(scanner)) != NULL) {
     if (context->config->use_delete) {
       mtx_lock(&context->mutex_scanner);
-      for (int i = 0; i < current_chunk->element_count; i++) {
-        const char* p = current_chunk->items[i]->path;
-        if (*p == '/')
-          p++;
-        char* manifest_entry = str_dup(p);
-        if (!manifest_entry) {
-          log_message(LOG_LEVEL_ERROR, "Failed to allocate manifest entry");
-          mtx_unlock(&context->mutex_scanner);
-          pipeline_cancel(context);
-          parallel_scanner_destroy(scanner);
-          return thrd_error;
-        }
-        if (!array_list_add(context->manifest, manifest_entry)) {
-          free(manifest_entry);
+      bool manifest_ok = add_chunk_to_manifest(context->manifest, current_chunk);
       mtx_unlock(&context->mutex_scanner);
+      if (!manifest_ok) {
         pipeline_cancel(context);
         chunk_destroy(current_chunk);
         parallel_scanner_destroy(scanner);
         return thrd_error;
       }
     }
-      mtx_unlock(&context->mutex_scanner);
-    }
     if (!queue_enqueue_multithreaded_cancel(
             context->queue_scanner, current_chunk, &context->mutex_scanner,
             &context->condition_not_empty_scanner, &context->condition_not_full_scanner,
@@ -540,61 +565,37 @@ int send_files(Config* config) {
   if (config->dry_run)
     return send_dry_run_manifest(config);
 
-  Client* client;
-  if (config->transport == TRANSPORT_SSH) {
-    if (config->use_sendfile) {
-      fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
+  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" : "");
     return 1;
   }
-    client =
-        client_connect_ssh(config->ssh_destination, config->ssh_port, config->fastsync_server_path);
-    if (!client)
-      return 1;
-  } else if (config->use_tls) {
-    client = client_create();
-    if (!client || !client_connect_tls(client, config->server_host, config->server_port,
-                                       config->tls_cert, config->tls_key, config->tls_ca)) {
-      if (client) {
-        client_disconnect(client);
-        client_delete(client);
-      }
-      fprintf(stderr, "Error: could not connect to server via TLS\n");
-      return 1;
-    }
-  } else {
-    client = client_create();
-    if (!client || !client_connect(client, config->server_host, config->server_port)) {
-      if (client) {
-        client_disconnect(client);
-        client_delete(client);
-      }
-      fprintf(stderr, "Error: could not connect to server\n");
-      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)) {
-    client_disconnect(client);
-    client_delete(client);
+    disconnect_transfer_client(client);
+    protocol_session_unbind();
     return 1;
   }
-  DirectoryScanner* scanner = directory_scanner_create(
-      config->send_directory, config->use_metadata, config->chunk_size, config->exclude_patterns,
-      config->exclude_count, config->include_patterns, config->include_count, config->max_size,
-      config->min_size, config->max_depth, config->follow_symlinks, config->copy_links,
-      config->safe_links, config->copy_unsafe_links, config->checksum);
+  ScannerOptions scanner_options = scanner_options_from_config(config, 0);
+  DirectoryScanner* scanner =
+      directory_scanner_create_with_options(config->send_directory, &scanner_options);
   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 = config->use_delete ? array_list_create(free) : 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);
-    client_disconnect(client);
-    client_delete(client);
+    disconnect_transfer_client(client);
+    protocol_session_unbind();
     return 1;
   }
   while ((current_chunk = directory_scanner_next(scanner)) != NULL) {
@@ -602,30 +603,10 @@ int send_files(Config* config) {
     for (int i = 0; i < current_chunk->element_count; i++) {
       chunk_bytes += current_chunk->items[i]->data->size;
       total_files++;
-      if (manifest) {
-        const char* p = current_chunk->items[i]->path;
-        if (*p == '/')
-          p++;
-        char* manifest_entry = str_dup(p);
-        if (!manifest_entry) {
-          log_message(LOG_LEVEL_ERROR, "Failed to allocate manifest entry");
+    }
+    if (!add_chunk_to_manifest(manifest, current_chunk)) {
       chunk_destroy(current_chunk);
-          array_list_delete(manifest);
-          directory_scanner_destroy(scanner);
-          client_disconnect(client);
-          client_delete(client);
-          return 1;
-        }
-        if (!array_list_add(manifest, manifest_entry)) {
-          free(manifest_entry);
-          chunk_destroy(current_chunk);
-          array_list_delete(manifest);
-          directory_scanner_destroy(scanner);
-          client_disconnect(client);
-          client_delete(client);
-          return 1;
-        }
-      }
+      goto send_fail;
     }
     if (!config->use_sendfile) {
       for (int i = 0; i < current_chunk->element_count; i++) {
@@ -669,10 +650,7 @@ int send_files(Config* config) {
     array_list_delete(manifest);
     manifest = NULL;
   }
-  if (!send_status(client->file_descriptor, STATUS_FINISHED))
-    goto send_fail;
-  Status s;
-  int ok = receive_status(client->file_descriptor, &s) && s == STATUS_OK;
+  bool ok = finalize_transfer(client);
   double elapsed_total = difftime(time(NULL), start);
   if (config->show_progress) {
     double rate = elapsed_total > 0 ? total_bytes / (1048576.0 * elapsed_total) : 0;
@@ -684,16 +662,16 @@ int send_files(Config* config) {
             rate);
   }
   directory_scanner_destroy(scanner);
-  client_disconnect(client);
-  client_delete(client);
+  disconnect_transfer_client(client);
+  protocol_session_unbind();
   return ok ? 0 : 1;
 
 send_fail:
   if (manifest)
     array_list_delete(manifest);
   directory_scanner_destroy(scanner);
-  client_disconnect(client);
-  client_delete(client);
+  disconnect_transfer_client(client);
+  protocol_session_unbind();
   return 1;
 }
 
@@ -729,7 +707,7 @@ int send_files_multithreaded(Config* config) {
     return 1;
   }
   if (config->use_delete)
-    context->manifest = array_list_create(free);
+    context->manifest = create_transfer_manifest(config);
 
   thrd_t scanner, loader, sender;
   bool scanner_created = false;

src/client/client_validation.c

@@ -0,0 +1,50 @@
+#include "client_validation.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");
+    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 "
+                    "serialization)\n");
+    return false;
+  }
+  if (config->transport == TRANSPORT_SSH && config->use_sendfile) {
+    fprintf(stderr, "Error: -f/--sendfile is not supported with SSH transport\n");
+    return false;
+  }
+  if (config->use_incremental && config->use_chunk_serialization) {
+    fprintf(stderr, "Error: --incremental is not supported with -s (chunk serialization)\n");
+    return false;
+  }
+  if (config->use_delta && !config->use_incremental) {
+    fprintf(stderr, "Error: --delta requires --incremental\n");
+    return false;
+  }
+  if (config->use_delta && config->use_chunk_serialization) {
+    fprintf(stderr, "Error: --delta cannot be combined with -s (chunk serialization)\n");
+    return false;
+  }
+  if (config->use_delta && config->use_sendfile) {
+    fprintf(stderr, "Error: --delta cannot be combined with -f (sendfile)\n");
+    return false;
+  }
+  if (config->append || config->append_verify) {
+    fprintf(
+        stderr,
+        "Error: --append and --append-verify are not supported yet; refusing to ignore option\n");
+    return false;
+  }
+  if (config->use_tls) {
+    if (!config->tls_cert || !config->tls_key) {
+      fprintf(stderr, "Error: --tls requires --cert and --key\n");
+      return false;
+    }
+  }
+  return true;
+}

src/client/client_validation.h

@@ -0,0 +1,9 @@
+#ifndef CLIENT_VALIDATION_H
+#define CLIENT_VALIDATION_H
+
+#include "config.h"
+#include <stdbool.h>
+
+bool validate_config(const Config* config);
+
+#endif

src/client/scanner.c

@@ -52,13 +52,84 @@ static bool safe_relative_link(const char* source_root, const char* containing_d
   return safe;
 }
 
-DirectoryScanner* directory_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,
-                                           bool follow_symlinks, bool copy_links, bool safe_links,
-                                           bool copy_unsafe_links, bool checksum) {
+typedef struct {
+  char* path;
+  struct stat stats;
+  bool is_directory;
+} ScannerEntry;
+
+/* Inspect symlinks, resolve the entry type, and apply file filters once for both scanners. */
+static int scanner_inspect_entry(const ScannerOptions* options, const char* source_root,
+                                 const char* containing_dir, const char* name,
+                                 ScannerEntry* entry) {
+  entry->path = path_cat(containing_dir, name);
+  if (!entry->path)
+    return -1;
+
+  struct stat link_stats;
+  if (lstat(entry->path, &link_stats) != 0) {
+    free(entry->path);
+    return 0;
+  }
+  bool is_symlink = S_ISLNK(link_stats.st_mode);
+  if (is_symlink && !options->follow_symlinks && !options->copy_links && !options->safe_links &&
+      !options->copy_unsafe_links)
+    goto skip;
+
+  if (is_symlink && options->safe_links) {
+    char link_target[4096];
+    ssize_t length = readlink(entry->path, link_target, sizeof(link_target) - 1);
+    if (length < 0)
+      goto skip;
+    link_target[length] = '\0';
+    if (link_target[0] == '/' || !safe_relative_link(source_root, containing_dir, link_target))
+      goto skip;
+  }
+
+  if (is_symlink && options->copy_unsafe_links && !options->copy_links) {
+    char link_target[4096];
+    ssize_t length = readlink(entry->path, link_target, sizeof(link_target) - 1);
+    if (length < 0)
+      goto skip;
+    link_target[length] = '\0';
+    if (link_target[0] != '/')
+      goto skip;
+  }
+
+  if (is_symlink && options->follow_symlinks && !options->copy_links)
+    entry->stats = link_stats;
+  else if (stat(entry->path, &entry->stats) != 0)
+    goto skip;
+
+  entry->is_directory = S_ISDIR(entry->stats.st_mode);
+  if (entry->is_directory)
+    return 1;
+  for (int i = 0; i < options->exclude_count; i++)
+    if (glob_match(options->exclude_patterns[i], name))
+      goto skip;
+  if (options->include_count > 0) {
+    bool included = false;
+    for (int i = 0; i < options->include_count; i++)
+      if (glob_match(options->include_patterns[i], name))
+        included = true;
+    if (!included)
+      goto skip;
+  }
+  if ((options->max_size > 0 && (unsigned long long)entry->stats.st_size > options->max_size) ||
+      (options->min_size > 0 && (unsigned long long)entry->stats.st_size < options->min_size))
+    goto skip;
+  return 1;
+
+skip:
+  free(entry->path);
+  entry->path = NULL;
+  return 0;
+}
+
+DirectoryScanner* directory_scanner_create_with_options(const char* root_directory,
+                                                        const ScannerOptions* options) {
+  if (!root_directory || !options)
+    return NULL;
   DirectoryScanner* scanner = calloc(1, sizeof(DirectoryScanner));
   if (scanner == NULL)
     return NULL;
@@ -69,21 +140,21 @@ DirectoryScanner* directory_scanner_create(const char* root_directory, bool use_
   }
   scanner->current_dir = NULL;
   scanner->current_path = NULL;
-  scanner->use_metadata = use_metadata;
-  scanner->chunk_size = chunk_size > 0 ? chunk_size : DESIRED_CHUNK_SIZE;
-  scanner->exclude_patterns = exclude_patterns;
-  scanner->exclude_count = exclude_count;
-  scanner->include_patterns = include_patterns;
-  scanner->include_count = include_count;
-  scanner->max_size = max_size;
-  scanner->min_size = min_size;
-  scanner->max_depth = max_depth;
+  scanner->use_metadata = options->use_metadata;
+  scanner->chunk_size = options->chunk_size > 0 ? options->chunk_size : DESIRED_CHUNK_SIZE;
+  scanner->exclude_patterns = options->exclude_patterns;
+  scanner->exclude_count = options->exclude_count;
+  scanner->include_patterns = options->include_patterns;
+  scanner->include_count = options->include_count;
+  scanner->max_size = options->max_size;
+  scanner->min_size = options->min_size;
+  scanner->max_depth = options->max_depth;
   scanner->current_depth = 0;
-  scanner->follow_symlinks = follow_symlinks;
-  scanner->copy_links = copy_links;
-  scanner->safe_links = safe_links;
-  scanner->copy_unsafe_links = copy_unsafe_links;
-  scanner->checksum = checksum;
+  scanner->follow_symlinks = options->follow_symlinks;
+  scanner->copy_links = options->copy_links;
+  scanner->safe_links = options->safe_links;
+  scanner->copy_unsafe_links = options->copy_unsafe_links;
+  scanner->checksum = options->checksum;
   scanner->failed = false;
   DirEntry* root = dir_entry_create(root_directory, 0);
   if (!root) {
@@ -100,6 +171,20 @@ DirectoryScanner* directory_scanner_create(const char* root_directory, bool use_
   return scanner;
 }
 
+DirectoryScanner* directory_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,
+                                           bool follow_symlinks, bool copy_links, bool safe_links,
+                                           bool copy_unsafe_links, bool checksum) {
+  ScannerOptions options = {
+      use_metadata,    chunk_size, exclude_patterns, exclude_count,     include_patterns,
+      include_count,   max_size,   min_size,         max_depth,         0,
+      follow_symlinks, copy_links, safe_links,       copy_unsafe_links, checksum};
+  return directory_scanner_create_with_options(root_directory, &options);
+}
+
 void directory_scanner_destroy(DirectoryScanner* scanner) {
   if (scanner == NULL)
     return;
@@ -167,7 +252,7 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
         break;
     }
 
-    struct dirent* entry = readdir(scanner->current_dir);
+    const struct dirent* entry = readdir(scanner->current_dir);
     if (entry == NULL) {
       closedir(scanner->current_dir);
       scanner->current_dir = NULL;
@@ -179,67 +264,27 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
     if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
       continue;
 
-    char* cur_path = path_cat(scanner->current_path, entry->d_name);
-    if (!cur_path) {
+    ScannerOptions options = {scanner->use_metadata,     scanner->chunk_size,
+                              scanner->exclude_patterns, scanner->exclude_count,
+                              scanner->include_patterns, scanner->include_count,
+                              scanner->max_size,         scanner->min_size,
+                              scanner->max_depth,        0,
+                              scanner->follow_symlinks,  scanner->copy_links,
+                              scanner->safe_links,       scanner->copy_unsafe_links,
+                              scanner->checksum};
+    ScannerEntry inspected;
+    int inspection = scanner_inspect_entry(&options, scanner->current_path, scanner->current_path,
+                                           entry->d_name, &inspected);
+    if (inspection < 0) {
       scanner->failed = true;
       break;
     }
-    struct stat stats;
-    struct stat lstats;
-    bool is_symlink = false;
-    if (lstat(cur_path, &lstats) != 0) {
-      free(cur_path);
+    if (inspection == 0)
       continue;
-    }
-    is_symlink = S_ISLNK(lstats.st_mode);
+    char* cur_path = inspected.path;
+    struct stat stats = inspected.stats;
 
-    if (is_symlink && !scanner->follow_symlinks && !scanner->copy_links && !scanner->safe_links &&
-        !scanner->copy_unsafe_links) {
-      free(cur_path);
-      continue;
-    }
-
-    if (is_symlink && scanner->safe_links) {
-      char link_target[4096];
-      ssize_t len = readlink(cur_path, link_target, sizeof(link_target) - 1);
-      if (len < 0) {
-        free(cur_path);
-        continue;
-      }
-      link_target[len] = '\0';
-      if (link_target[0] == '/' ||
-          !safe_relative_link(scanner->current_path, scanner->current_path, link_target)) {
-        free(cur_path);
-        continue;
-      }
-    }
-
-    if (is_symlink && scanner->copy_unsafe_links && !scanner->copy_links) {
-      char link_target[4096];
-      ssize_t len = readlink(cur_path, link_target, sizeof(link_target) - 1);
-      if (len < 0) {
-        free(cur_path);
-        continue;
-      }
-      link_target[len] = '\0';
-      bool unsafe = (link_target[0] == '/');
-      if (!unsafe) {
-        free(cur_path);
-        continue;
-      }
-    }
-
-    bool use_lstat = is_symlink && scanner->follow_symlinks && !scanner->copy_links;
-    if (use_lstat) {
-      stats = lstats;
-    } else {
-      if (stat(cur_path, &stats) != 0) {
-        free(cur_path);
-        continue;
-      }
-    }
-
-    if (S_ISDIR(stats.st_mode)) {
+    if (inspected.is_directory) {
       int next_depth = scanner->current_depth + 1;
       if (scanner->max_depth <= 0 || next_depth < scanner->max_depth) {
         DirEntry* de = dir_entry_create(cur_path, next_depth);
@@ -254,38 +299,6 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
         free(cur_path);
         continue;
       }
-      bool excluded = false;
-      for (int i = 0; i < scanner->exclude_count; i++) {
-        if (glob_match(scanner->exclude_patterns[i], entry->d_name)) {
-          excluded = true;
-          break;
-        }
-      }
-      if (excluded) {
-        free(cur_path);
-        continue;
-      }
-
-      if (scanner->include_count > 0) {
-        bool included = false;
-        for (int i = 0; i < scanner->include_count; i++) {
-          if (glob_match(scanner->include_patterns[i], entry->d_name)) {
-            included = true;
-            break;
-          }
-        }
-        if (!included) {
-          free(cur_path);
-          continue;
-        }
-      }
-
-      if ((scanner->max_size > 0 && (unsigned long long)stats.st_size > scanner->max_size) ||
-          (scanner->min_size > 0 && (unsigned long long)stats.st_size < scanner->min_size)) {
-        free(cur_path);
-        continue;
-      }
-
       File* file = file_create(cur_path);
       if (file == NULL) {
         free(cur_path);
@@ -336,29 +349,13 @@ typedef struct {
   ParallelScanner* ps;
   char** dirs;
   int dir_count;
-  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;
-  bool follow_symlinks;
-  bool copy_links;
-  bool safe_links;
-  bool copy_unsafe_links;
-  bool checksum;
+  ScannerOptions options;
 } ParallelWorkerArg;
 
 static int parallel_worker_thread(void* arg) {
   ParallelWorkerArg* wa = (ParallelWorkerArg*)arg;
   for (int i = 0; i < wa->dir_count; i++) {
-    DirectoryScanner* ds = directory_scanner_create(
-        wa->dirs[i], wa->use_metadata, wa->chunk_size, wa->exclude_patterns, wa->exclude_count,
-        wa->include_patterns, wa->include_count, wa->max_size, wa->min_size, wa->max_depth,
-        wa->follow_symlinks, wa->copy_links, wa->safe_links, wa->copy_unsafe_links, wa->checksum);
+    DirectoryScanner* ds = directory_scanner_create_with_options(wa->dirs[i], &wa->options);
     if (!ds) {
       mtx_lock(&wa->ps->result_mutex);
       wa->ps->failed = true;
@@ -401,13 +398,10 @@ static int parallel_worker_thread(void* arg) {
   return thrd_success;
 }
 
-ParallelScanner* parallel_scanner_create(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) {
+  if (!root_directory || !options)
+    return NULL;
   ParallelScanner* ps = calloc(1, sizeof(ParallelScanner));
   if (!ps)
     return NULL;
@@ -460,105 +454,27 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
     parallel_scanner_destroy(ps);
     return NULL;
   }
-  struct dirent* entry;
+  const struct dirent* entry;
   while ((entry = readdir(dir)) != NULL) {
     if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
       continue;
-    char* cur_path = path_cat(root_directory, entry->d_name);
-    if (!cur_path)
-      continue;
-    struct stat lstats;
-    if (lstat(cur_path, &lstats) != 0) {
-      free(cur_path);
+    ScannerEntry inspected;
+    int inspection =
+        scanner_inspect_entry(options, root_directory, root_directory, entry->d_name, &inspected);
+    if (inspection < 0) {
+      ps->failed = true;
       continue;
     }
-    bool is_symlink = S_ISLNK(lstats.st_mode);
-
-    // Skip symlinks unless the user explicitly enabled following/copying them.
-    if (is_symlink && !follow_symlinks && !copy_links && !safe_links && !copy_unsafe_links) {
-      free(cur_path);
+    if (inspection == 0)
       continue;
-    }
-
-    // --safe-links: reject symlinks pointing outside the source tree.
-    if (is_symlink && safe_links) {
-      char link_target[4096];
-      ssize_t len = readlink(cur_path, link_target, sizeof(link_target) - 1);
-      if (len < 0) {
-        free(cur_path);
-        continue;
-      }
-      link_target[len] = 0;
-      if (link_target[0] == '/' ||
-          !safe_relative_link(root_directory, root_directory, link_target)) {
-        free(cur_path);
-        continue;
-      }
-    }
-
-    // --copy-unsafe-links (without --copy-links): only copy absolute symlinks.
-    if (is_symlink && copy_unsafe_links && !copy_links) {
-      char link_target[4096];
-      ssize_t len = readlink(cur_path, link_target, sizeof(link_target) - 1);
-      if (len < 0) {
-        free(cur_path);
-        continue;
-      }
-      link_target[len] = 0;
-      bool unsafe = (link_target[0] == '/');
-      if (!unsafe) {
-        free(cur_path);
-        continue;
-      }
-    }
-
-    // Determine whether to use lstat or stat results for the entry.
-    struct stat st;
-    bool use_lstat_res = is_symlink && follow_symlinks && !copy_links;
-    if (use_lstat_res) {
-      st = lstats;
-    } else {
-      if (stat(cur_path, &st) != 0) {
-        free(cur_path);
-        continue;
-      }
-    }
-
-    if (S_ISDIR(st.st_mode)) {
+    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 {
-      bool excluded = false;
-      for (int i = 0; i < exclude_count; i++) {
-        if (glob_match(exclude_patterns[i], entry->d_name)) {
-          excluded = true;
-          break;
-        }
-      }
-      if (excluded) {
-        free(cur_path);
-        continue;
-      }
-      if (include_count > 0) {
-        bool included = false;
-        for (int i = 0; i < include_count; i++) {
-          if (glob_match(include_patterns[i], entry->d_name)) {
-            included = true;
-            break;
-          }
-        }
-        if (!included) {
-          free(cur_path);
-          continue;
-        }
-      }
-      if ((max_size > 0 && (unsigned long long)st.st_size > max_size) ||
-          (min_size > 0 && (unsigned long long)st.st_size < min_size)) {
-        free(cur_path);
-        continue;
-      }
       File* file = file_create(cur_path);
       free(cur_path);
       if (!file) {
@@ -566,9 +482,9 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
         continue;
       }
       file->data->size = st.st_size;
-      if (use_metadata)
+      if (options->use_metadata)
         file->metadata = file_metadata_create(&st);
-      if (use_metadata && !file->metadata) {
+      if (options->use_metadata && !file->metadata) {
         file_destroy(file);
         ps->failed = true;
         continue;
@@ -581,7 +497,7 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
   }
   closedir(dir);
 
-  unsigned long long cs = chunk_size > 0 ? chunk_size : DESIRED_CHUNK_SIZE;
+  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) {
@@ -648,7 +564,7 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
   }
   array_list_delete(root_files);
 
-  int n = num_threads > 0 ? num_threads : 4;
+  int n = options->num_threads > 0 ? options->num_threads : 4;
   if (n > subdirs->size)
     n = subdirs->size > 0 ? subdirs->size : 1;
 
@@ -696,20 +612,8 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
         break;
       }
       wa->dir_count = count;
-      wa->use_metadata = use_metadata;
-      wa->chunk_size = cs;
-      wa->exclude_patterns = exclude_patterns;
-      wa->exclude_count = exclude_count;
-      wa->include_patterns = include_patterns;
-      wa->include_count = include_count;
-      wa->max_size = max_size;
-      wa->min_size = min_size;
-      wa->max_depth = max_depth;
-      wa->follow_symlinks = follow_symlinks;
-      wa->copy_links = copy_links;
-      wa->safe_links = safe_links;
-      wa->copy_unsafe_links = copy_unsafe_links;
-      wa->checksum = checksum;
+      wa->options = *options;
+      wa->options.chunk_size = cs;
       start += count;
       if (thrd_create(&ps->threads[t], parallel_worker_thread, wa) != thrd_success) {
         for (int j = 0; j < count; j++)
@@ -733,6 +637,20 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
   return ps;
 }
 
+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) {
+  ScannerOptions options = {use_metadata,     chunk_size,        exclude_patterns, exclude_count,
+                            include_patterns, include_count,     max_size,         min_size,
+                            max_depth,        num_threads,       follow_symlinks,  copy_links,
+                            safe_links,       copy_unsafe_links, checksum};
+  return parallel_scanner_create_with_options(root_directory, &options);
+}
+
 Chunk* parallel_scanner_next(ParallelScanner* ps) {
   if (ps->initial_chunk) {
     Chunk* c = ps->initial_chunk;

src/client/scanner.h

@@ -8,6 +8,24 @@
 #include <threads.h>
 #include <stdatomic.h>
 
+typedef struct {
+  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;
+} ScannerOptions;
+
 typedef struct {
   Queue* directories;
   DIR* current_dir;
@@ -53,17 +71,21 @@ DirectoryScanner* directory_scanner_create(const char* root_directory, bool use_
                                            unsigned long long min_size, int max_depth,
                                            bool follow_symlinks, bool copy_links, bool safe_links,
                                            bool copy_unsafe_links, bool checksum);
+DirectoryScanner* directory_scanner_create_with_options(const char* root_directory,
+                                                        const ScannerOptions* options);
 Chunk* directory_scanner_next(DirectoryScanner* scanner);
 bool directory_scanner_failed(const DirectoryScanner* scanner);
 void directory_scanner_destroy(DirectoryScanner* scanner);
 
-ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata,
+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);
 bool parallel_scanner_failed(const ParallelScanner* scanner);
 void parallel_scanner_destroy(ParallelScanner* scanner);

src/client/usage.c

@@ -1,8 +1,9 @@
-#include "stdio.h"
+#include "usage.h"
+#include <stdio.h>
 #include <delta.h>
 #include <chunk.h>
 
-static __attribute__((unused)) void print_usage() {
+void print_usage(void) {
   printf("Usage:\n");
   printf("  fastsync [options] <source> <destination>\n");
   printf("  fastsync [options] --source-dir <src> --dest-dir <dst>\n");

src/client/usage.h

@@ -0,0 +1,6 @@
+#ifndef USAGE_H
+#define USAGE_H
+
+void print_usage(void);
+
+#endif

src/server/receiver.c

@@ -0,0 +1,126 @@
+#include "receiver.h"
+
+#include "chunk.h"
+#include "log.h"
+#include "protocol.h"
+#include "utils.h"
+#include <stdlib.h>
+#include <sys/stat.h>
+
+static bool receiver_process_chunk(Chunk* chunk, const ReceiverSink* sink) {
+  for (int i = 0; i < chunk->element_count; i++) {
+    File* file = chunk->items[i];
+    chunk->items[i] = NULL;
+    if (!sink->store_file(file, sink->context)) {
+      chunk_destroy(chunk);
+      return false;
+    }
+  }
+  chunk_destroy(chunk);
+  return true;
+}
+
+static bool receiver_process_batch(Config* config, int file_descriptor) {
+  int count;
+  if (config->checksum || !receive_int(file_descriptor, &count) || count < 0 ||
+      count > MAX_MANIFEST_ENTRIES)
+    return false;
+  for (int i = 0; i < count; i++) {
+    char* check_path = receive_str(file_descriptor);
+    if (!check_path)
+      return false;
+    unsigned long long check_size;
+    long long check_mtime;
+    if (!receive_n_data(file_descriptor, &check_size, sizeof(check_size)) ||
+        !receive_n_data(file_descriptor, &check_mtime, sizeof(check_mtime))) {
+      free(check_path);
+      return false;
+    }
+    if (!utils_valid_batch_path(check_path)) {
+      free(check_path);
+      send_status(file_descriptor, STATUS_ERROR);
+      return false;
+    }
+    char* full_path = path_cat(config->receive_root_directory, check_path);
+    struct stat st;
+    bool has_old = full_path && lstat(full_path, &st) == 0;
+    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);
+    free(full_path);
+    free(check_path);
+    if (!sent)
+      return false;
+  }
+  return true;
+}
+
+int receiver_process(Config* config, int file_descriptor, const ReceiverSink* sink) {
+  Status status;
+  if (!receive_status(file_descriptor, &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) {
+      if (!send_status(file_descriptor, STATUS_KEEPALIVE))
+        return -1;
+      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(file_descriptor, config, &skipped);
+      if (!skipped && (!file || !sink->store_file(file, sink->context)))
+        goto receive_error;
+    } else if (status == STATUS_CHUNK) {
+      Chunk* chunk = receive_chunk_data(file_descriptor, config);
+      if (!chunk || !receiver_process_chunk(chunk, sink))
+        goto receive_error;
+    } else if (status == STATUS_CHECK_BATCH) {
+      if (!receiver_process_batch(config, file_descriptor))
+        return -1;
+      goto next;
+    } else {
+      File* file = file_receive(config, file_descriptor);
+      if (!file) {
+        log_message(LOG_LEVEL_ERROR, "Failed to receive file");
+        goto receive_error;
+      }
+      if (!sink->store_file(file, sink->context))
+        goto receive_error;
+    }
+  next:
+    if (!receive_status(file_descriptor, &status))
+      goto receive_error;
+  }
+  if (status == STATUS_MANIFEST && receive_manifest(file_descriptor, config, &status) != 0)
+    return -1;
+  if (status != STATUS_FINISHED) {
+    log_message(LOG_LEVEL_ERROR, "Did not receive FINISHED Status");
+    goto receive_error;
+  }
+  if (sink->send_success && !send_status(file_descriptor, STATUS_OK))
+    return -1;
+  return 0;
+
+receive_error:
+  if (sink->send_error)
+    send_status(file_descriptor, STATUS_ERROR);
+  return -1;
+}
+
+static bool receiver_save_file(File* file, void* context) {
+  Config* config = context;
+  bool success =
+      !config->save_to_disk || file_save_to_disk(config->receive_root_directory, file, config);
+  file_destroy(file);
+  return success;
+}
+
+int receiver_receive_files(Config* config, int file_descriptor) {
+  ReceiverSink sink = {receiver_save_file, config, true, true};
+  return receiver_process(config, file_descriptor, &sink);
+}

src/server/receiver.h

@@ -0,0 +1,19 @@
+#ifndef RECEIVER_H
+#define RECEIVER_H
+
+#include "config.h"
+#include "file.h"
+
+typedef bool (*ReceiverFileSink)(File* file, void* context);
+
+typedef struct {
+  ReceiverFileSink store_file;
+  void* context;
+  bool send_error;
+  bool send_success;
+} ReceiverSink;
+
+int receiver_process(Config* config, int file_descriptor, const ReceiverSink* sink);
+int receiver_receive_files(Config* config, int file_descriptor);
+
+#endif

src/server/server.c

@@ -1,24 +1,20 @@
-#include "array_list.h"
-#include "chunk.h"
 #include "config.h"
-#include "data.h"
 #include "file.h"
 #include "log.h"
 #include "multiprocessing.h"
-#include "protocol.h"
 #include "queue.h"
+#include "receiver.h"
 #include "transport_tcp.h"
 #include "transport_tls.h"
 #include "unistd.h"
 #include "utils.h"
+#include <fcntl.h>
+#include <limits.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <limits.h>
-#include <fcntl.h>
 #include <sys/stat.h>
-#include <unistd.h>
 
 static char* authorized_root;
 static int authorized_root_fd = -1;
@@ -29,11 +25,6 @@ static bool path_is_within(const char* root, const char* path) {
   return strncmp(root, path, n) == 0 && (path[n] == '\0' || path[n] == '/');
 }
 
-static bool valid_batch_path(const char* path) {
-  return path && path[0] != '\0' && path[0] != '/' && !has_path_traversal(path) &&
-         strchr(path, '\0') == path + strlen(path);
-}
-
 static bool __attribute__((unused)) configure_authorization(const char* root) {
   char resolved[PATH_MAX];
   if (!root || !realpath(root, resolved))
@@ -52,131 +43,24 @@ static bool __attribute__((unused)) configure_authorization(const char* root) {
   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 (!valid_batch_path(check_path)) {
-          free(check_path);
-          send_status(fd, STATUS_ERROR);
-          return -1;
-        }
-        char* full_path = path_cat(config->receive_root_directory, check_path);
-        struct stat st;
-        bool has_old = full_path && lstat(full_path, &st) == 0;
-        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;
+  protocol_session_init(&session, file_descriptor, file_descriptor);
+  protocol_session_set_ssl(&session, ssl);
+  protocol_session_bind(&session);
   Config* config = config_receive(file_descriptor);
   if (config == NULL) {
     log_message(LOG_LEVEL_ERROR, "Failed to receive config");
     close(file_descriptor);
+    protocol_session_unbind();
     return;
   }
   if (!authorized_root) {
     log_message(LOG_LEVEL_ERROR, "No server-side destination root configured");
     config_delete(config);
     close(file_descriptor);
+    protocol_session_unbind();
     return;
   }
   char resolved_destination[PATH_MAX];
@@ -188,6 +72,7 @@ void handler(int file_descriptor) {
     free(canonical_destination);
     config_delete(config);
     close(file_descriptor);
+    protocol_session_unbind();
     return;
   }
   if (canonical_destination)
@@ -200,6 +85,7 @@ void handler(int file_descriptor) {
   if (!config->receive_root_directory) {
     config_delete(config);
     close(file_descriptor);
+    protocol_session_unbind();
     return;
   }
   config->use_delete = config->use_delete && allow_delete;
@@ -208,6 +94,7 @@ void handler(int file_descriptor) {
     if (q == NULL) {
       config_delete(config);
       close(file_descriptor);
+      protocol_session_unbind();
       return;
     }
     PipelineContextReceiver* context =
@@ -216,14 +103,14 @@ void handler(int file_descriptor) {
       queue_destroy(q);
       config_delete(config);
       close(file_descriptor);
+      protocol_session_unbind();
       return;
     }
     thrd_t receiver, writer;
-    bool receiver_created = false;
+    bool receiver_created = thrd_create(&receiver, receive_thread, context) == thrd_success;
     bool writer_created = false;
-    receiver_created = (thrd_create(&receiver, receive_thread, context) == thrd_success);
     if (receiver_created)
-      writer_created = (thrd_create(&writer, write_thread, context) == thrd_success);
+      writer_created = thrd_create(&writer, write_thread, context) == thrd_success;
     if (!receiver_created || !writer_created) {
       perror("Error creating Threads");
       if (receiver_created) {
@@ -240,22 +127,23 @@ void handler(int file_descriptor) {
       if (writer_created)
         thrd_join(writer, NULL);
       pipeline_context_receiver_destroy(context);
+      protocol_session_unbind();
       return;
     }
     int receiver_result;
     int writer_result;
     thrd_join(receiver, &receiver_result);
     thrd_join(writer, &writer_result);
-    if (receiver_result == thrd_success && writer_result == thrd_success)
-      send_status(file_descriptor, STATUS_OK);
-    else
-      send_status(file_descriptor, STATUS_ERROR);
+    send_status(file_descriptor, receiver_result == thrd_success && writer_result == thrd_success
+                                     ? STATUS_OK
+                                     : STATUS_ERROR);
     pipeline_context_receiver_destroy(context);
   } else {
-    if (receive_files(config, file_descriptor) != 0)
+    if (receiver_receive_files(config, file_descriptor) != 0)
       log_message(LOG_LEVEL_ERROR, "Transfer failed");
     config_delete(config);
   }
+  protocol_session_unbind();
   close(file_descriptor);
 }
 
@@ -264,16 +152,14 @@ static Server* g_server = NULL;
 
 static void cleanup(int sig) {
   (void)sig;
-  if (g_server) {
+  if (g_server)
     server_delete(&g_server);
-  }
   _exit(0);
 }
 
 static void print_server_usage(void) {
   printf("FastSync Server\n");
-  printf("Usage: fastsync-server [options]\n");
-  printf("\n");
+  printf("Usage: fastsync-server [options]\n\n");
   printf("Options:\n");
   printf("  --stdio             Run in stdio mode (SSH transport)\n");
   printf("  -p <port>           TCP port (default: 8080, range: 1-65535)\n");
@@ -289,9 +175,7 @@ static void print_server_usage(void) {
 
 int main(int argc, char* argv[]) {
   bool use_tls = false;
-  char* tls_cert = NULL;
-  char* tls_key = NULL;
-  char* tls_ca = NULL;
+  char *tls_cert = NULL, *tls_key = NULL, *tls_ca = NULL;
   int port = 8080;
   const char* destination_root = ".";
   bool stdio_mode = false;
@@ -331,11 +215,8 @@ int main(int argc, char* argv[]) {
       return 1;
     }
   }
-
-  if (tls_ca && !use_tls) {
+  if (tls_ca && !use_tls)
     log_message(LOG_LEVEL_WARNING, "--ca has no effect without --tls");
-  }
-
   signal(SIGINT, cleanup);
   signal(SIGTERM, cleanup);
   if (!configure_authorization(destination_root)) {
@@ -352,7 +233,7 @@ int main(int argc, char* argv[]) {
     return 0;
   }
   g_server = server_create(port);
-  if (g_server == NULL) {
+  if (!g_server) {
     log_message(LOG_LEVEL_ERROR, "Failed to create server");
     return 1;
   }
@@ -374,4 +255,4 @@ int main(int argc, char* argv[]) {
   }
   return 0;
 }
-#endif /* !FASTSYNC_SERVER_AS_LIB */
+#endif

src/shared/config.c

@@ -173,102 +173,157 @@ void config_delete(Config* config) {
   free(config);
 }
 
-/* Wire format order (must match config_receive and be updated when PROTOCOL_VERSION bumps):
- * version, send_directory, receive_root_directory, save_to_disk, use_multithreading,
- * use_chunk_serialization, use_compression, use_metadata, compression_level, chunk_size,
- * use_sendfile, use_delete, use_incremental, use_delta, delta_block_size, delta_max_file_size,
- * backup, backup_dir, follow_symlinks, copy_links, safe_links, copy_unsafe_links,
- * preserve_hard_links, preserve_acls, preserve_xattrs, preserve_devices, preserve_sparse,
- * update, inplace, append, append_verify, delete_excluded, delete_after, max_delete, relative,
- * prune_empty_dirs, temp_dir, partial, partial_dir, suffix, delete_before, checksum,
- * compress_choice, status
- */
+/* Each helper is deliberately ordered to match the wire format. Keep the
+ * helper call order in config_send and config_receive unchanged when adding
+ * fields. */
+static bool send_core_fields(int fd, const Config* c) {
+  return send_str(fd, c->version) && send_str(fd, c->send_directory) &&
+         send_str(fd, c->receive_root_directory) && send_int(fd, c->save_to_disk) &&
+         send_int(fd, c->use_multithreading) && send_int(fd, c->use_chunk_serialization) &&
+         send_int(fd, c->use_compression) && send_int(fd, c->use_metadata) &&
+         send_int(fd, c->compression_level) &&
+         send_n_data(fd, &c->chunk_size, sizeof(c->chunk_size)) && send_int(fd, c->use_sendfile);
+}
+
+static bool send_delta_fields(int fd, const Config* c) {
+  return send_int(fd, c->use_delete) && send_int(fd, c->use_incremental) &&
+         send_int(fd, c->use_delta) &&
+         send_n_data(fd, &c->delta_block_size, sizeof(c->delta_block_size)) &&
+         send_n_data(fd, &c->delta_max_file_size, sizeof(unsigned long long));
+}
+
+static bool send_file_options(int fd, const Config* c) {
+  return send_int(fd, c->backup) && send_str(fd, c->backup_dir ? c->backup_dir : "") &&
+         send_int(fd, c->follow_symlinks) && send_int(fd, c->copy_links) &&
+         send_int(fd, c->safe_links) && send_int(fd, c->copy_unsafe_links) &&
+         send_int(fd, c->preserve_hard_links) && send_int(fd, c->preserve_acls) &&
+         send_int(fd, c->preserve_xattrs) && send_int(fd, c->preserve_devices) &&
+         send_int(fd, c->preserve_sparse);
+}
+
+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) &&
+         send_int(fd, c->append_verify) && send_int(fd, c->delete_excluded) &&
+         send_int(fd, c->delete_after) && 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) {
+  return send_str(fd, c->temp_dir ? c->temp_dir : "") && send_int(fd, c->partial) &&
+         send_str(fd, c->partial_dir ? c->partial_dir : "") &&
+         send_str(fd, c->suffix ? c->suffix : "") && send_int(fd, c->delete_before) &&
+         send_int(fd, c->checksum) && send_str(fd, c->compress_choice ? c->compress_choice : "");
+}
+
+static bool receive_core_fields(int fd, Config* c) {
+  int value;
+  c->send_directory = receive_str(fd);
+  c->receive_root_directory = receive_str(fd);
+  if (!c->send_directory || !c->receive_root_directory)
+    return false;
+  if (!receive_int(fd, &value))
+    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))
+    return false;
+  c->use_sendfile = value;
+  return true;
+}
+
+static bool receive_delta_fields(int fd, Config* c) {
+  int value;
+  if (!receive_int(fd, &value))
+    return false;
+  c->use_delete = value;
+  if (!receive_int(fd, &value))
+    return false;
+  c->use_incremental = value;
+  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_int(fd, &value))
+    return false;
+  c->backup = value;
+  c->backup_dir = receive_str(fd);
+  if (!c->backup_dir)
+    return false;
+  bool* flags[] = {&c->follow_symlinks,   &c->copy_links,          &c->safe_links,
+                   &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))
+      return false;
+    *flags[i] = value;
+  }
+  return true;
+}
+
+static bool receive_selection_options(int fd, Config* c) {
+  int value;
+  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))
+      return false;
+    *flags[i] = value;
+  }
+  if (!receive_n_data(fd, &c->max_delete, sizeof(c->max_delete)))
+    return false;
+  if (!receive_int(fd, &value))
+    return false;
+  c->relative = value;
+  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))
+    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))
+    return false;
+  c->delete_before = value;
+  if (!receive_int(fd, &value))
+    return false;
+  c->checksum = value;
+  c->compress_choice = receive_str(fd);
+  return c->compress_choice != NULL;
+}
+
 bool config_send(int file_descriptor, const Config* config) {
-  if (!send_str(file_descriptor, config->version))
-    return false;
-  if (!send_str(file_descriptor, config->send_directory))
-    return false;
-  if (!send_str(file_descriptor, config->receive_root_directory))
-    return false;
-  if (!send_int(file_descriptor, config->save_to_disk))
-    return false;
-  if (!send_int(file_descriptor, config->use_multithreading))
-    return false;
-  if (!send_int(file_descriptor, config->use_chunk_serialization))
-    return false;
-  if (!send_int(file_descriptor, config->use_compression))
-    return false;
-  if (!send_int(file_descriptor, config->use_metadata))
-    return false;
-  if (!send_int(file_descriptor, config->compression_level))
-    return false;
-  if (!send_n_data(file_descriptor, &config->chunk_size, sizeof(config->chunk_size)))
-    return false;
-  if (!send_int(file_descriptor, config->use_sendfile))
-    return false;
-  if (!send_int(file_descriptor, config->use_delete))
-    return false;
-  if (!send_int(file_descriptor, config->use_incremental))
-    return false;
-  if (!send_int(file_descriptor, config->use_delta))
-    return false;
-  if (!send_n_data(file_descriptor, &config->delta_block_size, sizeof(config->delta_block_size)))
-    return false;
-  if (!send_n_data(file_descriptor, &config->delta_max_file_size, sizeof(unsigned long long)))
-    return false;
-  if (!send_int(file_descriptor, config->backup))
-    return false;
-  if (!send_str(file_descriptor, config->backup_dir ? config->backup_dir : ""))
-    return false;
-  if (!send_int(file_descriptor, config->follow_symlinks))
-    return false;
-  if (!send_int(file_descriptor, config->copy_links))
-    return false;
-  if (!send_int(file_descriptor, config->safe_links))
-    return false;
-  if (!send_int(file_descriptor, config->copy_unsafe_links))
-    return false;
-  if (!send_int(file_descriptor, config->preserve_hard_links))
-    return false;
-  if (!send_int(file_descriptor, config->preserve_acls))
-    return false;
-  if (!send_int(file_descriptor, config->preserve_xattrs))
-    return false;
-  if (!send_int(file_descriptor, config->preserve_devices))
-    return false;
-  if (!send_int(file_descriptor, config->preserve_sparse))
-    return false;
-  if (!send_int(file_descriptor, config->update))
-    return false;
-  if (!send_int(file_descriptor, config->inplace))
-    return false;
-  if (!send_int(file_descriptor, config->append))
-    return false;
-  if (!send_int(file_descriptor, config->append_verify))
-    return false;
-  if (!send_int(file_descriptor, config->delete_excluded))
-    return false;
-  if (!send_int(file_descriptor, config->delete_after))
-    return false;
-  if (!send_n_data(file_descriptor, &config->max_delete, sizeof(config->max_delete)))
-    return false;
-  if (!send_int(file_descriptor, config->relative))
-    return false;
-  if (!send_int(file_descriptor, config->prune_empty_dirs))
-    return false;
-  if (!send_str(file_descriptor, config->temp_dir ? config->temp_dir : ""))
-    return false;
-  if (!send_int(file_descriptor, config->partial))
-    return false;
-  if (!send_str(file_descriptor, config->partial_dir ? config->partial_dir : ""))
-    return false;
-  if (!send_str(file_descriptor, config->suffix ? config->suffix : ""))
-    return false;
-  if (!send_int(file_descriptor, config->delete_before))
-    return false;
-  if (!send_int(file_descriptor, config->checksum))
-    return false;
-  if (!send_str(file_descriptor, config->compress_choice ? config->compress_choice : ""))
+  if (!send_core_fields(file_descriptor, config) || !send_delta_fields(file_descriptor, config) ||
+      !send_file_options(file_descriptor, config) ||
+      !send_selection_options(file_descriptor, config) ||
+      !send_resume_options(file_descriptor, config))
     return false;
   Status status;
   if (!receive_status(file_descriptor, &status))
@@ -280,159 +335,25 @@ bool config_send(int file_descriptor, const Config* config) {
   return true;
 }
 
-/* Wire format order: see the comment above config_send. */
 Config* config_receive(int file_descriptor) {
-  Config* config = (Config*)malloc(sizeof(Config));
-  if (config == NULL)
+  Config* config = config_create();
+  if (!config)
     return NULL;
-  config_set_defaults(config);
   free(config->version);
   config->version = receive_str(file_descriptor);
-  if (!config->version) {
-    free(config->server_host);
-    free(config);
-    return NULL;
-  }
+  if (!config->version)
+    goto error;
   if (strcmp(config->version, PROTOCOL_VERSION) != 0) {
     fprintf(stderr, "Protocol version mismatch: client=%s, server=%s\n", config->version,
             PROTOCOL_VERSION);
-    free(config->version);
-    free(config->server_host);
-    free(config);
     send_status(file_descriptor, STATUS_ERROR);
-    return NULL;
+    goto error;
   }
-  config->send_directory = receive_str(file_descriptor);
-  if (!config->send_directory) {
-    free(config->version);
-    free(config->server_host);
-    free(config);
-    return NULL;
-  }
-  config->receive_root_directory = receive_str(file_descriptor);
-  if (!config->receive_root_directory) {
-    free(config->version);
-    free(config->send_directory);
-    free(config->server_host);
-    free(config);
-    return NULL;
-  }
-  int tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->save_to_disk = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_multithreading = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_chunk_serialization = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_compression = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_metadata = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->compression_level = tmp;
-  if (!receive_n_data(file_descriptor, &config->chunk_size, sizeof(config->chunk_size)))
-    goto error;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_sendfile = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_delete = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_incremental = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->use_delta = tmp;
-  if (!receive_n_data(file_descriptor, &config->delta_block_size, sizeof(config->delta_block_size)))
-    goto error;
-  if (!receive_n_data(file_descriptor, &config->delta_max_file_size, sizeof(unsigned long long)))
-    goto error;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->backup = tmp;
-  config->backup_dir = receive_str(file_descriptor);
-  if (config->backup_dir == NULL)
-    goto error;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->follow_symlinks = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->copy_links = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->safe_links = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->copy_unsafe_links = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->preserve_hard_links = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->preserve_acls = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->preserve_xattrs = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->preserve_devices = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->preserve_sparse = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->update = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->inplace = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->append = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->append_verify = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->delete_excluded = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->delete_after = tmp;
-  if (!receive_n_data(file_descriptor, &config->max_delete, sizeof(config->max_delete)))
-    goto error;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->relative = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->prune_empty_dirs = tmp;
-  config->temp_dir = receive_str(file_descriptor);
-  if (config->temp_dir == NULL)
-    goto error;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->partial = tmp;
-  config->partial_dir = receive_str(file_descriptor);
-  if (config->partial_dir == NULL)
-    goto error;
-  config->suffix = receive_str(file_descriptor);
-  if (config->suffix == NULL)
-    goto error;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->delete_before = tmp;
-  if (!receive_int(file_descriptor, &tmp))
-    goto error;
-  config->checksum = tmp;
-  config->compress_choice = receive_str(file_descriptor);
-  if (config->compress_choice == NULL)
+  if (!receive_core_fields(file_descriptor, config) ||
+      !receive_delta_fields(file_descriptor, config) ||
+      !receive_file_options(file_descriptor, config) ||
+      !receive_selection_options(file_descriptor, config) ||
+      !receive_resume_options(file_descriptor, config))
     goto error;
   if (config->compress_choice[0] != '\0' && strcmp(config->compress_choice, "zstd") != 0 &&
       strcmp(config->compress_choice, "none") != 0) {
@@ -445,15 +366,6 @@ Config* config_receive(int file_descriptor) {
   return config;
 
 error:
-  free(config->version);
-  free(config->send_directory);
-  free(config->receive_root_directory);
-  free(config->server_host);
-  free(config->backup_dir);
-  free(config->temp_dir);
-  free(config->partial_dir);
-  free(config->suffix);
-  free(config->compress_choice);
-  free(config);
+  config_delete(config);
   return NULL;
 }

src/shared/file.c

@@ -19,6 +19,7 @@
 #include "config.h"
 #include "data.h"
 #include "file.h"
+#include "file_store.h"
 #include "metadata.h"
 #include "protocol.h"
 #include "utils.h"
@@ -153,17 +154,8 @@ bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
   return true;
 }
 
-static bool to_disk_secure(const char* path, const void* data, unsigned long long data_size,
-                           bool inplace, bool sparse, const FileMetadata* metadata);
-static int open_secure_parent(const char* path, char** leaf_out);
-static bool rename_secure(const char* old_path, const char* new_path);
-static int authorized_root_fd = -1;
-static char* authorized_root_path;
-
 void file_set_authorized_root(int fd, const char* canonical_path) {
-  authorized_root_fd = fd;
-  free(authorized_root_path);
-  authorized_root_path = canonical_path ? str_dup(canonical_path) : NULL;
+  file_store_set_authorized_root(fd, canonical_path);
 }
 
 static bool path_is_within_root(const char* root, const char* path) {
@@ -280,7 +272,7 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
           mkdir_r(bdir);
           free(backup_dir_path);
         }
-        if (!rename_secure(disk_path, backup_path)) {
+        if (!file_store_rename_secure(disk_path, backup_path)) {
           free(backup_path);
           free(resolved_root);
           free(confined_backup);
@@ -335,7 +327,7 @@ bool file_save_to_disk(const char* root_directory, const File* file, const Confi
   free(resolved_dir);
   free(resolved_root);
 
-  bool ok = to_disk_secure(disk_path, file->data->data, file->data->size, inplace, sparse,
+  bool ok = file_store_write_secure(disk_path, file->data->data, file->data->size, inplace, sparse,
                                     file->metadata);
   free(confined_backup);
   free(confined_partial);
@@ -532,7 +524,7 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   int old_fd = -1;
   if (full_path) {
     char* leaf = NULL;
-    int parent_fd = open_secure_parent(full_path, &leaf);
+    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);
@@ -651,243 +643,11 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   return file;
 }
 
-static int open_secure_parent(const char* path, char** leaf_out) {
-  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;
-  }
-  int fd;
-  if (authorized_root_fd >= 0 && authorized_root_path && path[0] == '/' &&
-      path_is_within_root(authorized_root_path, path)) {
-    fd = dup(authorized_root_fd);
-    size_t root_len = strlen(authorized_root_path);
-    char* relative = str_dup(path + root_len);
-    if (!relative) {
-      free(copy);
-      free(leaf);
-      close(fd);
-      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;
-  }
-  char* save = NULL;
-  char* component = strtok_r(parent, "/", &save);
-  while (component) {
-    if (strcmp(component, ".") != 0 && 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)
-        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;
-}
-
-static bool rename_secure(const char* old_path, const char* new_path) {
-  char *old_leaf = NULL, *new_leaf = NULL;
-  int old_parent = open_secure_parent(old_path, &old_leaf);
-  int new_parent = open_secure_parent(new_path, &new_leaf);
-  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;
-}
-
-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;
-}
-
-static bool 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 = open_secure_parent(path, &leaf);
-  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);
-    }
-  }
-  if (fd >= 0)
-    close(fd);
-  close(dirfd);
-  free(leaf);
-  return ok;
-}
-
 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 to_disk_secure(path, data, data_size, inplace, sparse, NULL);
-  /* Kept below only as historical context; all writes use descriptor-relative operations. */
-  char* tmp_path = NULL;
-  char* directory = NULL;
-
-  char* path_dup = str_dup(path);
-  if (!path_dup)
-    return false;
-  const char* dir_result = dirname(path_dup);
-  directory = str_dup(dir_result);
-  free(path_dup);
-  if (!directory)
-    return false;
-
-  bool ok = true;
-  if (!mkdir_r(directory))
-    goto done;
-
-  if (inplace) {
-    FILE* file_pointer = fopen(path, "wb");
-    if (file_pointer == NULL) {
-      perror("Could not open file for inplace write");
-      ok = false;
-      goto done;
-    }
-    if (sparse && data_size > 0) {
-      if (fseek(file_pointer, data_size - 1, SEEK_SET) != 0) {
-        perror("Failed to seek for sparse file");
-        fclose(file_pointer);
-        ok = false;
-        goto done;
-      }
-      if (fwrite("", 1, 1, file_pointer) != 1) {
-        perror("Failed to write sparse file");
-        fclose(file_pointer);
-        ok = false;
-        goto done;
-      }
-      rewind(file_pointer);
-    }
-    if (data_size > 0 && fwrite(data, 1, data_size, file_pointer) != data_size) {
-      perror("Failed to write all data to file");
-      fclose(file_pointer);
-      ok = false;
-      goto done;
-    }
-    fclose(file_pointer);
-    free(directory);
-    return true;
-  }
-
-  size_t path_len = strlen(path);
-  tmp_path = malloc(path_len + 5);
-  if (!tmp_path) {
-    ok = false;
-    goto done;
-  }
-  memcpy(tmp_path, path, path_len);
-  memcpy(tmp_path + path_len, ".tmp", 5);
-
-  FILE* file_pointer = fopen(tmp_path, "wb");
-  if (file_pointer == NULL) {
-    perror("Could not open temporary file");
-    ok = false;
-    goto done;
-  }
-  if (sparse && data_size > 0) {
-    if (fseek(file_pointer, data_size - 1, SEEK_SET) != 0) {
-      perror("Failed to seek for sparse file");
-      fclose(file_pointer);
-      ok = false;
-      goto done;
-    }
-    if (fwrite("", 1, 1, file_pointer) != 1) {
-      perror("Failed to write sparse file");
-      fclose(file_pointer);
-      ok = false;
-      goto done;
-    }
-    rewind(file_pointer);
-  }
-  if (fwrite(data, 1, data_size, file_pointer) != data_size) {
-    perror("Failed to write all data to temporary file");
-    fclose(file_pointer);
-    unlink(tmp_path);
-    ok = false;
-    goto done;
-  }
-  fclose(file_pointer);
-
-  if (rename(tmp_path, path) != 0) {
-    perror("Failed to atomically rename temporary file");
-    unlink(tmp_path);
-    ok = false;
-    goto done;
-  }
-
-done:
-  free(tmp_path);
-  free(directory);
-  return ok;
+  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,

src/shared/file_store.c

@@ -0,0 +1,159 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "file_store.h"
+#include "metadata.h"
+#include "utils.h"
+
+static int authorized_root_fd = -1;
+static char* authorized_root_path;
+
+static bool path_is_within_root(const char* root, const char* path) {
+  size_t root_length = strlen(root);
+  return strncmp(root, path, root_length) == 0 &&
+         (path[root_length] == '\0' || path[root_length] == '/');
+}
+
+void file_store_set_authorized_root(int fd, const char* canonical_path) {
+  authorized_root_fd = fd;
+  free(authorized_root_path);
+  authorized_root_path = canonical_path ? str_dup(canonical_path) : NULL;
+}
+
+int file_store_open_secure_parent(const char* path, char** leaf_out) {
+  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;
+  }
+  int fd;
+  if (authorized_root_fd >= 0 && authorized_root_path && path[0] == '/' &&
+      path_is_within_root(authorized_root_path, path)) {
+    fd = dup(authorized_root_fd);
+    size_t root_length = strlen(authorized_root_path);
+    char* relative = str_dup(path + root_length);
+    if (!relative) {
+      free(copy);
+      free(leaf);
+      close(fd);
+      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;
+  }
+  char* save = NULL;
+  char* component = strtok_r(parent, "/", &save);
+  while (component) {
+    if (strcmp(component, ".") != 0 && 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)
+        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_store_rename_secure(const char* old_path, const char* new_path) {
+  char *old_leaf = NULL, *new_leaf = NULL;
+  int old_parent = file_store_open_secure_parent(old_path, &old_leaf);
+  int new_parent = file_store_open_secure_parent(new_path, &new_leaf);
+  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;
+}
+
+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_store_write_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_store_open_secure_parent(path, &leaf);
+  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);
+    }
+  }
+  if (fd >= 0)
+    close(fd);
+  close(dirfd);
+  free(leaf);
+  return ok;
+}

src/shared/file_store.h

@@ -0,0 +1,13 @@
+#ifndef FILE_STORE_H
+#define FILE_STORE_H
+
+#include "file.h"
+#include <stdbool.h>
+
+void 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,
+                             bool inplace, bool sparse, const FileMetadata* metadata);
+
+#endif

src/shared/multiprocessing.c

@@ -1,4 +1,5 @@
 #include "multiprocessing.h"
+#include "receiver.h"
 
 #include "array_list.h"
 #include "chunk.h"
@@ -14,10 +15,6 @@
 #include <string.h>
 #include <threads.h>
 
-static bool valid_batch_path(const char* path) {
-  return path && path[0] != '\0' && path[0] != '/' && !has_path_traversal(path);
-}
-
 PipelineContextSender* pipeline_context_sender_create(Config* config, Queue* queue_scanner,
                                                       Queue* queue_loader) {
   PipelineContextSender* context = malloc(sizeof(PipelineContextSender));
@@ -101,6 +98,8 @@ PipelineContextReceiver* pipeline_context_receiver_create(Config* config, Queue*
   context->queue = queue;
   context->file_descriptor = file_descriptor;
   context->ssl = ssl;
+  protocol_session_init(&context->session, file_descriptor, file_descriptor);
+  protocol_session_set_ssl(&context->session, ssl);
   context->receiver_done = false;
   atomic_init(&context->cancelled, false);
   int init = 0;
@@ -137,24 +136,14 @@ void pipeline_context_receiver_destroy(PipelineContextReceiver* context) {
   free(context);
 }
 
-static bool receive_chunk_enqueue(int file_descriptor, PipelineContextReceiver* context) {
-  Chunk* chunk = receive_chunk_data(file_descriptor, context->config);
-  if (chunk == NULL)
-    return false;
-
-  for (int i = 0; i < chunk->element_count; i++) {
-    File* file = chunk->items[i];
-    chunk->items[i] = NULL;
-    if (!queue_enqueue_multithreaded_cancel(context->queue, file, &context->mutex,
+static bool receiver_enqueue_file(File* file, void* context_pointer) {
+  PipelineContextReceiver* context = context_pointer;
+  if (queue_enqueue_multithreaded_cancel(context->queue, file, &context->mutex,
                                          &context->condition_not_empty,
-                                            &context->condition_not_full, &context->cancelled)) {
-      file_destroy(file);
-      chunk_destroy(chunk);
-      return false;
-    }
-  }
-  chunk_destroy(chunk);
+                                         &context->condition_not_full, &context->cancelled))
     return true;
+  file_destroy(file);
+  return false;
 }
 
 static void receiver_thread_fail(PipelineContextReceiver* context) {
@@ -167,119 +156,29 @@ static void receiver_thread_fail(PipelineContextReceiver* context) {
 }
 
 int receive_thread(void* pipeline_context) {
-#define RECEIVE_THREAD_FAIL()                                                                      \
-  do {                                                                                             \
-    receiver_thread_fail(context);                                                                 \
-    return thrd_error;                                                                             \
-  } while (0)
   PipelineContextReceiver* context = (PipelineContextReceiver*)pipeline_context;
-  if (context->ssl)
-    io_set_ssl(context->ssl);
+  protocol_session_bind(&context->session);
   mtx_lock(&context->mutex);
   int file_descriptor = context->file_descriptor;
   const Config* config = context->config;
   mtx_unlock(&context->mutex);
 
-  Status status;
-  if (!receive_status(file_descriptor, &status))
-    RECEIVE_THREAD_FAIL();
-  while (status == STATUS_NEXT || status == STATUS_CHUNK || status == STATUS_CHECK ||
-         status == STATUS_KEEPALIVE || status == STATUS_ABORT || status == STATUS_CHECK_BATCH) {
-    if (status == STATUS_KEEPALIVE) {
-      if (!send_status(file_descriptor, STATUS_KEEPALIVE))
-        RECEIVE_THREAD_FAIL();
-      goto next;
-    }
-    if (status == STATUS_ABORT) {
-      log_message(LOG_LEVEL_INFO, "Received abort from client, cleaning up");
-      RECEIVE_THREAD_FAIL();
-    }
-    if (status == STATUS_CHECK) {
-      bool skipped;
-      File* file = receive_incremental_check(file_descriptor, config, &skipped);
-      if (!skipped) {
-        if (file == NULL)
-          RECEIVE_THREAD_FAIL();
-        if (!queue_enqueue_multithreaded_cancel(
-                context->queue, file, &context->mutex, &context->condition_not_empty,
-                &context->condition_not_full, &context->cancelled)) {
-          file_destroy(file);
-          RECEIVE_THREAD_FAIL();
-        }
-      }
-    } else if (status == STATUS_CHUNK) {
-      if (!receive_chunk_enqueue(file_descriptor, context))
-        RECEIVE_THREAD_FAIL();
-    } else if (status == STATUS_CHECK_BATCH) {
-      int count;
-      if (config->checksum || !receive_int(file_descriptor, &count) || count < 0 ||
-          count > MAX_MANIFEST_ENTRIES)
-        RECEIVE_THREAD_FAIL();
-      for (int i = 0; i < count; i++) {
-        char* check_path = receive_str(file_descriptor);
-        if (!check_path)
-          RECEIVE_THREAD_FAIL();
-        unsigned long long check_size;
-        long long check_mtime;
-        if (!receive_n_data(file_descriptor, &check_size, sizeof(check_size)) ||
-            !receive_n_data(file_descriptor, &check_mtime, sizeof(check_mtime))) {
-          free(check_path);
-          RECEIVE_THREAD_FAIL();
-        }
-        if (!valid_batch_path(check_path)) {
-          free(check_path);
-          if (!send_status(file_descriptor, STATUS_ERROR))
-            RECEIVE_THREAD_FAIL();
-          RECEIVE_THREAD_FAIL();
-        }
-        char* full_path = path_cat(config->receive_root_directory, check_path);
-        struct stat st;
-        bool has_old = full_path && lstat(full_path, &st) == 0;
-        bool match = has_old && (unsigned long long)st.st_size == check_size &&
-                     (long long)st.st_mtime == check_mtime;
-        if (!send_status(file_descriptor, match ? STATUS_OK : STATUS_NEXT))
-          RECEIVE_THREAD_FAIL();
-        free(full_path);
-        free(check_path);
-      }
-      goto next;
-    } else {
-      File* file = file_receive(config, file_descriptor);
-      if (file) {
-        if (!queue_enqueue_multithreaded_cancel(
-                context->queue, file, &context->mutex, &context->condition_not_empty,
-                &context->condition_not_full, &context->cancelled)) {
-          file_destroy(file);
+  ReceiverSink sink = {receiver_enqueue_file, context, false, false};
+  if (receiver_process((Config*)config, file_descriptor, &sink) != 0) {
     receiver_thread_fail(context);
+    protocol_session_unbind();
     return thrd_error;
   }
-      } else {
-        log_message(LOG_LEVEL_ERROR, "Failed to receive file");
-        RECEIVE_THREAD_FAIL();
-      }
-    }
-  next:
-    if (!receive_status(file_descriptor, &status))
-      RECEIVE_THREAD_FAIL();
-  }
-  if (status == STATUS_MANIFEST) {
-    if (receive_manifest(file_descriptor, config, &status) != 0)
-      RECEIVE_THREAD_FAIL();
-  }
-  if (status != STATUS_FINISHED)
-    RECEIVE_THREAD_FAIL();
   mtx_lock(&context->mutex);
   context->receiver_done = true;
   cnd_signal(&context->condition_not_empty);
   mtx_unlock(&context->mutex);
-#undef RECEIVE_THREAD_FAIL
+  protocol_session_unbind();
   return thrd_success;
 }
 
 int write_thread(void* pipeline_context) {
   PipelineContextReceiver* context = (PipelineContextReceiver*)pipeline_context;
-  if (context->ssl)
-    io_set_ssl(context->ssl);
   mtx_lock(&context->mutex);
   bool save_to_disk = context->config->save_to_disk;
   char* root_directory = str_dup(context->config->receive_root_directory);

src/shared/multiprocessing.h

@@ -35,6 +35,7 @@ typedef struct PipelineContextReceiver {
   Config* config;
   int file_descriptor;
   SSL* ssl;
+  ProtocolSession session;
   mtx_t mutex;
   cnd_t condition_not_full;
   cnd_t condition_not_empty;

src/shared/protocol.c

@@ -18,6 +18,7 @@
 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 unsigned long long io_bwlimit = 0;
 static long long bw_tokens = 0;
@@ -28,6 +29,7 @@ static once_flag bw_mutex_once = ONCE_FLAG_INIT;
 static __thread unsigned long long total_allocated_bytes = 0;
 
 void io_set_fds(int read_fd, int write_fd) {
+  bound_session = NULL;
   io_read_fd = read_fd;
   io_write_fd = write_fd;
   /* A descriptor switch starts a new transport; never reuse a TLS object
@@ -36,6 +38,29 @@ void io_set_fds(int read_fd, int write_fd) {
   total_allocated_bytes = 0;
 }
 
+void protocol_session_init(ProtocolSession* session, int read_fd, int write_fd) {
+  if (!session)
+    return;
+  memset(session, 0, sizeof(*session));
+  session->read_fd = read_fd;
+  session->write_fd = write_fd;
+  if (io_bwlimit)
+    protocol_session_set_bwlimit(session, io_bwlimit);
+}
+
+void protocol_session_bind(ProtocolSession* session) {
+  bound_session = session;
+}
+
+void protocol_session_unbind(void) {
+  bound_session = NULL;
+}
+
+void protocol_session_set_ssl(ProtocolSession* session, SSL* ssl) {
+  if (session)
+    session->ssl = ssl;
+}
+
 static void bw_mutex_init(void) {
   mtx_init(&bw_mutex, mtx_plain);
 }
@@ -49,39 +74,51 @@ void io_set_bwlimit(unsigned long long bytes_per_sec) {
   mtx_unlock(&bw_mutex);
 }
 
-static void bw_throttle(size_t bytes_written) {
-  if (io_bwlimit == 0)
+void protocol_session_set_bwlimit(ProtocolSession* session, unsigned long long bytes_per_sec) {
+  if (!session)
+    return;
+  session->bwlimit = bytes_per_sec;
+  session->bw_tokens = (long long)bytes_per_sec;
+  struct timespec now;
+  clock_gettime(CLOCK_MONOTONIC, &now);
+  session->bw_last_refill_sec = now.tv_sec;
+  session->bw_last_refill_nsec = now.tv_nsec;
+}
+
+static void bw_throttle_session(ProtocolSession* session, size_t bytes_written) {
+  if (session->bwlimit == 0)
     return;
-  call_once(&bw_mutex_once, bw_mutex_init);
-  mtx_lock(&bw_mutex);
 
   struct timespec now;
   clock_gettime(CLOCK_MONOTONIC, &now);
 
-  long long elapsed_ns =
-      (now.tv_sec - bw_last_refill.tv_sec) * 1000000000LL + (now.tv_nsec - bw_last_refill.tv_nsec);
-  bw_last_refill = now;
+  long long elapsed_ns = (now.tv_sec - session->bw_last_refill_sec) * 1000000000LL +
+                         (now.tv_nsec - session->bw_last_refill_nsec);
+  session->bw_last_refill_sec = now.tv_sec;
+  session->bw_last_refill_nsec = now.tv_nsec;
 
-  long long tokens_to_add = (long long)((double)io_bwlimit * elapsed_ns / 1000000000.0);
-  bw_tokens += tokens_to_add;
-  if (bw_tokens > (long long)io_bwlimit)
-    bw_tokens = (long long)io_bwlimit;
+  long long tokens_to_add = (long long)((double)session->bwlimit * elapsed_ns / 1000000000.0);
+  session->bw_tokens += tokens_to_add;
+  if (session->bw_tokens > (long long)session->bwlimit)
+    session->bw_tokens = (long long)session->bwlimit;
 
-  bw_tokens -= (long long)bytes_written;
+  session->bw_tokens -= bytes_written;
 
-  if (bw_tokens < 0) {
-    long long deficit_us = (long long)((double)(-bw_tokens) / io_bwlimit * 1000000.0);
+  if (session->bw_tokens < 0) {
+    long long deficit_us =
+        (long long)((double)(-session->bw_tokens) / session->bwlimit * 1000000.0);
     if (deficit_us >= 1000)
       poll(NULL, 0, (int)(deficit_us / 1000));
     else
       usleep((useconds_t)deficit_us);
-    bw_tokens = 0;
-    clock_gettime(CLOCK_MONOTONIC, &bw_last_refill);
+    session->bw_tokens = 0;
+    session->bw_last_refill_sec = now.tv_sec;
+    session->bw_last_refill_nsec = now.tv_nsec;
   }
-  mtx_unlock(&bw_mutex);
 }
 
 void io_set_ssl(SSL* ssl) {
+  bound_session = NULL;
   io_ssl = ssl;
 }
 
@@ -89,8 +126,32 @@ SSL* io_get_ssl(void) {
   return io_ssl;
 }
 
-static int io_fd(int dir_fd, int file_descriptor) {
-  return (dir_fd != -1) ? dir_fd : file_descriptor;
+static ProtocolSession* legacy_session(void) {
+  static __thread ProtocolSession session;
+  if (bound_session)
+    return bound_session;
+  session.read_fd = io_read_fd;
+  session.write_fd = io_write_fd;
+  session.ssl = io_ssl;
+  session.bwlimit = io_bwlimit;
+  session.bw_tokens = (unsigned long long)(bw_tokens < 0 ? 0 : bw_tokens);
+  session.bw_last_refill_sec = bw_last_refill.tv_sec;
+  session.bw_last_refill_nsec = bw_last_refill.tv_nsec;
+  return &session;
+}
+
+bool send_n_data(int file_descriptor, const void* data, size_t data_size) {
+  ProtocolSession* session = legacy_session();
+  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();
+  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) {
@@ -104,9 +165,11 @@ static int deadline_remaining_ms(const struct timespec* deadline) {
   return ms > INT_MAX ? INT_MAX : (int)ms;
 }
 
-bool send_n_data(int file_descriptor, const void* data, size_t data_size) {
+bool protocol_send_n_data(ProtocolSession* session, const void* data, size_t data_size) {
   log_message(LOG_LEVEL_DEBUG, "    Sending n Data: %zu", data_size);
-  int fd = io_fd(io_write_fd, file_descriptor);
+  if (!session)
+    return false;
+  int fd = session->write_fd;
   struct timespec deadline;
   clock_gettime(CLOCK_MONOTONIC, &deadline);
   deadline.tv_sec += SEND_TIMEOUT_SEC;
@@ -114,7 +177,7 @@ bool send_n_data(int file_descriptor, const void* data, size_t data_size) {
   ssize_t total_bytes_send = 0;
   while ((size_t)total_bytes_send < data_size) {
     size_t chunk = data_size - total_bytes_send;
-    if (io_bwlimit > 0 && chunk > 65536)
+    if (session->bwlimit > 0 && chunk > 65536)
       chunk = 65536;
     struct pollfd pfd = {.fd = fd, .events = wait_events};
     int poll_result = poll(&pfd, 1, deadline_remaining_ms(&deadline));
@@ -127,13 +190,13 @@ bool send_n_data(int file_descriptor, const void* data, size_t data_size) {
     if (pfd.revents & (POLLERR | POLLNVAL))
       return false;
     ssize_t bytes_send;
-    if (io_ssl)
-      bytes_send = SSL_write(io_ssl, (const char*)data + total_bytes_send, chunk);
+    if (session->ssl)
+      bytes_send = SSL_write(session->ssl, (const char*)data + total_bytes_send, chunk);
     else
       bytes_send = write(fd, (const char*)data + total_bytes_send, chunk);
     if (bytes_send <= 0) {
-      if (io_ssl) {
-        int ssl_err = SSL_get_error(io_ssl, (int)bytes_send);
+      if (session->ssl) {
+        int ssl_err = SSL_get_error(session->ssl, (int)bytes_send);
         if (ssl_err == SSL_ERROR_WANT_WRITE || ssl_err == SSL_ERROR_WANT_READ) {
           wait_events = ssl_err == SSL_ERROR_WANT_WRITE ? POLLOUT : POLLIN;
           continue;
@@ -142,16 +205,18 @@ bool send_n_data(int file_descriptor, const void* data, size_t data_size) {
       log_message(LOG_LEVEL_ERROR, "Could not send data");
       return false;
     }
-    bw_throttle((size_t)bytes_send);
+    bw_throttle_session(session, (size_t)bytes_send);
     total_bytes_send += bytes_send;
   }
   log_message(LOG_LEVEL_DEBUG, "    Send n Data: %zu", total_bytes_send);
   return true;
 }
 
-bool receive_n_data(int file_descriptor, void* data, size_t data_size) {
+bool protocol_receive_n_data(ProtocolSession* session, void* data, size_t data_size) {
   log_message(LOG_LEVEL_DEBUG, "    Receiving n Data: %zu", data_size);
-  int fd = io_fd(io_read_fd, file_descriptor);
+  if (!session)
+    return false;
+  int fd = session->read_fd;
 
   struct timespec deadline;
   clock_gettime(CLOCK_MONOTONIC, &deadline);
@@ -176,15 +241,15 @@ bool receive_n_data(int file_descriptor, void* data, size_t data_size) {
       return false;
 
     ssize_t bytes_received;
-    if (io_ssl)
-      bytes_received =
-          SSL_read(io_ssl, (char*)data + total_bytes_received, data_size - total_bytes_received);
+    if (session->ssl)
+      bytes_received = SSL_read(session->ssl, (char*)data + total_bytes_received,
+                                data_size - total_bytes_received);
     else
       bytes_received =
           read(fd, (char*)data + total_bytes_received, data_size - total_bytes_received);
     if (bytes_received <= 0) {
-      if (io_ssl) {
-        int ssl_err = SSL_get_error(io_ssl, (int)bytes_received);
+      if (session->ssl) {
+        int ssl_err = SSL_get_error(session->ssl, (int)bytes_received);
         if (ssl_err == SSL_ERROR_WANT_WRITE || ssl_err == SSL_ERROR_WANT_READ) {
           wait_events = ssl_err == SSL_ERROR_WANT_WRITE ? POLLOUT : POLLIN;
           continue;
@@ -231,24 +296,24 @@ static const char* status_to_string(Status status) {
   }
 }
 
-bool send_str(int file_descriptor, const char* data) {
+bool protocol_send_str(ProtocolSession* session, const char* data) {
   if (data == NULL)
     return false;
   size_t size = strlen(data);
-  if (!send_n_data(file_descriptor, &size, sizeof(size_t)))
+  if (!protocol_send_n_data(session, &size, sizeof(size_t)))
     return false;
-  if (!send_n_data(file_descriptor, data, size))
+  if (!protocol_send_n_data(session, data, size))
     return false;
   log_message(LOG_LEVEL_DEBUG, "Send String: %s", data);
   return true;
 }
 
-char* receive_str(int file_descriptor) {
+char* protocol_receive_str(ProtocolSession* session) {
   size_t size;
-  if (!receive_n_data(file_descriptor, &size, sizeof(size_t)))
+  if (!protocol_receive_n_data(session, &size, sizeof(size_t)))
     return NULL;
   if (size > MAX_STRING_SIZE || size > SIZE_MAX - 1 ||
-      size + 1 > MAX_CONNECTION_MEMORY - total_allocated_bytes) {
+      size + 1 > MAX_CONNECTION_MEMORY - session->total_allocated_bytes) {
     log_message(LOG_LEVEL_ERROR, "String size %zu exceeds maximum %llu", size,
                 (unsigned long long)MAX_STRING_SIZE);
     return NULL;
@@ -256,29 +321,29 @@ char* receive_str(int file_descriptor) {
   char* data = (char*)malloc(size + 1);
   if (data == NULL)
     return NULL;
-  if (!receive_n_data(file_descriptor, data, size)) {
+  if (!protocol_receive_n_data(session, data, size)) {
     free(data);
     return NULL;
   }
   data[size] = '\0';
-  total_allocated_bytes += size + 1;
+  session->total_allocated_bytes += size + 1;
   log_message(LOG_LEVEL_DEBUG, "Received String: %s", data);
   return data;
 }
 
-bool send_data(int file_descriptor, const Data* data) {
+bool protocol_send_data(ProtocolSession* session, const Data* data) {
   unsigned long long data_size = data->size;
-  if (!send_n_data(file_descriptor, &data_size, sizeof(unsigned long long)))
+  if (!protocol_send_n_data(session, &data_size, sizeof(unsigned long long)))
     return false;
-  if (!send_n_data(file_descriptor, data->data, data_size))
+  if (!protocol_send_n_data(session, data->data, data_size))
     return false;
   log_message(LOG_LEVEL_DEBUG, "Send %lld data", data_size);
   return true;
 }
 
-Data* receive_data(int file_descriptor) {
+Data* protocol_receive_data(ProtocolSession* session) {
   unsigned long long size = 0;
-  if (!receive_n_data(file_descriptor, &size, sizeof(unsigned long long)))
+  if (!protocol_receive_n_data(session, &size, sizeof(unsigned long long)))
     return NULL;
   if (size > MAX_DATA_PAYLOAD_SIZE) {
     log_message(LOG_LEVEL_ERROR, "Data size %llu exceeds maximum %llu", size,
@@ -286,53 +351,86 @@ Data* receive_data(int file_descriptor) {
     return NULL;
   }
   size_t allocation_size = size == 0 ? 1 : (size_t)size;
-  if (allocation_size > MAX_CONNECTION_MEMORY - total_allocated_bytes) {
+  if (allocation_size > MAX_CONNECTION_MEMORY - session->total_allocated_bytes) {
     log_message(LOG_LEVEL_ERROR, "Per-connection memory limit exceeded (%llu + %llu > %llu)",
-                (unsigned long long)total_allocated_bytes, size,
+                (unsigned long long)session->total_allocated_bytes, size,
                 (unsigned long long)MAX_CONNECTION_MEMORY);
     return NULL;
   }
   void* data = malloc(allocation_size);
   if (data == NULL)
     return NULL;
-  if (!receive_n_data(file_descriptor, data, (size_t)size)) {
+  if (!protocol_receive_n_data(session, data, (size_t)size)) {
     free(data);
     return NULL;
   }
-  total_allocated_bytes += allocation_size;
+  session->total_allocated_bytes += allocation_size;
   log_message(LOG_LEVEL_DEBUG, "Received %lld data", size);
   Data* result = data_create(data, (size_t)size);
   if (!result) {
     free(data);
-    total_allocated_bytes -= allocation_size;
+    session->total_allocated_bytes -= allocation_size;
   }
   return result;
 }
 
-bool send_int(int file_descriptor, int data) {
-  if (!send_n_data(file_descriptor, &data, sizeof(int)))
+bool protocol_send_int(ProtocolSession* session, int data) {
+  if (!protocol_send_n_data(session, &data, sizeof(int)))
     return false;
   log_message(LOG_LEVEL_DEBUG, "Send Int: %d", data);
   return true;
 }
 
-bool receive_int(int file_descriptor, int* data) {
-  if (!receive_n_data(file_descriptor, data, sizeof(int)))
+bool protocol_receive_int(ProtocolSession* session, int* data) {
+  if (!protocol_receive_n_data(session, data, sizeof(int)))
     return false;
   log_message(LOG_LEVEL_DEBUG, "Received Int: %d", *data);
   return true;
 }
 
-bool send_status(int file_descriptor, Status status) {
-  if (!send_n_data(file_descriptor, &status, sizeof(Status)))
+bool protocol_send_status(ProtocolSession* session, Status status) {
+  if (!protocol_send_n_data(session, &status, sizeof(Status)))
     return false;
   log_message(LOG_LEVEL_DEBUG, "Send Status: %s", status_to_string(status));
   return true;
 }
 
-bool receive_status(int file_descriptor, Status* status) {
-  if (!receive_n_data(file_descriptor, status, sizeof(Status)))
+bool protocol_receive_status(ProtocolSession* session, Status* status) {
+  if (!protocol_receive_n_data(session, status, sizeof(Status)))
     return false;
   log_message(LOG_LEVEL_DEBUG, "Received Status: %s", status_to_string(*status));
   return true;
 }
+
+bool send_str(int fd, const char* data) {
+  (void)fd;
+  return protocol_send_str(legacy_session(), data);
+}
+char* receive_str(int fd) {
+  (void)fd;
+  return protocol_receive_str(legacy_session());
+}
+bool send_data(int fd, const Data* data) {
+  (void)fd;
+  return protocol_send_data(legacy_session(), data);
+}
+Data* receive_data(int fd) {
+  (void)fd;
+  return protocol_receive_data(legacy_session());
+}
+bool send_int(int fd, int data) {
+  (void)fd;
+  return protocol_send_int(legacy_session(), data);
+}
+bool receive_int(int fd, int* data) {
+  (void)fd;
+  return protocol_receive_int(legacy_session(), data);
+}
+bool send_status(int fd, Status status) {
+  (void)fd;
+  return protocol_send_status(legacy_session(), status);
+}
+bool receive_status(int fd, Status* status) {
+  (void)fd;
+  return protocol_receive_status(legacy_session(), status);
+}

src/shared/protocol.h

@@ -19,6 +19,23 @@
 
 typedef struct ssl_st SSL;
 
+/*
+ * Explicit owner of protocol I/O.  A session does not own the descriptors or
+ * SSL object; it only describes the transport used by a transfer.  This makes
+ * it safe to pass the transport to a worker without relying on inherited
+ * thread-local state.
+ */
+typedef struct ProtocolSession {
+  int read_fd;
+  int write_fd;
+  SSL* ssl;
+  unsigned long long bwlimit;
+  long long bw_tokens;
+  long long bw_last_refill_sec;
+  long bw_last_refill_nsec;
+  unsigned long long total_allocated_bytes;
+} ProtocolSession;
+
 typedef int Status;
 enum NET_STATUS {
   STATUS_OK,
@@ -39,6 +56,23 @@ void io_set_fds(int read_fd, int write_fd);
 void io_set_bwlimit(unsigned long long bytes_per_sec);
 void io_set_ssl(SSL* ssl);
 SSL* io_get_ssl(void);
+
+void protocol_session_init(ProtocolSession* session, int read_fd, int write_fd);
+/* Transitional bridge for helpers whose signatures still carry only an fd. */
+void protocol_session_bind(ProtocolSession* session);
+void protocol_session_unbind(void);
+void protocol_session_set_ssl(ProtocolSession* session, SSL* ssl);
+void protocol_session_set_bwlimit(ProtocolSession* session, unsigned long long bytes_per_sec);
+bool protocol_send_n_data(ProtocolSession* session, const void* data, size_t data_size);
+bool protocol_receive_n_data(ProtocolSession* session, void* data, size_t data_size);
+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);
+bool protocol_send_int(ProtocolSession* session, int data);
+bool protocol_receive_int(ProtocolSession* session, int* data);
+bool protocol_send_status(ProtocolSession* session, Status status);
+bool protocol_receive_status(ProtocolSession* session, Status* status);
 bool send_n_data(int file_descriptor, const void* data, size_t data_size);
 bool receive_n_data(int file_descriptor, void* data, size_t data_size);
 

src/shared/transport_tcp.c

@@ -173,7 +173,7 @@ Client* client_create() {
   return client;
 }
 
-bool client_connect(Client* client, char* host, int port) {
+bool tcp_connect_socket(Client* client, char* host, int port) {
   struct addrinfo hints;
   struct addrinfo* result;
   memset(&hints, 0, sizeof(hints));
@@ -222,6 +222,12 @@ bool client_connect(Client* client, char* host, int port) {
     return false;
   }
 
+  return true;
+}
+
+bool client_connect(Client* client, char* host, int port) {
+  if (!tcp_connect_socket(client, host, port))
+    return false;
   tcp_apply_socket_timeout(client->file_descriptor);
   return true;
 }

src/shared/transport_tcp.h

@@ -30,6 +30,7 @@ void server_accept_loop(Server* server, void (*child_fn)(int, void*), void* chil
 void server_delete(Server** server);
 Client* client_create();
 bool client_connect(Client* client, char* host, int port);
+bool tcp_connect_socket(Client* client, char* host, int port);
 void client_disconnect(Client* client);
 void client_delete(Client* client);
 void tcp_set_timeouts(int timeout_sec, int contimeout_sec);

src/shared/transport_tls.c

@@ -3,15 +3,12 @@
 #include "protocol.h"
 #include "transport_tcp.h"
 #include <arpa/inet.h>
-#include <netdb.h>
 #include <openssl/err.h>
 #include <openssl/ssl.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <sys/socket.h>
-#include <sys/wait.h>
 #include <unistd.h>
 
 bool tls_global_init(void) {
@@ -149,53 +146,8 @@ 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) {
-  struct addrinfo hints;
-  struct addrinfo* result;
-  memset(&hints, 0, sizeof(hints));
-  hints.ai_family = AF_UNSPEC;
-  hints.ai_socktype = SOCK_STREAM;
-  hints.ai_protocol = IPPROTO_TCP;
-
-  char port_str[16];
-  snprintf(port_str, sizeof(port_str), "%d", port);
-
-  int err = getaddrinfo(host, port_str, &hints, &result);
-  if (err != 0 || result == NULL) {
-    fprintf(stderr, "Could not resolve host: %s (%s)\n", host, gai_strerror(err));
+  if (!tcp_connect_socket(client, host, port))
     return false;
-  }
-
-  struct addrinfo* rp;
-  bool connected = false;
-  for (rp = result; rp != NULL; rp = rp->ai_next) {
-    if (client->file_descriptor >= 0)
-      close(client->file_descriptor);
-
-    client->file_descriptor = socket(rp->ai_family, rp->ai_socktype, rp->ai_protocol);
-    if (client->file_descriptor < 0)
-      continue;
-
-    struct timeval ct;
-    ct.tv_sec = tcp_get_contimeout_sec();
-    ct.tv_usec = 0;
-    setsockopt(client->file_descriptor, SOL_SOCKET, SO_RCVTIMEO, &ct, sizeof(ct));
-    setsockopt(client->file_descriptor, SOL_SOCKET, SO_SNDTIMEO, &ct, sizeof(ct));
-
-    memcpy(&client->address, rp->ai_addr, rp->ai_addrlen);
-    client->address_length = rp->ai_addrlen;
-
-    if (connect(client->file_descriptor, (struct sockaddr*)&client->address,
-                client->address_length) == 0) {
-      connected = true;
-      break;
-    }
-  }
-  freeaddrinfo(result);
-
-  if (!connected) {
-    perror("Could not connect to Server!");
-    return false;
-  }
 
   SSL_CTX* ctx = create_ssl_ctx(false, cert_path, key_path, ca_path);
   if (!ctx)

src/shared/utils.c

@@ -237,6 +237,10 @@ bool has_path_traversal(const char* path) {
   return false;
 }
 
+bool utils_valid_batch_path(const char* path) {
+  return path && path[0] != '\0' && path[0] != '/' && !has_path_traversal(path);
+}
+
 char* path_cat(const char* path1, const char* path2) {
   if (path1 == NULL || *path1 == '\0')
     return str_dup(path2);

src/shared/utils.h

@@ -11,5 +11,6 @@ bool glob_match(const char* pattern, const char* str);
 bool delete_extras(const char* dest_root, ArrayList* manifest);
 void utils_set_authorized_root_fd(int fd);
 bool has_path_traversal(const char* path);
+bool utils_valid_batch_path(const char* path);
 
 #endif

tests/test_client_cli.c

@@ -1,4 +1,5 @@
 #include "test_client_cli.h"
+#include "client_validation.h"
 #include "config.h"
 #include "test_utils.h"
 #include "utils.h"
@@ -9,6 +10,56 @@
 /* Declaration of parse_args from client_cli.c */
 int parse_args(Config* config, int argc, char* argv[], int* positional_args, int* positional_count);
 
+static Config* valid_client_config() {
+  Config* cfg = config_create();
+  if (!cfg)
+    return NULL;
+  cfg->send_directory = str_dup("/src");
+  cfg->receive_root_directory = str_dup("/dst");
+  return cfg;
+}
+
+static void test_validate_config_required_paths() {
+  Config* cfg = config_create();
+  EXPECT_FALSE(validate_config(cfg));
+  cfg->send_directory = str_dup("/src");
+  EXPECT_FALSE(validate_config(cfg));
+  config_delete(cfg);
+}
+
+static void test_validate_config_incompatible_options() {
+  Config* cfg = valid_client_config();
+  cfg->use_sendfile = true;
+  cfg->use_compression = true;
+  EXPECT_FALSE(validate_config(cfg));
+  cfg->use_compression = false;
+  cfg->use_incremental = true;
+  cfg->use_chunk_serialization = true;
+  EXPECT_FALSE(validate_config(cfg));
+  config_delete(cfg);
+}
+
+static void test_validate_config_tls_requirements() {
+  Config* cfg = valid_client_config();
+  cfg->use_tls = true;
+  EXPECT_FALSE(validate_config(cfg));
+  cfg->tls_cert = str_dup("cert.pem");
+  EXPECT_FALSE(validate_config(cfg));
+  cfg->tls_key = str_dup("key.pem");
+  EXPECT_TRUE(validate_config(cfg));
+  config_delete(cfg);
+}
+
+static void test_validate_config_delta_sendfile_constraints() {
+  Config* cfg = valid_client_config();
+  cfg->use_delta = true;
+  EXPECT_FALSE(validate_config(cfg));
+  cfg->use_incremental = true;
+  cfg->use_sendfile = true;
+  EXPECT_FALSE(validate_config(cfg));
+  config_delete(cfg);
+}
+
 /* Test main() with --help flag (early return path, no server connection needed) */
 static void test_cli_help() {
   /* We can't easily call main() because it calls send_files which needs a server.
@@ -287,6 +338,10 @@ static void test_parse_args_archive() {
 }
 
 void test_client_cli() {
+  test_validate_config_required_paths();
+  test_validate_config_incompatible_options();
+  test_validate_config_tls_requirements();
+  test_validate_config_delta_sendfile_constraints();
   test_cli_help();
   test_cli_archive_flags();
   test_cli_dry_run();

tests/test_config.c

@@ -224,6 +224,26 @@ static void test_config_send_receive_version_mismatch() {
   }
 }
 
+static void test_config_receive_truncated() {
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  io_set_fds(p[0], p[0]);
+  io_set_bwlimit(0);
+
+  /* A valid prefix exercises cleanup after allocated wire strings and a
+   * partially received scalar field. */
+  EXPECT_TRUE(send_str(p[1], PROTOCOL_VERSION));
+  EXPECT_TRUE(send_str(p[1], "/src"));
+  EXPECT_TRUE(send_str(p[1], "/dst"));
+  EXPECT_TRUE(send_int(p[1], 1));
+  shutdown(p[1], SHUT_WR);
+
+  const Config* cfg = config_receive(p[0]);
+  EXPECT_NULL(cfg);
+  close(p[0]);
+  close(p[1]);
+}
+
 static void test_is_remote_dest() {
   /* Valid SSH-style destinations */
   EXPECT_TRUE(is_remote_dest("user@host:/path"));
@@ -256,6 +276,7 @@ void test_config() {
   if (!is_running_under_valgrind()) {
     test_config_send_receive();
     test_config_send_receive_version_mismatch();
+    test_config_receive_truncated();
   }
   test_is_remote_dest();
 }

tests/test_protocol.c

@@ -38,6 +38,23 @@ static void test_send_receive_n_data_zero() {
   close(p[1]);
 }
 
+static void test_explicit_session_context() {
+  int p[2];
+  EXPECT_EQ_INT(pipe(p), 0);
+  ProtocolSession session;
+  protocol_session_init(&session, p[0], p[1]);
+  protocol_session_set_bwlimit(&session, 0);
+
+  const char payload[] = "explicit context";
+  char received[sizeof(payload)] = {0};
+  EXPECT_TRUE(protocol_send_n_data(&session, payload, sizeof(payload)));
+  EXPECT_TRUE(protocol_receive_n_data(&session, received, sizeof(received)));
+  EXPECT_EQ_INT(memcmp(payload, received, sizeof(payload)), 0);
+
+  close(p[0]);
+  close(p[1]);
+}
+
 static void test_send_receive_str() {
   int p[2];
   EXPECT_EQ_INT(pipe(p), 0);
@@ -173,6 +190,7 @@ static void test_receive_str_truncated() {
 void test_protocol() {
   test_send_receive_n_data();
   test_send_receive_n_data_zero();
+  test_explicit_session_context();
   test_send_receive_str();
   test_send_receive_str_normal();
   test_send_receive_data();

tests/test_server.c

@@ -11,11 +11,7 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
-/* Include server.c but rename main to avoid conflict with test runner's main */
-#define main server_main_
-#define FASTSYNC_SERVER_AS_LIB
-#include "server.c"
-#undef main
+#include "receiver.h"
 
 /* Test receive_files with immediate FINISHED status */
 static void test_receive_files_finished() {
@@ -36,7 +32,7 @@ static void test_receive_files_finished() {
     /* Child: use p[0] for both read and write */
     close(p[1]);
     io_set_fds(p[0], p[0]);
-    int ret = receive_files(cfg, p[0]);
+    int ret = receiver_receive_files(cfg, p[0]);
     close(p[0]);
     config_delete(cfg);
     _exit(ret == 0 ? 0 : 1);
@@ -88,7 +84,7 @@ static void test_receive_files_single_file() {
     /* Child: use p[0] for both read and write */
     close(p[1]);
     io_set_fds(p[0], p[0]);
-    int ret = receive_files(cfg, p[0]);
+    int ret = receiver_receive_files(cfg, p[0]);
     close(p[0]);
     config_delete(cfg);
     _exit(ret == 0 ? 0 : 1);
@@ -149,7 +145,7 @@ static void test_receive_files_abort() {
   if (pid == 0) {
     close(p[1]);
     io_set_fds(p[0], p[0]);
-    int ret = receive_files(cfg, p[0]);
+    int ret = receiver_receive_files(cfg, p[0]);
     close(p[0]);
     config_delete(cfg);
     /* Should return -1 on abort */