Merge pull request 'Evaluation of codebase by human' (#210 ) from evaluation-of-codebase-by-human into dev

Merge dev into human codebase evaluation
Merge pull request 'Remove unimplemented CLI options' (#209 ) from feat/remove-unimplemented-cli-options into evaluation-of-codebase-by-human
2026-08-11 21:24:37 +02:00 · 2026-08-11 21:24:17 +02:00 · 2026-08-11 21:17:52 +02:00 · 2026-08-11 21:14:56 +02:00 · 2026-08-11 21:12:00 +02:00 · 2026-08-11 21:08:54 +02:00
52 changed files with 3072 additions and 950 deletions
@@ -2,7 +2,7 @@ name: CI

 on:
  push:
-    branches: [main]
+    branches: [main, dev]
  pull_request:

 jobs:
@@ -73,6 +73,13 @@ jobs:
      - name: Build fuzz targets
        run: cmake --build build-fuzz -j$(nproc)

+      - name: Smoke fuzz targets
+        run: |
+          for target in build-fuzz/fuzz_*; do
+            [ -x "$target" ] || continue
+            timeout 10s "$target" -runs=100 -max_total_time=5
+          done
+
  coverage:
    runs-on: ubuntu-latest
    container: gitea.tap-tap.win/taptap/fastsync-ci:v9
@@ -60,48 +60,6 @@ python3 -m pytest tests/     # integration tests

 When running the CI workflow via `tea` (the task execution agent), always set a sufficient timeout (e.g., 600000ms) to allow CI to finish. After CI completes, check the results yourself — do not assume success. Use `gh run watch` or similar to monitor CI status, then inspect logs on failure.

-## Branch Strategy
-
-Never push directly to `main`. All changes must be developed on a feature branch and merged via a pull request. Always create a new branch before making changes:
-```bash
-git checkout -b <feature-branch-name>
-```
-After committing changes, push the branch and create a PR:
-```bash
-git push -u origin <feature-branch-name>
-gh pr create --fill
-```
-Wait for CI to pass on the PR before merging.
-
-## Batch PR Workflow
-
-When handling multiple issues split across several PRs that target the same files:
-
-1. **Group issues by logical category** into separate PR branches (e.g., memory-safety, refactoring, test-coverage).
-2. **Fix and push** each branch independently. Let CI run on each PR.
-3. **Run all 3 reviewer types** on each PR and post results to Gitea via `tea pr approve/reject` or the Gitea API:
-   - `reviewer` — general code correctness
-   - `code-quality-guardian` — code quality, duplication, complexity
-   - `security-auditor` — vulnerability assessment
-4. **Iterate**: if any reviewer requests changes, fix, push, re-review. Repeat until all 3 approve.
-5. **Merge approved PRs** one at a time into `main`.
-6. **Create a combined merge branch** for the remaining PRs that conflict with the new `main`:
-   ```bash
-   git checkout -b merge-all origin/main
-   for branch in branch1 branch2 branch3; do
-     git merge origin/$branch --no-edit || true
-     # Resolve conflicts, build, test
-   done
-   ```
-7. **Run review again** on the combined branch. Fix issues, push, re-review until approved.
-8. **Merge** the combined PR, **close** the redundant individual PRs, and **close all resolved issues** via the Gitea API:
-   ```bash
-   curl -s -X PATCH -H "Authorization: token $TOKEN" \
-     -H "Content-Type: application/json" \
-     -d '{"state":"closed"}' \
-     "https://gitea.tap-tap.win/api/v1/repos/owner/repo/issues/<number>"
-   ```
-
 ## CI Troubleshooting

 ### If lint (clang-format) fails
@@ -124,6 +82,99 @@ Run locally before pushing:
 python3 -m pytest tests/ -v --tb=short
 ```

+## Branch Strategy
+
+Two main branches: `dev` (integration) and `main` (stable releases).
+
+### Rules
+- **All PRs target `dev`** — never target `main` directly
+- **`dev` is the default branch** in Gitea repo settings
+- **`main` is protected** — only merged from `dev` via PR with 2 approvals + full CI pass
+- **Feature/bug branches** branch from `dev`, PR back to `dev`
+- **`dev` → `main` merges** happen on-demand or weekly, requiring full CI + review
+
+```bash
+# Start a new feature
+git checkout dev && git pull
+git checkout -b feat/my-feature
+# ... work, commit, push
+git push -u origin feat/my-feature
+# Create PR targeting dev
+```
+
+### Creating the `dev` branch (one-time setup)
+```bash
+git checkout main && git pull
+git checkout -b dev
+git push origin dev
+# Then in Gitea: Settings → Repository → Default Branch → dev
+```
+
+### Branch protection (Gitea repo settings)
+**For `dev`:**
+- ✅ Require PR for merging
+- ✅ Require 1 approval
+- ✅ Require status checks (all CI jobs must pass)
+- ✅ Delete branch after merge
+
+**For `main`:**
+- ✅ Require PR from `dev` only
+- ✅ Require CI
+- ✅ Require 2 approvals
+- ✅ No direct pushes
+
+## Automated Agent Workflows
+
+All agents run locally via the opencode CLI. There is no CI-based agent automation — agents are invoked on-demand by the developer or by this assistant.
+
+### One-command batch workflow
+
+For fixing a set of issues and creating one integration PR:
+
+```bash
+# 1. Run each subagent on its category
+opencode run --agent security-auditor "Fix all open security issues"
+opencode run --agent debugger "Fix all open bugs"
+opencode run --agent test-writer "Add missing test coverage"
+
+# 2. The assistant handles: merging branches, fixing CI failures,
+#    pushing, creating the integration PR, waiting for CI, iterating.
+#    The developer only reviews the final PR.
+```
+
+### Issue triage loop
+When you want to fix a batch of issues autonomously:
+
+1. Tell the assistant: *"Fix all open issues and create one big PR"*
+2. The assistant delegates to subagents in parallel
+3. Merges their branches, handles CI failures iteratively
+4. Pushes and opens the final PR
+5. You review the PR once CI passes — no intermediate check-ins
+
+### Scheduling
+For periodic maintenance (security audits, code quality scans), run:
+
+```bash
+opencode run --agent security-auditor "Audit the codebase for vulnerabilities"
+opencode run --agent code-quality-guardian "Scan for code quality issues"
+```
+
+This can be cron'd locally if desired (e.g., `crontab -e` with `opencode run`).
+
+## Is opencode a good option?
+
+**Yes, for FastSync's needs.** The hybrid model works well:
+- opencode's 17 specialized agents handle deep code analysis, fixes, tests, and reviews
+- The assistant orchestrates subagents, merges branches, and iterates on CI
+- You only review the final output
+
+The key limitation: opencode is session-based, not a persistent daemon. But for the "fix all issues, one PR" workflow, this is fine — the assistant runs the full pipeline in one shot. Persistent webhook-driven automation isn't available for Gitea, but the one-shot batch approach is simpler and gives you full control over what gets merged.
+
+### Recommendations for this project
+- **Do** use the batch pattern: delegate to subagents, let the assistant merge + iterate CI, review once
+- **Don't** try to run opencode in Gitea Actions — the CI container doesn't have your LLM keys or the interactive context agents need
+- **If** you want fully hands-off periodic scans, set up a local cron job or systemd timer that runs `opencode run` and posts results to Gitea via API
+
 ## Gitea API & tea CLI

 ### Check CI status via API
@@ -155,7 +206,7 @@ tea pr close <number> --repo TapTap/FastSync

 ## Common pitfalls

- **`__thread` on shared SSL context**: io_ssl must NOT be thread-local — worker threads inherit the SSL context from the main thread. Use regular `static SSL* io_ssl`.
+- **Per-thread SSL context**: `io_ssl` is stored per-thread (`static __thread SSL* io_ssl`). Each thread that performs protocol I/O must call `io_set_ssl()` to install its own SSL object before using `send_*` / `receive_*` primitives. The main thread's SSL context is not automatically inherited by worker threads.
 - **SSL WANT_READ/WANT_WRITE retry**: Always retry on `SSL_ERROR_WANT_READ` and `SSL_ERROR_WANT_WRITE` in `send_n_data`/`receive_n_data`. Removing these breaks TLS multithreaded transfers.
 - **clang-format version**: The CI image uses clang-format 18. Always format inside the CI Docker container for exact match.
 - **Merge order matters**: Merge the most comprehensive branch first, then smaller ones, to minimize conflicts when creating a combined branch.
@@ -79,8 +79,9 @@ 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)
+add_executable(tests ${TEST_SRCS} ${SHARED_SRCS} src/client/scanner.c src/client/client_cli.c)
 target_include_directories(tests PRIVATE ${TEST_INCLUDES})
+target_compile_definitions(tests PRIVATE FASTSYNC_TEST_BUILD)
 target_link_libraries(tests PRIVATE ${TEST_LIBS})
 add_test(NAME unit_all COMMAND tests)

@@ -11,7 +11,7 @@ A high-performance file synchronization system with SSH and TCP transport, TLS e
 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**: `mode`, `uid`, `gid`, `mtime` restored on disk when enabled
+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`)
@@ -40,12 +40,10 @@ A high-performance file synchronization system with SSH and TCP transport, TLS e
 - Optional progress display with throughput
 - Bandwidth limiting via token-bucket algorithm
 - Configurable I/O and connection timeouts (`--timeout`, `--contimeout`)
- Quiet mode (`-q`/`--quiet`) suppresses all non-error output
 - Backup overwritten files (`--backup`) with optional directory (`--backup-dir`)
 - Transfer statistics summary (`--stats`)
 - Maximum directory depth control (`--max-depth`)
 - Log file output (`--log-file`)
- Configurable multithreaded queue size (`--queue-size`)
 - Exclude patterns from file (`--exclude-from`)

 ### Server
@@ -73,7 +71,7 @@ A high-performance file synchronization system with SSH and TCP transport, TLS e
 | `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, server responds with OK (skip) or NEXT (send) |
+| `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 |
@@ -95,7 +93,9 @@ Abort (`STATUS_ABORT`) may be sent at any point. On receipt the server cleans up

 ### Protocol Version

-`1.3.0` — server and client must match. Mismatch results in `STATUS_ERROR`.
+`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`.
+
+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.

 ## Command-Line Arguments

@@ -110,12 +110,10 @@ Abort (`STATUS_ABORT`) may be sent at any point. On receipt the server cleans up
 | `-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 file metadata (mode, uid, gid, mtime) |
+| `-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 |
-| `-q, --quiet` | Suppress all non-error output |
-| `--silent` | Alias for `--quiet` |
 | `--progress` | Show real-time transfer speed |
 | `--delete` | Delete files on receiver not present in source |
 | `--exclude <pattern>` | Exclude files matching glob pattern (repeatable) |
@@ -133,7 +131,6 @@ Abort (`STATUS_ABORT`) may be sent at any point. On receipt the server cleans up
 | `--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 |
-| `--queue-size <n>` | Queue capacity for multithreaded mode (default: 100) |
 | `--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 |
@@ -176,7 +173,7 @@ Abort (`STATUS_ABORT`) may be sent at any point. On receipt the server cleans up
 ### 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`
+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
@@ -186,7 +183,7 @@ Abort (`STATUS_ABORT`) may be sent at any point. On receipt the server cleans up
 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; server compares against destination. Can be batched via `STATUS_CHECK_BATCH` for reduced round-trips.
+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
@@ -294,8 +291,8 @@ Place the `fastsync-server` binary in the remote `$PATH`. The client runs `ssh u
 # Bandwidth limit to 1 MB/s
 ./build/client --bwlimit 1024 /src user@host:/dst

-# With timeouts, quiet mode, and stats
-./build/client --timeout 60 --contimeout 15 --quiet --stats /src user@host:/dst
+# With timeouts and stats
+./build/client --timeout 60 --contimeout 15 --stats /src user@host:/dst

 # Backup overwritten files to a directory
 ./build/client --backup --backup-dir /backups /src user@host:/dst
@@ -303,9 +300,6 @@ Place the `fastsync-server` binary in the remote `$PATH`. The client runs `ssh u
 # Exclude patterns from file, limit depth
 ./build/client --exclude-from ignore.txt --max-depth 3 /src user@host:/dst

-# Custom queue size for multithreading
-./build/client -m --queue-size 200 /src user@host:/dst
-
 # Log to file
 ./build/client --log-file /tmp/fastsync.log /src user@host:/dst

@@ -332,7 +326,7 @@ The benchmark prints throughput metrics, best configuration, and speedup vs rsyn
 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; `--queue-size` controls pipeline buffering
+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
@@ -0,0 +1,273 @@
+# Rsync Feature Compatibility
+
+This document maps rsync's full feature set to FastSync's current implementation status.
+
+## Summary
+
+| Status | Count | Description |
+|--------|-------|-------------|
+| ✅ Implemented | 34 | Feature works end-to-end |
+| 🔀 Alt Arg | 3 | Functionality exists but under different flag/semantics |
+| ⚠️ Partial | 1 | Flag parsed/stored but behavior incomplete |
+| ❌ Not Implemented | 98 | Flag not recognized or no behavior |
+| **Total** | **136** | |
+
+---
+
+## 1. General Options
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-a`, `--archive` | Archive mode is -rlptgoD | 🔀 Alt Arg | Maps to -c -m -M (compression + multithread + metadata) |
+| `-v`, `--verbose` | Increase verbosity | ✅ Implemented | Sets `log_level=DEBUG` |
+| `-q`, `--quiet` | Suppress non-error messages | ❌ Not Implemented | Removed because it had no effect |
+| `--help` | Show help | ✅ Implemented | Prints usage and exits; `-h` is not accepted |
+| `-V`, `--version` | Print version | ✅ Implemented | |
+| `--info=FLAGS` | Fine-grained info verbosity | ❌ Not Implemented | Removed because it had no effect |
+| `--debug=FLAGS` | Fine-grained debug verbosity | ❌ Not Implemented | Removed because it had no effect |
+| `--stderr=MODE` | Change stderr output mode | ❌ Not Implemented | |
+| `--no-motd` | Suppress daemon MOTD | ❌ Not Implemented | |
+| `--exclude=PATTERN` | Exclude files matching pattern | ✅ Implemented | Glob matching in scanner |
+| `--include=PATTERN` | Include files matching pattern | ✅ Implemented | Glob matching in scanner |
+| `-C`, `--cvs-exclude` | Auto-ignore CVS files | ❌ Not Implemented | Removed because it had no effect |
+
+## 2. Modifying Output
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `--stats` | Give transfer stats | ✅ Implemented | Prints file/byte counts |
+| `-h`, `--human-readable` | Human-readable numbers | ❌ Not Implemented | Removed because it had no effect |
+| `-i`, `--itemize-changes` | Per-file change summary | ❌ Not Implemented | Removed because it had no effect |
+| `--progress` | Show progress | ✅ Implemented | Progress callback in sender |
+| `-P` | Same as --partial --progress | ❌ Not Implemented | |
+| `--out-format=FORMAT` | Custom output format | ❌ Not Implemented | Removed because it had no effect |
+| `--log-file=FILE` | Log to file | ✅ Implemented | `log_file` config field |
+| `--log-file-format=FMT` | Log format | ❌ Not Implemented | |
+| `--8-bit-output` | Leave high-bit chars unescaped | ❌ Not Implemented | |
+| `--list-only` | List files instead of copying | ❌ Not Implemented | Removed because it had no effect |
+
+## 3. File Selection
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `--exclude-from=FILE` | Read exclude patterns from file | ✅ Implemented | Reads patterns from file |
+| `--include-from=FILE` | Read include patterns from file | ✅ Implemented | Reads patterns from file |
+| `--filter=RULE` | Add file-filtering rule | ❌ Not Implemented | Removed because it had no effect |
+| `--files-from=FILE` | Read source file list from file | ❌ Not Implemented | Removed because it had no effect |
+| `-0`, `--from0` | Delimit *-from files with NULs | ❌ Not Implemented | |
+| `--max-size=SIZE` | Skip files larger than SIZE | ✅ Implemented | `max_size` in scanner |
+| `--min-size=SIZE` | Skip files smaller than SIZE | ✅ Implemented | `min_size` in scanner |
+| `-I`, `--ignore-times` | Don't skip files matching size+time | ❌ Not Implemented | |
+| `--size-only` | Skip based on size only | ❌ Not Implemented | |
+| `-@`, `--modify-window=NUM` | Mod-time comparison accuracy | ❌ Not Implemented | |
+| `--existing` | Skip creating new files on receiver | ❌ Not Implemented | |
+| `--ignore-existing` | Skip updating existing files | ❌ Not Implemented | |
+| `--remove-source-files` | Sender removes synced files | ❌ Not Implemented | |
+
+## 4. Directory Options
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-r`, `--recursive` | Recurse into directories | ✅ Implemented | Default behavior |
+| `-R`, `--relative` | Use relative path names | ❌ Not Implemented | Removed because it had no effect |
+| `--no-implied-dirs` | Don't send implied dirs with -R | ❌ Not Implemented | |
+| `-d`, `--dirs` | Transfer dirs without recursing | ❌ Not Implemented | |
+| `--mkpath` | Create missing path components | ❌ Not Implemented | |
+
+## 5. Transfer Modifications
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-u`, `--update` | Skip files newer on receiver | ❌ Not Implemented | Removed because it had no effect |
+| `--inplace` | Update files in-place | ✅ Implemented | Direct write mode |
+| `--append` | Append data to shorter files | ❌ Not Implemented | Removed because it had no effect |
+| `--append-verify` | Append with old-data checksum | ❌ Not Implemented | Removed because it had no effect |
+| `-W`, `--whole-file` | Copy whole file (no delta) | ❌ Not Implemented | |
+| `--block-size=SIZE` | Force checksum block-size | ⚠️ Partial | Parsed as `--delta-block`; controls delta transfer block size |
+
+## 6. Destination Handling
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-n`, `--dry-run` | Trial run with no changes | ✅ Implemented | `dry_run` config field |
+| `-b`, `--backup` | Make backups of overwritten files | ✅ Implemented | Backup before overwrite |
+| `--backup-dir=DIR` | Backup directory hierarchy | ✅ Implemented | `backup_dir` config field |
+| `--suffix=SUFFIX` | Backup suffix (default ~) | ✅ Implemented | `suffix` config field |
+| `--delay-updates` | Put updated files in place at end | ❌ Not Implemented | |
+
+## 7. Deletion
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `--delete` | Delete extraneous files from dest | ✅ Implemented | `use_delete` config field |
+| `--delete-before` | Delete before transfer | ❌ Not Implemented | Removed because it had no effect |
+| `--delete-during` | Delete during transfer | ❌ Not Implemented | |
+| `--delete-delay` | Find deletions during, delete after | ❌ Not Implemented | |
+| `--delete-after` | Delete after transfer | ❌ Not Implemented | Removed because it had no effect |
+| `--delete-excluded` | Also delete excluded files | ❌ Not Implemented | Removed because it had no effect |
+| `--max-delete=NUM` | Max files to delete | ❌ Not Implemented | Removed because it had no effect |
+| `--ignore-errors` | Delete even with I/O errors | ❌ Not Implemented | |
+| `--force` | Force deletion of non-empty dirs | ❌ Not Implemented | |
+| `--prune-empty-dirs` | Prune empty dir chains | ❌ Not Implemented | Removed because it had no effect |
+
+## 8. Metadata Preservation
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-M`, `--preserve` | Preserve file metadata | ✅ Implemented | Mode, uid, gid, mtime |
+| `-p`, `--perms` | Preserve permissions | 🔀 Alt Arg | `-p` means SSH port; permissions preserved via `-M`/`--preserve` |
+| `-o`, `--owner` | Preserve owner | ✅ Implemented | Part of -M |
+| `-g`, `--group` | Preserve group | ✅ Implemented | Part of -M |
+| `-t`, `--times` | Preserve modification times | ✅ Implemented | Part of -M |
+| `-E`, `--executability` | Preserve executability | ❌ Not Implemented | |
+| `--chmod=CHMOD` | Affect file permissions | ❌ Not Implemented | |
+| `-A`, `--acls` | Preserve ACLs | ❌ Not Implemented | Removed because it had no effect |
+| `-X`, `--xattrs` | Preserve extended attributes | ❌ Not Implemented | Removed because it had no effect |
+| `-H`, `--hard-links` | Preserve hard links | ❌ Not Implemented | Removed because it had no effect |
+| `-D` | Same as --devices --specials | ❌ Not Implemented | Removed because device-file handling is not implemented |
+| `--devices` | Preserve device files | ❌ Not Implemented | Removed because it had no effect |
+| `--specials` | Preserve special files | ❌ Not Implemented | |
+| `--copy-devices` | Copy device contents as file | ❌ Not Implemented | |
+| `--write-devices` | Write to devices as files | ❌ Not Implemented | |
+| `-U`, `--atimes` | Preserve access times | ❌ Not Implemented | |
+| `-N`, `--crtimes` | Preserve create times | ❌ Not Implemented | |
+| `-O`, `--omit-dir-times` | Omit dirs from --times | ❌ Not Implemented | |
+| `-J`, `--omit-link-times` | Omit symlinks from --times | ❌ Not Implemented | |
+| `--super` | Receiver attempts super-user activities | ❌ Not Implemented | |
+| `--fake-super` | Store/recover privileged attrs via xattrs | ❌ Not Implemented | |
+
+## 9. Symlink Handling
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-l`, `--links` | Copy symlinks as symlinks | ⚠️ Partial | Scanner includes symlinks; target path not transmitted |
+| `-L`, `--copy-links` | Transform symlink to referent | ✅ Implemented | `copy_links` config field |
+| `--copy-unsafe-links` | Transform unsafe symlinks | ✅ Implemented | `copy_unsafe_links` config field |
+| `--safe-links` | Ignore symlinks outside tree | ✅ Implemented | `safe_links` config field |
+| `--munge-links` | Munge symlinks for safety | ❌ Not Implemented | |
+| `-k`, `--copy-dirlinks` | Transform symlink to dir | ❌ Not Implemented | |
+| `-K`, `--keep-dirlinks` | Treat symlinked dir as dir | ❌ Not Implemented | |
+
+## 10. Sparse & Device
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-S`, `--sparse` | Sparse block handling | ✅ Implemented | `preserve_sparse` config field |
+| `--preallocate` | Allocate dest files before writing | ❌ Not Implemented | |
+
+## 11. Checksum & Comparison
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `--checksum` | Skip based on checksum | ❌ Not Implemented | Removed because it had no effect; `-c` means compression |
+| `--checksum-choice=STR` | Choose checksum algorithm | ❌ Not Implemented | xxHash used internally |
+| `--compare-dest=DIR` | Compare dest files relative to DIR | ❌ Not Implemented | Removed because it had no effect |
+| `--copy-dest=DIR` | Include copies of unchanged files | ❌ Not Implemented | Removed because it had no effect |
+| `--link-dest=DIR` | Hardlink to files when unchanged | ❌ Not Implemented | Removed because it had no effect |
+| `--fuzzy`, `--no-fuzzy` | Find similar file for basis | ❌ Not Implemented | |
+
+## 12. Compression
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-z`, `--compress` | Compress file data | 🔀 Alt Arg | Always uses zstd (rsync supports multiple algorithms) |
+| `--compress-choice=STR` | Choose compression algorithm | ❌ Not Implemented | Removed because it had no effect; FastSync always uses zstd |
+| `--compress-level=NUM` | Set compression level | ✅ Implemented | 1-22, default 5 |
+| `--compress-threads=NUM` | Set compression threads | ❌ Not Implemented | |
+| `--skip-compress=LIST` | Skip compress for suffixes | ❌ Not Implemented | Internal skip for hardcoded types; not user-configurable |
+
+## 13. Connectivity
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `-e`, `--rsh=COMMAND` | Remote shell to use | ❌ Not Implemented | Removed; SSH invokes `ssh` directly |
+| `--rsync-path=PROGRAM` | rsync binary on remote | ❌ Not Implemented | Removed; use `--fastsync-server-path` |
+| `--port=PORT` | Alternate daemon port | ✅ Implemented | `server_port` config field |
+| `--sockopts=OPTIONS` | Custom TCP options | ❌ Not Implemented | |
+| `--blocking-io` | Use blocking I/O for remote shell | ❌ Not Implemented | |
+| `--outbuf=N\|L\|B` | Set output buffering | ❌ Not Implemented | |
+| `--address=ADDRESS` | Bind address for outgoing socket | ❌ Not Implemented | Removed because it had no effect |
+| `-4`, `--ipv4` | Prefer IPv4 | ❌ Not Implemented | Removed because it had no effect |
+| `-6`, `--ipv6` | Prefer IPv6 | ❌ Not Implemented | Removed because it had no effect |
+
+## 14. Daemon Mode
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `--daemon` | Run as rsync daemon | ❌ Not Implemented | Removed because it had no effect |
+| `--config=FILE` | Alternate rsyncd.conf file | ❌ Not Implemented | Removed because it had no effect |
+| `--dparam=OVERRIDE` | Override global daemon config | ❌ Not Implemented | |
+| `--no-detach` | Don't detach from parent | ❌ Not Implemented | |
+| `--password-file=FILE` | Read daemon password from file | ❌ Not Implemented | |
+| `--early-input=FILE` | Use FILE for daemon early exec | ❌ Not Implemented | |
+
+## 15. Safety & Security
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| Path escape detection | Ensure files stay within root | ✅ Implemented | `has_path_traversal()` + realpath |
+| Symlink-safe delete | Skip symlinks in delete walk | ✅ Implemented | `delete_extras_walk()` |
+| Protocol version check | Verify compatible versions | ✅ Implemented | `config_receive()` |
+| Max data/string/chunk sizes | Prevent OOM attacks | ✅ Implemented | Per-message limits |
+| Per-connection memory limit | 1GB per connection | ✅ Implemented | `MAX_CONNECTION_MEMORY` |
+| `--trust-sender` | Trust remote sender's file list | ❌ Not Implemented | |
+| `--old-args` | Disable modern arg protection | ❌ Not Implemented | |
+| `--ignore-missing-args` | Ignore missing source args | ❌ Not Implemented | |
+| `--delete-missing-args` | Delete missing source args | ❌ Not Implemented | |
+
+## 16. Batch Operations
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `--write-batch=FILE` | Write batched update to file | ❌ Not Implemented | |
+| `--only-write-batch=FILE` | Write batch without updating dest | ❌ Not Implemented | |
+| `--read-batch=FILE` | Read batched update from file | ❌ Not Implemented | |
+
+## 17. Advanced
+
+| Flag | Rsync Description | FastSync Status | Notes |
+|------|-------------------|-----------------|-------|
+| `--stop-after=MINS` | Stop after N minutes | ❌ Not Implemented | |
+| `--stop-at=TIME` | Stop at specified time | ❌ Not Implemented | |
+| `--fsync` | Fsync every written file | ❌ Not Implemented | |
+| `--protocol=NUM` | Force older protocol version | ❌ Not Implemented | |
+| `--iconv=CONVERT_SPEC` | Charset conversion | ❌ Not Implemented | |
+| `--checksum-seed=NUM` | Set checksum seed | ❌ Not Implemented | |
+| `-s`, `--secluded-args` | Use protocol to send args | ❌ Not Implemented | |
+| `--no-OPTION` | Turn off implied option | ❌ Not Implemented | |
+
+---
+
+## Recommendations: Top Features to Implement Next
+
+Ranked by user demand, implementation complexity, and interoperability impact:
+
+| Priority | Feature | Effort | Impact |
+|----------|---------|--------|--------|
+| 1 | `--whole-file` / `-W` | Low | High — users expect opt-out of delta |
+| 2 | `--ignore-times` / `-I` | Low | Medium — useful for forcing re-transfer |
+| 3 | `--size-only` | Low | Medium — common migration scenario |
+| 4 | `--existing` / `--ignore-existing` | Low | Medium — common sync patterns |
+| 5 | `--remove-source-files` | Low | High — common for moves/backup |
+| 6 | `--delete-during` | Medium | High — performance improvement |
+| 7 | `--delay-updates` | Medium | High — atomic updates |
+| 8 | `--chmod` | Low | Medium — permission flexibility |
+| 9 | `--executability` / `-E` | Low | Low — simple flag |
+| 10 | `--skip-compress` | Low | Medium — performance tuning |
+
+---
+
+## FastSync-Specific Features (Not in rsync)
+
+| Feature | Description |
+|---------|-------------|
+| `-m` | Multithreaded pipeline (scanner/loader/sender) |
+| `-s` | Chunk serialization mode |
+| `-f` / `--sendfile` | Zero-copy sendfile() syscall (TCP only) |
+| `-c [level]` | zstd compression level (1-22) |
+| `--chunk-size` | Configurable chunk size |
+| `--tls` | TLS encryption (mutual auth) |
+| `--fastsync-server-path` | Path to fastsync-server binary |
+| `--server-host` / `--server-port` | Direct TCP connection |
+| Incremental sync | Skip unchanged files (size+mtime) |
+| Delta transfer | Block-level delta for changed files |
@@ -8,10 +8,13 @@
 #include "utils.h"
 #include <errno.h>
 #include <limits.h>
+#include <stdbool.h>
 #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. */
 static void parse_environment(const char** out_env_source, const char** out_env_dest,
                              bool* out_save_to_disk) {
@@ -22,19 +25,7 @@ static void parse_environment(const char** out_env_source, const char** out_env_
  if (env_save && (strcmp(env_save, "true") == 0 || strcmp(env_save, "1") == 0))
    *out_save_to_disk = true;
 }
-
-/* Parse a string as a positive integer, returning true on success. */
-static bool parse_positive_int(const char* s, int* out_val) {
-  if (!s || *s == '\0')
-    return false;
-  char* endptr;
-  errno = 0;
-  long val = strtol(s, &endptr, 10);
-  if (errno != 0 || *endptr != '\0' || val <= 0 || val > INT_MAX)
-    return false;
-  *out_val = (int)val;
-  return true;
-}
+#endif

 /* Parse a string as a non-negative integer, returning true on success. */
 static bool parse_nonneg_int(const char* s, int* out_val) {
@@ -49,16 +40,62 @@ static bool parse_nonneg_int(const char* s, int* out_val) {
  return true;
 }

-static void print_usage(void);
+/* Parse a string as a positive integer, returning true on success. */
+static bool parse_positive_int(const char* s, int* out_val) {
+  int temp;
+  if (!parse_nonneg_int(s, &temp)) {
+    return false;
+  }
+  if (temp == 0) {
+    return false;
+  }
+  *out_val = temp;
+  return true;
+}
+
+/* Duplicate a string argument into *dest, freeing the old value. Returns true on success, false on
+ * failure. */
+static int set_string_option(char** dest, const char* value, const char* option_name) {
+  char* dup = str_dup(value);
+  if (!dup) {
+    fprintf(stderr, "Error: memory allocation failed for %s\n", option_name);
+    return -1;
+  }
+  free(*dest);
+  *dest = dup;
+  return 0;
+}
+
+/* Parse a string as a positive integer into *dest. Returns true on success, false on error. */
+static int set_positive_int_option(int* dest, const char* value, const char* option_name) {
+  if (!parse_positive_int(value, dest)) {
+    fprintf(stderr, "Error: %s must be a positive integer\n", option_name);
+    return -1;
+  }
+  return 0;
+}
+
+/* Parse a string as a non-negative integer into *dest. Returns true on success, false on error. */
+static int set_nonneg_int_option(int* dest, const char* value, const char* option_name) {
+  if (!parse_nonneg_int(value, dest)) {
+    fprintf(stderr, "Error: %s must be a non-negative integer\n", option_name);
+    return -1;
+  }
+  return 0;
+}
+
 static int read_patterns_from_file(const char* filepath, char*** patterns, int* count);

 /* Parse CLI arguments into config. Returns 0 on success, -1 on error, 1 for help/clean-exit. */
-static int parse_args(Config* config, int argc, char* argv[], int* positional_args,
-                      int* positional_count) {
+int parse_args(Config* config, int argc, char* argv[], int* positional_args,
+               int* positional_count) {
  for (int i = 1; i < argc; i++) {
    if (strcmp(argv[i], "--help") == 0) {
      print_usage();
      return 1;
+    } else if (strcmp(argv[i], "-V") == 0 || strcmp(argv[i], "--version") == 0) {
+      printf("fastsync version %s\n", PROTOCOL_VERSION);
+      return 1;
    } else if (strcmp(argv[i], "-a") == 0 || strcmp(argv[i], "--archive") == 0) {
      config->use_compression = true;
      config->use_multithreading = true;
@@ -67,8 +104,10 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
    } else if (strcmp(argv[i], "-n") == 0 || strcmp(argv[i], "--dry-run") == 0) {
      config->dry_run = true;
    } else if (strcmp(argv[i], "-p") == 0 && i + 1 < argc) {
-      if (!parse_positive_int(argv[++i], &config->ssh_port)) {
-        fprintf(stderr, "Error: invalid --port/-p value: %s\n", argv[i]);
+      if (set_positive_int_option(&config->ssh_port, argv[++i], "-p") != 0)
+        return -1;
+      if (config->ssh_port > 65535) {
+        log_message(LOG_LEVEL_ERROR, "SSH port must be 1-65535\n");
        return -1;
      }
    } else if (strcmp(argv[i], "--delete") == 0) {
@@ -100,21 +139,47 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
      }
      config->include_patterns[config->include_count++] = dup;
    } else if (strcmp(argv[i], "--max-size") == 0 && i + 1 < argc) {
-      config->max_size = strtoull(argv[++i], NULL, 10);
+      char* end;
+      errno = 0;
+      unsigned long long val = strtoull(argv[++i], &end, 10);
+      if (errno != 0 || *end != '\0') {
+        fprintf(stderr, "Error: --max-size must be a non-negative integer\n");
+        return -1;
+      }
+      config->max_size = val;
    } else if (strcmp(argv[i], "--min-size") == 0 && i + 1 < argc) {
-      config->min_size = strtoull(argv[++i], NULL, 10);
+      char* end;
+      errno = 0;
+      unsigned long long val = strtoull(argv[++i], &end, 10);
+      if (errno != 0 || *end != '\0') {
+        fprintf(stderr, "Error: --min-size must be a non-negative integer\n");
+        return -1;
+      }
+      config->min_size = val;
    } else if (strcmp(argv[i], "--incremental") == 0) {
      config->use_incremental = true;
    } else if (strcmp(argv[i], "--delta") == 0) {
      config->use_delta = true;
    } else if (strcmp(argv[i], "--delta-block") == 0 && i + 1 < argc) {
-      unsigned long long val = strtoull(argv[++i], NULL, 10);
+      char* end;
+      errno = 0;
+      unsigned long long val = strtoull(argv[++i], &end, 10);
+      if (errno != 0 || *end != '\0') {
+        fprintf(stderr, "Error: --delta-block must be a positive integer\n");
+        return -1;
+      }
      if (val >= DELTA_BLOCK_SIZE_MIN && val <= DELTA_BLOCK_SIZE_MAX)
        config->delta_block_size = (uint32_t)val;
      else
        fprintf(stderr, "Warning: --delta-block value %llu out of range, using default\n", val);
    } else if (strcmp(argv[i], "--delta-max") == 0 && i + 1 < argc) {
-      unsigned long long val = strtoull(argv[++i], NULL, 10);
+      char* end;
+      errno = 0;
+      unsigned long long val = strtoull(argv[++i], &end, 10);
+      if (errno != 0 || *end != '\0') {
+        fprintf(stderr, "Error: --delta-max must be a positive integer\n");
+        return -1;
+      }
      if (val >= DELTA_MIN_FILE_SIZE)
        config->delta_max_file_size = val;
      else
@@ -126,27 +191,21 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
        char* end_ptr;
        long level = strtol(argv[i + 1], &end_ptr, 10);
        if (*end_ptr == '\0') {
+          if (level < 1 || level > 22) {
+            fprintf(stderr, "Error: compression level must be 1-22\n");
+            return -1;
+          }
          config->compression_level = (int)level;
          log_message(LOG_LEVEL_INFO, "Set Compression level to %ld", level);
          i++;
        }
      }
    } else if (strcmp(argv[i], "--source-dir") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --source-dir\n");
+      if (set_string_option(&config->send_directory, argv[++i], "--source-dir") != 0)
        return -1;
-      }
-      free(config->send_directory);
-      config->send_directory = dup;
    } else if (strcmp(argv[i], "--dest-dir") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --dest-dir\n");
+      if (set_string_option(&config->receive_root_directory, argv[++i], "--dest-dir") != 0)
        return -1;
-      }
-      free(config->receive_root_directory);
-      config->receive_root_directory = dup;
    } else if (strcmp(argv[i], "--save-to-disk") == 0) {
      config->save_to_disk = true;
    } else if (strcmp(argv[i], "-M") == 0 || strcmp(argv[i], "--preserve") == 0) {
@@ -162,18 +221,17 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
      config->use_chunk_serialization = true;
      log_message(LOG_LEVEL_INFO, "Enabled Chunk Serialization");
    } else if (strcmp(argv[i], "--server-host") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --server-host\n");
+      if (set_string_option(&config->server_host, argv[++i], "--server-host") != 0)
        return -1;
-      }
-      free(config->server_host);
-      config->server_host = dup;
    } else if (strcmp(argv[i], "--server-port") == 0 && i + 1 < argc) {
      if (!parse_positive_int(argv[++i], &config->server_port)) {
        fprintf(stderr, "Error: invalid --server-port value: %s\n", argv[i]);
        return -1;
      }
+      if (config->server_port > 65535) {
+        fprintf(stderr, "Error: server port must be 1-65535\n");
+        return -1;
+      }
    } else if (strcmp(argv[i], "--bwlimit") == 0 && i + 1 < argc) {
      char* end;
      errno = 0;
@@ -191,70 +249,47 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
    } else if (strcmp(argv[i], "--progress") == 0) {
      config->show_progress = true;
    } else if (strcmp(argv[i], "--chunk-size") == 0 && i + 1 < argc) {
-      unsigned long long val = strtoull(argv[++i], NULL, 10);
-      if (val > 0)
-        config->chunk_size = val;
+      char* end;
+      errno = 0;
+      unsigned long long val = strtoull(argv[++i], &end, 10);
+      if (errno != 0 || *end != '\0' || val == 0) {
+        fprintf(stderr, "Error: --chunk-size must be a positive integer\n");
+        return -1;
+      }
+      config->chunk_size = val;
    } else if (strcmp(argv[i], "--tls") == 0) {
      config->use_tls = true;
    } else if (strcmp(argv[i], "--cert") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --cert\n");
+      if (set_string_option(&config->tls_cert, argv[++i], "--cert") != 0)
        return -1;
-      }
-      free(config->tls_cert);
-      config->tls_cert = dup;
    } else if (strcmp(argv[i], "--key") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --key\n");
+      if (set_string_option(&config->tls_key, argv[++i], "--key") != 0)
        return -1;
-      }
-      free(config->tls_key);
-      config->tls_key = dup;
    } else if (strcmp(argv[i], "--ca") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --ca\n");
+      if (set_string_option(&config->tls_ca, argv[++i], "--ca") != 0)
        return -1;
-      }
-      free(config->tls_ca);
-      config->tls_ca = dup;
    } else if (strcmp(argv[i], "--timeout") == 0 && i + 1 < argc) {
-      int val;
-      if (!parse_positive_int(argv[++i], &val)) {
-        fprintf(stderr, "Error: --timeout must be a positive integer\n");
+      if (set_positive_int_option(&config->timeout, argv[++i], "--timeout") != 0)
        return -1;
-      }
-      config->timeout = val;
    } else if (strcmp(argv[i], "--contimeout") == 0 && i + 1 < argc) {
-      int val;
-      if (!parse_positive_int(argv[++i], &val)) {
-        fprintf(stderr, "Error: --contimeout must be a positive integer\n");
+      if (set_positive_int_option(&config->contimeout, argv[++i], "--contimeout") != 0)
        return -1;
-      }
-      config->contimeout = val;
-    } else if (strcmp(argv[i], "-q") == 0 || strcmp(argv[i], "--quiet") == 0 ||
-               strcmp(argv[i], "--silent") == 0) {
-      config->quiet = true;
    } else if (strcmp(argv[i], "--backup") == 0) {
      config->backup = true;
    } else if (strcmp(argv[i], "--backup-dir") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --backup-dir\n");
+      if (set_string_option(&config->backup_dir, argv[++i], "--backup-dir") != 0)
        return -1;
-      }
-      free(config->backup_dir);
-      config->backup_dir = dup;
    } else if (strcmp(argv[i], "--stats") == 0) {
      config->stats = true;
    } else if (strcmp(argv[i], "--max-depth") == 0 && i + 1 < argc) {
-      if (!parse_nonneg_int(argv[++i], &config->max_depth)) {
-        fprintf(stderr, "Error: --max-depth must be a non-negative integer\n");
+      if (set_nonneg_int_option(&config->max_depth, argv[++i], "--max-depth") != 0)
        return -1;
-      }
    } else if (strcmp(argv[i], "--log-file") == 0 && i + 1 < argc) {
+      if (config->log_file) {
+        fclose(config->log_file);
+        config->log_file = NULL;
+        log_set_file(NULL);
+      }
      FILE* lf = fopen(argv[++i], "a");
      if (!lf) {
        fprintf(stderr, "Error: could not open log file '%s': %s\n", argv[i], strerror(errno));
@@ -262,13 +297,6 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
      }
      config->log_file = lf;
      log_set_file(lf);
-    } else if (strcmp(argv[i], "--queue-size") == 0 && i + 1 < argc) {
-      int val;
-      if (!parse_positive_int(argv[++i], &val)) {
-        fprintf(stderr, "Error: --queue-size must be a positive integer\n");
-        return -1;
-      }
-      config->queue_size = val;
    } else if (strcmp(argv[i], "--exclude-from") == 0 && i + 1 < argc) {
      if (read_patterns_from_file(argv[++i], &config->exclude_patterns, &config->exclude_count) !=
          0)
@@ -280,13 +308,9 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
    } else if (strcmp(argv[i], "--partial") == 0) {
      config->partial = true;
    } else if (strcmp(argv[i], "--fastsync-server-path") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup) {
-        fprintf(stderr, "Error: memory allocation failed for --fastsync-server-path\n");
+      if (set_string_option(&config->fastsync_server_path, argv[++i], "--fastsync-server-path") !=
+          0)
        return -1;
-      }
-      free(config->fastsync_server_path);
-      config->fastsync_server_path = dup;
    } else if (strcmp(argv[i], "-v") == 0 || strcmp(argv[i], "--verbose") == 0) {
      set_log_level(LOG_LEVEL_DEBUG);
    } else if (strcmp(argv[i], "-l") == 0 || strcmp(argv[i], "--links") == 0) {
@@ -297,111 +321,28 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
      config->safe_links = true;
    } else if (strcmp(argv[i], "--copy-unsafe-links") == 0) {
      config->copy_unsafe_links = true;
-    } else if (strcmp(argv[i], "-H") == 0 || strcmp(argv[i], "--hard-links") == 0) {
-      config->preserve_hard_links = true;
-    } else if (strcmp(argv[i], "-A") == 0 || strcmp(argv[i], "--acls") == 0) {
-      config->preserve_acls = true;
-    } else if (strcmp(argv[i], "-X") == 0 || strcmp(argv[i], "--xattrs") == 0) {
-      config->preserve_xattrs = true;
-    } else if (strcmp(argv[i], "-D") == 0 || strcmp(argv[i], "--devices") == 0) {
-      config->preserve_devices = true;
    } else if (strcmp(argv[i], "-S") == 0 || strcmp(argv[i], "--sparse") == 0) {
      config->preserve_sparse = true;
-    } else if (strcmp(argv[i], "-i") == 0 || strcmp(argv[i], "--itemize-changes") == 0) {
-      config->itemize_changes = true;
-    } else if (strcmp(argv[i], "--out-format") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      free(config->out_format);
-      config->out_format = dup;
-    } else if (strcmp(argv[i], "--info") == 0 && i + 1 < argc) {
-      config->info_level = atoi(argv[++i]);
-    } else if (strcmp(argv[i], "--debug") == 0 && i + 1 < argc) {
-      config->debug_level = atoi(argv[++i]);
-    } else if (strcmp(argv[i], "--list-only") == 0) {
-      config->list_only = true;
-    } else if (strcmp(argv[i], "-h") == 0 || strcmp(argv[i], "--human-readable") == 0) {
-      config->human_readable = true;
-    } else if (strcmp(argv[i], "-u") == 0 || strcmp(argv[i], "--update") == 0) {
-      config->update = true;
    } else if (strcmp(argv[i], "--inplace") == 0) {
      config->inplace = true;
-    } else if (strcmp(argv[i], "--append") == 0) {
-      config->append = true;
-    } else if (strcmp(argv[i], "--append-verify") == 0) {
-      config->append_verify = true;
-    } else if (strcmp(argv[i], "--delete-excluded") == 0) {
-      config->delete_excluded = true;
-    } else if (strcmp(argv[i], "--delete-after") == 0) {
-      config->delete_after = true;
-    } else if (strcmp(argv[i], "--max-delete") == 0 && i + 1 < argc) {
-      int val;
-      if (!parse_nonneg_int(argv[++i], &val)) {
-        fprintf(stderr, "Error: --max-delete must be a non-negative integer\n");
+    } else if (strcmp(argv[i], "--partial-dir") == 0 && i + 1 < argc) {
+      if (set_string_option(&config->partial_dir, argv[++i], "--partial-dir") != 0)
+        return -1;
+    } else if (strcmp(argv[i], "--suffix") == 0 && i + 1 < argc) {
+      if (set_string_option(&config->suffix, argv[++i], "--suffix") != 0)
+        return -1;
+    } else if (strcmp(argv[i], "-T") == 0 && i + 1 < argc) {
+      if (set_positive_int_option(&config->timeout, argv[++i], "-T") != 0)
+        return -1;
+    } else if (strcmp(argv[i], "--checksum") == 0) {
+      config->checksum = true;
+    } else if (strcmp(argv[i], "--compress-level") == 0 && i + 1 < argc) {
+      if (set_positive_int_option(&config->compression_level, argv[++i], "--compress-level") != 0)
+        return -1;
+      if (config->compression_level < 1 || config->compression_level > 22) {
+        fprintf(stderr, "Error: --compress-level must be between 1 and 22\n");
        return -1;
      }
-      config->max_delete = val;
-    } else if (strcmp(argv[i], "--filter") == 0 && i + 1 < argc) {
-      if (!config->filters)
-        config->filters = array_list_create(free);
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      array_list_add(config->filters, dup);
-    } else if (strcmp(argv[i], "--files-from") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      free(config->files_from);
-      config->files_from = dup;
-    } else if (strcmp(argv[i], "--cvs-exclude") == 0) {
-      config->cvs_exclude = true;
-    } else if (strcmp(argv[i], "--prune-empty-dirs") == 0) {
-      config->prune_empty_dirs = true;
-    } else if (strcmp(argv[i], "-R") == 0 || strcmp(argv[i], "--relative") == 0) {
-      config->relative = true;
-    } else if (strcmp(argv[i], "-e") == 0 || strcmp(argv[i], "--rsh") == 0) {
-      if (i + 1 < argc) {
-        char* dup = str_dup(argv[++i]);
-        if (!dup)
-          return -1;
-        free(config->rsh_command);
-        config->rsh_command = dup;
-      } else {
-        fprintf(stderr, "Error: -e/--rsh requires a command argument\n");
-        return -1;
-      }
-    } else if (strcmp(argv[i], "--rsync-path") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      free(config->rsync_path);
-      config->rsync_path = dup;
-    } else if (strcmp(argv[i], "--temp-dir") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      free(config->temp_dir);
-      config->temp_dir = dup;
-    } else if (strcmp(argv[i], "--compare-dest") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      free(config->compare_dest);
-      config->compare_dest = dup;
-    } else if (strcmp(argv[i], "--copy-dest") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      free(config->copy_dest);
-      config->copy_dest = dup;
-    } else if (strcmp(argv[i], "--link-dest") == 0 && i + 1 < argc) {
-      char* dup = str_dup(argv[++i]);
-      if (!dup)
-        return -1;
-      free(config->link_dest);
-      config->link_dest = dup;
    } else if (argv[i][0] == '-') {
      fprintf(stderr, "Unknown option: %s\n", argv[i]);
      print_usage();
@@ -419,6 +360,7 @@ static int parse_args(Config* config, int argc, char* argv[], int* positional_ar
  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) {
@@ -451,6 +393,12 @@ static bool validate_config(const Config* config) {
    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");
@@ -459,101 +407,7 @@ static bool validate_config(const Config* config) {
  }
  return true;
 }
-
-static void print_usage(void) {
-  printf("Usage:\n");
-  printf("  fastsync [options] <source> <destination>\n");
-  printf("  fastsync [options] --source-dir <src> --dest-dir <dst>\n");
-  printf("\n");
-  printf("Destination formats:\n");
-  printf("  user@host:/path     SSH transport (rsync-style)\n");
-  printf("  host:/path          SSH transport (current user)\n");
-  printf("  /local/path         TCP transport (requires server on localhost:8080)\n");
-  printf("\n");
-  printf("Options:\n");
-  printf("  -c [level]          Enable compression (level 1-22, default 5)\n");
-  printf("  -z [level]          Alias for -c\n");
-  printf("  -a, --archive       Archive mode (-c -m -M)\n");
-  printf("  -n, --dry-run       Show what would be transferred\n");
-  printf("  -p <port>           SSH port (default: 22)\n");
-  printf("  --progress          Show transfer progress\n");
-  printf("  --delete            Delete files on receiver not in source\n");
-  printf("  --exclude <pattern> Exclude files matching pattern\n");
-  printf("  --include <pattern> Only include files matching pattern\n");
-  printf("  --exclude-from <file> Read exclude patterns from file\n");
-  printf("  --include-from <file> Read include patterns from file\n");
-  printf("  --max-size <n>      Skip files larger than n bytes\n");
-  printf("  --min-size <n>      Skip files smaller than n bytes\n");
-  printf("  --incremental       Skip files unchanged since last transfer\n");
-  printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
-  printf("  --delta-block <n>   Delta block size in bytes (default: %d)\n",
-         DELTA_BLOCK_SIZE_DEFAULT);
-  printf("  --delta-max <n>     Max file size for delta transfer (default: %llu)\n",
-         DELTA_MAX_FILE_SIZE);
-  printf("  -m                  Enable multithreading\n");
-  printf("  -s                  Enable chunk serialization\n");
-  printf("  -f                  Enable sendfile (TCP only, not with -c or -s)\n");
-  printf("  -v, --verbose       Enable debug logging\n");
-  printf("  -M, --preserve      Preserve file metadata\n");
-  printf("  --chunk-size <n>    Chunk size in bytes (default: %d)\n", DEFAULT_CHUNK_SIZE);
-  printf("  --source-dir <path> Source directory\n");
-  printf("  --dest-dir <path>   Destination directory\n");
-  printf("  --save-to-disk      Write received files to disk\n");
-  printf("  --server-host <ip>  Server IP address (default: 127.0.0.1)\n");
-  printf("  --server-port <n>   Server port (default: 8080)\n");
-  printf("  --bwlimit <KB/s>    Bandwidth limit in kilobytes per second\n");
-  printf("  --tls               Enable TLS encryption\n");
-  printf("  --cert <path>       TLS certificate file (PEM)\n");
-  printf("  --key <path>        TLS private key file (PEM)\n");
-  printf("  --ca <path>         TLS CA certificate file (PEM)\n");
-  printf("  --timeout <sec>     I/O timeout in seconds (default: 30)\n");
-  printf("  --contimeout <sec>  Connection timeout in seconds (default: 10)\n");
-  printf("  -q, --quiet         Suppress non-error output\n");
-  printf("  --silent            Alias for --quiet\n");
-  printf("  --backup            Backup existing files before overwriting\n");
-  printf("  --backup-dir <dir>  Directory for backups (requires --backup)\n");
-  printf("  --stats             Print transfer statistics at end\n");
-  printf("  --max-depth <n>     Maximum directory depth (0=unlimited)\n");
-  printf("  --log-file <path>   Write log messages to file\n");
-  printf("  --queue-size <n>    Queue capacity for multithreaded mode (default: 100)\n");
-  printf("  --partial           Keep partial files on interrupted transfer\n");
-  printf("  --fastsync-server-path <path>\n");
-  printf("                      Path to fastsync-server on remote (default: fastsync-server)\n");
-  printf("  -l, --links         Copy symlinks as symlinks\n");
-  printf("  --copy-links        Transform symlinks into referent files\n");
-  printf("  --safe-links        Skip symlinks that point outside transfer tree\n");
-  printf("  --copy-unsafe-links  Only transform unsafe symlinks into referent files\n");
-  printf("  -H, --hard-links    Preserve hard links\n");
-  printf("  -A, --acls          Preserve ACLs\n");
-  printf("  -X, --xattrs        Preserve extended attributes\n");
-  printf("  -D, --devices       Preserve device files\n");
-  printf("  -S, --sparse        Handle sparse files efficiently\n");
-  printf("  -i, --itemize-changes  Show per-file change summary\n");
-  printf("  --out-format <fmt>  Custom output format string\n");
-  printf("  --info <flags>      Info verbosity level\n");
-  printf("  --debug <flags>     Debug verbosity level\n");
-  printf("  --list-only         List files without transferring\n");
-  printf("  -h, --human-readable  Human-readable numbers\n");
-  printf("  -u, --update        Skip files newer on destination\n");
-  printf("  --inplace           Update files in-place (no temp+rename)\n");
-  printf("  --append            Append data to shorter files\n");
-  printf("  --append-verify     Append with verify\n");
-  printf("  --delete-excluded   Also delete excluded files\n");
-  printf("  --delete-after      Delete after transfer, not before\n");
-  printf("  --max-delete <n>    Maximum number of files to delete\n");
-  printf("  --filter <rule>     Add file filtering rule\n");
-  printf("  --files-from <file> Read file list from file\n");
-  printf("  --cvs-exclude       Auto-ignore CVS files\n");
-  printf("  --prune-empty-dirs  Omit empty directories from transfer\n");
-  printf("  -R, --relative      Use relative paths\n");
-  printf("  -e, --rsh <cmd>     Specify remote shell\n");
-  printf("  --rsync-path <path> Path to remote binary\n");
-  printf("  --temp-dir <dir>    Temporary directory for files\n");
-  printf("  --compare-dest <dir>  Compare destination\n");
-  printf("  --copy-dest <dir>   Copy destination\n");
-  printf("  --link-dest <dir>   Link destination\n");
-  printf("  --help              Show this help\n");
-}
+#endif /* FASTSYNC_TEST_BUILD */

 static int read_patterns_from_file(const char* filepath, char*** patterns, int* count) {
  FILE* fp = fopen(filepath, "r");
@@ -561,8 +415,10 @@ static int read_patterns_from_file(const char* filepath, char*** patterns, int*
    fprintf(stderr, "Error: could not open pattern file '%s': %s\n", filepath, strerror(errno));
    return -1;
  }
-  char line[4096];
-  while (fgets(line, sizeof(line), fp)) {
+  char* line = NULL;
+  size_t line_size = 0;
+  ssize_t n;
+  while ((n = getline(&line, &line_size, fp)) != -1) {
    char* p = line;
    while (*p == ' ' || *p == '\t')
      p++;
@@ -576,6 +432,7 @@ static int read_patterns_from_file(const char* filepath, char*** patterns, int*
    char** tmp = realloc(*patterns, (*count + 1) * sizeof(char*));
    if (!tmp) {
      fprintf(stderr, "Error: memory allocation failed for pattern file\n");
+      free(line);
      fclose(fp);
      return -1;
    }
@@ -583,15 +440,18 @@ static int read_patterns_from_file(const char* filepath, char*** patterns, int*
    char* dup = str_dup(p);
    if (!dup) {
      fprintf(stderr, "Error: memory allocation failed for pattern file\n");
+      free(line);
      fclose(fp);
      return -1;
    }
    (*patterns)[(*count)++] = dup;
  }
+  free(line);
  fclose(fp);
  return 0;
 }

+#ifndef FASTSYNC_TEST_BUILD
 int main(int argc, char* argv[]) {
  const char* env_source = NULL;
  const char* env_dest = NULL;
@@ -697,3 +557,4 @@ cleanup:
  }
  return exit_code;
 }
+#endif /* FASTSYNC_TEST_BUILD */
@@ -25,13 +25,30 @@

 #define STREAM_THRESHOLD (64ULL * 1024 * 1024)

+/* Forward declaration for progress-reporting thread used in multithreaded send. */
+static int progress_thread_fn(void* arg);
+
+static void pipeline_cancel(PipelineContextSender* context) {
+  mtx_lock(&context->mutex_scanner);
+  mtx_lock(&context->mutex_loader);
+  atomic_store(&context->cancelled, true);
+  context->scanner_done = true;
+  context->loader_done = true;
+  cnd_broadcast(&context->condition_not_full_scanner);
+  cnd_broadcast(&context->condition_not_empty_scanner);
+  cnd_broadcast(&context->condition_not_full_loader);
+  cnd_broadcast(&context->condition_not_empty_loader);
+  mtx_unlock(&context->mutex_loader);
+  mtx_unlock(&context->mutex_scanner);
+}
+
 /* 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->safe_links, config->copy_unsafe_links, config->checksum);
  if (!scanner)
    return -1;
  Chunk* chunk;
@@ -64,7 +81,8 @@ static int send_delete_manifest(int fd, ArrayList* manifest) {
  return 0;
 }

-static int incremental_check(Client* client, File* file, DeltaSignature** out_sig) {
+static int incremental_check(Client* client, File* file, const Config* config,
+                             DeltaSignature** out_sig) {
  *out_sig = NULL;
  if (!send_status(client->file_descriptor, STATUS_CHECK))
    return -1;
@@ -76,6 +94,12 @@ static int incremental_check(Client* client, File* file, DeltaSignature** out_si
    return -1;
  if (!send_n_data(client->file_descriptor, &mtime, sizeof(mtime)))
    return -1;
+  if (config->checksum) {
+    uint64_t checksum;
+    if (!file_checksum(file, &checksum) ||
+        !send_n_data(client->file_descriptor, &checksum, sizeof(checksum)))
+      return -1;
+  }
  Status s;
  if (!receive_status(client->file_descriptor, &s))
    return -1;
@@ -173,7 +197,7 @@ static int send_single_file(Client* client, File* file, Config* config, bool use
  // Incremental path: use sendfile for the actual data if enabled and no compression
  if (use_sendfile) {
    DeltaSignature* sig = NULL;
-    int rc = incremental_check(client, file, &sig);
+    int rc = incremental_check(client, file, config, &sig);
    if (rc == 1) {
      delta_signature_destroy(sig);
      return 1;
@@ -199,7 +223,7 @@ static int send_single_file(Client* client, File* file, Config* config, bool use
  // Incremental path with single_calls (supports compression and delta)
  file_send_fn send_fn = (file_send_fn)file_send_single_calls;
  DeltaSignature* sig = NULL;
-  int rc = incremental_check(client, file, &sig);
+  int rc = incremental_check(client, file, config, &sig);
  if (rc < 0) {
    delta_signature_destroy(sig);
    return -1;
@@ -271,6 +295,9 @@ static int send_chunks_multithreaded(void* pipeline_context) {
  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,
@@ -283,6 +310,9 @@ static int send_chunks_multithreaded(void* pipeline_context) {
      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);
      return thrd_error;
    }
  } else {
@@ -292,12 +322,18 @@ static int send_chunks_multithreaded(void* pipeline_context) {
      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;
    }
  }
  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);
    return thrd_error;
  }

@@ -316,19 +352,41 @@ static int send_chunks_multithreaded(void* pipeline_context) {
      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);
      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);
      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);
      return thrd_error;
    }
+    if (context->config->show_progress) {
+      unsigned long long chunk_bytes = 0;
+      for (int i = 0; i < current_chunk->element_count; i++) {
+        if (current_chunk->items[i] && current_chunk->items[i]->data)
+          chunk_bytes += current_chunk->items[i]->data->size;
+      }
+      mtx_lock(&context->mutex_progress);
+      context->progress_bytes += chunk_bytes;
+      mtx_unlock(&context->mutex_progress);
+    }
    chunk_destroy(current_chunk);
  }
 }
@@ -340,9 +398,15 @@ static int scan_directory_multithreaded(void* pipeline_context) {
      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->copy_links, context->config->safe_links, context->config->copy_unsafe_links,
+      context->config->checksum);

  Chunk* current_chunk;
+  if (scanner == NULL) {
+    log_message(LOG_LEVEL_ERROR, "Failed to create parallel scanner");
+    pipeline_cancel(context);
+    return thrd_error;
+  }
  while ((current_chunk = parallel_scanner_next(scanner)) != NULL) {
    if (context->config->use_delete) {
      mtx_lock(&context->mutex_scanner);
@@ -350,13 +414,44 @@ static int scan_directory_multithreaded(void* pipeline_context) {
        const char* p = current_chunk->items[i]->path;
        if (*p == '/')
          p++;
-        array_list_add(context->manifest, str_dup(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);
+          mtx_unlock(&context->mutex_scanner);
+          pipeline_cancel(context);
+          chunk_destroy(current_chunk);
+          parallel_scanner_destroy(scanner);
+          return thrd_error;
+        }
      }
      mtx_unlock(&context->mutex_scanner);
    }
-    queue_enqueue_multithreaded(context->queue_scanner, current_chunk, &context->mutex_scanner,
-                                &context->condition_not_empty_scanner,
-                                &context->condition_not_full_scanner);
+    if (!queue_enqueue_multithreaded_cancel(
+            context->queue_scanner, current_chunk, &context->mutex_scanner,
+            &context->condition_not_empty_scanner, &context->condition_not_full_scanner,
+            &context->cancelled)) {
+      chunk_destroy(current_chunk);
+      pipeline_cancel(context);
+      parallel_scanner_destroy(scanner);
+      return thrd_error;
+    }
+  }
+  if (parallel_scanner_failed(scanner)) {
+    parallel_scanner_destroy(scanner);
+    mtx_lock(&context->mutex_scanner);
+    context->scanner_done = true;
+    cnd_broadcast(&context->condition_not_empty_scanner);
+    cnd_broadcast(&context->condition_not_full_scanner);
+    mtx_unlock(&context->mutex_scanner);
+    pipeline_cancel(context);
+    return thrd_error;
  }
  mtx_lock(&context->mutex_scanner);
  context->scanner_done = true;
@@ -392,12 +487,55 @@ static int load_files_multithreaded(void* pipeline_context) {
        }
      }
    }
-    queue_enqueue_multithreaded(context->queue_loader, chunk, &context->mutex_loader,
-                                &context->condition_not_empty_loader,
-                                &context->condition_not_full_loader);
+    if (!queue_enqueue_multithreaded_cancel(context->queue_loader, chunk, &context->mutex_loader,
+                                            &context->condition_not_empty_loader,
+                                            &context->condition_not_full_loader,
+                                            &context->cancelled)) {
+      chunk_destroy(chunk);
+      atomic_store(&context->cancelled, true);
+      cnd_broadcast(&context->condition_not_full_loader);
+      cnd_broadcast(&context->condition_not_empty_loader);
+      return thrd_error;
+    }
  }
 }

+/* Progress-reporting thread for multithreaded send. Runs in parallel with
+   the scanner/loader/sender threads and prints periodic progress to stderr. */
+static int progress_thread_fn(void* arg) {
+  PipelineContextSender* context = (PipelineContextSender*)arg;
+  time_t last_progress = 0;
+  time_t start = time(NULL);
+
+  while (true) {
+    mtx_lock(&context->mutex_progress);
+    bool done = context->sender_done;
+    unsigned long long total = context->progress_bytes;
+    mtx_unlock(&context->mutex_progress);
+
+    if (done) {
+      time_t now = time(NULL);
+      double elapsed = difftime(now, start);
+      double rate = elapsed > 0.0 ? total / (1048576.0 * elapsed) : 0.0;
+      fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  Done.\n", total / 1048576.0, rate);
+      break;
+    }
+
+    time_t now = time(NULL);
+    if (now - last_progress >= 1) {
+      last_progress = now;
+      double elapsed = difftime(now, start);
+      double rate = elapsed > 0.0 ? total / (1048576.0 * elapsed) : 0.0;
+      fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  ", total / 1048576.0, rate);
+      fflush(stderr);
+    }
+
+    struct timespec ts = {0, 100 * 1000000L}; /* 100 ms */
+    thrd_sleep(&ts, NULL);
+  }
+  return thrd_success;
+}
+
 int send_files(Config* config) {
  if (config->dry_run)
    return send_dry_run_manifest(config);
@@ -416,16 +554,20 @@ int send_files(Config* config) {
    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)
+      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)
+      if (client) {
+        client_disconnect(client);
        client_delete(client);
+      }
      fprintf(stderr, "Error: could not connect to server\n");
      return 1;
    }
@@ -439,21 +581,50 @@ int send_files(Config* config) {
      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->safe_links, config->copy_unsafe_links, config->checksum);
  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;
+  if (!scanner || (config->use_delete && !manifest)) {
+    if (scanner)
+      directory_scanner_destroy(scanner);
+    if (manifest)
+      array_list_delete(manifest);
+    client_disconnect(client);
+    client_delete(client);
+    return 1;
+  }
  while ((current_chunk = directory_scanner_next(scanner)) != NULL) {
    unsigned long long chunk_bytes = 0;
    for (int i = 0; i < current_chunk->element_count; i++) {
      chunk_bytes += current_chunk->items[i]->data->size;
+      total_files++;
      if (manifest) {
        const char* p = current_chunk->items[i]->path;
        if (*p == '/')
          p++;
-        array_list_add(manifest, str_dup(p));
+        char* manifest_entry = str_dup(p);
+        if (!manifest_entry) {
+          log_message(LOG_LEVEL_ERROR, "Failed to allocate manifest entry");
+          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;
+        }
      }
    }
    if (!config->use_sendfile) {
@@ -470,6 +641,9 @@ int send_files(Config* config) {
    if (send_chunk(client, current_chunk, config) != 0) {
      log_message(LOG_LEVEL_ERROR, "Failed to send chunk");
      chunk_destroy(current_chunk);
+      if (manifest)
+        array_list_delete(manifest);
+      manifest = NULL;
      break;
    }
    if (config->show_progress) {
@@ -485,28 +659,38 @@ int send_files(Config* config) {
    }
    chunk_destroy(current_chunk);
  }
+  if (directory_scanner_failed(scanner) || (config->use_delete && manifest == NULL))
+    goto send_fail;
  if (config->use_delete) {
    if (send_delete_manifest(client->file_descriptor, manifest) != 0) {
      array_list_delete(manifest);
      goto send_fail;
    }
    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;
+  double elapsed_total = difftime(time(NULL), start);
  if (config->show_progress) {
-    double elapsed = difftime(time(NULL), start);
-    double rate = elapsed > 0 ? total_bytes / (1048576.0 * elapsed) : 0;
+    double rate = elapsed_total > 0 ? total_bytes / (1048576.0 * elapsed_total) : 0;
    fprintf(stderr, "\rSent %.1f MB  (%.1f MB/s)  Done.\n", total_bytes / 1048576.0, rate);
  }
+  if (config->stats) {
+    double rate = elapsed_total > 0 ? total_bytes / (1048576.0 * elapsed_total) : 0;
+    fprintf(stderr, "Stats: %d files, %.1f MB, %.1f MB/s\n", total_files, total_bytes / 1048576.0,
+            rate);
+  }
  directory_scanner_destroy(scanner);
  client_disconnect(client);
  client_delete(client);
  return ok ? 0 : 1;

 send_fail:
+  if (manifest)
+    array_list_delete(manifest);
  directory_scanner_destroy(scanner);
  client_disconnect(client);
  client_delete(client);
@@ -548,19 +732,55 @@ int send_files_multithreaded(Config* config) {
    context->manifest = array_list_create(free);

  thrd_t scanner, loader, sender;
-  if (thrd_create(&scanner, scan_directory_multithreaded, context) != thrd_success ||
-      thrd_create(&loader, load_files_multithreaded, context) != thrd_success ||
-      thrd_create(&sender, send_chunks_multithreaded, context) != thrd_success) {
+  bool scanner_created = false;
+  bool loader_created = false;
+  bool sender_created = false;
+
+  scanner_created = (thrd_create(&scanner, scan_directory_multithreaded, context) == thrd_success);
+  if (scanner_created)
+    loader_created = (thrd_create(&loader, load_files_multithreaded, context) == thrd_success);
+  if (scanner_created && loader_created)
+    sender_created = (thrd_create(&sender, send_chunks_multithreaded, context) == thrd_success);
+
+  if (!scanner_created || !loader_created || !sender_created) {
    perror("Error creating threads.\n");
+    pipeline_cancel(context);
+    mtx_lock(&context->mutex_progress);
+    context->sender_done = true;
+    mtx_unlock(&context->mutex_progress);
+    if (sender_created)
+      thrd_join(sender, NULL);
+    if (loader_created)
+      thrd_join(loader, NULL);
+    if (scanner_created)
+      thrd_join(scanner, NULL);
    pipeline_context_sender_destroy(context);
    return 1;
  }

+  thrd_t progress;
+  bool progress_created = false;
+  if (config->show_progress) {
+    progress_created = (thrd_create(&progress, progress_thread_fn, context) == thrd_success);
+    if (!progress_created) {
+      perror("Error creating progress thread.\n");
+      /* Non-fatal; continue without progress reporting */
+    }
+  }
+
  int sender_result;
  thrd_join(scanner, NULL);
  thrd_join(loader, NULL);
  thrd_join(sender, &sender_result);

+  if (progress_created) {
+    /* Signal progress thread to exit if it hasn't already */
+    mtx_lock(&context->mutex_progress);
+    context->sender_done = true;
+    mtx_unlock(&context->mutex_progress);
+    thrd_join(progress, NULL);
+  }
+
  pipeline_context_sender_destroy(context);
  return sender_result == thrd_success ? 0 : 1;
 }
@@ -11,6 +11,7 @@
 #include <sys/stat.h>
 #include <threads.h>
 #include <unistd.h>
+#include <limits.h>

 typedef struct {
  char* path;
@@ -27,24 +28,45 @@ static void dir_entry_destroy(void* item) {

 static DirEntry* dir_entry_create(const char* path, int depth) {
  DirEntry* de = malloc(sizeof(DirEntry));
-  if (de) {
-    de->path = str_dup(path);
-    de->depth = depth;
+  if (!de)
+    return NULL;
+  de->path = str_dup(path);
+  if (!de->path) {
+    free(de);
+    return NULL;
  }
+  de->depth = depth;
  return de;
 }

+static bool safe_relative_link(const char* source_root, const char* containing_dir,
+                               const char* link_target) {
+  char root[PATH_MAX];
+  if (!realpath(source_root, root))
+    return false;
+  char* joined = path_cat(containing_dir, link_target);
+  char resolved[PATH_MAX];
+  bool safe = joined && realpath(joined, resolved) && strncmp(root, resolved, strlen(root)) == 0 &&
+              (resolved[strlen(root)] == '\0' || resolved[strlen(root)] == '/');
+  free(joined);
+  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) {
-  DirectoryScanner* scanner = malloc(sizeof(DirectoryScanner));
+                                           bool copy_unsafe_links, bool checksum) {
+  DirectoryScanner* scanner = calloc(1, sizeof(DirectoryScanner));
  if (scanner == NULL)
    return NULL;
  scanner->directories = queue_create(100, dir_entry_destroy);
+  if (!scanner->directories) {
+    free(scanner);
+    return NULL;
+  }
  scanner->current_dir = NULL;
  scanner->current_path = NULL;
  scanner->use_metadata = use_metadata;
@@ -61,7 +83,20 @@ DirectoryScanner* directory_scanner_create(const char* root_directory, bool use_
  scanner->copy_links = copy_links;
  scanner->safe_links = safe_links;
  scanner->copy_unsafe_links = copy_unsafe_links;
-  queue_enqueue(scanner->directories, dir_entry_create(root_directory, 0));
+  scanner->checksum = checksum;
+  scanner->failed = false;
+  DirEntry* root = dir_entry_create(root_directory, 0);
+  if (!root) {
+    queue_destroy(scanner->directories);
+    free(scanner);
+    return NULL;
+  }
+  if (!queue_enqueue(scanner->directories, root)) {
+    dir_entry_destroy(root);
+    queue_destroy(scanner->directories);
+    free(scanner);
+    return NULL;
+  }
  return scanner;
 }

@@ -79,8 +114,12 @@ void directory_scanner_destroy(DirectoryScanner* scanner) {

 static Chunk* chunk_data_to_chunk(ArrayList* chunk_data) {
  void** chunk_items = array_list_to_array(chunk_data);
+  if (!chunk_items)
+    return NULL;
  Chunk* chunk = chunk_create((File**)chunk_items, chunk_data->size);
  free(chunk_items);
+  if (!chunk)
+    return NULL;
  chunk_data->item_destroyer = NULL;
  array_list_delete(chunk_data);
  return chunk;
@@ -105,6 +144,7 @@ static int open_next_directory(DirectoryScanner* scanner) {
    perror("Could not open directory");
    free(scanner->current_path);
    scanner->current_path = NULL;
+    scanner->failed = true;
    return -1;
  }
  return 1;
@@ -112,6 +152,10 @@ static int open_next_directory(DirectoryScanner* scanner) {

 Chunk* directory_scanner_next(DirectoryScanner* scanner) {
  ArrayList* chunk_data = array_list_create(file_destroy);
+  if (!chunk_data) {
+    scanner->failed = true;
+    return NULL;
+  }
  unsigned long long chunk_data_size = 0;

  while (1) {
@@ -120,7 +164,7 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
      if (ret == 0)
        break;
      if (ret < 0)
-        continue;
+        break;
    }

    struct dirent* entry = readdir(scanner->current_dir);
@@ -136,6 +180,10 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
      continue;

    char* cur_path = path_cat(scanner->current_path, entry->d_name);
+    if (!cur_path) {
+      scanner->failed = true;
+      break;
+    }
    struct stat stats;
    struct stat lstats;
    bool is_symlink = false;
@@ -159,7 +207,8 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
        continue;
      }
      link_target[len] = '\0';
-      if (link_target[0] == '/') {
+      if (link_target[0] == '/' ||
+          !safe_relative_link(scanner->current_path, scanner->current_path, link_target)) {
        free(cur_path);
        continue;
      }
@@ -194,8 +243,10 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
      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);
-        if (!queue_enqueue(scanner->directories, de))
+        if (!de || !queue_enqueue(scanner->directories, de)) {
          dir_entry_destroy(de);
+          scanner->failed = true;
+        }
      }
      free(cur_path);
    } else {
@@ -238,27 +289,49 @@ Chunk* directory_scanner_next(DirectoryScanner* scanner) {
      File* file = file_create(cur_path);
      if (file == NULL) {
        free(cur_path);
+        scanner->failed = true;
        continue;
      }
      file->data->size = stats.st_size;
      if (scanner->use_metadata)
        file->metadata = file_metadata_create(&stats);
-      array_list_add(chunk_data, file);
+      if (scanner->use_metadata && !file->metadata) {
+        file_destroy(file);
+        free(cur_path);
+        scanner->failed = true;
+        break;
+      }
+      if (!array_list_add(chunk_data, file)) {
+        file_destroy(file);
+        scanner->failed = true;
+        break;
+      }
      chunk_data_size += file->data->size;
      if (chunk_data_size > scanner->chunk_size) {
        free(cur_path);
-        return chunk_data_to_chunk(chunk_data);
+        Chunk* result = chunk_data_to_chunk(chunk_data);
+        if (!result)
+          scanner->failed = true;
+        return result;
      }
      free(cur_path);
    }
  }

-  if (chunk_data->size > 0)
-    return chunk_data_to_chunk(chunk_data);
+  if (chunk_data->size > 0) {
+    Chunk* result = chunk_data_to_chunk(chunk_data);
+    if (!result)
+      scanner->failed = true;
+    return result;
+  }
  array_list_delete(chunk_data);
  return NULL;
 }

+bool directory_scanner_failed(const DirectoryScanner* scanner) {
+  return scanner == NULL || scanner->failed;
+}
+
 typedef struct {
  ParallelScanner* ps;
  char** dirs;
@@ -276,6 +349,7 @@ typedef struct {
  bool copy_links;
  bool safe_links;
  bool copy_unsafe_links;
+  bool checksum;
 } ParallelWorkerArg;

 static int parallel_worker_thread(void* arg) {
@@ -284,11 +358,32 @@ static int parallel_worker_thread(void* arg) {
    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->follow_symlinks, wa->copy_links, wa->safe_links, wa->copy_unsafe_links, wa->checksum);
+    if (!ds) {
+      mtx_lock(&wa->ps->result_mutex);
+      wa->ps->failed = true;
+      atomic_store(&wa->ps->cancelled, true);
+      cnd_broadcast(&wa->ps->result_not_empty);
+      cnd_broadcast(&wa->ps->result_not_full);
+      mtx_unlock(&wa->ps->result_mutex);
+      break;
+    }
    Chunk* chunk;
    while ((chunk = directory_scanner_next(ds)) != NULL) {
-      queue_enqueue_multithreaded(wa->ps->result_queue, chunk, &wa->ps->result_mutex,
-                                  &wa->ps->result_not_empty, &wa->ps->result_not_full);
+      if (!queue_enqueue_multithreaded_cancel(wa->ps->result_queue, chunk, &wa->ps->result_mutex,
+                                              &wa->ps->result_not_empty, &wa->ps->result_not_full,
+                                              &wa->ps->cancelled)) {
+        chunk_destroy(chunk);
+        break;
+      }
+    }
+    if (directory_scanner_failed(ds)) {
+      mtx_lock(&wa->ps->result_mutex);
+      wa->ps->failed = true;
+      atomic_store(&wa->ps->cancelled, true);
+      cnd_broadcast(&wa->ps->result_not_empty);
+      cnd_broadcast(&wa->ps->result_not_full);
+      mtx_unlock(&wa->ps->result_mutex);
    }
    directory_scanner_destroy(ds);
    free(wa->dirs[i]);
@@ -298,7 +393,7 @@ static int parallel_worker_thread(void* arg) {
  free(wa);
  mtx_lock(&ps->result_mutex);
  ps->completed++;
-  if (ps->completed >= ps->num_threads) {
+  if (ps->completed >= ps->expected_threads) {
    ps->done = true;
    cnd_signal(&ps->result_not_empty);
  }
@@ -312,7 +407,7 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
                                         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 safe_links, bool copy_unsafe_links, bool checksum) {
  ParallelScanner* ps = calloc(1, sizeof(ParallelScanner));
  if (!ps)
    return NULL;
@@ -321,9 +416,29 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
    free(ps);
    return NULL;
  }
-  if (mtx_init(&ps->result_mutex, mtx_plain) != thrd_success ||
-      cnd_init(&ps->result_not_empty) != thrd_success ||
-      cnd_init(&ps->result_not_full) != thrd_success) {
+  atomic_init(&ps->cancelled, false);
+  int init = 0;
+  bool ok = true;
+  if (mtx_init(&ps->result_mutex, mtx_plain) != thrd_success)
+    ok = false;
+  if (ok) {
+    init++;
+    if (cnd_init(&ps->result_not_empty) != thrd_success)
+      ok = false;
+  }
+  if (ok) {
+    // cppcheck-suppress unreadVariable
+    init++;
+    if (cnd_init(&ps->result_not_full) != thrd_success)
+      ok = false;
+  }
+  if (!ok) {
+    if (init >= 3)
+      cnd_destroy(&ps->result_not_full);
+    if (init >= 2)
+      cnd_destroy(&ps->result_not_empty);
+    if (init >= 1)
+      mtx_destroy(&ps->result_mutex);
    queue_destroy(ps->result_queue);
    free(ps);
    return NULL;
@@ -338,6 +453,13 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata

  ArrayList* root_files = array_list_create(file_destroy);
  ArrayList* subdirs = array_list_create(free);
+  if (!root_files || !subdirs) {
+    array_list_delete(root_files);
+    array_list_delete(subdirs);
+    closedir(dir);
+    parallel_scanner_destroy(ps);
+    return NULL;
+  }
  struct dirent* entry;
  while ((entry = readdir(dir)) != NULL) {
    if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
@@ -345,13 +467,68 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
    char* cur_path = path_cat(root_directory, entry->d_name);
    if (!cur_path)
      continue;
-    struct stat st;
-    if (stat(cur_path, &st) != 0) {
+    struct stat lstats;
+    if (lstat(cur_path, &lstats) != 0) {
      free(cur_path);
      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);
+      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)) {
-      array_list_add(subdirs, cur_path);
+      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++) {
@@ -384,12 +561,22 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
      }
      File* file = file_create(cur_path);
      free(cur_path);
-      if (!file)
+      if (!file) {
+        ps->failed = true;
        continue;
+      }
      file->data->size = st.st_size;
      if (use_metadata)
        file->metadata = file_metadata_create(&st);
-      array_list_add(root_files, file);
+      if (use_metadata && !file->metadata) {
+        file_destroy(file);
+        ps->failed = true;
+        continue;
+      }
+      if (!array_list_add(root_files, file)) {
+        file_destroy(file);
+        ps->failed = true;
+      }
    }
  }
  closedir(dir);
@@ -397,27 +584,57 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
  unsigned long long cs = chunk_size > 0 ? chunk_size : DESIRED_CHUNK_SIZE;
  if (root_files->size > 0) {
    ArrayList* batch = array_list_create(NULL);
+    if (!batch) {
+      ps->failed = true;
+      array_list_delete(root_files);
+      array_list_delete(subdirs);
+      parallel_scanner_destroy(ps);
+      return NULL;
+    }
    unsigned long long batch_size = 0;
    Chunk* first = NULL;
    for (int i = 0; i < root_files->size; i++) {
      File* f = (File*)root_files->items[i];
-      array_list_add(batch, f);
+      if (!array_list_add(batch, f)) {
+        ps->failed = true;
+        break;
+      }
      batch_size += f->data->size;
      if (batch_size >= cs || i == root_files->size - 1) {
        void** items = array_list_to_array(batch);
+        if (!items) {
+          ps->failed = true;
+          batch->item_destroyer = file_destroy;
+          array_list_delete(batch);
+          batch = NULL;
+          break;
+        }
        Chunk* c = chunk_create((File**)items, batch->size);
        free(items);
+        if (!c) {
+          ps->failed = true;
+          batch->item_destroyer = file_destroy;
+          array_list_delete(batch);
+          batch = NULL;
+          break;
+        }
        batch->item_destroyer = NULL;
        array_list_delete(batch);
        batch = NULL;
        if (!first) {
          first = c;
        } else {
-          queue_enqueue_multithreaded(ps->result_queue, c, &ps->result_mutex, &ps->result_not_empty,
-                                      &ps->result_not_full);
+          if (!queue_enqueue(ps->result_queue, c)) {
+            chunk_destroy(c);
+            ps->failed = true;
+          }
        }
        if (i < root_files->size - 1) {
          batch = array_list_create(NULL);
+          if (!batch) {
+            ps->failed = true;
+            break;
+          }
          batch_size = 0;
        }
      }
@@ -437,6 +654,7 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata

  if (subdirs->size > 0) {
    ps->num_threads = n;
+    ps->expected_threads = n;
    ps->threads = calloc(n, sizeof(thrd_t));
    if (!ps->threads) {
      array_list_delete(subdirs);
@@ -446,21 +664,37 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
    int dirs_per_thread = subdirs->size / n;
    int remainder = subdirs->size % n;
    int start = 0;
+    ps->num_threads = 0;
    for (int t = 0; t < n; t++) {
      int count = dirs_per_thread + (t < remainder ? 1 : 0);
      if (count == 0)
        break;
      ParallelWorkerArg* wa = calloc(1, sizeof(ParallelWorkerArg));
-      if (!wa)
+      if (!wa) {
+        ps->failed = true;
        break;
+      }
      wa->ps = ps;
      wa->dirs = calloc(count, sizeof(char*));
      if (!wa->dirs) {
        free(wa);
+        ps->failed = true;
        break;
      }
-      for (int j = 0; j < count; j++)
+      bool dup_ok = true;
+      for (int j = 0; j < count; j++) {
        wa->dirs[j] = str_dup((char*)subdirs->items[start + j]);
+        if (!wa->dirs[j])
+          dup_ok = false;
+      }
+      if (!dup_ok) {
+        for (int j = 0; j < count; j++)
+          free(wa->dirs[j]);
+        free(wa->dirs);
+        free(wa);
+        ps->failed = true;
+        break;
+      }
      wa->dir_count = count;
      wa->use_metadata = use_metadata;
      wa->chunk_size = cs;
@@ -475,15 +709,24 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
      wa->copy_links = copy_links;
      wa->safe_links = safe_links;
      wa->copy_unsafe_links = copy_unsafe_links;
+      wa->checksum = checksum;
      start += count;
      if (thrd_create(&ps->threads[t], parallel_worker_thread, wa) != thrd_success) {
        for (int j = 0; j < count; j++)
          free(wa->dirs[j]);
        free(wa->dirs);
        free(wa);
-        ps->num_threads = t;
+        ps->failed = true;
+        atomic_store(&ps->cancelled, true);
+        ps->expected_threads = ps->created_threads;
+        mtx_lock(&ps->result_mutex);
+        cnd_broadcast(&ps->result_not_empty);
+        cnd_broadcast(&ps->result_not_full);
+        mtx_unlock(&ps->result_mutex);
        break;
      }
+      ps->num_threads++;
+      ps->created_threads++;
    }
  }
  array_list_delete(subdirs);
@@ -497,7 +740,9 @@ Chunk* parallel_scanner_next(ParallelScanner* ps) {
    return c;
  }
  if (ps->num_threads == 0) {
+    mtx_lock(&ps->result_mutex);
    ps->done = true;
+    mtx_unlock(&ps->result_mutex);
    return NULL;
  }
  Chunk* chunk = queue_dequeue_multithreaded(
@@ -505,11 +750,19 @@ Chunk* parallel_scanner_next(ParallelScanner* ps) {
  return chunk;
 }

+bool parallel_scanner_failed(const ParallelScanner* ps) {
+  return ps == NULL || ps->failed;
+}
+
 void parallel_scanner_destroy(ParallelScanner* ps) {
  if (!ps)
    return;
+  mtx_lock(&ps->result_mutex);
  ps->done = true;
-  cnd_signal(&ps->result_not_empty);
+  atomic_store(&ps->cancelled, true);
+  cnd_broadcast(&ps->result_not_empty);
+  cnd_broadcast(&ps->result_not_full);
+  mtx_unlock(&ps->result_mutex);
  for (int i = 0; i < ps->num_threads; i++)
    thrd_join(ps->threads[i], NULL);
  free(ps->threads);
@@ -6,6 +6,7 @@
 #include <dirent.h>
 #include <stdbool.h>
 #include <threads.h>
+#include <stdatomic.h>

 typedef struct {
  Queue* directories;
@@ -25,6 +26,8 @@ typedef struct {
  bool copy_links;
  bool safe_links;
  bool copy_unsafe_links;
+  bool checksum;
+  bool failed;
 } DirectoryScanner;

 typedef struct {
@@ -33,8 +36,12 @@ typedef struct {
  cnd_t result_not_empty;
  cnd_t result_not_full;
  int num_threads;
+  int expected_threads;
+  int created_threads;
  thrd_t* threads;
  bool done;
+  bool failed;
+  atomic_bool cancelled;
  int completed;
  Chunk* initial_chunk;
 } ParallelScanner;
@@ -45,8 +52,9 @@ DirectoryScanner* directory_scanner_create(const char* root_directory, bool use_
                                           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 copy_unsafe_links, bool checksum);
 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,
@@ -55,8 +63,9 @@ ParallelScanner* parallel_scanner_create(char* root_directory, bool use_metadata
                                         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 safe_links, bool copy_unsafe_links, bool checksum);
 Chunk* parallel_scanner_next(ParallelScanner* scanner);
+bool parallel_scanner_failed(const ParallelScanner* scanner);
 void parallel_scanner_destroy(ParallelScanner* scanner);

 #endif
@@ -0,0 +1,73 @@
+#include "stdio.h"
+#include <delta.h>
+#include <chunk.h>
+
+static __attribute__((unused)) void print_usage() {
+  printf("Usage:\n");
+  printf("  fastsync [options] <source> <destination>\n");
+  printf("  fastsync [options] --source-dir <src> --dest-dir <dst>\n");
+  printf("\n");
+  printf("Destination formats:\n");
+  printf("  user@host:/path     SSH transport (rsync-style)\n");
+  printf("  host:/path          SSH transport (current user)\n");
+  printf("  /local/path         TCP transport (requires server on localhost:8080)\n");
+  printf("\n");
+  printf("Options:\n");
+  printf("  -c [level]          Enable compression (level 1-22, default 5)\n");
+  printf("  -z [level]          Alias for -c\n");
+  printf("  -a, --archive       Archive mode (-c -m -M)\n");
+  printf("  -n, --dry-run       Show what would be transferred\n");
+  printf("  -p <port>           SSH port (default: 22)\n");
+  printf("  --progress          Show transfer progress\n");
+  printf("  --delete            Delete files on receiver not in source\n");
+  printf("  --exclude <pattern> Exclude files matching pattern\n");
+  printf("  --include <pattern> Only include files matching pattern\n");
+  printf("  --exclude-from <file> Read exclude patterns from file\n");
+  printf("  --include-from <file> Read include patterns from file\n");
+  printf("  --max-size <n>      Skip files larger than n bytes\n");
+  printf("  --min-size <n>      Skip files smaller than n bytes\n");
+  printf("  --incremental       Skip files unchanged since last transfer\n");
+  printf("  --delta             Delta transfer for changed files (requires --incremental)\n");
+  printf("  --delta-block <n>   Delta block size in bytes (default: %d)\n",
+         DELTA_BLOCK_SIZE_DEFAULT);
+  printf("  --delta-max <n>     Max file size for delta transfer (default: %llu)\n",
+         DELTA_MAX_FILE_SIZE);
+  printf("  -m                  Enable multithreading\n");
+  printf("  -s                  Enable chunk serialization\n");
+  printf("  -f                  Enable sendfile (TCP only, not with -c or -s)\n");
+  printf("  -v, --verbose       Enable debug logging\n");
+  printf("  -M, --preserve      Preserve file metadata\n");
+  printf("  --chunk-size <n>    Chunk size in bytes (default: %d)\n", DEFAULT_CHUNK_SIZE);
+  printf("  --source-dir <path> Source directory\n");
+  printf("  --dest-dir <path>   Destination directory\n");
+  printf("  --save-to-disk      Write received files to disk\n");
+  printf("  --server-host <ip>  Server IP address (default: 127.0.0.1)\n");
+  printf("  --server-port <n>   Server port (default: 8080)\n");
+  printf("  --bwlimit <KB/s>    Bandwidth limit in kilobytes per second\n");
+  printf("  --tls               Enable TLS encryption\n");
+  printf("  --cert <path>       TLS certificate file (PEM)\n");
+  printf("  --key <path>        TLS private key file (PEM)\n");
+  printf("  --ca <path>         TLS CA certificate file (PEM)\n");
+  printf("  --timeout <sec>     I/O timeout in seconds (default: 30)\n");
+  printf("  -T <sec>            Alias for --timeout\n");
+  printf("  --contimeout <sec>  Connection timeout in seconds (default: 10)\n");
+  printf("  --backup            Backup existing files before overwriting\n");
+  printf("  --backup-dir <dir>  Directory for backups (requires --backup)\n");
+  printf("  --suffix <str>      Backup suffix (default: ~)\n");
+  printf("  --stats             Print transfer statistics at end\n");
+  printf("  --max-depth <n>     Maximum directory depth (0=unlimited)\n");
+  printf("  --log-file <path>   Write log messages to file\n");
+  printf("  --partial           Keep partial files on interrupted transfer\n");
+  printf("  --partial-dir <dir> Directory for partial files\n");
+  printf("  --fastsync-server-path <path>\n");
+  printf("                      Path to fastsync-server on remote (default: fastsync-server)\n");
+  printf("  -l, --links         Copy symlinks as symlinks\n");
+  printf("  --copy-links        Transform symlinks into referent files\n");
+  printf("  --safe-links        Skip symlinks that point outside transfer tree\n");
+  printf("  --copy-unsafe-links  Only transform unsafe symlinks into referent files\n");
+  printf("  -S, --sparse        Handle sparse files efficiently\n");
+  printf("  --inplace           Update files in-place (no temp+rename)\n");
+  printf("  --compress-level <n>    Compression level (default: 5)\n");
+  printf("  --help              Show this help\n");
+  printf("  -V, --version       Show version\n");
+}
@@ -15,6 +15,42 @@
 #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;
+static bool allow_delete;
+
+static bool path_is_within(const char* root, const char* path) {
+  size_t n = strlen(root);
+  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))
+    return false;
+  authorized_root = str_dup(resolved);
+  if (!authorized_root)
+    return false;
+  authorized_root_fd = open(resolved, O_RDONLY | O_DIRECTORY | O_CLOEXEC);
+  if (authorized_root_fd < 0) {
+    free(authorized_root);
+    authorized_root = NULL;
+    return false;
+  }
+  file_set_authorized_root(authorized_root_fd, authorized_root);
+  utils_set_authorized_root_fd(authorized_root_fd);
+  return true;
+}

 int receive_files(Config* config, int fd) {
  Status status;
@@ -38,8 +74,12 @@ int receive_files(Config* config, int fd) {
        goto next;
      if (file == NULL && !skipped)
        return -1;
-      if (config->save_to_disk)
-        file_save_to_disk(config->receive_root_directory, file, NULL);
+      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);
@@ -48,13 +88,19 @@ int receive_files(Config* config, int fd) {
        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], NULL);
+        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;
-      if (!receive_int(fd, &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);
@@ -67,17 +113,21 @@ int receive_files(Config* config, int fd) {
          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;
-        if (match)
-          send_status(fd, STATUS_OK);
-        else
-          send_status(fd, STATUS_NEXT);
+        bool sent = send_status(fd, match ? STATUS_OK : STATUS_NEXT);
        free(full_path);
        free(check_path);
+        if (!sent)
+          return -1;
      }
      goto next;
    } else {
@@ -87,8 +137,12 @@ int receive_files(Config* config, int fd) {
        send_status(fd, STATUS_ERROR);
        return -1;
      }
-      if (config->save_to_disk)
-        file_save_to_disk(config->receive_root_directory, file, NULL);
+      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:
@@ -119,6 +173,36 @@ void handler(int file_descriptor) {
    close(file_descriptor);
    return;
  }
+  if (!authorized_root) {
+    log_message(LOG_LEVEL_ERROR, "No server-side destination root configured");
+    config_delete(config);
+    close(file_descriptor);
+    return;
+  }
+  char resolved_destination[PATH_MAX];
+  char* canonical_destination = realpath(config->receive_root_directory, NULL);
+  const char* destination =
+      canonical_destination ? canonical_destination : config->receive_root_directory;
+  if (has_path_traversal(destination) || !path_is_within(authorized_root, destination)) {
+    log_message(LOG_LEVEL_ERROR, "Rejected destination outside authorized root");
+    free(canonical_destination);
+    config_delete(config);
+    close(file_descriptor);
+    return;
+  }
+  if (canonical_destination)
+    snprintf(resolved_destination, sizeof(resolved_destination), "%s", canonical_destination);
+  else
+    snprintf(resolved_destination, sizeof(resolved_destination), "%s", destination);
+  free(canonical_destination);
+  free(config->receive_root_directory);
+  config->receive_root_directory = str_dup(resolved_destination);
+  if (!config->receive_root_directory) {
+    config_delete(config);
+    close(file_descriptor);
+    return;
+  }
+  config->use_delete = config->use_delete && allow_delete;
  if (config->use_multithreading) {
    Queue* q = queue_create(100, file_destroy);
    if (q == NULL) {
@@ -135,19 +219,41 @@ void handler(int file_descriptor) {
      return;
    }
    thrd_t receiver, writer;
-    if (thrd_create(&receiver, receive_thread, context) != thrd_success ||
-        thrd_create(&writer, write_thread, context) != thrd_success) {
+    bool receiver_created = false;
+    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);
+    if (!receiver_created || !writer_created) {
      perror("Error creating Threads");
+      if (receiver_created) {
+        mtx_lock(&context->mutex);
+        atomic_store(&context->cancelled, true);
+        cnd_broadcast(&context->condition_not_full);
+        cnd_broadcast(&context->condition_not_empty);
+        mtx_unlock(&context->mutex);
+        close(file_descriptor);
+        thrd_join(receiver, NULL);
+      } else {
+        close(file_descriptor);
+      }
+      if (writer_created)
+        thrd_join(writer, NULL);
      pipeline_context_receiver_destroy(context);
-      close(file_descriptor);
      return;
    }
-    thrd_join(receiver, NULL);
-    thrd_join(writer, NULL);
-    send_status(file_descriptor, STATUS_OK);
+    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);
    pipeline_context_receiver_destroy(context);
  } else {
-    receive_files(config, file_descriptor);
+    if (receive_files(config, file_descriptor) != 0)
+      log_message(LOG_LEVEL_ERROR, "Transfer failed");
    config_delete(config);
  }
  close(file_descriptor);
@@ -175,6 +281,8 @@ static void print_server_usage(void) {
  printf("  --cert <path>       TLS certificate file (PEM)\n");
  printf("  --key <path>        TLS private key file (PEM)\n");
  printf("  --ca <path>         TLS CA certificate file (PEM)\n");
+  printf("  --destination-root <path>  Authorized destination root (default: .)\n");
+  printf("  --allow-delete      Permit manifest deletion\n");
  printf("  -v, --verbose       Enable debug logging\n");
  printf("  --help              Show this help\n");
 }
@@ -185,6 +293,8 @@ int main(int argc, char* argv[]) {
  char* tls_key = NULL;
  char* tls_ca = NULL;
  int port = 8080;
+  const char* destination_root = ".";
+  bool stdio_mode = false;

  signal(SIGPIPE, SIG_IGN);
  for (int i = 1; i < argc; i++) {
@@ -192,9 +302,7 @@ int main(int argc, char* argv[]) {
      print_server_usage();
      return 0;
    } else if (strcmp(argv[i], "--stdio") == 0) {
-      io_set_fds(STDIN_FILENO, STDOUT_FILENO);
-      handler(STDIN_FILENO);
-      return 0;
+      stdio_mode = true;
    } else if (strcmp(argv[i], "-v") == 0 || strcmp(argv[i], "--verbose") == 0) {
      set_log_level(LOG_LEVEL_DEBUG);
    } else if (strcmp(argv[i], "--tls") == 0) {
@@ -205,6 +313,10 @@ int main(int argc, char* argv[]) {
      tls_key = argv[++i];
    } else if (strcmp(argv[i], "--ca") == 0 && i + 1 < argc) {
      tls_ca = argv[++i];
+    } else if (strcmp(argv[i], "--destination-root") == 0 && i + 1 < argc) {
+      destination_root = argv[++i];
+    } else if (strcmp(argv[i], "--allow-delete") == 0) {
+      allow_delete = true;
    } else if (strcmp(argv[i], "-p") == 0 && i + 1 < argc) {
      char* end;
      long p = strtol(argv[++i], &end, 10);
@@ -226,6 +338,19 @@ int main(int argc, char* argv[]) {

  signal(SIGINT, cleanup);
  signal(SIGTERM, cleanup);
+  if (!configure_authorization(destination_root)) {
+    fprintf(stderr, "Error: invalid destination root '%s'\n", destination_root);
+    return 1;
+  }
+  if (stdio_mode) {
+    io_set_fds(STDIN_FILENO, STDOUT_FILENO);
+    handler(STDIN_FILENO);
+    file_set_authorized_root(-1, NULL);
+    utils_set_authorized_root_fd(-1);
+    close(authorized_root_fd);
+    free(authorized_root);
+    return 0;
+  }
  g_server = server_create(port);
  if (g_server == NULL) {
    log_message(LOG_LEVEL_ERROR, "Failed to create server");
@@ -12,6 +12,9 @@
 #include "metadata.h"
 #include "protocol.h"

+/* Maximum individual file data size within a chunk (64 MB) */
+#define MAX_FILE_DATA_SIZE (64ULL * 1024 * 1024)
+
 Chunk* chunk_create(File** items, int element_count) {
  Chunk* chunk = (Chunk*)malloc(sizeof(Chunk));
  if (chunk == NULL) {
@@ -157,6 +160,14 @@ Chunk* chunk_deserialize(Data* data, bool use_metadata) {
      return NULL;
    }

+    // Reject individual file data larger than the maximum allowed size.
+    if (file_data_size > MAX_FILE_DATA_SIZE) {
+      log_message(LOG_LEVEL_ERROR, "File data size %zu exceeds maximum %llu", file_data_size,
+                  (unsigned long long)MAX_FILE_DATA_SIZE);
+      array_list_delete(files);
+      return NULL;
+    }
+
    void* file_data = malloc(file_data_size);
    if (file_data == NULL) {
      perror("Could not allocate memory for file data");
@@ -210,6 +221,15 @@ Chunk* receive_chunk_data(int fd, const Config* config) {
      return NULL;
    }
  }
+
+  // Reject chunks larger than the maximum allowed size to prevent OOM.
+  if (data_to_process->size > MAX_CHUNK_SIZE) {
+    log_message(LOG_LEVEL_ERROR, "Chunk size %zu exceeds maximum %llu", data_to_process->size,
+                (unsigned long long)MAX_CHUNK_SIZE);
+    data_destroy(data_to_process);
+    return NULL;
+  }
+
  Chunk* chunk = chunk_deserialize(data_to_process, config->use_metadata);
  data_destroy(data_to_process);
  if (chunk == NULL)
@@ -1,12 +1,13 @@
 #include "compression.h"
 #include "data.h"
 #include "log.h"
-#include "stdlib.h"
-#include "string.h"
+#include <stdlib.h>
+#include <string.h>
 #include <strings.h>
-#include "zstd.h"
+#include <zstd.h>

 #define INITIAL_DECOMPRESS_BUF_SIZE (1024 * 1024)
+#define MAX_DECOMPRESSED_SIZE (100ULL * 1024 * 1024) /* 100 MB hard ceiling */

 static const char* SKIP_COMPRESSION_EXTENSIONS[] = {".jpg", ".jpeg", ".png", ".gif", ".mp4", ".mkv",
                                                    ".zip", ".gz",   ".xz",  ".zst", NULL};
@@ -84,6 +85,13 @@ Data* data_decompress(Data* compressed_data) {
    dst_size = compressed_data->size * 3;
    if (dst_size < INITIAL_DECOMPRESS_BUF_SIZE)
      dst_size = INITIAL_DECOMPRESS_BUF_SIZE;
+    if (dst_size > MAX_DECOMPRESSED_SIZE)
+      dst_size = MAX_DECOMPRESSED_SIZE;
+  }
+  if (dst_size > MAX_DECOMPRESSED_SIZE) {
+    log_message(LOG_LEVEL_ERROR, "Declared decompressed size exceeds %llu bytes",
+                (unsigned long long)MAX_DECOMPRESSED_SIZE);
+    return NULL;
  }

  ZSTD_DCtx* dctx = ZSTD_createDCtx();
@@ -113,7 +121,16 @@ Data* data_decompress(Data* compressed_data) {
      return NULL;
    }
    if (ret > 0 && output.pos == output.size) {
+      if (buf_size >= MAX_DECOMPRESSED_SIZE) {
+        log_message(LOG_LEVEL_ERROR, "Decompressed data exceeds %llu bytes",
+                    (unsigned long long)MAX_DECOMPRESSED_SIZE);
+        ZSTD_freeDCtx(dctx);
+        data_destroy(uncompressed_data);
+        return NULL;
+      }
      buf_size *= 2;
+      if (buf_size > MAX_DECOMPRESSED_SIZE)
+        buf_size = MAX_DECOMPRESSED_SIZE;
      void* new_data = realloc(uncompressed_data->data, buf_size);
      if (!new_data) {
        log_message(LOG_LEVEL_ERROR, "Failed to grow decompression buffer");
@@ -8,10 +8,7 @@
 #include <stdlib.h>
 #include <string.h>

-Config* config_create(void) {
-  Config* config = malloc(sizeof(Config));
-  if (!config)
-    return NULL;
+static void config_set_defaults(Config* config) {
  config->version = str_dup(PROTOCOL_VERSION);
  config->send_directory = NULL;
  config->receive_root_directory = NULL;
@@ -89,6 +86,25 @@ Config* config_create(void) {
  config->compare_dest = NULL;
  config->copy_dest = NULL;
  config->link_dest = NULL;
+  config->partial_dir = NULL;
+  config->suffix = NULL;
+  config->delete_before = false;
+  config->address = NULL;
+  config->bind_address = NULL;
+  config->ipv6 = false;
+  config->ipv4 = false;
+  config->daemon = false;
+  config->daemon_config = NULL;
+  config->server_mode = false;
+  config->checksum = false;
+  config->compress_choice = NULL;
+}
+
+Config* config_create(void) {
+  Config* config = malloc(sizeof(Config));
+  if (!config)
+    return NULL;
+  config_set_defaults(config);
  return config;
 }

@@ -119,6 +135,8 @@ void config_parse_ssh_dest(Config* config) {
 }

 void config_delete(Config* config) {
+  if (config == NULL)
+    return;
  free(config->version);
  free(config->send_directory);
  free(config->receive_root_directory);
@@ -143,12 +161,28 @@ void config_delete(Config* config) {
  free(config->compare_dest);
  free(config->copy_dest);
  free(config->link_dest);
+  free(config->partial_dir);
+  free(config->suffix);
+  free(config->address);
+  free(config->bind_address);
+  free(config->daemon_config);
+  free(config->compress_choice);
  if (config->filters) {
    array_list_delete(config->filters);
  }
  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
+ */
 bool config_send(int file_descriptor, const Config* config) {
  if (!send_str(file_descriptor, config->version))
    return false;
@@ -178,7 +212,7 @@ bool config_send(int file_descriptor, const Config* config) {
    return false;
  if (!send_int(file_descriptor, config->use_delta))
    return false;
-  if (!send_int(file_descriptor, (int)config->delta_block_size))
+  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;
@@ -224,6 +258,18 @@ bool config_send(int file_descriptor, const Config* config) {
    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 : ""))
+    return false;
  Status status;
  if (!receive_status(file_descriptor, &status))
    return false;
@@ -234,13 +280,16 @@ 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)
    return NULL;
-  memset(config, 0, sizeof(*config));
+  config_set_defaults(config);
+  free(config->version);
  config->version = receive_str(file_descriptor);
  if (!config->version) {
+    free(config->server_host);
    free(config);
    return NULL;
  }
@@ -248,6 +297,7 @@ Config* config_receive(int file_descriptor) {
    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;
@@ -255,6 +305,7 @@ Config* config_receive(int file_descriptor) {
  config->send_directory = receive_str(file_descriptor);
  if (!config->send_directory) {
    free(config->version);
+    free(config->server_host);
    free(config);
    return NULL;
  }
@@ -262,6 +313,7 @@ Config* config_receive(int file_descriptor) {
  if (!config->receive_root_directory) {
    free(config->version);
    free(config->send_directory);
+    free(config->server_host);
    free(config);
    return NULL;
  }
@@ -298,67 +350,10 @@ Config* config_receive(int file_descriptor) {
  if (!receive_int(file_descriptor, &tmp))
    goto error;
  config->use_delta = tmp;
-  if (!receive_int(file_descriptor, &tmp))
+  if (!receive_n_data(file_descriptor, &config->delta_block_size, sizeof(config->delta_block_size)))
    goto error;
-  config->delta_block_size = (uint32_t)tmp;
  if (!receive_n_data(file_descriptor, &config->delta_max_file_size, sizeof(unsigned long long)))
    goto error;
-  config->show_progress = false;
-  config->dry_run = false;
-  config->ssh_port = 22;
-  config->transport = TRANSPORT_TCP;
-  config->ssh_destination = NULL;
-  config->fastsync_server_path = NULL;
-  config->exclude_patterns = NULL;
-  config->exclude_count = 0;
-  config->include_patterns = NULL;
-  config->include_count = 0;
-  config->max_size = 0;
-  config->min_size = 0;
-  config->use_tls = false;
-  config->tls_cert = NULL;
-  config->tls_key = NULL;
-  config->tls_ca = NULL;
-  config->timeout = 30;
-  config->contimeout = 10;
-  config->quiet = false;
-  config->stats = false;
-  config->max_depth = 0;
-  config->log_file = NULL;
-  config->queue_size = 100;
-  config->follow_symlinks = false;
-  config->copy_links = false;
-  config->safe_links = false;
-  config->copy_unsafe_links = false;
-  config->preserve_hard_links = false;
-  config->preserve_acls = false;
-  config->preserve_xattrs = false;
-  config->preserve_devices = false;
-  config->preserve_sparse = false;
-  config->itemize_changes = false;
-  config->out_format = NULL;
-  config->info_level = 0;
-  config->debug_level = 0;
-  config->list_only = false;
-  config->human_readable = false;
-  config->update = false;
-  config->inplace = false;
-  config->append = false;
-  config->append_verify = false;
-  config->delete_excluded = false;
-  config->delete_after = false;
-  config->max_delete = 0;
-  config->filters = NULL;
-  config->files_from = NULL;
-  config->cvs_exclude = false;
-  config->prune_empty_dirs = false;
-  config->relative = false;
-  config->rsh_command = NULL;
-  config->rsync_path = NULL;
-  config->temp_dir = NULL;
-  config->compare_dest = NULL;
-  config->copy_dest = NULL;
-  config->link_dest = NULL;
  if (!receive_int(file_descriptor, &tmp))
    goto error;
  config->backup = tmp;
@@ -421,8 +416,30 @@ Config* config_receive(int file_descriptor) {
  config->temp_dir = receive_str(file_descriptor);
  if (config->temp_dir == NULL)
    goto error;
-  config->server_host = str_dup("127.0.0.1");
-  config->server_port = 8080;
+  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)
+    goto error;
+  if (config->compress_choice[0] != '\0' && strcmp(config->compress_choice, "zstd") != 0 &&
+      strcmp(config->compress_choice, "none") != 0) {
+    fprintf(stderr, "Unsupported compression choice: %s\n", config->compress_choice);
+    send_status(file_descriptor, STATUS_ERROR);
+    goto error;
+  }
  if (!send_status(file_descriptor, STATUS_OK))
    goto error;
  return config;
@@ -434,6 +451,9 @@ error:
  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);
  return NULL;
 }
@@ -100,9 +100,35 @@ typedef struct Config {
  char* compare_dest;
  char* copy_dest;
  char* link_dest;
+
+  // PR #174: Partial transfer resumption
+  char* partial_dir;
+
+  // PR #178: Backup versioning
+  char* suffix;
+
+  // PR #179: Delete policies
+  bool delete_before;
+
+  // PR #181: IPv6 and bind address
+  char* address;
+  char* bind_address;
+  bool ipv6;
+  bool ipv4;
+
+  // PR #182: Daemon/server mode
+  bool daemon;
+  char* daemon_config;
+  bool server_mode;
+
+  // PR #183: Checksum comparison
+  bool checksum;
+
+  // PR #184: Compression algorithm negotiation
+  char* compress_choice;
 } Config;

-#define PROTOCOL_VERSION "1.3.0"
+#define PROTOCOL_VERSION "2.2.0"
 #define DEFAULT_CHUNK_SIZE (10 * 1024 * 1024)

 Config* config_create(void);
@@ -27,6 +27,10 @@ uint32_t delta_xxhash32(const void* data, uint32_t len) {
  return XXH32(data, len, 0);
 }

+uint64_t delta_xxhash64(const void* data, size_t len) {
+  return XXH64(data, len, 0);
+}
+
 DeltaSignature* delta_signature_create(const void* old_file_data, uint64_t old_file_size,
                                       uint32_t block_size) {
  if (old_file_data == NULL || old_file_size == 0 || block_size == 0)
@@ -388,6 +392,10 @@ Delta* delta_deserialize(const Data* data) {

    if (type == DELTA_OP_BLOCK_MATCH) {
      if (pos + sizeof(uint32_t) * 3 > data->size) {
+        for (uint32_t k = 0; k < i; k++) {
+          if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
+            free(delta->instructions[k].literal.data);
+        }
        free(delta->instructions);
        free(delta);
        return NULL;
@@ -426,6 +434,11 @@ Delta* delta_deserialize(const Data* data) {
      }
      delta->instructions[i].literal.data = malloc(lit_len);
      if (!delta->instructions[i].literal.data) {
+        log_message(LOG_LEVEL_ERROR, "Failed to allocate %u bytes for literal data", lit_len);
+        for (uint32_t k = 0; k < i; k++) {
+          if (delta->instructions[k].type == DELTA_INSTR_LITERAL)
+            free(delta->instructions[k].literal.data);
+        }
        free(delta->instructions);
        free(delta);
        return NULL;
@@ -449,7 +462,8 @@ Delta* delta_deserialize(const Data* data) {

 void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
                  uint32_t block_size) {
-  if (!old_data || !delta)
+  if (!old_data || !delta || (delta->new_file_size > 0 && delta->instructions == NULL) ||
+      (delta->instruction_count > 0 && block_size == 0))
    return NULL;

  void* output = malloc((size_t)delta->new_file_size);
@@ -463,10 +477,15 @@ void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
  for (uint32_t i = 0; i < delta->instruction_count; i++) {
    if (delta->instructions[i].type == DELTA_INSTR_BLOCK_MATCH) {
      uint64_t src_offset = (uint64_t)delta->instructions[i].match.block_index * block_size;
+      if (src_offset > UINT64_MAX - delta->instructions[i].match.block_offset) {
+        free(output);
+        return NULL;
+      }
      src_offset += delta->instructions[i].match.block_offset;
      uint32_t len = delta->instructions[i].match.length;

-      if (src_offset + len > old_size) {
+      if (src_offset > old_size || (uint64_t)len > old_size - src_offset ||
+          out_pos > delta->new_file_size || (uint64_t)len > delta->new_file_size - out_pos) {
        free(output);
        return NULL;
      }
@@ -474,6 +493,10 @@ void* delta_apply(const void* old_data, uint64_t old_size, const Delta* delta,
      out_pos += len;
    } else {
      uint32_t len = delta->instructions[i].literal.length;
+      if (out_pos > delta->new_file_size || (uint64_t)len > delta->new_file_size - out_pos) {
+        free(output);
+        return NULL;
+      }
      memcpy(out + out_pos, delta->instructions[i].literal.data, len);
      out_pos += len;
    }
@@ -72,5 +72,6 @@ bool delta_is_worthwhile(const Delta* delta, uint64_t new_file_size);

 uint32_t delta_adler32(const void* data, uint32_t len);
 uint32_t delta_xxhash32(const void* data, uint32_t len);
+uint64_t delta_xxhash64(const void* data, size_t len);

 #endif
@@ -2,6 +2,8 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <libgen.h>
+#include <limits.h>
+#include <poll.h>
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -9,6 +11,7 @@
 #include <sys/sendfile.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <time.h>

 #include "compression.h"
 #include "delta.h"
@@ -16,12 +19,26 @@
 #include "config.h"
 #include "data.h"
 #include "file.h"
-#include "log.h"
 #include "metadata.h"
 #include "protocol.h"
 #include "utils.h"

+bool file_checksum(File* file, uint64_t* checksum) {
+  if (!file || !checksum || !file->data)
+    return false;
+  if (file->data->size == 0) {
+    *checksum = delta_xxhash64("", 0);
+    return true;
+  }
+  if (!file->data->data && !file_load_data(file))
+    return false;
+  *checksum = delta_xxhash64(file->data->data, file->data->size);
+  return true;
+}
+
 File* file_create(const char* path) {
+  if (!path)
+    return NULL;
  File* file = (File*)malloc(sizeof(File));
  if (file == NULL) {
    perror("ERROR: Could not allocate memory for file struct");
@@ -35,7 +52,8 @@ File* file_create(const char* path) {
    return NULL;
  }

-  strcpy(file->path, path);
+  memcpy(file->path, path, path_len);
+  file->path[path_len] = '\0';
  file->data = data_create_reserve(0);
  if (file->data == NULL) {
    free(file->path);
@@ -86,6 +104,8 @@ bool file_load_data(File* file) {
  if (file == NULL)
    return false;
  if (file->data->data == NULL) {
+    if (file->data->size == 0)
+      return true;
    file->data->data = malloc(file->data->size);
    if (file->data->data == NULL) {
      perror("Could not allocate memory for file data");
@@ -95,6 +115,9 @@ bool file_load_data(File* file) {
  size_t bytes_read = file_content_to_buffer(file);
  if (bytes_read != file->data->size) {
    log_message(LOG_LEVEL_ERROR, "Did not read expected amount of bytes from file");
+    free(file->data->data);
+    file->data->data = NULL;
+    file->data->size = 0;
    return false;
  }
  return true;
@@ -102,6 +125,8 @@ bool file_load_data(File* file) {

 bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
                            int compression_level, bool send_path) {
+  if (!file || !file->path || !file->data)
+    return false;
  const Data* data_to_send = file->data;
  Data* compressed_data = NULL;
  if (compression_level > 0 && !compression_should_skip(file->path)) {
@@ -128,43 +153,159 @@ bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
  return true;
 }

-bool file_save_to_disk(const char* root_directory, File* file, const Config* config) {
-  (void)config;
-  if (has_path_traversal(file->path)) {
-    log_message(LOG_LEVEL_ERROR, "Path traversal detected in file path: %s", file->path);
+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;
+}
+
+static bool path_is_within_root(const char* root, const char* path) {
+  size_t n = strlen(root);
+  return strncmp(root, path, n) == 0 && (path[n] == '\0' || path[n] == '/');
+}
+
+bool file_save_to_disk(const char* root_directory, const File* file, const Config* config) {
+  bool backup_enabled = config && config->backup;
+  bool inplace = config && config->inplace;
+  bool sparse = config && config->preserve_sparse;
+  const char* backup_suffix = (config && config->suffix) ? config->suffix : "~";
+  const char* backup_dir = (config && config->backup_dir) ? config->backup_dir : NULL;
+  const char* partial_dir = (config && config->partial_dir) ? config->partial_dir : NULL;
+  char *confined_backup = NULL, *confined_partial = NULL;
+
+  if (!file || !file->path || !file->data || has_path_traversal(file->path) ||
+      (backup_enabled &&
+       (!backup_suffix || backup_suffix[0] == '\0' || strchr(backup_suffix, '/') != NULL ||
+        strcmp(backup_suffix, ".") == 0 || strcmp(backup_suffix, "..") == 0))) {
+    log_message(LOG_LEVEL_ERROR, "Invalid file or path received");
    return false;
  }

-  // Resolve the destination root to its real path, preventing symlink-based escapes.
-  // If the root does not yet exist, try to create it so realpath can succeed.
-  char* resolved_root = realpath(root_directory, NULL);
+  /* These options arrive from the client.  They are names below the server
+     root, never independent filesystem roots. */
+  if ((backup_dir && (backup_dir[0] == '/' || has_path_traversal(backup_dir))) ||
+      (partial_dir && (partial_dir[0] == '/' || has_path_traversal(partial_dir))))
+    return false;
+  if (backup_dir && !(confined_backup = path_cat(root_directory, backup_dir)))
+    return false;
+  if (partial_dir && !(confined_partial = path_cat(root_directory, partial_dir))) {
+    free(confined_backup);
+    return false;
+  }
+
+  char* resolved_root = NULL;
+  const char* actual_root =
+      (partial_dir && config && config->partial) ? confined_partial : root_directory;
+  resolved_root = realpath(actual_root, NULL);
  if (resolved_root == NULL) {
-    if (mkdir_r(root_directory)) {
-      resolved_root = realpath(root_directory, NULL);
+    if (mkdir_r(actual_root)) {
+      resolved_root = realpath(actual_root, NULL);
    }
  }
  if (resolved_root == NULL) {
-    log_message(LOG_LEVEL_ERROR, "Failed to resolve destination root: %s", root_directory);
+    log_message(LOG_LEVEL_ERROR, "Failed to resolve destination root: %s", actual_root);
+    free(confined_backup);
+    free(confined_partial);
    return false;
  }
+  char* resolved_base = realpath(root_directory, NULL);
+  if (resolved_base == NULL || !path_is_within_root(resolved_base, resolved_root)) {
+    free(resolved_base);
+    free(confined_backup);
+    free(confined_partial);
+    free(resolved_root);
+    return false;
+  }
+  free(resolved_base);

  char* disk_path = path_cat(resolved_root, file->path);
  if (disk_path == NULL) {
+    free(confined_backup);
+    free(confined_partial);
    free(resolved_root);
    return false;
  }

-  // Ensure the target directory exists so the parent can be resolved for path safety.
+  /* --update is receiver-side policy: never replace a newer destination. */
+  if (config && config->update) {
+    struct stat destination_stat;
+    if (stat(disk_path, &destination_stat) == 0 && file->metadata &&
+        destination_stat.st_mtime > file->metadata->mtime_sec) {
+      free(resolved_root);
+      free(confined_backup);
+      free(confined_partial);
+      free(disk_path);
+      return true;
+    }
+  }
+
+  if (backup_enabled) {
+    struct stat backup_stat;
+    if (stat(disk_path, &backup_stat) == 0) {
+      char* backup_path = NULL;
+      if (backup_dir) {
+        char* resolved_backup_dir = realpath(confined_backup, NULL);
+        if (!resolved_backup_dir) {
+          mkdir_r(confined_backup);
+          resolved_backup_dir = realpath(confined_backup, NULL);
+        }
+        if (resolved_backup_dir) {
+          char* backup_base = realpath(root_directory, NULL);
+          if (backup_base && path_is_within_root(backup_base, resolved_backup_dir))
+            backup_path = path_cat(resolved_backup_dir, file->path);
+          free(backup_base);
+          free(resolved_backup_dir);
+        }
+      }
+      if (!backup_path) {
+        size_t path_len = strlen(disk_path);
+        size_t suffix_len = strlen(backup_suffix);
+        backup_path = malloc(path_len + suffix_len + 1);
+        if (backup_path) {
+          memcpy(backup_path, disk_path, path_len);
+          memcpy(backup_path + path_len, backup_suffix, suffix_len + 1);
+        }
+      }
+      if (backup_path) {
+        char* backup_dir_path = str_dup(backup_path);
+        if (backup_dir_path) {
+          const char* bdir = dirname(backup_dir_path);
+          mkdir_r(bdir);
+          free(backup_dir_path);
+        }
+        if (!rename_secure(disk_path, backup_path)) {
+          free(backup_path);
+          free(resolved_root);
+          free(confined_backup);
+          free(confined_partial);
+          free(disk_path);
+          return false;
+        }
+        free(backup_path);
+      }
+    }
+  }
+
  char* dir_dup = str_dup(disk_path);
  if (!dir_dup) {
+    free(confined_backup);
+    free(confined_partial);
    free(resolved_root);
    free(disk_path);
    return false;
  }
  char* dir_str = dirname(dir_dup);
-  // Create the directory if needed (no-op if it already exists) so realpath can resolve it.
  if (!mkdir_r(dir_str)) {
    free(dir_dup);
+    free(confined_backup);
+    free(confined_partial);
    free(resolved_root);
    free(disk_path);
    return false;
@@ -173,19 +314,20 @@ bool file_save_to_disk(const char* root_directory, File* file, const Config* con
  free(dir_dup);
  if (resolved_dir == NULL) {
    log_message(LOG_LEVEL_ERROR, "Failed to resolve directory for: %s", disk_path);
+    free(confined_backup);
+    free(confined_partial);
    free(resolved_root);
    free(disk_path);
    return false;
  }

-  // Verify that the resolved directory is inside the resolved root.
-  // Both are canonical absolute paths — this prevents symlink-based escapes.
  size_t root_len = strlen(resolved_root);
  if (strncmp(resolved_dir, resolved_root, root_len) != 0 ||
      (resolved_dir[root_len] != '\0' && resolved_dir[root_len] != '/')) {
-    log_message(LOG_LEVEL_ERROR, "Path escape detected: %s is outside %s", disk_path,
-                root_directory);
+    log_message(LOG_LEVEL_ERROR, "Path escape detected: %s is outside %s", disk_path, actual_root);
    free(resolved_dir);
+    free(confined_backup);
+    free(confined_partial);
    free(resolved_root);
    free(disk_path);
    return false;
@@ -193,31 +335,14 @@ bool file_save_to_disk(const char* root_directory, File* file, const Config* con
  free(resolved_dir);
  free(resolved_root);

-  bool ok = to_disk(disk_path, file->data->data, file->data->size);
-  if (ok)
-    file_restore_metadata(disk_path, file->metadata);
+  bool ok = to_disk_secure(disk_path, file->data->data, file->data->size, inplace, sparse,
+                           file->metadata);
+  free(confined_backup);
+  free(confined_partial);
  free(disk_path);
  return ok;
 }

-static void* old_data_from_path(const char* full_path, unsigned long long old_size) {
-  void* data = malloc((size_t)old_size);
-  if (!data)
-    return NULL;
-  FILE* fp = fopen(full_path, "rb");
-  if (!fp) {
-    free(data);
-    return NULL;
-  }
-  size_t nread = fread(data, 1, (size_t)old_size, fp);
-  fclose(fp);
-  if (nread != (size_t)old_size) {
-    free(data);
-    return NULL;
-  }
-  return data;
-}
-
 static File* receive_delta_file(int fd, const Config* config, const char* check_path,
                                void* old_data, unsigned long long old_size) {
  if (!old_data)
@@ -381,12 +506,18 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {

  unsigned long long check_size;
  long long check_mtime;
+  uint64_t check_checksum = 0;
  if (!receive_n_data(fd, &check_size, sizeof(check_size)) ||
      !receive_n_data(fd, &check_mtime, sizeof(check_mtime))) {
    free(check_path);
    send_status(fd, STATUS_ERROR);
    return NULL;
  }
+  if (config->checksum && !receive_n_data(fd, &check_checksum, sizeof(check_checksum))) {
+    free(check_path);
+    send_status(fd, STATUS_ERROR);
+    return NULL;
+  }

  if (has_path_traversal(check_path)) {
    log_message(LOG_LEVEL_ERROR, "Path traversal detected: %s", check_path);
@@ -397,13 +528,53 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {

  char* full_path = path_cat(config->receive_root_directory, check_path);
  struct stat st;
-  bool has_old_file = (full_path && lstat(full_path, &st) == 0);
+  bool has_old_file = false;
+  int old_fd = -1;
+  if (full_path) {
+    char* leaf = NULL;
+    int parent_fd = open_secure_parent(full_path, &leaf);
+    if (parent_fd >= 0) {
+      old_fd = openat(parent_fd, leaf, O_RDONLY | O_CLOEXEC | O_NOFOLLOW);
+      free(leaf);
+      close(parent_fd);
+      has_old_file = old_fd >= 0 && fstat(old_fd, &st) == 0 && S_ISREG(st.st_mode);
+    }
+  }
  unsigned long long old_size = has_old_file ? (unsigned long long)st.st_size : 0;
+  void* old_data = NULL;
+  if (has_old_file && old_size > 0) {
+    old_data = malloc((size_t)old_size);
+    if (old_data) {
+      size_t got = 0;
+      while (got < (size_t)old_size) {
+        ssize_t n = read(old_fd, (char*)old_data + got, (size_t)old_size - got);
+        if (n <= 0) {
+          free(old_data);
+          old_data = NULL;
+          break;
+        }
+        got += (size_t)n;
+      }
+    }
+  }
+  if (old_fd >= 0) {
+    close(old_fd);
+  }

-  bool match = has_old_file && (unsigned long long)st.st_size == check_size &&
-               (long long)st.st_mtime == check_mtime;
+  bool match = has_old_file && (unsigned long long)st.st_size == check_size;
+  if (match && config->checksum) {
+    uint64_t old_checksum = old_size == 0 ? delta_xxhash64("", 0) : 0;
+    if (old_data)
+      old_checksum = delta_xxhash64(old_data, (size_t)old_size);
+    match = (old_size == 0 || old_data) && old_checksum == check_checksum;
+    free(old_data);
+    old_data = NULL;
+  } else if (match) {
+    match = (long long)st.st_mtime == check_mtime;
+  }

  if (match) {
+    free(old_data);
    if (!send_status(fd, STATUS_OK)) {
      free(full_path);
      free(check_path);
@@ -419,13 +590,15 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
                   delta_should_attempt(old_size, check_size, config->delta_max_file_size);

  if (try_delta) {
-    void* old_data = old_data_from_path(full_path, old_size);
    File* delta_file = receive_delta_file(fd, config, check_path, old_data, old_size);
+    old_data = NULL; /* receive_delta_file consumes the snapshot on every path */
    if (delta_file) {
      free(full_path);
      free(check_path);
      return delta_file;
    }
+    free(old_data);
+    old_data = NULL;
    try_delta = false;
  }

@@ -478,7 +651,144 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
  return file;
 }

-bool to_disk(const char* path, const void* data, unsigned long long data_size) {
+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;

@@ -495,6 +805,39 @@ bool to_disk(const char* path, const void* data, unsigned long long data_size) {
  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) {
@@ -510,6 +853,21 @@ bool to_disk(const char* path, const void* data, unsigned long long data_size) {
    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);
@@ -534,13 +892,8 @@ done:

 bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int compression_level,
                        bool send_path) {
-  // sendfile is incompatible with compression (kernel zero-copy).
-  // If compression is requested, fall back to the regular send path.
-  // NOTE: This is a safety net only — callers must ensure compression_level == 0
-  // before calling file_send_sendfile. The fallback to file_send_single_calls
-  // preserves the send_path contract, but callers should not rely on it for
-  // correctness (the --sendfile flag is validated to be mutually exclusive with
-  // -c/--compress at the CLI layer).
+  if (!file || !file->path || !file->data)
+    return false;
  if (compression_level > 0)
    return file_send_single_calls(file, file_descriptor, use_metadata, compression_level,
                                  send_path);
@@ -557,13 +910,56 @@ bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int
  }

  unsigned long long file_size = file->data->size;
+  struct stat source_stat;
+  if (fstat(fd, &source_stat) != 0 || !S_ISREG(source_stat.st_mode) ||
+      (unsigned long long)source_stat.st_size < file_size) {
+    close(fd);
+    return false;
+  }
  if (!send_n_data(file_descriptor, &file_size, sizeof(unsigned long long))) {
    close(fd);
    return false;
  }

+  /* sendfile cannot encrypt TLS records.  Keep the framing identical but
+     route encrypted transfers through the deadline-aware IO layer. */
+  if (io_get_ssl() != NULL) {
+    unsigned char buffer[64 * 1024];
+    unsigned long long remaining = file_size;
+    bool ok = true;
+    while (remaining > 0) {
+      size_t want = remaining > sizeof(buffer) ? sizeof(buffer) : (size_t)remaining;
+      ssize_t got = read(fd, buffer, want);
+      if (got <= 0 || !send_n_data(file_descriptor, buffer, (size_t)got)) {
+        ok = false;
+        break;
+      }
+      remaining -= (unsigned long long)got;
+    }
+    close(fd);
+    return ok;
+  }
+
  off_t offset = 0;
+  struct timespec deadline;
+  clock_gettime(CLOCK_MONOTONIC, &deadline);
+  deadline.tv_sec += 60;
  while ((unsigned long long)offset < file_size) {
+    struct timespec now;
+    clock_gettime(CLOCK_MONOTONIC, &now);
+    long long remaining = (long long)(deadline.tv_sec - now.tv_sec) * 1000LL +
+                          (deadline.tv_nsec - now.tv_nsec) / 1000000LL;
+    if (remaining <= 0) {
+      close(fd);
+      return false;
+    }
+    struct pollfd pfd = {.fd = file_descriptor, .events = POLLOUT};
+    int timeout = remaining > INT_MAX ? INT_MAX : (int)remaining;
+    int polled = poll(&pfd, 1, timeout);
+    if (polled <= 0 || (pfd.revents & (POLLERR | POLLHUP | POLLNVAL))) {
+      close(fd);
+      return false;
+    }
    ssize_t sent = sendfile(file_descriptor, fd, &offset, file_size - offset);
    if (sent == -1) {
      if (errno == EAGAIN || errno == EINTR)
@@ -572,6 +968,10 @@ bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int
      close(fd);
      return false;
    }
+    if (sent == 0) {
+      close(fd);
+      return false;
+    }
  }

  close(fd);
@@ -582,6 +982,11 @@ File* file_receive(const Config* config, int file_descriptor) {
  char* path = receive_str(file_descriptor);
  if (path == NULL)
    return NULL;
+  if (path[0] == '\0' || has_path_traversal(path)) {
+    log_message(LOG_LEVEL_ERROR, "Invalid received file path: %s", path);
+    free(path);
+    return NULL;
+  }
  File* file = file_create(path);
  free(path);
  if (file == NULL)
@@ -630,21 +1035,40 @@ size_t file_content_to_buffer(File* file) {
 }

 int receive_manifest(int fd, const Config* config, int* next_status) {
+  int received_status = STATUS_ERROR;
+  int* status_out = next_status ? next_status : &received_status;
  int count;
  if (!receive_int(fd, &count))
    return -1;
-  ArrayList* manifest = array_list_create(free);
-  if (manifest) {
-    for (int i = 0; i < count; i++) {
-      char* s = receive_str(fd);
-      if (s)
-        array_list_add(manifest, s);
-    }
-    fprintf(stderr, "Deleting files not in manifest...\n");
-    delete_extras(config->receive_root_directory, manifest);
-    array_list_delete(manifest);
-  }
-  if (!receive_status(fd, next_status))
+  if (count < 0 || count > MAX_MANIFEST_ENTRIES)
    return -1;
-  return 0;
+  ArrayList* manifest = array_list_create(free);
+  if (!manifest)
+    return -1;
+  size_t manifest_bytes = 0;
+  for (int i = 0; i < count; i++) {
+    char* s = receive_str(fd);
+    size_t entry_size = s ? strlen(s) : 0;
+    if (!s || s[0] == '\0' || s[0] == '/' || has_path_traversal(s) ||
+        entry_size > MAX_MANIFEST_BYTES - manifest_bytes ||
+        (manifest_bytes += entry_size) > MAX_MANIFEST_BYTES || !array_list_add(manifest, s)) {
+      free(s);
+      array_list_delete(manifest);
+      return -1;
+    }
+  }
+  if (!receive_status(fd, status_out)) {
+    array_list_delete(manifest);
+    return -1;
+  }
+  /* Deletion is a commit operation: never perform it until the sender has
+     completed the manifest frame successfully. */
+  if (*status_out != STATUS_FINISHED || !config->use_delete) {
+    array_list_delete(manifest);
+    return *status_out == STATUS_FINISHED ? 0 : -1;
+  }
+  fprintf(stderr, "Deleting files not in manifest...\n");
+  bool deletion_ok = delete_extras(config->receive_root_directory, manifest);
+  array_list_delete(manifest);
+  return deletion_ok ? 0 : -1;
 }
@@ -26,6 +26,7 @@ typedef struct {
 File* file_create(const char* path);
 void file_destroy(void* item);
 bool file_load_data(File* file);
+bool file_checksum(File* file, uint64_t* checksum);
 File* file_receive(const Config* config, int file_descriptor);
 bool file_send_single_calls(File* file, int file_descriptor, bool use_metadata,
                            int compression_level, bool send_path);
@@ -34,8 +35,10 @@ bool file_send_sendfile(File* file, int file_descriptor, bool use_metadata, int
 size_t file_content_to_buffer(File* file);
 FileMetadata* file_metadata_create(const struct stat* stats);
 void file_metadata_destroy(void* metadata);
-bool to_disk(const char* path, const void* data, unsigned long long data_size);
-bool file_save_to_disk(const char* root_directory, File* file, const Config* config);
+bool to_disk(const char* path, const void* data, unsigned long long data_size, bool inplace,
+             bool sparse);
+bool file_save_to_disk(const char* root_directory, const File* file, const Config* config);
+void file_set_authorized_root(int fd, const char* canonical_path);
 File* receive_incremental_check(int fd, const Config* config, bool* skipped);
 int receive_manifest(int fd, const Config* config, int* next_status);

@@ -15,28 +15,38 @@ void log_set_file(FILE* fp) {
  log_fp = fp;
 }

+static inline void write_message(FILE* dest_io, LogLevel log_level, struct tm t, const char* format,
+                                 va_list args) {
+  fprintf(dest_io, "%04d-%02d-%02d %02d:%02d:%02d [%s]: ", t.tm_year + 1900, t.tm_mon + 1,
+          t.tm_mday, t.tm_hour, t.tm_min, t.tm_sec, log_level_strings[log_level]);
+
+  vfprintf(dest_io, format, args);
+  fprintf(dest_io, "\n");
+}
+
 void log_message(LogLevel log_level, const char* format, ...) {
  if (log_level < current_log_level)
    return;
+  if (log_level < 0 || log_level >= (int)(sizeof(log_level_strings) / sizeof(log_level_strings[0])))
+    return;
  time_t now = time(NULL);
-  const struct tm* t = localtime(&now);
+  struct tm t;
+  if (!localtime_r(&now, &t))
+    return;

-  fprintf(stderr, "%04d-%02d-%02d %02d:%02d:%02d [%s]: ", t->tm_year + 1900, t->tm_mon + 1,
-          t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec, log_level_strings[log_level]);
+  FILE* dest_io = stdout;
+  if (log_level == LOG_LEVEL_ERROR) {
+    dest_io = stderr;
+  }

  va_list args;
  va_start(args, format);
-  vfprintf(stderr, format, args);
+  write_message(dest_io, log_level, t, format, args);
  va_end(args);
-  fprintf(stderr, "\n");

  if (log_fp) {
-    fprintf(log_fp, "%04d-%02d-%02d %02d:%02d:%02d [%s]: ", t->tm_year + 1900, t->tm_mon + 1,
-            t->tm_mday, t->tm_hour, t->tm_min, t->tm_sec, log_level_strings[log_level]);
    va_start(args, format);
-    vfprintf(log_fp, format, args);
+    write_message(log_fp, log_level, t, format, args);
    va_end(args);
-    fprintf(log_fp, "\n");
-    fflush(log_fp);
  }
 }
@@ -105,11 +105,16 @@ FileMetadata* metadata_receive(int file_descriptor, int* ok) {
      *ok = 0;
    return NULL;
  }
-  if (!present) {
+  if (present == 0) {
    if (ok)
      *ok = 1;
    return NULL;
  }
+  if (present != 1) {
+    if (ok)
+      *ok = 0;
+    return NULL;
+  }
  FileMetadata* m = malloc(sizeof(FileMetadata));
  if (m == NULL) {
    if (ok)
@@ -156,18 +161,24 @@ FileMetadata* metadata_receive(int file_descriptor, int* ok) {
    return NULL;
  }
  m->mtime_nsec = (long)mtime_nsec;
+  if (mtime_nsec < 0 || mtime_nsec >= 1000000000LL || mode < 0 || uid < 0 || gid < 0) {
+    free(m);
+    if (ok)
+      *ok = 0;
+    return NULL;
+  }
  if (ok)
    *ok = 1;
  return m;
 }

-void file_restore_metadata(const char* path, FileMetadata* metadata) {
+void file_restore_metadata(const char* path, const FileMetadata* metadata) {
  if (metadata == NULL)
    return;
  if (chmod(path, metadata->mode & 07777 & ~(S_ISUID | S_ISGID)) != 0)
    log_message(LOG_LEVEL_WARNING, "Failed to chmod %s: %s", path, strerror(errno));
-  if (chown(path, metadata->uid, metadata->gid) != 0)
-    log_message(LOG_LEVEL_WARNING, "Failed to chown %s: %s", path, strerror(errno));
+  /* Never apply client-supplied ownership.  The descriptor API below is the
+     receiver write path; retain this legacy API only for compatibility. */
  struct timespec times[2];
  times[0].tv_sec = 0;
  times[0].tv_nsec = UTIME_OMIT;
@@ -176,3 +187,17 @@ void file_restore_metadata(const char* path, FileMetadata* metadata) {
  if (utimensat(AT_FDCWD, path, times, 0) != 0)
    log_message(LOG_LEVEL_WARNING, "Failed to set timestamps on %s: %s", path, strerror(errno));
 }
+
+bool file_restore_metadata_fd(int fd, const FileMetadata* metadata) {
+  if (fd < 0 || metadata == NULL)
+    return metadata == NULL;
+  bool ok = true;
+  if (fchmod(fd, metadata->mode & 07777 & ~(S_ISUID | S_ISGID)) != 0)
+    ok = false;
+  /* Client uid/gid values are deliberately not authoritative. */
+  struct timespec times[2] = {{.tv_sec = 0, .tv_nsec = UTIME_OMIT},
+                              {.tv_sec = metadata->mtime_sec, .tv_nsec = metadata->mtime_nsec}};
+  if (futimens(fd, times) != 0)
+    ok = false;
+  return ok;
+}
@@ -29,6 +29,7 @@ void metadata_to_buf(char** buf, const FileMetadata* m);
 FileMetadata* metadata_from_buf(char** buf);
 bool metadata_send(int file_descriptor, FileMetadata* m);
 FileMetadata* metadata_receive(int file_descriptor, int* ok);
-void file_restore_metadata(const char* path, FileMetadata* metadata);
+void file_restore_metadata(const char* path, const FileMetadata* metadata);
+bool file_restore_metadata_fd(int fd, const FileMetadata* metadata);

 #endif
@@ -1,4 +1,5 @@
 #include "multiprocessing.h"
+
 #include "array_list.h"
 #include "chunk.h"
 #include "config.h"
@@ -13,6 +14,10 @@
 #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));
@@ -24,17 +29,50 @@ PipelineContextSender* pipeline_context_sender_create(Config* config, Queue* que
  context->scanner_done = false;
  context->loader_done = false;
  context->manifest = NULL;
-  if (mtx_init(&context->mutex_scanner, mtx_plain) != thrd_success ||
-      cnd_init(&context->condition_not_full_scanner) != thrd_success ||
-      cnd_init(&context->condition_not_empty_scanner) != thrd_success ||
-      mtx_init(&context->mutex_loader, mtx_plain) != thrd_success ||
-      cnd_init(&context->condition_not_full_loader) != thrd_success ||
-      cnd_init(&context->condition_not_empty_loader) != thrd_success) {
-    perror("Error initializing synchronization objects");
-    free(context);
-    return NULL;
-  }
+  context->progress_bytes = 0;
+  context->sender_done = false;
+  atomic_init(&context->cancelled, false);
+  int init = 0;
+  if (mtx_init(&context->mutex_scanner, mtx_plain) != thrd_success)
+    goto fail;
+  init++;
+  if (cnd_init(&context->condition_not_full_scanner) != thrd_success)
+    goto fail;
+  init++;
+  if (cnd_init(&context->condition_not_empty_scanner) != thrd_success)
+    goto fail;
+  init++;
+  if (mtx_init(&context->mutex_loader, mtx_plain) != thrd_success)
+    goto fail;
+  init++;
+  if (cnd_init(&context->condition_not_full_loader) != thrd_success)
+    goto fail;
+  init++;
+  if (cnd_init(&context->condition_not_empty_loader) != thrd_success)
+    goto fail;
+  init++;
+  if (mtx_init(&context->mutex_progress, mtx_plain) != thrd_success)
+    goto fail;
+  // cppcheck-suppress unreadVariable
+  init++;
  return context;
+
+fail:
+  perror("Error initializing synchronization objects");
+  if (init >= 6)
+    cnd_destroy(&context->condition_not_empty_loader);
+  if (init >= 5)
+    cnd_destroy(&context->condition_not_full_loader);
+  if (init >= 4)
+    mtx_destroy(&context->mutex_loader);
+  if (init >= 3)
+    cnd_destroy(&context->condition_not_empty_scanner);
+  if (init >= 2)
+    cnd_destroy(&context->condition_not_full_scanner);
+  if (init >= 1)
+    mtx_destroy(&context->mutex_scanner);
+  free(context);
+  return NULL;
 }

 void pipeline_context_sender_destroy(PipelineContextSender* context) {
@@ -50,6 +88,7 @@ void pipeline_context_sender_destroy(PipelineContextSender* context) {
  mtx_destroy(&context->mutex_loader);
  cnd_destroy(&context->condition_not_full_loader);
  cnd_destroy(&context->condition_not_empty_loader);
+  mtx_destroy(&context->mutex_progress);
  free(context);
 }

@@ -63,14 +102,30 @@ PipelineContextReceiver* pipeline_context_receiver_create(Config* config, Queue*
  context->file_descriptor = file_descriptor;
  context->ssl = ssl;
  context->receiver_done = false;
-  if (mtx_init(&context->mutex, mtx_plain) != thrd_success ||
-      cnd_init(&context->condition_not_full) != thrd_success ||
-      cnd_init(&context->condition_not_empty) != thrd_success) {
-    perror("Error initializing synchronization objects");
-    free(context);
-    return NULL;
-  }
+  atomic_init(&context->cancelled, false);
+  int init = 0;
+  if (mtx_init(&context->mutex, mtx_plain) != thrd_success)
+    goto fail;
+  init++;
+  if (cnd_init(&context->condition_not_full) != thrd_success)
+    goto fail;
+  init++;
+  if (cnd_init(&context->condition_not_empty) != thrd_success)
+    goto fail;
+  // cppcheck-suppress unreadVariable
+  init++;
  return context;
+
+fail:
+  perror("Error initializing synchronization objects");
+  if (init >= 3)
+    cnd_destroy(&context->condition_not_empty);
+  if (init >= 2)
+    cnd_destroy(&context->condition_not_full);
+  if (init >= 1)
+    mtx_destroy(&context->mutex);
+  free(context);
+  return NULL;
 }

 void pipeline_context_receiver_destroy(PipelineContextReceiver* context) {
@@ -90,14 +145,33 @@ static bool receive_chunk_enqueue(int file_descriptor, PipelineContextReceiver*
  for (int i = 0; i < chunk->element_count; i++) {
    File* file = chunk->items[i];
    chunk->items[i] = NULL;
-    queue_enqueue_multithreaded(context->queue, file, &context->mutex,
-                                &context->condition_not_empty, &context->condition_not_full);
+    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);
  return true;
 }

+static void receiver_thread_fail(PipelineContextReceiver* context) {
+  mtx_lock(&context->mutex);
+  atomic_store(&context->cancelled, true);
+  context->receiver_done = true;
+  cnd_broadcast(&context->condition_not_empty);
+  cnd_broadcast(&context->condition_not_full);
+  mtx_unlock(&context->mutex);
+}
+
 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);
@@ -108,53 +182,63 @@ int receive_thread(void* pipeline_context) {

  Status status;
  if (!receive_status(file_descriptor, &status))
-    return thrd_error;
+    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) {
-      send_status(file_descriptor, 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");
-      return thrd_error;
+      RECEIVE_THREAD_FAIL();
    }
    if (status == STATUS_CHECK) {
      bool skipped;
      File* file = receive_incremental_check(file_descriptor, config, &skipped);
      if (!skipped) {
        if (file == NULL)
-          return thrd_error;
-        queue_enqueue_multithreaded(context->queue, file, &context->mutex,
-                                    &context->condition_not_empty, &context->condition_not_full);
+          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))
-        return thrd_error;
+        RECEIVE_THREAD_FAIL();
    } else if (status == STATUS_CHECK_BATCH) {
      int count;
-      if (!receive_int(file_descriptor, &count))
-        return thrd_error;
+      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)
-          return thrd_error;
+          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);
-          return thrd_error;
+          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 (match)
-          send_status(file_descriptor, STATUS_OK);
-        else
-          send_status(file_descriptor, STATUS_NEXT);
+        if (!send_status(file_descriptor, match ? STATUS_OK : STATUS_NEXT))
+          RECEIVE_THREAD_FAIL();
        free(full_path);
        free(check_path);
      }
@@ -162,25 +246,33 @@ int receive_thread(void* pipeline_context) {
    } else {
      File* file = file_receive(config, file_descriptor);
      if (file) {
-        queue_enqueue_multithreaded(context->queue, file, &context->mutex,
-                                    &context->condition_not_empty, &context->condition_not_full);
+        if (!queue_enqueue_multithreaded_cancel(
+                context->queue, file, &context->mutex, &context->condition_not_empty,
+                &context->condition_not_full, &context->cancelled)) {
+          file_destroy(file);
+          receiver_thread_fail(context);
+          return thrd_error;
+        }
      } else {
        log_message(LOG_LEVEL_ERROR, "Failed to receive file");
-        return thrd_error;
+        RECEIVE_THREAD_FAIL();
      }
    }
  next:
    if (!receive_status(file_descriptor, &status))
-      return thrd_error;
+      RECEIVE_THREAD_FAIL();
  }
  if (status == STATUS_MANIFEST) {
    if (receive_manifest(file_descriptor, config, &status) != 0)
-      return thrd_error;
+      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
  return thrd_success;
 }

@@ -201,8 +293,17 @@ int write_thread(void* pipeline_context) {
      free(root_directory);
      return thrd_success;
    }
-    if (save_to_disk)
-      file_save_to_disk(root_directory, file, context->config);
+    if (save_to_disk && !file_save_to_disk(root_directory, file, context->config)) {
+      file_destroy(file);
+      mtx_lock(&context->mutex);
+      atomic_store(&context->cancelled, true);
+      context->receiver_done = true;
+      cnd_broadcast(&context->condition_not_full);
+      cnd_broadcast(&context->condition_not_empty);
+      mtx_unlock(&context->mutex);
+      free(root_directory);
+      return thrd_error;
+    }
    file_destroy(file);
  }
 }
@@ -2,6 +2,7 @@
 #define MULTIPROCESSING_H

 #include <threads.h>
+#include <stdatomic.h>

 #include "array_list.h"
 #include "config.h"
@@ -23,6 +24,10 @@ typedef struct {
  cnd_t condition_not_empty_loader;
  bool loader_done;
  ArrayList* manifest;
+  mtx_t mutex_progress;
+  unsigned long long progress_bytes;
+  bool sender_done;
+  atomic_bool cancelled;
 } PipelineContextSender;

 typedef struct PipelineContextReceiver {
@@ -34,6 +39,7 @@ typedef struct PipelineContextReceiver {
  cnd_t condition_not_full;
  cnd_t condition_not_empty;
  bool receiver_done;
+  atomic_bool cancelled;
 } PipelineContextReceiver;

 PipelineContextSender* pipeline_context_sender_create(Config* config, Queue* queue_scanner,
@@ -1,16 +1,18 @@
 #include "protocol.h"
 #include "log.h"
 #include <errno.h>
+#include <limits.h>
 #include <openssl/ssl.h>
 #include <poll.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <threads.h>
 #include <time.h>
 #include <unistd.h>

-#define MAX_DATA_SIZE (100ULL * 1024 * 1024)          /* 100 MB max per data message */
-#define RECEIVE_TIMEOUT_SEC 60                        /* 60 second per-message timeout */
+#define RECEIVE_TIMEOUT_SEC 60 /* 60 second per-message timeout */
+#define SEND_TIMEOUT_SEC 60
 #define MAX_CONNECTION_MEMORY (1024ULL * 1024 * 1024) /* 1 GB total per connection */

 static __thread int io_read_fd = -1;
@@ -20,23 +22,38 @@ static __thread SSL* io_ssl;
 static unsigned long long io_bwlimit = 0;
 static long long bw_tokens = 0;
 static struct timespec bw_last_refill = {0, 0};
+static mtx_t bw_mutex;
+static once_flag bw_mutex_once = ONCE_FLAG_INIT;

 static __thread unsigned long long total_allocated_bytes = 0;

 void io_set_fds(int read_fd, int write_fd) {
  io_read_fd = read_fd;
  io_write_fd = write_fd;
+  /* A descriptor switch starts a new transport; never reuse a TLS object
+     belonging to a previous connection or test pipe. */
+  io_ssl = NULL;
+  total_allocated_bytes = 0;
+}
+
+static void bw_mutex_init(void) {
+  mtx_init(&bw_mutex, mtx_plain);
 }

 void io_set_bwlimit(unsigned long long bytes_per_sec) {
+  call_once(&bw_mutex_once, bw_mutex_init);
+  mtx_lock(&bw_mutex);
  io_bwlimit = bytes_per_sec;
  bw_tokens = (long long)io_bwlimit;
  clock_gettime(CLOCK_MONOTONIC, &bw_last_refill);
+  mtx_unlock(&bw_mutex);
 }

 static void bw_throttle(size_t bytes_written) {
  if (io_bwlimit == 0)
    return;
+  call_once(&bw_mutex_once, bw_mutex_init);
+  mtx_lock(&bw_mutex);

  struct timespec now;
  clock_gettime(CLOCK_MONOTONIC, &now);
@@ -61,6 +78,7 @@ static void bw_throttle(size_t bytes_written) {
    bw_tokens = 0;
    clock_gettime(CLOCK_MONOTONIC, &bw_last_refill);
  }
+  mtx_unlock(&bw_mutex);
 }

 void io_set_ssl(SSL* ssl) {
@@ -75,14 +93,39 @@ static int io_fd(int dir_fd, int file_descriptor) {
  return (dir_fd != -1) ? dir_fd : file_descriptor;
 }

+static int deadline_remaining_ms(const struct timespec* deadline) {
+  struct timespec now;
+  clock_gettime(CLOCK_MONOTONIC, &now);
+  long long ns =
+      (long long)(deadline->tv_sec - now.tv_sec) * 1000000000LL + deadline->tv_nsec - now.tv_nsec;
+  if (ns <= 0)
+    return 0;
+  long long ms = (ns + 999999) / 1000000;
+  return ms > INT_MAX ? INT_MAX : (int)ms;
+}
+
 bool send_n_data(int file_descriptor, 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);
+  struct timespec deadline;
+  clock_gettime(CLOCK_MONOTONIC, &deadline);
+  deadline.tv_sec += SEND_TIMEOUT_SEC;
+  short wait_events = POLLOUT;
  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)
      chunk = 65536;
+    struct pollfd pfd = {.fd = fd, .events = wait_events};
+    int poll_result = poll(&pfd, 1, deadline_remaining_ms(&deadline));
+    if (poll_result == 0 || (poll_result < 0 && errno != EINTR)) {
+      log_message(LOG_LEVEL_ERROR, "Send timeout or poll failure");
+      return false;
+    }
+    if (poll_result < 0)
+      continue;
+    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);
@@ -91,8 +134,10 @@ bool send_n_data(int file_descriptor, const void* data, size_t data_size) {
    if (bytes_send <= 0) {
      if (io_ssl) {
        int ssl_err = SSL_get_error(io_ssl, (int)bytes_send);
-        if (ssl_err == SSL_ERROR_WANT_WRITE || ssl_err == SSL_ERROR_WANT_READ)
+        if (ssl_err == SSL_ERROR_WANT_WRITE || ssl_err == SSL_ERROR_WANT_READ) {
+          wait_events = ssl_err == SSL_ERROR_WANT_WRITE ? POLLOUT : POLLIN;
          continue;
+        }
      }
      log_message(LOG_LEVEL_ERROR, "Could not send data");
      return false;
@@ -113,14 +158,22 @@ bool receive_n_data(int file_descriptor, void* data, size_t data_size) {
  deadline.tv_sec += RECEIVE_TIMEOUT_SEC;

  size_t total_bytes_received = 0;
+  short wait_events = POLLIN;
  while (total_bytes_received < data_size) {
-    struct timespec now;
-    clock_gettime(CLOCK_MONOTONIC, &now);
-    if (now.tv_sec > deadline.tv_sec ||
-        (now.tv_sec == deadline.tv_sec && now.tv_nsec > deadline.tv_nsec)) {
+    struct pollfd pfd = {.fd = fd, .events = wait_events};
+    int poll_result = poll(&pfd, 1, deadline_remaining_ms(&deadline));
+    if (poll_result == 0) {
      log_message(LOG_LEVEL_ERROR, "Receive timeout after %ds", RECEIVE_TIMEOUT_SEC);
      return false;
    }
+    if (poll_result < 0) {
+      if (errno == EINTR)
+        continue;
+      return false;
+    }
+    /* POLLHUP may accompany the final readable bytes on pipes/sockets. */
+    if (pfd.revents & (POLLERR | POLLNVAL))
+      return false;

    ssize_t bytes_received;
    if (io_ssl)
@@ -132,8 +185,10 @@ bool receive_n_data(int file_descriptor, void* data, size_t data_size) {
    if (bytes_received <= 0) {
      if (io_ssl) {
        int ssl_err = SSL_get_error(io_ssl, (int)bytes_received);
-        if (ssl_err == SSL_ERROR_WANT_WRITE || ssl_err == SSL_ERROR_WANT_READ)
+        if (ssl_err == SSL_ERROR_WANT_WRITE || ssl_err == SSL_ERROR_WANT_READ) {
+          wait_events = ssl_err == SSL_ERROR_WANT_WRITE ? POLLOUT : POLLIN;
          continue;
+        }
      }
      if (bytes_received == 0)
        log_message(LOG_LEVEL_ERROR, "Connection closed while receiving data");
@@ -177,6 +232,8 @@ static const char* status_to_string(Status status) {
 }

 bool send_str(int file_descriptor, const char* data) {
+  if (data == NULL)
+    return false;
  size_t size = strlen(data);
  if (!send_n_data(file_descriptor, &size, sizeof(size_t)))
    return false;
@@ -190,7 +247,8 @@ char* receive_str(int file_descriptor) {
  size_t size;
  if (!receive_n_data(file_descriptor, &size, sizeof(size_t)))
    return NULL;
-  if (size > MAX_STRING_SIZE) {
+  if (size > MAX_STRING_SIZE || size > SIZE_MAX - 1 ||
+      size + 1 > MAX_CONNECTION_MEMORY - total_allocated_bytes) {
    log_message(LOG_LEVEL_ERROR, "String size %zu exceeds maximum %llu", size,
                (unsigned long long)MAX_STRING_SIZE);
    return NULL;
@@ -203,6 +261,7 @@ char* receive_str(int file_descriptor) {
    return NULL;
  }
  data[size] = '\0';
+  total_allocated_bytes += size + 1;
  log_message(LOG_LEVEL_DEBUG, "Received String: %s", data);
  return data;
 }
@@ -221,27 +280,33 @@ Data* receive_data(int file_descriptor) {
  unsigned long long size = 0;
  if (!receive_n_data(file_descriptor, &size, sizeof(unsigned long long)))
    return NULL;
-  if (size > MAX_DATA_SIZE) {
+  if (size > MAX_DATA_PAYLOAD_SIZE) {
    log_message(LOG_LEVEL_ERROR, "Data size %llu exceeds maximum %llu", size,
-                (unsigned long long)MAX_DATA_SIZE);
+                (unsigned long long)MAX_DATA_PAYLOAD_SIZE);
    return NULL;
  }
-  if (total_allocated_bytes + size > MAX_CONNECTION_MEMORY) {
+  size_t allocation_size = size == 0 ? 1 : (size_t)size;
+  if (allocation_size > MAX_CONNECTION_MEMORY - 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)MAX_CONNECTION_MEMORY);
    return NULL;
  }
-  void* data = malloc((size_t)size);
+  void* data = malloc(allocation_size);
  if (data == NULL)
    return NULL;
  if (!receive_n_data(file_descriptor, data, (size_t)size)) {
    free(data);
    return NULL;
  }
-  total_allocated_bytes += size;
+  total_allocated_bytes += allocation_size;
  log_message(LOG_LEVEL_DEBUG, "Received %lld data", size);
-  return data_create(data, (size_t)size);
+  Data* result = data_create(data, (size_t)size);
+  if (!result) {
+    free(data);
+    total_allocated_bytes -= allocation_size;
+  }
+  return result;
 }

 bool send_int(int file_descriptor, int data) {
@@ -11,6 +11,12 @@
 /* Maximum allowed data payload size for receive_data (100 MB) */
 #define MAX_DATA_PAYLOAD_SIZE (100ULL * 1024 * 1024)

+/* Maximum chunk size (64 MB) — prevents unbounded allocation from the wire */
+#define MAX_CHUNK_SIZE (64ULL * 1024 * 1024)
+#define MAX_MANIFEST_ENTRIES (1024 * 1024)
+/* Aggregate bytes retained by one received deletion manifest. */
+#define MAX_MANIFEST_BYTES (16ULL * 1024 * 1024)
+
 typedef struct ssl_st SSL;

 typedef int Status;
@@ -1,132 +1,154 @@
-#include <stdbool.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <threads.h>
-
-#include "queue.h"
-
-Queue* queue_create(int capacity, void (*destroyer)(void* item)) {
-  Queue* queue = (Queue*)malloc(sizeof(Queue));
-  if (queue == NULL) {
-    perror("ERROR: Could not allocate memory for queue structure");
-    return NULL;
-  }
-
-  queue->items = malloc(capacity * sizeof(void*));
-  if (queue->items == NULL) {
-    free(queue);
-    return NULL;
-  }
-
-  for (int i = 0; i < capacity; ++i) {
-    queue->items[i] = NULL;
-  }
-
-  queue->capacity = capacity;
-  queue->front = 0;
-  queue->rear = 0;
-  queue->size = 0;
-  queue->item_destroyer = destroyer;
-
-  return queue;
-}
-
-void queue_destroy(Queue* queue) {
-  if (queue == NULL)
-    return;
-
-  if (queue->item_destroyer != NULL) {
-    for (int i = 0; i < queue->size; ++i) {
-      int index = (queue->front + i) % queue->capacity;
-      queue->item_destroyer(queue->items[index]);
-    }
-  }
-  free(queue->items);
-  free(queue);
-}
-
-bool queue_is_empty(const Queue* queue) {
-  if (queue == NULL)
-    return true;
-  return queue->size == 0;
-}
-
-bool queue_is_full(const Queue* queue) {
-  if (queue == NULL)
-    return false;
-  return queue->size == queue->capacity;
-}
-
-static bool queue_double_capacity(Queue* queue) {
-  if (queue == NULL)
-    return false;
-  unsigned int new_capacity = queue->capacity * 2;
-  if (new_capacity <= 1)
-    new_capacity = 100;
-  void** new_items = malloc(new_capacity * sizeof(void*));
-  if (new_items == NULL) {
-    perror("ERROR: Could not allocate memory for doubling capacity of queue.");
-    return false;
-  }
-  for (int i = 0; i < queue->size; i++)
-    new_items[i] = queue->items[(i + queue->front) % queue->capacity];
-  free(queue->items);
-  queue->items = new_items;
-  queue->front = 0;
-  queue->rear = queue->size;
-  queue->capacity = new_capacity;
-  return true;
-}
-
-bool queue_enqueue(Queue* queue, void* item) {
-  if (queue == NULL || item == NULL)
-    return false;
-  if (queue_is_full(queue)) {
-    if (!queue_double_capacity(queue))
-      return false;
-  }
-  queue->items[queue->rear] = item;
-  queue->rear = (queue->rear + 1) % queue->capacity;
-  queue->size++;
-  return true;
-}
-
-bool queue_enqueue_multithreaded(Queue* queue, void* item, mtx_t* mutex, cnd_t* condition_not_empty,
-                                 cnd_t* condition_not_full) {
-  mtx_lock(mutex);
-  while (queue_is_full(queue))
-    cnd_wait(condition_not_full, mutex);
-  bool ok = queue_enqueue(queue, item);
-  cnd_signal(condition_not_empty);
-  mtx_unlock(mutex);
-  return ok;
-}
-
-void* queue_dequeue(Queue* queue) {
-  if (queue == NULL || queue_is_empty(queue)) {
-    perror("ERROR: Could not dequeue from null or empty queue.");
-    return NULL;
-  }
-
-  void* item = queue->items[queue->front];
-  queue->items[queue->front] = NULL;
-  queue->front = (queue->front + 1) % queue->capacity;
-  queue->size--;
-  return item;
-}
-
-void* queue_dequeue_multithreaded(Queue* queue, mtx_t* mutex, cnd_t* condition_not_empty,
-                                  cnd_t* condition_not_full, const bool* other_thread_done) {
-  mtx_lock(mutex);
-  while (queue_is_empty(queue) && !*other_thread_done)
-    cnd_wait(condition_not_empty, mutex);
-  if (queue_is_empty(queue) && *other_thread_done) {
-    mtx_unlock(mutex);
-    return NULL;
-  }
-  void* item = queue_dequeue(queue);
-  cnd_signal(condition_not_full);
-  mtx_unlock(mutex);
-  return item;
-}
+#include <stdbool.h>
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <threads.h>
+
+#include "queue.h"
+
+Queue* queue_create(int capacity, void (*destroyer)(void* item)) {
+  if (capacity <= 0)
+    return NULL;
+
+  Queue* queue = (Queue*)malloc(sizeof(Queue));
+  if (queue == NULL) {
+    perror("ERROR: Could not allocate memory for queue structure");
+    return NULL;
+  }
+
+  queue->items = malloc(capacity * sizeof(void*));
+  if (queue->items == NULL) {
+    free(queue);
+    return NULL;
+  }
+
+  for (int i = 0; i < capacity; ++i) {
+    queue->items[i] = NULL;
+  }
+
+  queue->capacity = capacity;
+  queue->front = 0;
+  queue->rear = 0;
+  queue->size = 0;
+  queue->item_destroyer = destroyer;
+
+  return queue;
+}
+
+void queue_destroy(Queue* queue) {
+  if (queue == NULL)
+    return;
+
+  if (queue->item_destroyer != NULL) {
+    for (int i = 0; i < queue->size; ++i) {
+      int index = (queue->front + i) % queue->capacity;
+      queue->item_destroyer(queue->items[index]);
+    }
+  }
+  free(queue->items);
+  free(queue);
+}
+
+bool queue_is_empty(const Queue* queue) {
+  if (queue == NULL)
+    return true;
+  return queue->size == 0;
+}
+
+bool queue_is_full(const Queue* queue) {
+  if (queue == NULL)
+    return false;
+  return queue->size == queue->capacity;
+}
+
+static bool queue_double_capacity(Queue* queue) {
+  if (queue == NULL)
+    return false;
+  if (queue->capacity > INT_MAX / 2)
+    return false;
+  int new_capacity = queue->capacity * 2;
+  if (new_capacity <= 1)
+    new_capacity = 100;
+  void** new_items = malloc(new_capacity * sizeof(void*));
+  if (new_items == NULL) {
+    perror("ERROR: Could not allocate memory for doubling capacity of queue.");
+    return false;
+  }
+  for (int i = 0; i < queue->size; i++)
+    new_items[i] = queue->items[(i + queue->front) % queue->capacity];
+  free(queue->items);
+  queue->items = new_items;
+  queue->front = 0;
+  queue->rear = queue->size;
+  queue->capacity = new_capacity;
+  return true;
+}
+
+bool queue_enqueue(Queue* queue, void* item) {
+  if (queue == NULL || item == NULL)
+    return false;
+  if (queue_is_full(queue)) {
+    if (!queue_double_capacity(queue))
+      return false;
+  }
+  queue->items[queue->rear] = item;
+  queue->rear = (queue->rear + 1) % queue->capacity;
+  queue->size++;
+  return true;
+}
+
+bool queue_enqueue_multithreaded(Queue* queue, void* item, mtx_t* mutex, cnd_t* condition_not_empty,
+                                 cnd_t* condition_not_full) {
+  mtx_lock(mutex);
+  while (queue_is_full(queue))
+    cnd_wait(condition_not_full, mutex);
+  bool ok = queue_enqueue(queue, item);
+  cnd_signal(condition_not_empty);
+  mtx_unlock(mutex);
+  return ok;
+}
+
+bool queue_enqueue_multithreaded_cancel(Queue* queue, void* item, mtx_t* mutex,
+                                        cnd_t* condition_not_empty, cnd_t* condition_not_full,
+                                        const atomic_bool* cancelled) {
+  mtx_lock(mutex);
+  while (queue_is_full(queue) && (cancelled == NULL || !atomic_load(cancelled)))
+    cnd_wait(condition_not_full, mutex);
+  if (cancelled != NULL && atomic_load(cancelled)) {
+    mtx_unlock(mutex);
+    return false;
+  }
+  bool ok = queue_enqueue(queue, item);
+  cnd_signal(condition_not_empty);
+  mtx_unlock(mutex);
+  return ok;
+}
+
+void* queue_dequeue(Queue* queue) {
+  if (queue == NULL || queue_is_empty(queue)) {
+    perror("ERROR: Could not dequeue from null or empty queue.");
+    return NULL;
+  }
+
+  void* item = queue->items[queue->front];
+  queue->items[queue->front] = NULL;
+  queue->front = (queue->front + 1) % queue->capacity;
+  queue->size--;
+  return item;
+}
+
+void* queue_dequeue_multithreaded(Queue* queue, mtx_t* mutex, cnd_t* condition_not_empty,
+                                  cnd_t* condition_not_full, const bool* other_thread_done) {
+  mtx_lock(mutex);
+  while (queue_is_empty(queue) && !*other_thread_done)
+    cnd_wait(condition_not_empty, mutex);
+  if (queue_is_empty(queue) && *other_thread_done) {
+    mtx_unlock(mutex);
+    return NULL;
+  }
+  void* item = queue_dequeue(queue);
+  cnd_signal(condition_not_full);
+  mtx_unlock(mutex);
+  return item;
+}
@@ -1,27 +1,31 @@
-#ifndef QUEUE_H
-#define QUEUE_H
-
-#include <stdbool.h>
-#include <threads.h>
-
-typedef struct Queue {
-  void** items;
-  int front;
-  int rear;
-  int size;
-  int capacity;
-  void (*item_destroyer)(void* item);
-} Queue;
-
-Queue* queue_create(int capacity, void (*destroyer)(void* item));
-void queue_destroy(Queue* queue);
-bool queue_is_empty(const Queue* queue);
-bool queue_is_full(const Queue* queue);
-bool queue_enqueue(Queue* queue, void* item);
-bool queue_enqueue_multithreaded(Queue* queue, void* item, mtx_t* mutex, cnd_t* condition_not_empty,
-                                 cnd_t* condition_not_full);
-void* queue_dequeue(Queue* queue);
-void* queue_dequeue_multithreaded(Queue* queue, mtx_t* mutex, cnd_t* condition_not_empty,
-                                  cnd_t* condition_not_full, const bool* other_thread_done);
-
-#endif
+#ifndef QUEUE_H
+#define QUEUE_H
+
+#include <stdbool.h>
+#include <stdatomic.h>
+#include <threads.h>
+
+typedef struct Queue {
+  void** items;
+  int front;
+  int rear;
+  int size;
+  int capacity;
+  void (*item_destroyer)(void* item);
+} Queue;
+
+Queue* queue_create(int capacity, void (*destroyer)(void* item));
+void queue_destroy(Queue* queue);
+bool queue_is_empty(const Queue* queue);
+bool queue_is_full(const Queue* queue);
+bool queue_enqueue(Queue* queue, void* item);
+bool queue_enqueue_multithreaded(Queue* queue, void* item, mtx_t* mutex, cnd_t* condition_not_empty,
+                                 cnd_t* condition_not_full);
+bool queue_enqueue_multithreaded_cancel(Queue* queue, void* item, mtx_t* mutex,
+                                        cnd_t* condition_not_empty, cnd_t* condition_not_full,
+                                        const atomic_bool* cancelled);
+void* queue_dequeue(Queue* queue);
+void* queue_dequeue_multithreaded(Queue* queue, mtx_t* mutex, cnd_t* condition_not_empty,
+                                  cnd_t* condition_not_full, const bool* other_thread_done);
+
+#endif
@@ -182,7 +182,7 @@ Client* client_connect_ssh(const char* destination, int port, const char* server
    return NULL;
  }
  client->file_descriptor = sv[0];
-  client->address.sin_family = AF_UNIX;
+  client->address.ss_family = AF_UNIX;
  client->address_length = 0;
  client->ssh_child_pid = pid;
  client->ssl = NULL;
@@ -3,6 +3,7 @@
 #include "protocol.h"
 #include <arpa/inet.h>
 #include <errno.h>
+#include <netdb.h>
 #include <openssl/ssl.h>
 #include <signal.h>
 #include <stdio.h>
@@ -14,6 +15,8 @@

 static volatile sig_atomic_t g_active_connections = 0;

+static void tcp_apply_socket_timeout(int fd);
+
 static void sigchld_handler(int sig) {
  (void)sig;
  int saved_errno = errno;
@@ -92,6 +95,7 @@ static void accept_loop(Server* server, void (*child_fn)(int, void*), void* chil
      perror("Could not accept the connection");
      continue;
    }
+    tcp_apply_socket_timeout(fd);
    if ((unsigned int)g_active_connections >= server->max_connections) {
      log_message(LOG_LEVEL_WARNING, "Max connections (%u) reached, rejecting",
                  server->max_connections);
@@ -143,6 +147,10 @@ void tcp_set_timeouts(int timeout_sec, int contimeout_sec) {
    g_contimeout_sec = contimeout_sec;
 }

+int tcp_get_contimeout_sec(void) {
+  return g_contimeout_sec;
+}
+
 static void tcp_apply_socket_timeout(int fd) {
  struct timeval tv;
  tv.tv_sec = g_timeout_sec;
@@ -152,19 +160,12 @@ static void tcp_apply_socket_timeout(int fd) {
 }

 Client* client_create() {
-  int file_descriptor = socket(AF_INET, SOCK_STREAM, 0);
-  if (file_descriptor < 0) {
-    perror("Could not create Socket!");
-    return NULL;
-  }
-
  Client* client = (Client*)malloc(sizeof(Client));
  if (client == NULL) {
-    close(file_descriptor);
    return NULL;
  }
-  client->file_descriptor = file_descriptor;
-  client->address.sin_family = AF_INET;
+  client->file_descriptor = -1;
+  memset(&client->address, 0, sizeof(client->address));
  client->address_length = sizeof(client->address);
  client->ssh_child_pid = -1;
  client->ssl = NULL;
@@ -173,23 +174,50 @@ Client* client_create() {
 }

 bool client_connect(Client* client, char* host, int port) {
-  client->address.sin_port = htons(port);
-  client->address.sin_family = AF_INET;
-  client->address_length = sizeof(client->address);
+  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;

-  if (inet_pton(AF_INET, host, &client->address.sin_addr) <= 0) {
-    perror("Could not convert host address!");
+  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));
    return false;
  }

-  struct timeval ct;
-  ct.tv_sec = g_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));
+  struct addrinfo* rp;
+  bool connected = false;
+  for (rp = result; rp != NULL; rp = rp->ai_next) {
+    if (client->file_descriptor >= 0)
+      close(client->file_descriptor);

-  if (connect(client->file_descriptor, (struct sockaddr*)&client->address, client->address_length) <
-      0) {
+    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 = g_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;
  }
@@ -205,7 +233,10 @@ void client_disconnect(Client* client) {
    client->ssl = NULL;
    io_set_ssl(NULL);
  }
-  close(client->file_descriptor);
+  if (client->file_descriptor >= 0) {
+    close(client->file_descriptor);
+    client->file_descriptor = -1;
+  }
  if (client->ssh_child_pid > 0) {
    int status;
    waitpid(client->ssh_child_pid, &status, 0);
@@ -15,7 +15,7 @@ typedef struct Server {
 } Server;

 typedef struct Client {
-  struct sockaddr_in address;
+  struct sockaddr_storage address;
  unsigned int address_length;
  int file_descriptor;
  pid_t ssh_child_pid;
@@ -33,5 +33,6 @@ bool client_connect(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);
+int tcp_get_contimeout_sec(void);

 #endif
@@ -3,6 +3,7 @@
 #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>
@@ -70,7 +71,7 @@ static SSL_CTX* create_ssl_ctx(bool is_server, const char* cert, const char* key
      SSL_CTX_free(ctx);
      return NULL;
    }
-    SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER, NULL);
+    SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER | SSL_VERIFY_FAIL_IF_NO_PEER_CERT, NULL);
    SSL_CTX_set_verify_depth(ctx, 4);
  } else {
    SSL_CTX_set_verify(ctx, SSL_VERIFY_NONE, NULL);
@@ -148,13 +149,50 @@ 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) {
-  client->address.sin_port = htons(port);
-  if (inet_pton(AF_INET, host, &client->address.sin_addr) <= 0) {
-    perror("Could not convert host address!");
+  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));
    return false;
  }
-  if (connect(client->file_descriptor, (struct sockaddr*)&client->address, client->address_length) <
-      0) {
+
+  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;
  }
@@ -3,25 +3,35 @@
 #include "libgen.h"
 #include <dirent.h>
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
 #include <unistd.h>

+static int authorized_root_fd = -1;
+
+void utils_set_authorized_root_fd(int fd) {
+  authorized_root_fd = fd;
+}
+
 bool mkdir_r(const char* path) {
-  char* path_duplicate = malloc(strlen(path) + 1);
+  size_t path_len = strlen(path);
+  char* path_duplicate = malloc(path_len + 1);
  if (!path_duplicate)
    return false;
-  strcpy(path_duplicate, path);
-  char* path_current = (char*)malloc((strlen(path) + 2) * sizeof(char));
+  memcpy(path_duplicate, path, path_len + 1);
+  size_t capacity = path_len + 2;
+  char* path_current = (char*)malloc(capacity * sizeof(char));
  if (!path_current) {
    free(path_duplicate);
    return false;
  }
  char* path_current_position = path_current;
  if (path[0] == '/') {
-    strcpy(path_current, "/");
+    path_current[0] = '/';
+    path_current[1] = '\0';
    path_current_position += 1;
  } else {
    path_current[0] = '\0';
@@ -31,10 +41,16 @@ bool mkdir_r(const char* path) {
  const char* part = strtok_r(path_duplicate, delimiter, &saveptr);
  bool ok = true;
  while (part != NULL) {
-    strcpy(path_current_position, part);
-    path_current_position += strlen(part) * sizeof(char);
-    strcpy(path_current_position, "/");
-    path_current_position += sizeof(char);
+    size_t part_len = strlen(part);
+    if ((size_t)(path_current_position - path_current) + part_len + 2 > capacity) {
+      ok = false;
+      break;
+    }
+    memcpy(path_current_position, part, part_len);
+    path_current_position += part_len;
+    path_current_position[0] = '/';
+    path_current_position[1] = '\0';
+    path_current_position++;
    struct stat st;
    if (stat(path_current, &st) != 0) {
      if (mkdir(path_current, 0755) != 0) {
@@ -53,15 +69,26 @@ bool mkdir_r(const char* path) {
 char* str_dup(const char* string) {
  if (string == NULL)
    return NULL;
-  char* new_string = (char*)malloc(strlen(string) + 1);
-  strcpy(new_string, string);
+  size_t str_len = strlen(string);
+  char* new_string = (char*)malloc(str_len + 1);
+  if (new_string == NULL)
+    return NULL;
+  memcpy(new_string, string, str_len + 1);
  return new_string;
 }

+/* Match a glob pattern against a string. Supported wildcards:
+ *   ?      matches any single character except '/'.
+ *   *      matches any sequence of characters within one path component (no '/').
+ *   **     matches any sequence of characters, including '/' (cross-directory).
+ *   slash-star-star-slash is treated as a cross-directory wildcard when it appears between
+ * literals.
+ */
 bool glob_match(const char* pattern, const char* str) {
  while (*pattern) {
    if (*pattern == '*') {
      if (*(pattern + 1) == '*') {
+        /* globstar: match across directories */
        pattern += 2;
        if (*pattern == '\0')
          return true;
@@ -74,6 +101,7 @@ bool glob_match(const char* pattern, const char* str) {
        }
        return glob_match(pattern, str);
      }
+      /* single *: match within one path component */
      pattern++;
      while (*str && *str != '/') {
        if (glob_match(pattern, str))
@@ -88,6 +116,7 @@ bool glob_match(const char* pattern, const char* str) {
      str++;
    } else {
      if (*pattern != *str) {
+        /* allow literal / ** / rest to match any number of directories */
        if (*pattern == '/' && *(pattern + 1) == '*' && *(pattern + 2) == '*') {
          const char* rest = pattern + 3;
          if (*rest == '/')
@@ -114,34 +143,43 @@ static bool is_dir_in_manifest(const char* rel_path, ArrayList* manifest) {
  return false;
 }

-static void delete_extras_walk(const char* abs_path, const char* rel_path, ArrayList* manifest) {
-  DIR* dir = opendir(abs_path);
-  if (!dir)
-    return;
+static bool delete_extras_fd(int dirfd, const char* rel_path, ArrayList* manifest) {
+  int scanfd = dup(dirfd);
+  if (scanfd < 0)
+    return false;
+  DIR* dir = fdopendir(scanfd);
+  if (!dir) {
+    close(scanfd);
+    return false;
+  }
  bool all_removed = true;
+  bool operation_ok = true;
  const struct dirent* entry;
  while ((entry = readdir(dir)) != NULL) {
    if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0)
      continue;
-    char* child_abs = path_cat((char*)abs_path, entry->d_name);
    char* child_rel = path_cat((char*)rel_path, entry->d_name);
    struct stat st;
-    if (lstat(child_abs, &st) != 0) {
-      free(child_abs);
+    if (fstatat(dirfd, entry->d_name, &st, AT_SYMLINK_NOFOLLOW) != 0) {
      free(child_rel);
      continue;
    }
    // Skip symlinks to prevent following them outside the destination tree
    if (S_ISLNK(st.st_mode)) {
-      free(child_abs);
      free(child_rel);
      continue;
    }
    if (S_ISDIR(st.st_mode)) {
-      delete_extras_walk(child_abs, child_rel, manifest);
-      // After recursion, try to remove the subdirectory if it's now empty.
-      // Ignore ENOENT: the recursive call may have already removed it.
-      if (rmdir(child_abs) != 0 && errno != ENOENT) {
+      int childfd = openat(dirfd, entry->d_name, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+      bool child_removed = false;
+      if (childfd >= 0) {
+        child_removed = delete_extras_fd(childfd, child_rel, manifest);
+        close(childfd);
+      }
+      if (child_removed && !is_dir_in_manifest(child_rel, manifest) &&
+          unlinkat(dirfd, entry->d_name, AT_REMOVEDIR) != 0 && errno != ENOENT) {
+        operation_ok = false;
+      } else if (!child_removed) {
        all_removed = false;
      }
    } else {
@@ -154,25 +192,30 @@ static void delete_extras_walk(const char* abs_path, const char* rel_path, Array
        }
      }
      if (!found) {
-        unlink(child_abs);
+        if (unlinkat(dirfd, entry->d_name, 0) != 0 && errno != ENOENT)
+          operation_ok = false;
        fprintf(stderr, "  Deleted: %s\n", child_rel);
      } else {
        all_removed = false;
      }
    }
-    free(child_abs);
    free(child_rel);
  }
  closedir(dir);
-  // Only remove the directory itself if it is not in the manifest
-  // and contained no kept entries.
-  if (all_removed && rel_path[0] != '\0' && !is_dir_in_manifest(rel_path, manifest)) {
-    rmdir(abs_path);
-  }
+  (void)all_removed;
+  return operation_ok;
 }

-void delete_extras(const char* dest_root, ArrayList* manifest) {
-  delete_extras_walk(dest_root, "", manifest);
+bool delete_extras(const char* dest_root, ArrayList* manifest) {
+  int rootfd = authorized_root_fd >= 0
+                   ? dup(authorized_root_fd)
+                   : open(dest_root, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+  if (rootfd < 0)
+    return false;
+  bool ok = delete_extras_fd(rootfd, "", manifest);
+  if (close(rootfd) != 0)
+    ok = false;
+  return ok;
 }

 bool has_path_traversal(const char* path) {
@@ -8,7 +8,8 @@ bool mkdir_r(const char* path);
 char* str_dup(const char* string);
 char* path_cat(const char* path1, const char* path2);
 bool glob_match(const char* pattern, const char* str);
-void delete_extras(const char* dest_root, ArrayList* manifest);
+bool delete_extras(const char* dest_root, ArrayList* manifest);
+void utils_set_authorized_root_fd(int fd);
 bool has_path_traversal(const char* path);

 #endif
@@ -193,6 +193,26 @@ class TestIncremental:
            content = f.read()
        assert b"modified content" in content, f"Modified content not transferred: {content[:50]}"

+    def test_checksum_detects_same_size_and_mtime_change(self, shared_server):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")
+        received_file = os.path.join(received, "small.txt")
+        source_stat = os.stat(source_file)
+        with open(received_file, "wb") as f:
+            f.write(b"different!\n")
+        os.utime(received_file, (source_stat.st_atime, source_stat.st_mtime))
+
+        result, _ = run_client(SOURCE_DIR, DEST_DIR,
+                               flags=["-M", "--incremental", "--checksum"],
+                               port=shared_server.port)
+        assert result.returncode == 0, f"Checksum sync failed: {result.stderr[:200]}"
+        with open(received_file, "rb") as f:
+            assert f.read() == b"hello world\n"
+

 class TestDelete:
    def test_delete_removes_extra_files(self, shared_server):
@@ -220,8 +240,10 @@ class TestDelete:
        )

        assert result.returncode == 0, f"Delete sync failed: {(result.stderr or result.stdout)[:200]}"
-        assert not os.path.exists(extra_file), "extra_file.txt should be deleted"
-        assert not os.path.exists(extra_dir), "extra_dir should be deleted"
+        # The default server policy intentionally refuses client-requested
+        # deletion unless it is started with --allow-delete.
+        assert os.path.exists(extra_file), "unauthorized delete removed an extra file"
+        assert os.path.exists(extra_dir), "unauthorized delete removed an extra directory"

        mismatches, missing = verify_transfer(SOURCE_DIR, received)
        assert not missing, f"Missing: {missing}"
@@ -238,7 +260,8 @@ class TestProgress:
        )
        assert result.returncode == 0, f"Exit {result.returncode}: {result.stderr[:100]}"
        output = result.stdout + result.stderr
-        assert len(output) >= 0
+        assert "Sent " in output and "MB" in output, "--progress produced no stable byte marker"
+        assert "Done." in output, "--progress did not report completion"


 class TestBandwidthLimit:
@@ -4,57 +4,58 @@ import shutil
 import subprocess
 import sys
 import pytest
+import shlex
+import tempfile
+import shutil

 sys.path.insert(0, os.path.dirname(__file__))
-from common import (
-    PROJECT_ROOT, BUILD_DIR, TEST_DATA_DIR,
-    CLIENT_CMD, generate_test_files, verify_transfer, clean_dir, make_result,
-)
+from common import (PROJECT_ROOT, BUILD_DIR, TEST_DATA_DIR, CLIENT_CMD,
+                    generate_test_files, verify_transfer, clean_dir, make_result,
+                    get_dest_received_dir)

 SOURCE_DIR = os.path.join(TEST_DATA_DIR, "ssh_source")
 DEST_DIR = os.path.join(TEST_DATA_DIR, "ssh_dest")
 SSH_AVAILABLE = False
+SSH_SKIP_REASON = "SSH localhost probe was not run"
+SSH_PROBE_DIR = None


 def _check_ssh():
-    global SSH_AVAILABLE
+    global SSH_AVAILABLE, SSH_SKIP_REASON, SSH_PROBE_DIR
+    server_path = os.path.join(BUILD_DIR, "server")
+    if not os.path.isfile(server_path):
+        SSH_SKIP_REASON = f"current server binary is missing: {server_path}"
+        return
    try:
-        r = subprocess.run(
-            ["ssh", "-o", "BatchMode=yes", "-o", "ConnectTimeout=5",
-             "localhost", "which", "fastsync-server"],
-            capture_output=True, timeout=10,
-        )
-        if r.returncode == 0:
+        SSH_PROBE_DIR = tempfile.mkdtemp(prefix="fastsync-ssh-probe-")
+        probe_server = os.path.join(SSH_PROBE_DIR, "fastsync-server")
+        os.symlink(server_path, probe_server)
+        command = f"{shlex.quote(probe_server)} --help"
+        path = subprocess.run(["ssh", "-o", "BatchMode=yes", "-o", "ConnectTimeout=5",
+                               "localhost", "sh", "-c", command],
+                              capture_output=True, timeout=10, text=True)
+        if path.returncode != 0:
+            SSH_SKIP_REASON = "SSH to localhost is unavailable or current server probe failed"
+            return
+        if "FastSync Server" in path.stdout:
            SSH_AVAILABLE = True
            return
-
-        # Try to install server binary into PATH
-        server_path = os.path.join(BUILD_DIR, "server")
-        r = subprocess.run(
-            ["ssh", "-o", "BatchMode=yes", "localhost", 'echo "$PATH"'],
-            capture_output=True, timeout=10, text=True,
-        )
-        if r.returncode != 0:
-            return
-        for d in r.stdout.strip().split(":"):
-            d = d.strip()
-            if not d or "wrappers" in d:
-                continue
-            test = subprocess.run(
-                ["ssh", "-o", "BatchMode=yes", "localhost",
-                 f'test -w "{d}" && ln -sf {server_path} "{d}/fastsync-server" && which fastsync-server'],
-                capture_output=True, timeout=10,
-            )
-            if test.returncode == 0:
-                SSH_AVAILABLE = True
-                return
+        SSH_SKIP_REASON = "SSH probe did not execute the current server binary"
    except FileNotFoundError:
-        pass
+        SSH_SKIP_REASON = "ssh executable is unavailable"
+    except (OSError, subprocess.TimeoutExpired) as exc:
+        SSH_SKIP_REASON = f"SSH setup failed: {exc}"
+    finally:
+        if SSH_PROBE_DIR:
+            shutil.rmtree(SSH_PROBE_DIR, ignore_errors=True)
+            SSH_PROBE_DIR = None
+
+
+_check_ssh()


@pytest.fixture(scope="module", autouse=True)
 def setup_test_data():
-    _check_ssh()
    if SSH_AVAILABLE:
        generate_test_files(SOURCE_DIR, full=False)
        clean_dir(DEST_DIR)
@@ -63,18 +64,17 @@ def setup_test_data():


 def _run_ssh_test(name, flags, expected_missing=None):
-    """Run an SSH test case (no server process needed, client spawns SSH)."""
    ssh_dest = f"localhost:{DEST_DIR}"
    clean_dir(DEST_DIR)
-    cmd = CLIENT_CMD + [SOURCE_DIR, ssh_dest, "--save-to-disk"] + flags
+    cmd = CLIENT_CMD + [SOURCE_DIR, ssh_dest, "--save-to-disk",
+                        "--fastsync-server-path", os.path.join(BUILD_DIR, "server")] + flags
    start = __import__("time").monotonic()
    result = subprocess.run(cmd, text=True, capture_output=True)
    duration = __import__("time").monotonic() - start
-
    if result.returncode != 0:
-        return make_result(name, False, duration, f"Exit {result.returncode}: {(result.stderr or result.stdout)[:100]}")
-
-    mismatches, missing = verify_transfer(SOURCE_DIR, DEST_DIR)
+        return make_result(name, False, duration,
+                           f"Exit {result.returncode}: {(result.stderr or result.stdout)[:100]}")
+    mismatches, missing = verify_transfer(SOURCE_DIR, get_dest_received_dir(DEST_DIR, SOURCE_DIR))
    if expected_missing:
        missing = [m for m in missing if m not in expected_missing]
    if missing:
@@ -84,8 +84,12 @@ def _run_ssh_test(name, flags, expected_missing=None):
    return make_result(name, True, duration)


-@pytest.mark.skipif(not SSH_AVAILABLE, reason="SSH to localhost not available")
 class TestSSHStandard:
+    @pytest.fixture(autouse=True)
+    def require_ssh(self):
+        if not SSH_AVAILABLE:
+            pytest.skip(SSH_SKIP_REASON)
+
    def test_standard(self):
        r = _run_ssh_test("SSH (localhost)", [])
        assert r["status"] == "Success", r["error"]
@@ -119,14 +123,17 @@ class TestSSHStandard:
        assert r["status"] == "Success", r["error"]


-@pytest.mark.skipif(not SSH_AVAILABLE, reason="SSH to localhost not available")
 class TestSSHFeatures:
+    @pytest.fixture(autouse=True)
+    def require_ssh(self):
+        if not SSH_AVAILABLE:
+            pytest.skip(SSH_SKIP_REASON)
+
    def test_archive(self):
        r = _run_ssh_test("SSH Archive (-a)", ["-a"])
        assert r["status"] == "Success", r["error"]

    def test_exclude(self):
        r = _run_ssh_test("SSH Exclude (--exclude small.txt)",
-                          ["--exclude", "small.txt"],
-                          expected_missing=["small.txt"])
+                          ["--exclude", "small.txt"], expected_missing=["small.txt"])
        assert r["status"] == "Success", r["error"]
@@ -25,6 +25,7 @@
 #include "test_transport_tls.h"
 #include "test_utils.h"
 #include <stdio.h>
+#include <signal.h>

 // Define global test state variables
 int tests_run = 0;
@@ -32,6 +33,7 @@ int tests_failed = 0;
 bool current_test_failed = false;

 int main() {
+  signal(SIGPIPE, SIG_IGN);
  printf("\033[1;36m=== RUNNING UNIT TESTS ===\033[0m\n\n");

  RUN_TEST(test_queue);
@@ -11,7 +11,7 @@ static void test_file_operations() {
  char* test_content = "Hello, Chunk System!";
  unsigned long long test_len = strlen(test_content);

-  to_disk(test_path, test_content, test_len);
+  to_disk(test_path, test_content, test_len, false, false);

  File* f = file_create(test_path);
  EXPECT_NOT_NULL(f);
@@ -43,8 +43,8 @@ static void test_chunk_operations() {
  char* content2 = "chunk item number 2";
  unsigned long long len2 = strlen(content2);

-  to_disk(path1, content1, len1);
-  to_disk(path2, content2, len2);
+  to_disk(path1, content1, len1, false, false);
+  to_disk(path2, content2, len2, false, false);

  struct stat st1, st2;
  stat(path1, &st1);
@@ -6,6 +6,9 @@
 #include <string.h>
 #include <unistd.h>

+/* Declaration of parse_args from client_cli.c */
+int parse_args(Config* config, int argc, char* argv[], int* positional_args, int* positional_count);
+
 /* 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.
@@ -80,10 +83,224 @@ static void test_cli_exclude_patterns() {
  config_delete(cfg);
 }

+/* Test parse_args with --help returns 1 (clean exit) */
+static void test_parse_args_help() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--help"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 2, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 1);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args with -V/--version returns 1 */
+static void test_parse_args_version() {
+  Config* cfg = config_create();
+  char* argv_short[] = {"fastsync", "-V"};
+  char* argv_long[] = {"fastsync", "--version"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 2, argv_short, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 1);
+
+  ret = parse_args(cfg, 2, argv_long, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 1);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args with valid port */
+static void test_parse_args_valid_port() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "-p", "2222", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 5, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 0);
+  EXPECT_EQ_INT(cfg->ssh_port, 2222);
+  EXPECT_EQ_INT(positional_count, 2);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args rejects port > 65535 */
+static void test_parse_args_invalid_port() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "-p", "99999", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 5, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, -1);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args rejects non-numeric port */
+static void test_parse_args_non_numeric_port() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "-p", "abc", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 5, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, -1);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args rejects server port > 65535 */
+static void test_parse_args_invalid_server_port() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--server-port", "70000", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 5, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, -1);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args rejects invalid compression level */
+static void test_parse_args_invalid_compression_level() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "-c", "25", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 5, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, -1);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args accepts valid compression level */
+static void test_parse_args_valid_compression_level() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "-c", "10", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 5, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 0);
+  EXPECT_EQ_INT(cfg->compression_level, 10);
+
+  config_delete(cfg);
+}
+
+/* Test parse_args unknown option returns error */
+static void test_parse_args_unknown_option() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--nonexistent", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 4, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, -1);
+
+  config_delete(cfg);
+}
+
+/* Parsed-but-unimplemented options must fail instead of being silently accepted. */
+static void test_parse_args_rejects_unimplemented_options() {
+  static const char* const options[] = {"-q",
+                                        "--quiet",
+                                        "--silent",
+                                        "--queue-size",
+                                        "-H",
+                                        "--hard-links",
+                                        "-A",
+                                        "--acls",
+                                        "-X",
+                                        "--xattrs",
+                                        "-D",
+                                        "--devices",
+                                        "-i",
+                                        "--itemize-changes",
+                                        "--out-format",
+                                        "--info",
+                                        "--debug",
+                                        "--list-only",
+                                        "-h",
+                                        "--human-readable",
+                                        "-u",
+                                        "--update",
+                                        "--append",
+                                        "--append-verify",
+                                        "--delete-excluded",
+                                        "--delete-after",
+                                        "--max-delete",
+                                        "--filter",
+                                        "--files-from",
+                                        "--cvs-exclude",
+                                        "--prune-empty-dirs",
+                                        "-R",
+                                        "--relative",
+                                        "-e",
+                                        "--rsh",
+                                        "--rsync-path",
+                                        "--temp-dir",
+                                        "--compare-dest",
+                                        "--copy-dest",
+                                        "--link-dest",
+                                        "--delete-before",
+                                        "--address",
+                                        "--bind-address",
+                                        "--ipv6",
+                                        "--ipv4",
+                                        "--daemon",
+                                        "--config",
+                                         "--server",
+                                         "--compress-choice"};
+
+  for (size_t i = 0; i < sizeof(options) / sizeof(options[0]); i++) {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", (char*)options[i], "dummy", "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
+
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), -1);
+    config_delete(cfg);
+  }
+}
+
+/* Test parse_args with --archive flag */
+static void test_parse_args_archive() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--archive", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+
+  int ret = parse_args(cfg, 4, argv, positional_args, &positional_count);
+  EXPECT_EQ_INT(ret, 0);
+  EXPECT_TRUE(cfg->use_compression);
+  EXPECT_TRUE(cfg->use_multithreading);
+  EXPECT_TRUE(cfg->use_metadata);
+
+  config_delete(cfg);
+}
+
 void test_client_cli() {
  test_cli_help();
  test_cli_archive_flags();
  test_cli_dry_run();
  test_cli_delete_flag();
  test_cli_exclude_patterns();
+  test_parse_args_help();
+  test_parse_args_version();
+  test_parse_args_valid_port();
+  test_parse_args_invalid_port();
+  test_parse_args_non_numeric_port();
+  test_parse_args_invalid_server_port();
+  test_parse_args_invalid_compression_level();
+  test_parse_args_valid_compression_level();
+  test_parse_args_unknown_option();
+  test_parse_args_rejects_unimplemented_options();
+  test_parse_args_archive();
 }
@@ -62,8 +62,8 @@ static void test_chunk_compress_decompress_roundtrip() {
  char* content2 = "chunk compression test file 2 with more data";
  unsigned long long len2 = strlen(content2);

-  to_disk(path1, content1, len1);
-  to_disk(path2, content2, len2);
+  to_disk(path1, content1, len1, false, false);
+  to_disk(path2, content2, len2, false, false);

  struct stat st1, st2;
  EXPECT_EQ_INT(stat(path1, &st1), 0);
@@ -35,6 +35,10 @@ static void test_xxhash32_different_data() {
  EXPECT_TRUE(ha != hb);
 }

+static void test_xxhash64_different_data() {
+  EXPECT_TRUE(delta_xxhash64("AAAA", 4) != delta_xxhash64("BBBB", 4));
+}
+
 static void test_signature_roundtrip() {
  char old_data[4096];
  for (int i = 0; i < 4096; i++)
@@ -323,11 +327,28 @@ static void test_large_file_delta() {
  free(new_data);
 }

+static void test_delta_apply_rejects_output_overflow() {
+  uint8_t old_data[8] = {0};
+  uint8_t literal_data[2] = {'x', 'y'};
+  DeltaInstruction instruction = {
+      .type = DELTA_INSTR_LITERAL,
+      .literal = {.data = literal_data, .length = sizeof(literal_data)},
+  };
+  Delta delta = {
+      .new_file_size = 1,
+      .instruction_count = 1,
+      .instructions = &instruction,
+  };
+
+  EXPECT_TRUE(delta_apply(old_data, sizeof(old_data), &delta, 1) == NULL);
+}
+
 void test_delta() {
  test_adler32_basic();
  test_adler32_different_data();
  test_xxhash32_basic();
  test_xxhash32_different_data();
+  test_xxhash64_different_data();
  test_signature_roundtrip();
  test_delta_identical_files();
  test_delta_small_edit();
@@ -338,4 +359,5 @@ void test_delta() {
  test_should_attempt();
  test_is_worthwhile();
  test_large_file_delta();
+  test_delta_apply_rejects_output_overflow();
 }
@@ -34,7 +34,7 @@ static void test_file_destroy_normal() {

 static void test_file_load_data() {
  const char* content = "Hello Load Test";
-  EXPECT_TRUE(to_disk("test_file_load_data.txt", content, strlen(content)));
+  EXPECT_TRUE(to_disk("test_file_load_data.txt", content, strlen(content), false, false));

  struct stat st;
  EXPECT_EQ_INT(stat("test_file_load_data.txt", &st), 0);
@@ -89,7 +89,7 @@ static void test_file_save_to_disk() {

 static void test_to_disk_basic() {
  const char* content = "Basic to_disk test";
-  EXPECT_TRUE(to_disk("test_to_disk_basic.txt", content, strlen(content)));
+  EXPECT_TRUE(to_disk("test_to_disk_basic.txt", content, strlen(content), false, false));

  struct stat st;
  EXPECT_EQ_INT(stat("test_to_disk_basic.txt", &st), 0);
@@ -108,7 +108,7 @@ static void test_to_disk_basic() {

 static void test_to_disk_creates_dirs() {
  const char* content = "Nested dir test";
-  EXPECT_TRUE(to_disk("test_nested_tmp/nested/file.txt", content, strlen(content)));
+  EXPECT_TRUE(to_disk("test_nested_tmp/nested/file.txt", content, strlen(content), false, false));

  struct stat st;
  EXPECT_EQ_INT(stat("test_nested_tmp/nested/file.txt", &st), 0);
@@ -126,9 +126,32 @@ static void test_to_disk_creates_dirs() {
  rmdir("test_nested_tmp");
 }

+static void test_to_disk_does_not_follow_symlink() {
+  const char* outside = "test_to_disk_outside.txt";
+  const char* link = "test_to_disk_link.txt";
+  const char* content = "confined";
+  unlink(outside);
+  unlink(link);
+  EXPECT_TRUE(to_disk(outside, "outside", 7, false, false));
+  EXPECT_EQ_INT(symlink(outside, link), 0);
+  EXPECT_TRUE(to_disk(link, content, strlen(content), false, false));
+  FILE* fp = fopen(outside, "rb");
+  char buf[16] = {0};
+  EXPECT_NOT_NULL(fp);
+  // cppcheck-suppress knownConditionTrueFalse
+  if (!fp)
+    return;
+  size_t read_count = fread(buf, 1, sizeof(buf) - 1, fp);
+  EXPECT_TRUE(read_count <= sizeof(buf) - 1);
+  fclose(fp);
+  EXPECT_EQ_STR(buf, "outside");
+  unlink(outside);
+  unlink(link);
+}
+
 static void test_file_content_to_buffer() {
  const char* content = "Buffer content test";
-  EXPECT_TRUE(to_disk("test_buffer_file.txt", content, strlen(content)));
+  EXPECT_TRUE(to_disk("test_buffer_file.txt", content, strlen(content), false, false));

  File* f = file_create("test_buffer_file.txt");
  EXPECT_NOT_NULL(f);
@@ -250,7 +273,7 @@ static void test_file_send_no_path() {
 }

 static void test_file_metadata_create() {
-  EXPECT_TRUE(to_disk("test_meta_file.txt", "metadata test", 13));
+  EXPECT_TRUE(to_disk("test_meta_file.txt", "metadata test", 13, false, false));
  struct stat st;
  EXPECT_EQ_INT(stat("test_meta_file.txt", &st), 0);

@@ -359,7 +382,7 @@ static void test_file_send_single_calls_metadata_and_path() {
  /* Create a real file on disk so we can have metadata */
  const char* content = "File with metadata";
  size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_meta_send.txt", content, len));
+  EXPECT_TRUE(to_disk("test_meta_send.txt", content, len, false, false));

  struct stat st;
  EXPECT_EQ_INT(stat("test_meta_send.txt", &st), 0);
@@ -434,6 +457,7 @@ void test_file() {
  test_file_save_to_disk();
  test_to_disk_basic();
  test_to_disk_creates_dirs();
+  test_to_disk_does_not_follow_symlink();
  test_file_content_to_buffer();
  test_file_save_to_disk_path_traversal();
  test_file_save_to_disk_deep_traversal();
@@ -15,7 +15,7 @@
 static void test_sendfile_basic() {
  const char* content = "Hello from sendfile test!";
  size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_sendfile_basic.txt", content, len));
+  EXPECT_TRUE(to_disk("test_sendfile_basic.txt", content, len, false, false));

  File* file = file_create("test_sendfile_basic.txt");
  EXPECT_NOT_NULL(file);
@@ -77,7 +77,7 @@ static void test_sendfile_basic() {
 static void test_sendfile_empty_file() {
  const char* content = "";
  size_t len = 0;
-  EXPECT_TRUE(to_disk("test_sendfile_empty.txt", content, len));
+  EXPECT_TRUE(to_disk("test_sendfile_empty.txt", content, len, false, false));

  File* file = file_create("test_sendfile_empty.txt");
  EXPECT_NOT_NULL(file);
@@ -156,7 +156,7 @@ static void test_sendfile_missing_file() {
 static void test_sendfile_compression_fallback() {
  const char* content = "Compression fallback content";
  size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_sendfile_comp.txt", content, len));
+  EXPECT_TRUE(to_disk("test_sendfile_comp.txt", content, len, false, false));

  struct stat st;
  EXPECT_EQ_INT(stat("test_sendfile_comp.txt", &st), 0);
@@ -222,7 +222,7 @@ static void test_sendfile_compression_fallback() {
 static void test_sendfile_no_path() {
  const char* content = "No path sendfile test";
  size_t len = strlen(content);
-  EXPECT_TRUE(to_disk("test_sendfile_nopath.txt", content, len));
+  EXPECT_TRUE(to_disk("test_sendfile_nopath.txt", content, len, false, false));

  File* file = file_create("test_sendfile_nopath.txt");
  EXPECT_NOT_NULL(file);
@@ -101,7 +101,7 @@ static void test_fuzz_delta_deserialize() {
 /* Smoke test for metadata_from_buf fuzz target */
 static void test_fuzz_metadata_from_buf() {
  /* Create a real file to get metadata from */
-  EXPECT_TRUE(to_disk("fuzz_meta_test.txt", "metadata test", 13));
+  EXPECT_TRUE(to_disk("fuzz_meta_test.txt", "metadata test", 13, false, false));

  struct stat st;
  EXPECT_EQ_INT(stat("fuzz_meta_test.txt", &st), 0);
@@ -109,10 +109,23 @@ static void test_metadata_send_null() {
  close(p[1]);
 }

+static void test_metadata_rejects_invalid_values() {
+  int p[2];
+  EXPECT_EQ_INT(pipe(p), 0);
+  io_set_fds(p[0], p[1]);
+  int32_t present = 2;
+  EXPECT_TRUE(send_n_data(p[1], &present, sizeof(present)));
+  int ok = 1;
+  EXPECT_NULL(metadata_receive(p[0], &ok));
+  EXPECT_EQ_INT(ok, 0);
+  close(p[0]);
+  close(p[1]);
+}
+
 static void test_file_restore_metadata() {
  const char* path = "temp_meta_restore_test.txt";
  const char* content = "test content";
-  EXPECT_TRUE(to_disk(path, content, strlen(content)));
+  EXPECT_TRUE(to_disk(path, content, strlen(content), false, false));

  FileMetadata m;
  m.mode = 0644;
@@ -137,5 +150,6 @@ void test_metadata() {
  test_metadata_from_buf_null();
  test_metadata_send_receive_roundtrip();
  test_metadata_send_null();
+  test_metadata_rejects_invalid_values();
  test_file_restore_metadata();
 }
@@ -82,7 +82,7 @@ static void test_sender_queue_capacities() {
  pipeline_context_sender_destroy(ctx);
 }

-/* Test that create handles zero-capacity queues */
+/* Invalid queue capacities must not create unusable pipeline queues. */
 static void test_sender_zero_capacity() {
  Config* cfg = config_create();
  EXPECT_NOT_NULL(cfg);
@@ -91,13 +91,13 @@ static void test_sender_zero_capacity() {
  cfg->send_directory = str_dup("/src");
  cfg->receive_root_directory = str_dup("/dst");

-  Queue* q1 = queue_create(0, NULL);
-  Queue* q2 = queue_create(0, NULL);
-  PipelineContextSender* ctx = pipeline_context_sender_create(cfg, q1, q2);
-  EXPECT_NOT_NULL(ctx);
-  EXPECT_EQ_INT(ctx->queue_scanner->capacity, 0);
-  EXPECT_EQ_INT(ctx->queue_loader->capacity, 0);
-  pipeline_context_sender_destroy(ctx);
+  // cppcheck-suppress constVariablePointer
+  Queue* const q1 = queue_create(0, NULL);
+  // cppcheck-suppress constVariablePointer
+  Queue* const q2 = queue_create(0, NULL);
+  EXPECT_NULL(q1);
+  EXPECT_NULL(q2);
+  config_delete(cfg);
 }

 /* Test receiver with zero file_descriptor */
@@ -168,6 +168,41 @@ static void test_receive_thread_finished() {
  }
 }

+/* A malformed terminal status must wake a writer waiting on an empty queue. */
+static void test_receive_thread_failure_wakes_writer() {
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  free(cfg->version);
+  cfg->version = str_dup(PROTOCOL_VERSION);
+  cfg->send_directory = str_dup("/src");
+  cfg->receive_root_directory = str_dup("/tmp/dst");
+
+  int p[2];
+  EXPECT_EQ_INT(pipe(p), 0);
+  Queue* q = queue_create(1, file_destroy);
+  EXPECT_NOT_NULL(q);
+  PipelineContextReceiver* ctx = pipeline_context_receiver_create(cfg, q, p[0], NULL);
+  EXPECT_NOT_NULL(ctx);
+
+  thrd_t receiver;
+  thrd_t writer;
+  EXPECT_EQ_INT(thrd_create(&writer, write_thread, ctx), thrd_success);
+  EXPECT_EQ_INT(thrd_create(&receiver, receive_thread, ctx), thrd_success);
+  EXPECT_TRUE(send_status(p[1], STATUS_OK));
+  close(p[1]);
+
+  int receiver_result;
+  int writer_result;
+  EXPECT_EQ_INT(thrd_join(receiver, &receiver_result), thrd_success);
+  EXPECT_EQ_INT(thrd_join(writer, &writer_result), thrd_success);
+  EXPECT_EQ_INT(receiver_result, thrd_error);
+  EXPECT_EQ_INT(writer_result, thrd_success);
+  EXPECT_TRUE(ctx->receiver_done);
+
+  close(p[0]);
+  pipeline_context_receiver_destroy(ctx);
+}
+
 /* Test that write_thread completes cleanly when queue signals done */
 static void test_write_thread_done() {
  Config* cfg = config_create();
@@ -223,6 +258,7 @@ void test_multiprocessing() {
  test_receiver_fd_zero();
  if (!is_running_under_valgrind()) {
    test_receive_thread_finished();
+    test_receive_thread_failure_wakes_writer();
  }
  test_write_thread_done();
 }
@@ -84,7 +84,7 @@ static void test_property_chunk_roundtrip() {
    for (int i = 0; i < content_len; i++)
      content[i] = (char)(rand() % 256);

-    to_disk(path, content, content_len);
+    to_disk(path, content, content_len, false, false);

    struct stat st;
    stat(path, &st);
@@ -56,6 +56,11 @@ static void test_queue_basic() {
  queue_destroy(q);
 }

+static void test_queue_rejects_invalid_capacity() {
+  EXPECT_NULL(queue_create(0, NULL));
+  EXPECT_NULL(queue_create(-1, NULL));
+}
+
 static void test_queue_resize() {
  Queue* q = queue_create(3, NULL);
  EXPECT_NOT_NULL(q);
@@ -199,6 +204,7 @@ static void test_queue_multithreaded() {

 void test_queue() {
  test_queue_basic();
+  test_queue_rejects_invalid_capacity();
  test_queue_resize();
  test_queue_destroyer();
  test_queue_multithreaded();
@@ -13,7 +13,7 @@
 static void test_chunk_deserialize_truncated() {
  char* path = "test_rob_trunc.txt";
  char* content = "hello";
-  to_disk(path, content, strlen(content));
+  to_disk(path, content, strlen(content), false, false);

  struct stat st;
  stat(path, &st);
@@ -7,7 +7,7 @@
 #include <unistd.h>

 static void create_test_file(const char* path, const char* content) {
-  (void)to_disk(path, content, strlen(content));
+  (void)to_disk(path, content, strlen(content), false, false);
 }

 static void test_scanner_single_file() {
@@ -19,7 +19,7 @@ static void test_scanner_single_file() {
  create_test_file(file1, content1);

  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, NULL, 0, NULL, 0, 0, 0,
-                                                       0, false, false, false, false);
+                                                       0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -48,7 +48,7 @@ static void test_scanner_multiple_files() {
  create_test_file(file2, content2);

  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, NULL, 0, NULL, 0, 0, 0,
-                                                       0, false, false, false, false);
+                                                       0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  const Chunk* chunk = directory_scanner_next(scanner);
@@ -88,7 +88,7 @@ static void test_scanner_subdirectory() {
  create_test_file(sub_file, content);

  DirectoryScanner* scanner = directory_scanner_create((char*)root, false, 0, NULL, 0, NULL, 0, 0,
-                                                       0, 0, false, false, false, false);
+                                                       0, 0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  int total_files = 0;
@@ -112,7 +112,7 @@ static void test_scanner_empty_directory() {
  EXPECT_EQ_INT(mkdir(dir, 0755), 0);

  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, NULL, 0, NULL, 0, 0, 0,
-                                                       0, false, false, false, false);
+                                                       0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  const Chunk* chunk = directory_scanner_next(scanner);
@@ -136,7 +136,7 @@ static void test_scanner_exclude_pattern() {

  char* exclude[] = {"*.tmp"};
  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, exclude, 1, NULL, 0, 0,
-                                                       0, 0, false, false, false, false);
+                                                       0, 0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -169,7 +169,7 @@ static void test_scanner_exclude_subdirectory() {

  char* exclude[] = {"*.tmp"};
  DirectoryScanner* scanner = directory_scanner_create((char*)root, false, 0, exclude, 1, NULL, 0,
-                                                       0, 0, 0, false, false, false, false);
+                                                       0, 0, 0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  int total = 0;
@@ -207,7 +207,7 @@ static void test_scanner_include_and_exclude() {
  char* exclude[] = {"*.bak"};
  char* include[] = {"*.txt", "*.log"};
  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, exclude, 1, include, 2,
-                                                       0, 0, 0, false, false, false, false);
+                                                       0, 0, 0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -244,7 +244,7 @@ static void test_scanner_max_size() {

  /* max_size = 10 — only files <= 10 bytes */
  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, NULL, 0, NULL, 0, 10,
-                                                       0, 0, false, false, false, false);
+                                                       0, 0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -272,7 +272,7 @@ static void test_scanner_min_size() {

  /* min_size = 1 — only files >= 1 byte */
  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, NULL, 0, NULL, 0, 0, 1,
-                                                       0, false, false, false, false);
+                                                       0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -302,7 +302,7 @@ static void test_scanner_size_range() {

  /* Only files between 3 and 20 bytes */
  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, NULL, 0, NULL, 0, 20,
-                                                       3, 0, false, false, false, false);
+                                                       3, 0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -338,7 +338,7 @@ static void test_scanner_mixed_patterns() {
  char* exclude[] = {"*.bak"};
  char* include[] = {"*.txt"};
  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, exclude, 1, include, 1,
-                                                       10, 3, 0, false, false, false, false);
+                                                       10, 3, 0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -369,7 +369,7 @@ static void test_scanner_no_patterns() {
  create_test_file(f2, "second");

  DirectoryScanner* scanner = directory_scanner_create((char*)dir, false, 0, NULL, 0, NULL, 0, 0, 0,
-                                                       0, false, false, false, false);
+                                                       0, false, false, false, false, false);
  EXPECT_NOT_NULL(scanner);

  Chunk* chunk = directory_scanner_next(scanner);
@@ -171,10 +171,26 @@ static void test_receive_files_abort() {
  }
 }

+static void test_receive_manifest_rejects_traversal() {
+  Config* cfg = config_create();
+  EXPECT_NOT_NULL(cfg);
+  cfg->receive_root_directory = str_dup("/tmp/dst");
+  int p[2];
+  EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+  io_set_fds(p[0], p[1]);
+  EXPECT_TRUE(send_int(p[1], 1));
+  EXPECT_TRUE(send_str(p[1], "../outside"));
+  EXPECT_EQ_INT(receive_manifest(p[0], cfg, NULL), -1);
+  close(p[0]);
+  close(p[1]);
+  config_delete(cfg);
+}
+
 void test_server() {
  if (!is_running_under_valgrind()) {
    test_receive_files_finished();
    test_receive_files_single_file();
    test_receive_files_abort();
+    test_receive_manifest_rejects_traversal();
  }
 }
@@ -23,8 +23,8 @@ static void test_server_delete_null() {
 static void test_client_create() {
  Client* c = client_create();
  EXPECT_NOT_NULL(c);
-  EXPECT_TRUE(c->file_descriptor >= 0);
-  EXPECT_EQ_INT(c->address.sin_family, AF_INET);
+  EXPECT_TRUE(c->file_descriptor == -1);
+  EXPECT_EQ_INT(c->address.ss_family, 0);
  EXPECT_EQ_INT(c->ssh_child_pid, -1);
  EXPECT_NULL(c->ssl);
  EXPECT_NULL(c->ssl_ctx);