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test: cover -x cross-device skip without root; harden OneFileSystem tests 
Add a rootless unit test that reaches the actual st_dev skip branch in both
the sequential and parallel (-m) scanners: a symlink nested under the scan
root points at a directory on /dev/shm (a different device than the build
fs) and, under --copy-links semantics, -x must drop that subtree while a
plain scan includes it. Skips only when no cross-device target exists.
Integration OneFileSystem test now cleans both dest dirs up front and
reports a busy test mountpoint instead of ignoring the umount result.
RSYNC_COMPAT.md notes that cross-filesystem mount-point subdirectories are
dropped entirely (rsync parity).
2026-09-06 12:49:26 +02:00

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# Rsync Feature Compatibility
This document maps rsync's full feature set to FastSync's current implementation status.
## Summary
| Status | Count | Description |
|--------|-------|-------------|
| ✅ Implemented | 51 | Feature works end-to-end |
| 🔀 Alt Arg | 3 | Functionality exists but under different flag/semantics |
| ⚠️ Partial | 5 | Flag parsed/stored but behavior incomplete |
| 🔄 Compatibility No-op | 1 | Flag is accepted for CLI compatibility but has no effect |
| ❌ Not Implemented | 87 | Flag not recognized or no behavior |
| **Total** | **147** | |
---
## 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 | ✅ Implemented | Suppresses client output while preserving errors |
| `--help` | Show help | ✅ Implemented | Prints usage and exits; `-h` is not accepted |
| `-V`, `--version` | Print version | ✅ Implemented | |
| `--info=FLAGS` | Fine-grained info verbosity | ✅ Implemented | Supports `copy`, `misc`, `skip`, `stats`, `all`, and `none`; explicit flags override `--verbose`, and `none` suppresses info output; unsupported names are rejected |
| `--debug=FLAGS` | Fine-grained debug verbosity | ✅ Implemented | `io`, `proto`, `pack`, and `util` are supported; `--debug=help` lists flags; other rsync categories are rejected |
| `--stderr=MODE` | Change stderr output mode | ⚠️ Partial | `errors` (default) and `all` are supported; `client` is rejected because FastSync has no rsync message channel |
| `--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 | ✅ Implemented | Formats transfer byte sizes using binary units |
| `-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 | ⚠️ Partial | Parses and enables progress, but interrupted files are not retained for resumable transfers |
| `--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`, `-8` | Leave high-bit chars unescaped | ✅ Implemented | Applies to displayed paths and protocol debug output |
| `--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 | ✅ Implemented | With `--incremental`, ignores mtime |
| `-@`, `--modify-window=NUM` | Mod-time comparison accuracy | ✅ Implemented | Whole-second tolerance with nanosecond-aware comparisons |
| `--existing` | Skip creating new files on receiver | ✅ Implemented | Existing destination files continue through normal update handling |
| `--ignore-existing` | Skip updating existing files | ❌ Not Implemented | |
| `--remove-source-files` | Sender removes regular files after confirmed transfer | ✅ Implemented | |
| `-x`, `--one-file-system` | Do not cross filesystem boundaries | ✅ Implemented | Sender scanner captures the root device and skips descending into mount-point crossings (`st_dev` differs); cross-filesystem mount-point subdirectories are dropped entirely, matching rsync |
| `-F` | Add the default `.rsync-filter` rules | ❌ 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`, `--old-dirs`, `--old-d` | Transfer dirs without recursing | ❌ Not Implemented | The aliases are recognized and rejected explicitly; they depend on the unimplemented `--dirs` behavior |
| `--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 | |
| `-T`, `--temp-dir=DIR` | Create temporary files in DIR | ❌ Not Implemented | `-T` is FastSync's timeout alias |
## 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 |
| `--del`, `--delete-during` | Delete during transfer | ❌ Not Implemented | Both flags are recognized but rejected; delete timing is 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 | ✅ Implemented | Preserves executable permission bits (implies metadata preservation) |
| `--chmod=CHMOD` | Affect file permissions | ✅ Implemented | Supports numeric and symbolic `ugo` `rwx` changes; retains receiver safety masking |
| `-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 | |
| `--open-noatime` | Avoid changing access time when opening files | ❌ Not Implemented | |
| `--numeric-ids` | Do not map uid/gid by name | ❌ Not Implemented | |
| `--usermap=STRING` | Map usernames | ❌ Not Implemented | |
| `--groupmap=STRING` | Map group names | ❌ Not Implemented | |
| `--chown=USER:GROUP` | Map owner and group | ❌ Not Implemented | |
| `--copy-as=USER[:GROUP]` | Perform the copy as another user/group | ❌ 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 | ⚠️ Partial | Flag is accepted, but full hole preservation is not implemented |
| `--preallocate` | Allocate dest files before writing | ❌ Not Implemented | |
## 11. Checksum & Comparison
| Flag | Rsync Description | FastSync Status | Notes |
|------|-------------------|-----------------|-------|
| `--checksum` | Skip based on checksum | ✅ Implemented | With `--incremental`, compares xxHash64 content checksums; `-c` remains compression |
| `--checksum-choice=STR` | Choose checksum algorithm | ❌ Not Implemented | xxHash used internally |
| `--compare-dest=DIR` | Compare dest files relative to DIR | ❌ Not Implemented | Removed because it had no effect |
| `--copy-dest=DIR` | Include copies of unchanged files | ❌ Not Implemented | Removed because it had no effect |
| `--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`, `--zc=STR` | Choose compression algorithm | ✅ Implemented | FastSync supports `zstd` and `none` |
| `--compress-level=NUM`, `--zl=NUM` | Set compression level | ✅ Implemented | 1-22, default 5 |
| `--compress-threads=NUM` | Set compression threads | ❌ Not Implemented | |
| `--skip-compress=LIST` | Skip compress for suffixes | ✅ Implemented | Comma-separated, case-insensitive suffix list; empty list skips none; incompatible with FastSync chunk serialization (`-s`) |
## 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 |
| `--remote-option=OPT`, `-M` | Send an option only to the remote side | ❌ Not Implemented | `-M` is FastSync's metadata-preservation flag |
## 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` |
| `--max-alloc=SIZE` | Limit a single memory allocation | ✅ Implemented | Caps the largest single allocation; binary units, default 1G |
| `--trust-sender` | Trust remote sender's file list | ❌ Not Implemented | |
| `--old-args` | Disable modern arg protection | ✅ Implemented | SSH-only legacy mode; restores raw remote command construction and permits shell interpretation of the configured server path |
| `--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 before publication | ✅ Implemented | |
| `--protocol=NUM` | Force older protocol version | ❌ Not Implemented | |
| `--iconv=CONVERT_SPEC` | Charset conversion | ❌ Not Implemented | |
| `--checksum-seed=NUM` | Set checksum seed | ❌ Not Implemented | |
| `--secluded-args` | Use protocol to send args | 🔄 Compatibility No-op | Accepted for CLI compatibility; it does not change FastSync transport or protocol behavior. `-s` remains chunk serialization. |
| `--no-OPTION` | Turn off implied option | ✅ Supported | Supported boolean FastSync options and archive-implied options; unsafe or value-taking options are rejected. |
---
## Implementation Difficulty Plan
The estimates below cover the currently unimplemented features in this document. They assume one engineer familiar with the codebase, include implementation and focused tests, and exclude production rollout time. A feature should not be marked implemented until its behavior is tested in both local and SSH/TCP paths where applicable.
> **Note:** This plan is a superset snapshot written while several of the listed features were still outstanding. The Summary matrix above is the authoritative record of what is already shipped (for example quiet/info/debug output, `--existing`, `--remove-source-files`, `-h`, and `--size-only` are now implemented on `dev`). Treat the phases as sequencing guidance for the work that remains unimplemented.
| Effort | Typical duration | Meaning |
|--------|------------------|---------|
| XS | 0.5-1 day | CLI alias or a local formatting/validation change |
| S | 1-3 days | Isolated behavior with little or no protocol change |
| M | 3-7 days | Cross-cutting client, server, or scanner behavior |
| L | 1-3 weeks | Protocol, filesystem, privilege, or compatibility work |
| XL | 3+ weeks | New transfer mode, daemon subsystem, or broad interoperability effort |
### Phase 1: Low-Risk CLI and Local Behavior
These are the best first changes because they require limited wire-format work and can be tested with existing transfer fixtures.
| Features | Effort | Implementation plan |
|----------|--------|--------------------|
| `--quiet`, `-q`; `--human-readable`, `-h`; `--8-bit-output`, `-8`; `--stderr=MODE`; `--info=FLAGS`; `--debug=FLAGS` | S | Extend logging and output formatting without changing transferred data. |
| `--no-OPTION`; `--old-args`; `--secluded-args`, `-s` | M | Add option implication/negation and safely serialize or protect remote arguments. `-s` currently has FastSync-specific semantics and needs a compatibility decision. |
| `-P`; `--del`; `--old-dirs`, `--old-d`; `--cc`; `--zc`; `--zl` | XS | Add aliases and composed behaviors after the underlying options exist. |
| `--whole-file`, `-W`; `--ignore-times`, `-I`; `--size-only`; `--modify-window`, `-@`; `--update`, `-u` | S | Extend the existing incremental comparison decision. |
| `--existing`; `--ignore-existing`; `--remove-source-files` | S | Add scanner/receiver eligibility checks and remove successfully synchronized source files. |
| `--executability`, `-E`; `--chmod=CHMOD` | M | Apply permission transformations safely while preserving current metadata behavior. |
| `--skip-compress=LIST`; `--compress-threads=NUM` | S | Make compression selection configurable and validate the thread setting against zstd behavior. |
| `--max-alloc=SIZE`; `--fsync` | S | Reuse existing allocation limits and add an explicit durability step after file writes. |
### Phase 2: Filesystem Selection and Update Semantics
These features are moderate because they affect traversal, temporary files, manifests, or the receiver's update policy.
| Features | Effort | Implementation plan |
|----------|--------|--------------------|
| `--one-file-system`, `-x` | M | Track the source device during scanner traversal and skip mount-point crossings. |
| `--relative`, `-R`; `--no-implied-dirs`; `--dirs`, `-d`; `--mkpath` | M | Extend path-list construction and destination directory creation while preserving traversal safety. |
| `--temp-dir`, `-T` | M | Separate temporary-file placement from FastSync's timeout alias and define collision, permissions, and cleanup rules. |
| `--delay-updates` | L | Stage all successful updates and publish them at completion, including crash and cancellation cleanup. |
| `--files-from=FILE`; `--from0`, `-0`; `--filter=RULE`, `-f`; `-F`; `--cvs-exclude`, `-C` | L | Build a complete filter/parser layer and integrate it with scanner pruning, manifests, and delete behavior. `-f` conflicts with FastSync sendfile mode. |
| `--list-only`; `--itemize-changes`, `-i`; `--out-format=FORMAT`; `--log-file-format=FMT` | M | Add a structured change-event model so output modes share one source of truth. |
### Phase 3: Deletion, Comparison, and Delta Compatibility
These features require careful interaction with manifests, incremental checks, backups, and the existing delta protocol.
| Features | Effort | Implementation plan |
|----------|--------|--------------------|
| `--delete-during`; `--delete-before`; `--delete-after`; `--delete-delay`; `--del` | L | Add deletion timing to the transfer state machine and ensure failures cannot remove files unexpectedly. |
| `--delete-excluded`; `--max-delete=NUM`; `--ignore-errors`; `--force`; `--prune-empty-dirs`, `-m` | M | Extend delete walks with policy limits, error handling, empty-directory pruning, and the `-m` short-flag conflict. |
| `--ignore-missing-args`; `--delete-missing-args` | M | Distinguish missing source arguments from traversal errors and apply explicit deletion policy. |
| `--compare-dest=DIR`; `--copy-dest=DIR`; `--link-dest=DIR` | L | Add alternate basis roots and hard-link handling, including metadata and cross-filesystem failures. |
| `--fuzzy`, `-y`; `--no-fuzzy` | L | Index candidate files and select a safe similar basis without making transfer time unbounded. |
| `--append`; `--append-verify` | M | Negotiate file length and verify the retained prefix before resuming. |
| `--checksum-choice=STR`, `--cc`; `--checksum-seed=NUM` | M | Negotiate checksum algorithms/seeds and preserve compatibility with existing xxHash checks. |
### Phase 4: Metadata, Links, and Devices
These features are platform-sensitive and need Linux permission, ACL, xattr, and special-file integration tests.
| Features | Effort | Implementation plan |
|----------|--------|--------------------|
| `--numeric-ids`; `--usermap=STRING`; `--groupmap=STRING`; `--chown=USER:GROUP` | L | Define identity mapping, privilege failures, and wire representation before applying ownership. |
| `--open-noatime`; `--atimes`, `-U`; `--crtimes`, `-N`; `--omit-dir-times`, `-O`; `--omit-link-times`, `-J` | L | Extend metadata capture/apply with platform capability checks and explicit unsupported-attribute handling. |
| `--acls`, `-A`; `--xattrs`, `-X`; `--fake-super` | XL | Add portable serialization, size limits, privilege behavior, and security tests for ACL/xattr data. |
| `--hard-links`, `-H` | L | Preserve inode relationships across the file list and coordinate hard-link creation order. |
| `--munge-links`; `--copy-dirlinks`, `-k`; `--keep-dirlinks`, `-K` | L | Define symlink trust boundaries and receiver-side directory/link collision behavior. |
| `--devices`; `--specials`; `-D`; `--copy-devices`; `--write-devices` | XL | Add privileged special-file handling with strict type, path, and authorization checks. |
| `--super`; `--copy-as=USER[:GROUP]` | XL | Requires a deliberate privilege model, identity switching, and refusal paths; do not implement by blindly elevating the process. |
| `--preallocate` | S | Use platform allocation APIs before writes and fall back cleanly when unsupported. |
### Phase 5: Connectivity and Daemon Compatibility
These options affect process startup, authentication, sockets, and remote execution. They should follow the filesystem and protocol work rather than being added as parser-only flags.
| Features | Effort | Implementation plan |
|----------|--------|--------------------|
| `--rsh=COMMAND`, `-e`; `--rsync-path=PROGRAM`; `--blocking-io`; `--outbuf=N\|L\|B` | M | Generalize SSH command construction and subprocess I/O while retaining argument escaping and timeout guarantees. |
| `--address=ADDRESS`; `--ipv4`, `-4`; `--ipv6`, `-6`; `--sockopts=OPTIONS`; `--port=PORT` daemon semantics | M | Add explicit socket-family/bind configuration and validate it independently for TCP client and daemon modes. |
| `--remote-option=OPT`, `-M`; `--trust-sender` | L | Add authenticated remote-option/config negotiation and reject unsafe sender-controlled values. `-M` conflicts with FastSync metadata mode. |
| `--daemon`; `--config=FILE`; `--dparam=OVERRIDE`; `--no-detach`; `--password-file=FILE`; `--early-input=FILE`; `--no-motd` | XL | Implement a real daemon lifecycle, module configuration, authentication, privilege separation, and process management. |
### Phase 6: Batch, Encoding, and Protocol Interoperability
These are the hardest compatibility items because they require durable formats or behavior that must interoperate with rsync itself.
| Features | Effort | Implementation plan |
|----------|--------|--------------------|
| `--write-batch=FILE`; `--only-write-batch=FILE`; `--read-batch=FILE` | XL | Specify a versioned batch format, persist all required metadata, and test replay, corruption, and partial application. |
| `--protocol=NUM` | XL | Add protocol-version negotiation and compatibility branches without weakening current validation. |
| `--iconv=CONVERT_SPEC` | L | Convert filenames at the protocol boundary with invalid-sequence and normalization tests. |
| `--stop-after=MINS`; `--stop-at=TIME` | M | Add deadline propagation, interruptible I/O, and safe checkpoint/cleanup behavior. |
| `--early-input=FILE`; `--password-file=FILE` | M | Securely read startup credentials/input with permission checks and no secret disclosure in logs. |
### Recommended Delivery Order
1. Resolve short-option conflicts (`-m`, `-M`, `-T`, `-f`, `-s`) and define the compatibility contract.
2. Implement Phase 1 comparison, update, output, and alias features with unit and integration coverage.
3. Implement Phase 2 traversal/filtering and Phase 3 deletion semantics.
4. Implement metadata and link features that are safe on the supported platforms.
5. Treat daemon mode, special files, batch mode, and protocol-version compatibility as separate projects.
The existing priority list below is a feature shortlist, not an implementation schedule; this plan supersedes it for effort and sequencing.
---
## 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 | `--ignore-existing` | Low | Medium — common sync patterns |
| 5 | `--existing` | Low | Medium — common sync patterns |
| 6 | `--remove-source-files` | Low | High — common for moves/backup |
| 7 | `--delete-during` | Medium | High — performance improvement |
| 8 | `--delay-updates` | Medium | High — atomic updates |
| 9 | `--chmod` | Low | Medium — permission flexibility |
| 10 | `--executability` / `-E` | Low | Low — simple flag |
| 11 | `--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 |
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