FastSync/RSYNC_COMPAT.md

Rsync Feature Compatibility

This document maps rsync's full feature set to FastSync's current implementation status.

Summary

Status	Count	Description
✅ Implemented	67	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	71	Flag not recognized or no behavior
Total	147

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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	✅ Implemented	Applies the well-known rsync default exclude set as exclude rules during scanning (RCS SCCS CVS CVS.adm RCSLOG cvslog.* tags TAGS .make.state .nse_depinfo ~ # .#* ,* _* * *.old *.bak *.BAK *.orig .rej .del- *.a *.olb *.o *.obj *.so *.exe *.Z *.elc *.ln core .svn/ .git/ .hg/ .bzr/); .git/-style repo dirs are pruned without descending

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	✅ Implemented	Prints rsync-style >f+++++++++ lines to stdout only for files actually sent (also under -m); unchanged files print nothing, matching single--i behavior
--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	✅ Implemented	Per-transfer template on stdout; tokens %f %n %l %b %M %% (%b is the source length, always == %l; post-compression/delta wire bytes are not counted); unknown escapes preserved
--log-file=FILE	Log to file	✅ Implemented	log_file config field
--log-file-format=FMT	Log format	✅ Implemented	Requires --log-file; writes one template line per transferred file using the same token set as --out-format (including %b == source length)
--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	✅ Implemented	ls -l-style listing of files that would be transferred; scans the source only, contacts no server, writes nothing; also works with -n

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	✅ Implemented	Long option only: rsync's short -f conflicts with FastSync sendfile (see FastSync-specific list), so -f is not reassigned. Supported subset: +/- include/exclude, implicit-exclude patterns, include/exclude word forms, a leading / anchor (to the transfer root, or to a .rsync-filter file's directory), and a trailing / for dir-only rules; first match wins with a default of include inside the filter layer. Filters are an independent layer from --exclude/--include (an entry must pass both). Rejected with a clear error (no silent no-ops): merge/dir-merge/hide/show/protect/risk/clear words, rules that begin with :/./! (merge/dir-merge/list-clear shorthands), and include/exclude modifiers other than / (! C s r p x)
--files-from=FILE	Read source file list from file	✅ Implemented	Entries are paths relative to the source root (leading ./ stripped, ../absolute entries rejected at parse time, blank lines ignored; NUL-delimited with -0). A listed regular file is transferred; a listed directory transfers its whole subtree (FastSync recursion is always on, unlike rsync's non-recursive default). Non-listed paths and their subtrees are pruned by the scanner. A listed entry that does not exist under the source (and an empty list) is a hard error reported before any transfer; listing . (whole tree) and empty listed directories are fine. Scalability note: file_list_affects is O(list size) per scanned entry, so a very large --files-from list against a huge tree is quadratic; lists are typically small enough that this is acceptable, but it is the documented bound. The delete manifest still derives from what was actually sent, so --delete stays consistent with the subset
-0, --from0	Delimit *-from files with NULs	✅ Implemented	--files-from entries become NUL-delimited; the flag may appear before or after --files-from on the command line. NUL mode preserves entry bytes exactly (trailing CR/LF are part of the name; only newline mode trims them)
--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	✅ Implemented	Reads one filter rule per line from each directory's .rsync-filter file during traversal and applies it to that directory's subtree; the current directory's rules are evaluated before its ancestors', so deeper files override shallower ones and per-directory files override the command-line --filter/-C base by default (matching rsync's first-match-wins precedence); .rsync-filter files are never transferred. The rsync -FF behavior (also .cvsignore) is out of scope; unsupported rule types inside the file abort with a clear error

4. Directory Options

Flag	Rsync Description	FastSync Status	Notes
-r, --recursive	Recurse into directories	✅ Implemented	Default behavior
-R, --relative	Use relative path names	✅ Implemented	Meaningful together with --files-from (FastSync's default full-tree scan always mirrors the full source argument path below the destination root, so -R does not change it). With -R + --files-from each listed entry is transmitted under its bare relative destination path: an entry sub/x.txt lands at <dest>/sub/x.txt (its leading components preserved) instead of under the <dest>/<full source path> mirror. Only the path sent on the wire changes; the client still reads the absolute source path, and the delete manifest derives from the sent (relative) paths so --delete and --remove-source-files stay consistent in both layouts. Works single-threaded and under -m (including chunk serialization)
--no-implied-dirs	Don't send implied dirs with -R	✅ Implemented	Client-side, meaningful only with -R + --files-from. rsync would normally create the ancestor directories implied by a listed file so it can be written; with --no-implied-dirs a listed file whose parent directory is not itself (or via an ancestor) explicitly listed cannot be placed, and FastSync fails the whole run up front with a clear error (--no-implied-dirs: cannot place file '...': parent directory '...' is not explicitly listed). Listing the directory (or an ancestor of it, or the whole tree .) permits the file. In every other mode the option has no effect. FastSync has no per-entry skip channel, so the rsync "omit the file" case is surfaced as a hard pre-transfer error
-d, --dirs, --old-dirs, --old-d	Transfer dirs without recursing	✅ Implemented	-d <dir> transmits an explicit directory entry for the source-root directory, so the destination mirror is created empty and nothing is descended into. With --files-from exactly the listed items are transferred: a listed directory is created empty (no descent) and a listed file is transferred with its content; the dest layout follows the same -R rules as plain files. A new wire frame (STATUS_MKDIR) carries each directory entry (path only); the receiver creates it with the same confined mkdir-parent semantics as regular writes, in single-threaded and -m receivers (chunk serialization carries a per-entry type marker). Directory entries appear in the delete manifest so --delete prunes correctly. FastSync divergences: directory mtimes/modes are not transmitted, filter/--exclude rules are not re-applied to the listed dirs mode (there is no descent during which they would apply), and -d never creates the intermediate directories between the destination root and a listed file beyond the usual on-demand parent creation. Under --delay-updates only regular files are staged: directory entries are created immediately, so a delayed run that fails part way can leave the already-created empty directories behind (matching rsync, which also creates directories as it processes the file list and only delays regular-file data)
--mkpath	Create missing path components	✅ Implemented	Wire option (client → server). At connection start the server creates the client's destination root directory (and any missing leading components below its own authorized root) when --mkpath is set, failing the connection cleanly if it cannot. Without --mkpath a destination root that does not exist yet is rejected up front (rsync semantics), so the flag is the only way to transfer into a not-yet-created destination directory. Creation is confined by the same secure mkdir walk as file writes (O_NOFOLLOW, no ..)

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	✅ Implemented	Successfully received files are staged under a private 0700 .fastsync-stage dir inside the receive root and atomically renamed into their final destinations only after the whole transfer (manifest/delete handling included) succeeds, just before the success/outcome frame is sent. The delete walker deliberately skips the staging dir at the receive root, so --delete removes genuine extras but never the staged files (deletion runs before publication; rsync's delete-after ordering is not implemented). --existing/--ignore-existing/--update decide against the final destination path at stage time; --backup moves the old file aside at publication. Incompatible with --inplace and with --backup-dir=.fastsync-stage (the internal staging name is reserved; both are rejected). The staging dir name is fixed, so two simultaneous delayed transfers to the same destination root are serialized with an exclusive advisory lock held for the whole transfer: the second session fails cleanly instead of corrupting the first. Aborting or failing before publication installs nothing and removes the staging tree; a crash between stage and publish leaves staged leftovers that the next delayed run wipes at start (process death releases the lock). A stage→publish failure aborts the transfer (best-effort cleanup of the not-yet-published staged files; already-published files are not rolled back). Works in single-threaded and -m modes
-T, --temp-dir=DIR	Create temporary files in DIR	✅ Implemented	--temp-dir only; -T stays FastSync's --timeout alias. Scratch dir is resolved under the receive root; temp copies use a unique name there and are atomically renamed into place. If the scratch dir and destination are on different filesystems the atomic rename fails with EXDEV and the file save fails, which aborts the whole transfer (FastSync has no per-file skip/resume on a save error; rsync's non-atomic copy fallback is deliberately not used). --inplace and --partial-dir writes bypass the scratch dir

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.

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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

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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