Rsync parity: extended metadata preservation (hard links, ACLs, xattrs, devices, sparse files) #121

New Issue

2026-07-21T16:23:17+02:00

TapTap commented

2026-07-21 16:23:17 +02:00

FastSync currently preserves basic POSIX metadata (mode, uid, gid, mtime) via -M / --preserve. Rsync can preserve much more. FastSync should support:

Missing preservation flags

-H, --hard-links — preserve hard links (detect and recreate same-inode files)
-A, --acls — preserve POSIX ACLs
-X, --xattrs — preserve extended attributes
--devices — preserve device files (requires super-user)
--specials — preserve special files (named sockets, fifos, etc.)
-S, --sparse — handle sparse files efficiently (finding 19)
--preallocate — pre-allocate destination files before writing
-O, --omit-dir-times — omit directories from mtime preservation
--super / --fake-super — receiver attempts super-user activities

Current code

src/shared/metadata.c — only handles mode, uid, gid, mtime_sec, mtime_nsec
src/shared/metadata.h — struct FileMetadata has no fields for ACLs, xattrs, hard links, or device info
src/shared/file.c — no ACL/xattr restore logic

Impact

Hard links: currently each hard link is transferred as a separate file copy, wasting bandwidth and disk space
ACLs/xattrs: lost on transfer, breaking permission models on systems that rely on them (NFSv4, SELinux, NFSv4 ACLs)
Sparse files: expanded to full size on disk when transferred
Device files: silently skipped or cause errors

Implementation notes

Hard links: scanner needs to track inode/dev pairs and emit a special marker; receiver creates link instead of writing data
ACLs/xattrs: require platform-specific syscalls (acl_get_fd/acl_set_fd on Linux)
Sparse: receiver-side optimization using lseek(SEEK_HOLE) or detecting zero runs
Wire format: metadata struct needs new optional fields with backward-compatible parsing

FastSync currently preserves basic POSIX metadata (mode, uid, gid, mtime) via `-M / --preserve`. Rsync can preserve much more. FastSync should support: ## Missing preservation flags - `-H, --hard-links` — preserve hard links (detect and recreate same-inode files) - `-A, --acls` — preserve POSIX ACLs - `-X, --xattrs` — preserve extended attributes - `--devices` — preserve device files (requires super-user) - `--specials` — preserve special files (named sockets, fifos, etc.) - `-S, --sparse` — handle sparse files efficiently (finding 19) - `--preallocate` — pre-allocate destination files before writing - `-O, --omit-dir-times` — omit directories from mtime preservation - `--super` / `--fake-super` — receiver attempts super-user activities ## Current code - `src/shared/metadata.c` — only handles mode, uid, gid, mtime_sec, mtime_nsec - `src/shared/metadata.h` — struct `FileMetadata` has no fields for ACLs, xattrs, hard links, or device info - `src/shared/file.c` — no ACL/xattr restore logic ## Impact - Hard links: currently each hard link is transferred as a separate file copy, wasting bandwidth and disk space - ACLs/xattrs: lost on transfer, breaking permission models on systems that rely on them (NFSv4, SELinux, NFSv4 ACLs) - Sparse files: expanded to full size on disk when transferred - Device files: silently skipped or cause errors ## Implementation notes - Hard links: scanner needs to track inode/dev pairs and emit a special marker; receiver creates link instead of writing data - ACLs/xattrs: require platform-specific syscalls (acl_get_fd/acl_set_fd on Linux) - Sparse: receiver-side optimization using lseek(SEEK_HOLE) or detecting zero runs - Wire format: metadata struct needs new optional fields with backward-compatible parsing

TapTap referenced this issue from a commit

2026-07-29 19:24:48 +02:00

feat: add rsync parity flags - symlink, metadata, output modes, delete options, file selection, connection options (#120, #121, #122, #127, #128, #129, #130)

TapTap referenced this issue

2026-07-29 19:29:55 +02:00

Comprehensive fix: security, bugs, refactoring, tests, and rsync parity features #173

TapTap closed this issue

2026-07-30 18:17:29 +02:00

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