archive: cache parent directory fd during tar extraction

[thaJeztah]

• stacked on archive: harden tar extraction against path traversal #45
• replaces archive: cache parent directory fd during tar extraction #26

Each root.*(path) call re-walks every path component via doInRoot, costing 2D syscalls per call at path depth D. A typical file entry triggers ~5 root.* calls, so at depth 4 that is ~40 syscalls in path traversal alone.

This PR adds a dirCache that keeps one parent-directory fd open between entries. Consecutive entries in the same directory reuse the cached fd for all *at(2) operations, reducing path-traversal cost to ~5 syscalls per entry regardless of depth.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 62.50000% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.91%. Comparing base (216738e) to head (8b247ec).
⚠️ Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
dircache_unix.go	69.23%	10 Missing and 10 partials ⚠️
archive.go	42.10%	3 Missing and 8 partials ⚠️
diff.go	50.00%	3 Missing ⚠️
archive_unix.go	75.00%	1 Missing ⚠️
time_nonwindows.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #49      +/-   ##
==========================================
- Coverage   65.81%   63.91%   -1.91%     
==========================================
  Files          42       35       -7     
  Lines        2039     2203     +164     
==========================================
+ Hits         1342     1408      +66     
- Misses        519      597      +78     
- Partials      178      198      +20     

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[Copilot]

Pull request overview

This PR introduces a dirCache abstraction to reduce repeated path-walk overhead during tar extraction by caching the most recently used parent-directory handle and performing *at(2) operations relative to it (Unix), while keeping Windows behavior as a thin delegation to os.Root.

Changes:

• Add dirCache implementations for Unix (openat/*at) and Windows (delegation).
• Thread dirCache through Unpack, UnpackLayer, and createTarFile to reuse parent-directory context across consecutive entries.
• Adjust chmod/chtime/lchown/lchtime call sites to go through the cache, and update tests to use the new createTarFile signature.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
dircache_unix.go	Adds Unix dirCache using cached parent dir fd + *at(2) syscalls for extraction operations.
dircache_windows.go	Adds Windows dirCache delegating to os.Root (no fd caching).
archive.go	Updates createTarFile and Unpack to use dirCache for extraction operations.
diff.go	Updates UnpackLayer to use dirCache, including AUFS temp-root handling.
archive_unix.go	Updates handleLChmod to route chmod through dirCache.
archive_windows.go	Updates handleLChmod signature for the new dirCache plumbing.
archive_test.go	Adjusts tests to construct/close dirCache when calling createTarFile.
time_nonwindows.go	Marks lchtimes helper as unused after moving call sites to dirCache.
time_windows.go	Removes the Windows stub lchtimes helper.

Comments suppressed due to low confidence (2)

dircache_unix.go:147

• UtimesNanoAt is called with flags=0 (follows symlinks). If a path is swapped to a symlink concurrently, this can apply timestamps to an unintended target. Using AT_SYMLINK_NOFOLLOW avoids following symlinks for non-symlink entries while keeping the syscall count low.

	if err := unix.UtimesNanoAt(int(d.Fd()), filepath.Base(path), utimes[0:], 0); err != nil {

dircache_unix.go:132

• chmod currently uses fchmodat with flags=0, which follows symlinks; if the path is swapped to a symlink concurrently, this can chmod an unintended target (potentially outside the extraction root). Use AT_SYMLINK_NOFOLLOW with a fallback (like the previous chmodNoSymlink implementation) to avoid following symlinks.

	if err := unix.Fchmodat(int(d.Fd()), filepath.Base(path), fileModeToPerm(mode), 0); err != nil {

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[thaJeztah]

Last commit probably won't work on Windows (yet); working on that