[lennix1337]

Problem

On Windows, cacheDirLadder() calls os::chmod(dir, 0700) on every run, even when lstat already reports an owner-only directory. The Win32 chmod re-applies a protected DACL with inheritable ACEs, and Windows propagates that DACL to every file under the cache. The cost grows with the number of files in the cache: with 38,000 files inside it, a warm run on a 1-file fixture took ~2 s.

Fix

Skip the chmod when the mode is already 0700. Ownership, directory type and mode are still checked by the existing lstat after the guard, and a permissive directory is still repaired.

Measurement

Windows, Release clang-cl 22.1.8, p50 of 3, 1-file fixture. A and B are built from the same revision; the only C++ difference is this guard.

	A (before)	B (this PR)
warm cache, 38K unrelated files outside the cache	11.02 ms	10.15 ms
warm cache, 38K files inside the cache	2,024.08 ms	10.64 ms
--no-cache, 38K files inside the cache	46.11 ms	44.77 ms

Responses were byte-identical between A and B (SHA-256).

Regression

• test/cache_ladder_windows.py (new, Windows only): repairs an open DACL, does not rewrite an owner-only DACL on a warm run, refuses a junction.
• test/cacheisolationcheck.sh gains a Windows arm: the private directory is checked through its real DACL instead of the MSYS-emulated mode, and JSON paths on stdin are converted to native paths.

On this branch: cacheisolationcheck.sh ALL PASS on Windows (including the new arm).
On origin/main (7abad95, same Windows toolchain) with only the two test files copied in: cache_ladder_windows.py fails 2 of 3 (test_permissive_cache_is_repaired_once and test_warm_private_cache_does_not_reapply_acl: the directory change time moves on the second run); the junction test passes.

Changelog fragment: changelog.d/win32-cache-dir-acl-skip.md.

Summary by CodeRabbit

• Bug Fixes
  • Windows cache directories with overly permissive access are repaired, while junction-based cache paths are refused without changing their targets.
• Performance
  • Warm runs avoid reapplying permissions to cache directories that are already private, reducing unnecessary filesystem updates.
• Tests
  • Added Windows checks for cache permissions, stable warm-run metadata, and junction handling.

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Walkthrough

Changes

The cache-directory ladder skips chmod(0700) when an existing directory already has that mode. Windows regression checks cover warm-cache stability, repair of permissive ACLs, and refusal of junction caches. The shell check validates private-directory permissions using platform-appropriate checks.

Cache-directory ACL behavior

Layer / File(s)	Summary
Skip redundant permission repair src/quality.h, changelog.d/win32-cache-dir-acl-skip.md	cacheDirLadder skips chmod(0700) when the existing directory already has mode 0700. The changelog describes the guard and its reported timing comparison.
Validate Windows cache ACL behavior test/cache_ladder_windows.py, test/cacheisolationcheck.sh	The Windows tests check warm-cache metadata stability, repair and subsequent stability for a permissive ACL, and unchanged junction targets. The shell check validates private permissions by platform and runs the Windows regression helper.

Priority: Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: joyful-ii-v-i

Merge Risk: Moderate · previous review 3f1a1

Fix the Cygwin test invocation before merging. The ACL and junction tests also need small corrections to run reliably for valid Windows configurations and paths.

Pre-merge checks: 4

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: skipping repeated chmod operations for an already-private Windows cache directory.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 3

---

• 🪄 Fix comments: Fix CodeRabbit comments on this PR

Prompt to fix review comments

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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
minimal, and validate.

Inline comments:
Review comments at @test/cache_ladder_windows.py:
- Around line 142-143: Update the `subprocess.run` call that invokes `mklink` so
`cmd.exe` treats both `self.cache` and `target` as single paths, even when they
contain shell metacharacters such as `&`; quote or escape both paths for
`cmd.exe`, or create the junction without invoking a command shell.
- Around line 86-87: Update the owner-only DACL check used by readOwnerAndMode
to handle deny ACEs separately from allow ACEs, rather than rejecting them
because fields[0] is not “A”. Preserve the owner-class trustee restriction for
allow ACEs so valid owner-only DACLs containing deny ACEs are accepted.

Review comments at @test/cacheisolationcheck.sh:
- Line 17: Update the CYGWIN* branch in the uname-based platform check to keep
it on the Windows path, and use an explicit native Windows Python for both
helper calls so the platform guard passes and DACL validation remains active.

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Reviewing files that changed from the base of the PR and between 7abad95 and 3f1a11f.

Files selected for processing: 4

• changelog.d/win32-cache-dir-acl-skip.md
• src/quality.h
• test/cache_ladder_windows.py
• test/cacheisolationcheck.sh

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[joyful-ii-V-I]

@lennix1337: 2,024.08 ms → 10.64 ms. That's 190× from one if. 🎉

Our cache had been re-notarizing 38,000 files on every warm run, just in case. You told it to calm down. Same-revision A/B, SHA-256 identical output, a test that goes red on main: no notes. Thank you!

[joyful-ii-V-I]

@lennix1337 — thank you, this is a great fix and a careful one.

What we checked and liked:

• The guard is exactly as narrow as it should be. The chmod is skipped only when the first lstat has already
  shown a directory that this user owns with mode 0700. On Windows that 0700 reading comes from readOwnerAndMode:
  a protected DACL whose allow entries name only owner-class principals. Either way, the existing post-lstat still
  re-checks directory, owner and 0700 before the cache is used. So the change trusts nothing that the old code did
  not already accept right after its own chmod. A freshly created directory still gets the chmod (st is
  zero-initialised on that path), and a permissive one is still repaired.
• POSIX behaviour is unchanged where it matters. macOS and Linux get the same accept/refuse decision and the same
  output. The only difference is that a warm run no longer touches an already-private directory, so its ctime no
  longer moves.
• The test watches the cause, not a stopwatch. Asserting that NTFS ChangeTime stays put on a warm run (and does
  move when you grant Everyone with icacls) catches the regression without a flaky timing threshold. Running the
  two test files on origin/main and showing 2 of 3 go red there is exactly the evidence we ask for.
• All three CodeRabbit points were fixed in c8f9804: deny ACEs are skipped and this user's SID is in the trustee
  set, just as readOwnerAndMode does it. The junction is created with _winapi.CreateJunction, with no cmd.exe
  involved. Under Cygwin the native-Windows-Python probe is used.
• We cannot rerun the 2,024 ms → 10.6 ms Windows measurement on our macOS machines, so it stays your number. The
  same-revision A/B with SHA-256-identical responses is the right way to present it.
• On macOS we built your head and mutated the guard to also skip chmod on a group/other-readable directory.
  cacheisolationcheck.sh went red on its existing "repairs a 0755 directory" arm, so the POSIX side of the guard is
  pinned by CI.

What is left: nothing from you. On our side we will refresh CI against current main (the branch is ~500 commits
behind; it merges cleanly) and then merge. As a follow-up of our own, we may wire test/cache_ladder_windows.py into the
Windows CI leg. Today only your local run exercises it.

Thanks again. A 190× warm-run win from one well-argued if is a lovely contribution.

[joyful-ii-V-I]

Thank you again, @lennix1337. This landed through #468: your commits are in unchanged, with current main merged on top, which CI needed after the 0.6.9 release (we don't push to contributor branches).

We added one small follow-up, 80b65e7, with you as co-author: a deny entry on a required bit, or a missing FILE_DELETE_CHILD, now reads as not 0700. It will be in the next release's notes with your name on it.