fix: learn fs accuracy from directory mtimes too by alexander-akait · Pull Request #350 · webpack/watchpack

alexander-akait · 2026-10-13T16:24:11Z

Fixes #295.

FS_ACCURACY starts at 2000ms and is lowered by ensureFsAccuracy from the mtimes the watcher sees, but only for files. A scan that finds only directories keeps the 2000ms default, so safeTime = birthtime + 2000 lands after startTime for any directory created in the 2s before it, and it is reported as changed.

The fix feeds the directory's mtime to ensureFsAccuracy at the three places setDirectory gets its value from a stat (watch event, scan, symlink to a directory). It uses mtime, not birthtime: FAT keeps creation times at 10ms but modification times at 2s, so learning from birthtime could shrink the margin below what file mtimes have and miss real changes. The safety margin itself is unchanged, unlike #299.

The issue's script (subdirectory only, Linux):

Wait before `watch({ startTime: Date.now() })`	Before	After
0ms	`change subdir`	`change subdir` (inside the ~10ms margin, as for files)
500ms	`change subdir`	none
1500ms	`change subdir`	none

No extra rebuilds or missed changes

The change only makes FS_ACCURACY drop earlier, which lowers safeTime and can only remove events. A scan that sees any file already reached the same value.
A directory created after startTime is still reported, both before watch() and while watching, with and without polling.
webpack's WatchTestCases (2487 passed) and WatchCacheUnaffectedTestCases (2439 passed, 4 skipped) pass with this lib/.
No API or event changes.

Tests

New test in test/Watchpack.test.js loads a fresh module (jest.isolateModules) so FS_ACCURACY starts at 2000ms. It creates real directories and expects no change. It fails without the fix (dir/sub reported) and passes with it, also with WATCHPACK_POLLING=200.
Full suite 204/204, npm run lint clean, all added lines covered.

🤖 Generated with Claude Code

https://claude.ai/code/session_01D9gUmpCrb3uVLv6uR8av82

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Summary by CodeRabbit

Bug Fixes
- Fixed directory-only scans incorrectly reporting directories created within the previous two seconds as changed.

A scan that saw only directories kept FS_ACCURACY at its 2000ms default, so directories created up to 2s before startTime were reported as changed (#295). Use the directory's mtime, not its birthtime, since FAT keeps creation times finer than modification times. Claude-Session: https://claude.ai/code/session_01D9gUmpCrb3uVLv6uR8av82

changeset-bot · 2026-10-13T16:24:18Z

🦋 Changeset detected

Latest commit: 67324a2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
watchpack	Patch

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coderabbitai · 2026-10-13T16:24:27Z

Walkthrough

Directory modification times now refine filesystem timestamp accuracy during event handling, scans, and followed directory symlinks. A regression test checks that watching a recently created directory with a current startTime emits no changes during the initial scan. A changeset records the adjustment.

Priority: ⬇️ Low

Severity of issue fixed: Low

Merge Risk: 🔵 Low · up to 67324

The new regression test may fail on filesystems with coarse directory timestamps; make it resolution-aware before merging.

Pre-merge checks: 5

Passed checks: 5 passed

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: using directory modification times to improve filesystem accuracy detection.
Linked Issues check		The PR implements the coding requirement in `#295` for the directory-only failure. It calls `ensureFsAccuracy` with directory `mtime` during watch events, scans, and symlink-to-directory handling. The a…
Out of Scope Changes check		All changed files support `#295`. The `DirectoryWatcher.js` changes correct timestamp accuracy, the `Watchpack.test.js` change adds regression coverage, and the changeset documents the patch. No unrelat…
Docstring Coverage		No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…

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codspeed · 2026-10-13T16:25:38Z

Merging this PR will not alter performance

⚠️

1 benchmark spent significant time in system calls

System calls cannot be consistently instrumented, so they are not included in the measure, which understates the real cost. Please switch to the Walltime instrument to accurately measure system calls.

Measurement and system calls

ignored-cross-platform: regex against windows paths

✅ 36 untouched benchmarks

_{Comparing fix/directory-fs-accuracy (67324a2) with main (d5ab9bf)}

codecov · 2026-10-13T16:31:28Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.47%. Comparing base (d5ab9bf) to head (67324a2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #350      +/-   ##
==========================================
+ Coverage   92.45%   92.47%   +0.02%     
==========================================
  Files           8        8              
  Lines        1338     1342       +4     
  Branches      396      399       +3     
==========================================
+ Hits         1237     1241       +4     
  Misses         95       95              
  Partials        6        6

Flag	Coverage Δ
integration	`92.47% <100.00%> (+0.02%)`	⬆️

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alexander-akait · 2026-10-13T17:29:00Z

codecov/project is red (92.47%, −0.13%), but not because of this change:

codecov/patch passes, and every added line runs (local coverage hits: DirectoryWatcher.js:685 ×5, :1003 ×5767, :1049–1050 ×3).
A local coverage run on e0e867d (this branch's base) and on this branch misses the same lines: 19 in DirectoryWatcher.js on both, and identical counts in every other lib/ file.
Codecov compares against c5fb972, not this branch's base. With no codecov.yml, the default threshold fails on any drop, so 2 lines that only some CI runs hit are enough to turn it red.

All 41 test jobs, lint and the benchmark pass. Codecov can't be re-run from here.

Generated by Claude Code

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Actionable comments posted: 1

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Reviewing files that changed from the base of the PR and between d5ab9bf and 67324a2.

📒 Files selected for processing: 3

.changeset/directory-fs-accuracy.md
lib/DirectoryWatcher.js
test/Watchpack.test.js

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Merge remote-tracking branch 'origin/main' into fix/directory-fs-accu…

67324a2

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coderabbitai Bot reviewed Oct 15, 2026

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Comment thread test/Watchpack.test.js Resolved

alexander-akait merged commit 6695b9f into main Oct 16, 2026
51 checks passed

alexander-akait deleted the fix/directory-fs-accuracy branch October 16, 2026 00:56

alexander-akait mentioned this pull request Oct 16, 2026

fix: do not emit spurious initial-scan change events for items created within FS_ACCURACY of startTime (#295) #299

Closed

github-actions Bot mentioned this pull request Oct 16, 2026

chore(release): new release #357

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fix: learn fs accuracy from directory mtimes too - #350

fix: learn fs accuracy from directory mtimes too#350
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