fix: recover events macOS drops when a watcher starts or stops by alexander-akait · Pull Request #354 · webpack/watchpack

alexander-akait · 2026-10-15T10:49:57Z

Draft: investigation first, fix to follow in this PR.

webpack's macOS CI loses a file deletion between watch builds (webpack/webpack#22681, restored-commonjs-reexport): no build ever reports it, and the next build reuses the stale module. On Linux the same failure appears only when the deletion event is dropped, never when it is late.

Suspected cause: on macOS, libuv (src/unix/fsevents.c) shares one FSEventStream between every directory fs.watch handle in the process. Adding or removing any handle destroys the stream and creates a new one from kFSEventStreamEventIdSinceNow, so events not yet delivered are lost. watchpack adds and removes handles whenever the set of watched directories changes.

This commit adds a temporary fsevents-probe job (macOS, Node 20/22/24) that deletes a watched file, starts another watcher 0–100 ms later, and counts lost deletions, with plain fs.watch and with watchpack. It will be removed once the numbers are in.

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Summary by CodeRabbit

Bug Fixes
- Directory monitoring now rescans watched folders after filesystem event streams restart on macOS. Changes and removed files can be detected even when events are missed during a restart.
- Rescanning can also be enabled on other platforms through a configuration setting. The scan runs after a short delay and skips watchers that have been closed or are still completing their initial scan.

…porary)

changeset-bot · 2026-10-15T10:50:01Z

🦋 Changeset detected

Latest commit: 71146c4

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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codspeed · 2026-10-15T10:52:01Z

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/macos-lost-events (71146c4) with main (1c00baa)}

codecov · 2026-10-15T10:58:29Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.56%. Comparing base (d5ab9bf) to head (71146c4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #354      +/-   ##
==========================================
+ Coverage   92.45%   92.56%   +0.11%     
==========================================
  Files           8        8              
  Lines        1338     1359      +21     
  Branches      396      402       +6     
==========================================
+ Hits         1237     1258      +21     
  Misses         95       95              
  Partials        6        6

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libuv shares one FSEventStream between all directory watchers of a process and recreates it from "now" whenever one starts or stops, so events it had not delivered yet are lost. Every directory watcher now rescans once that settles, which reports the lost removals and changes.

Polling reports the removal before the second watcher starts; the handler then threw on the missing watcher, and the one started after it kept jest alive.

coderabbitai · 2026-10-15T23:24:08Z

Walkthrough

The event source now schedules rescan events after watcher starts or stops. Rescanning is enabled by default on macOS and can be enabled on other platforms with an environment setting. DirectoryWatcher responds to eligible rescan events with a non-initial scan. The change also adds event type declarations, a test for reporting a removed file, and a patch changeset.

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 71146

The change adds rescanning after macOS watcher restarts to recover dropped events. No concrete merge-blocking risk was found.

Pre-merge checks: 5

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: recovering filesystem events that macOS may drop when a watcher starts or stops.
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 4…
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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🧹 Nitpick comments: 1

lib/watchEventSource.js: 1
64-64: 🩺 Stability & Availability | 🔵 Trivial | 💤 Low value

Isolate listener errors in the rescan loop.

w.emit("rescan") runs inside a setTimeout callback. A throwing rescan listener propagates out of the timer callback. Node then raises an uncaught exception. The loop also stops, so the remaining watchers get no rescan. The current listener in DirectoryWatcher calls doScan, which defers its work with process.nextTick, so the risk is low today. A try/catch per watcher would make the loop robust if listeners change.
Proposed fix
-		for (const w of activeWatchers) w.emit("rescan");
+		for (const w of [...activeWatchers]) {
+			try {
+				w.emit("rescan");
+			} catch (err) {
+				process.nextTick(() => {
+					throw err;
+				});
+			}
+		}

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Reviewing files that changed from the base of the PR and between d5ab9bf and 71146c4.

📒 Files selected for processing: 5

.changeset/macos-lost-events.md
lib/DirectoryWatcher.js
lib/watchEventSource.js
test/RestartRescan.test.js
types/watchEventSource.d.ts

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ci: probe whether macOS drops events when another watcher starts (tem…

43a52d9

…porary)

alexander-akait added 5 commits October 15, 2026 11:04

chore: add changeset

178eb22

test: type the restart rescan test

2f22684

ci: drop the temporary macOS event-loss probe

74e9967

test: close every rescan test watcher, so polling cannot hang the run

71146c4

Polling reports the removal before the second watcher starts; the handler then threw on the missing watcher, and the one started after it kept jest alive.

alexander-akait marked this pull request as ready for review October 15, 2026 23:22

coderabbitai Bot reviewed Oct 15, 2026

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alexander-akait merged commit 26f591c into main Oct 15, 2026
51 checks passed

alexander-akait deleted the fix/macos-lost-events branch October 15, 2026 23:35

github-actions Bot mentioned this pull request Oct 15, 2026

chore(release): new release #357

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fix: recover events macOS drops when a watcher starts or stops - #354

fix: recover events macOS drops when a watcher starts or stops#354
alexander-akait merged 6 commits into
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