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…e as a prefix A watcher on <dir>/views must not receive events for <dir>/views2/a.txt or <dir>/views.bak when another watcher covers <dir>. The test passes on Linux and Windows. On macOS it needs the path boundary check in the FSEvents filter.
A watcher kept every event whose path starts with the watched path. All watchers share one FSEventStream, so a watcher on <dir>/views also received <dir>/views2/a.txt as "2/a.txt" and <dir>/views.bak as ".bak" once another watcher covered <dir>. Require the byte after the matched prefix to be '/', unless the paths are equal or the watched path already ends in '/' (the root). libuv has the same check in uv__fsevents_event_cb.
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Status: ready for review. Head is a5ade90, an empty commit on top of 38ea156 that ran CI again. The diff is green. CI is red only on tests that this PR does not touch. This PR changes one source file that is compiled for macOS only, and one test file. How the bug was reproduced: the first commit of this PR is the test alone. Build 120413 ran it against the code without the fix. On macOS 15.7.7 arm64 the test failed on the first run and on all 3 retries. The watchers on
With the fix,
The file also passes on every Linux and Windows lane. Failures in CI that this PR does not cause:
Build 120452 has no other failure. The macOS jobs of that build are red because of the two install tests. |
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Replace the four-line comment on the check with one line and a named condition. The behaviour does not change.
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Included review availability: Your plan provides up to 10 included reviews per hour; 0 remain after this review. WalkthroughFSEvents path matching now rejects paths that share the watched path’s byte prefix unless the path ends there or continues with a slash. Regression tests cover sibling paths and recursive watching of ChangesFSEvents path matching
Suggested reviewers: Priority: ⬇️ Low Severity of issue fixed: Low Merge Risk: ⚪ Minimal · up to The regression tests cover the path-boundary behavior, and no actionable merge-blocking risk remains after normal checks. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
In `@test/js/node/watch/fs.watch.test.ts`:
- Around line 264-265: Update writeUntilSeen to bound the wait for the watcher
event and clear the repeat interval in a finally block, so the interval is
stopped on both success and failure.
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Beyond the inline findings, I also checked the Linux (path_watcher.rs) and Windows (win_watcher.rs) backends for the same prefix-match defect — neither filters events by a watched-path prefix (names arrive relative to the watched handle), so the boundary check is macOS-only by design. I also traced the new ends_on_boundary check against the file-watch case (path == handle_path) and trailing-slash watch paths — both still pass, so no watcher loses its own events.
Extended reasoning...
The change adds a six-line path-boundary check to the shared FSEvents callback in src/runtime/node/fs_events.rs and one new test in test/js/node/watch/fs.watch.test.ts; it touches no security-sensitive surface. Inline findings already cover the test's missing rejection path, the FSEvents system-flag interaction, and the unfixed "/" root case, so a human look is warranted; this note only records that sibling backends and the exact-match/trailing-slash inputs were examined and are unaffected.
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…ed / The sibling test waited on a promise that nothing rejects, next to an interval that nothing clears on failure. It now polls in a bounded loop, records 'error' events, and closes the watchers before the assertion. Add a macOS test for the exemption of a watched path that ends in '/': a recursive watcher on / must report a file below it.
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The latest push replaces the unbounded promise wait in the sibling test with a bounded poll that records watcher errors and closes the watchers before the assertion, which resolves the earlier concern about a hung test leaking watchers. Beyond the open inline threads, I also checked the new "/" test: the endsWith(basename/a.txt) match tolerates FSEvents reporting /private/var/... for a /var/... tempDir, the watcher is closed in finally before the assertion, and the loop is capped at 100 x 20ms, so it cannot run away on a missed event. The remaining open inline thread on system-flag events reaching the exact toEqual is still the one thing worth a human look.
Extended reasoning...
The change adds a six-line path-boundary check to the macOS-only FSEvents filter in src/runtime/node/fs_events.rs, matching libuv's uv__fsevents_event_cb, and two tests in test/js/node/watch/fs.watch.test.ts. It touches no security-sensitive surface. The newest commit addressed the prior unbounded-wait finding; the earlier inline thread about MustScanSubDirs/KernelDropped events emitting the watched directory's basename and breaking the exact toEqual remains open and unaddressed by any commit. The native change cannot be exercised on this Linux checkout, so CI on the macOS lanes is the only runtime proof.
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Problem
fs.watchreports a sibling whose name starts with the watched name. A watcher on<dir>/viewsreports<dir>/views2/a.txtas2/a.txtand<dir>/views.bakas.bak.FSEventsLoop::events_cb(src/runtime/node/fs_events.rs:548) keeps each event whose path starts with the watched path. It does not check where the match ends.Fix
/, unless the paths are equal or the watched path ends in/.uv__fsevents_event_cb. The/exemption is libuv PR 2460: without it a watcher on/receives nothing.test/js/node/watch/fs.watch.test.ts. Without the fix the sibling test fails on macOS 15 arm64 (build 120413). With the fix both pass there and on macOS 14 x64 (build 120444).Background
FSEventStream, as in libuv. Each filters the events by its own path, so a sibling arrives when another watcher covers it.ReadDirectoryChangesWreport names relative to the watched handle, so only macOS has a prefix match.bun_paths::is_parent_or_equal: it ignores ASCII case on macOS, but FSEvents paths are exact bytes.Downsides
gdb), with no allocation or syscall.Notes
Origin. Found by reading the code. No issue reports it. The check is missing since the first
fs.watchport (#3249), so this is not a regression. I have no macOS machine, so the macOS lanes of this PR are the runtime proof.Observed on macOS without the fix. The first commit of this PR is the test alone. Build 120413 ran it on macOS 15.7.7 arm64. The test failed on the first run and on all 3 retries, each time with the same 3 extra names. Every Linux and Windows lane of that build passed.
The x64 macOS lanes of that build had not finished when the fix was pushed.
Observed on macOS with the fix. Build 120444 ran the final head on macOS 15.7.7 arm64, the version that failed without the fix, and on macOS 14.8.9 x64. The file reports 44 pass, 10 skip, 0 fail on both, on the first attempt. Without the fix it had 42 pass, 10 skip, 1 fail. The two added passes are the sibling test and the test for a watched
/. Build 120424 passed too, with the first shape of the sibling test, on macOS 26.6.2 arm64 and macOS 14.8.9 x64.Repro on a Mac.
mkdir -p /tmp/fse/views /tmp/fse/views2 bun -e 'const fs=require("fs"); fs.watch("/tmp/fse", ()=>{}); fs.watch("/tmp/fse/views", {recursive:true}, (e,f)=>console.log(e,f)); setTimeout(()=>fs.writeFileSync("/tmp/fse/views2/a.txt","x"), 500); setTimeout(()=>process.exit(0), 1500)'Node prints nothing. Bun without the fix is expected to print an event with the filename
2/a.txt. I did not run this script. The test above shows the same behaviour.Filter logic, before and after. A standalone copy of the path logic of
events_cb, run on Linux:/a/views/a/views/b.txtb.txtb.txt/a/views/a/views2/a.txt2/a.txt/a/views/a/views.bak.bak/a/views/a/views-old(dir)-old/a/f.js/a/f.jsf.jsf.js/a/f.js/a/f.jsxx//x/x/x/a/views//a/views/b.txtb.txtb.txtComparison with libuv. A copy of this filter and a copy of libuv's callback body (1.48 and 1.52), run over 1680 combinations of watched path, event path, recursion and flags. The change alters 110 results. All 110 are siblings (
f.jsx,f.js.map,views.bak,views2,views2/a.txt), and all 110 now equal libuv's result. No result moves away from libuv.Reach. The same filter serves file watchers. In the model a watcher on
f.jsreceivesf.jsxasx, so a watcher on.envfires for.env.localwhen both are watched. The test covers the two directory forms only.What the tests do. The sibling test watches
<dir>, which puts the siblings into the shared stream, and watches<dir>/viewstwice, recursive and not. Each round writesviews2/a.txtandviews.bakfirst, then one file insideviews. It repeats the round until both watchers report that file, at most 100 times with 20 ms between rounds. The test then expects each watcher to have reportedb.txtandc.txtonly, and noerrorevent. On Linux and Windows it passes with and without the fix.The second test runs on macOS only. It guards the exemption for a watched path that ends in
/: a recursive watcher on/must report a file below it. Without theends_with(b"/")clause that watcher reports nothing. libuv tests the same case infs_event_watch_dir_recursive.Build 120413 ran the first shape of the sibling test, which waited on a promise next to an interval. The writes, the watchers and the assertion are the same in the final shape. Review asked for a bounded wait, so the final shape polls.
Cost numbers. Counted with
gdbsingle-step over a loop of 100 events,rustc -O, with the filter inlined as inevents_cb. An event of the watcher: 28.27 to 38.27 instructions. An unrelated event: 31.27 to 33.29. A shorter path: 5.27 to 6.29. On arm64 macOS the hot path grows from 13 to 22 instructions and the loop from 128 to 248 bytes (rustc --emit asm).valgrindandperfare not available in my environment. The size in the real binary is not measured: I cannot link a macOS release build.Found in the same function, not changed here. Each one differs from libuv and predates this PR. They are tracked in #43932:
UserDropped,KernelDropped,EventIdsWrapped,HistoryDone,RootChanged,Mount,Unmount) are not ignored. libuv drops them.fs.watch("/")reports nothing. The path of a root watcher keeps its leading/, and the subdirectory filter then drops it.Self-review. Three concerns came up and all three are addressed. The bug was unseen at runtime, so the test went up alone first. The check indexed
path[len], which left an unreachable panic path in a C callback: it usespath.get(len)now. The second round of the test wrote once, so one lost event could hang it: it repeats the writes now.Suites run.
bun bd test test/js/node/watch/fs.watch.test.ts: 46 pass, 7 skip, 0 fail on Linux x64. The new test: 11 of 11 on Linux (debug, ASAN), 8 of 8 on Windows x64 (canary).cargo check -p bun_runtime --target aarch64-apple-darwinpasses with the fix, and a planted type error in the new lines fails it.no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/watch/fs.watch.test.ts