fs.watch: run nextTicks and microtasks between the events of one batch

[robobun]

Problem

• On POSIX, fs.watch calls the listener for every event of one native batch (up to 8) back to back. Nothing the listener queued runs between two events: no process.nextTick, no microtask, no await continuation. Node runs them after every event.
• So for (;;) await once(watcher, 'change') sees 5 to 25 of 40 events. Node v26.3.0 sees 40.
• Cause: FSWatchTaskPosix::run (src/runtime/node/node_fs_watcher.rs:212) calls run_callback per event. tick() holds entered_event_loop_count at 1 while tasks run (src/jsc/event_loop.rs:781), so run_callback never drains.

Fix

• Each task now delivers one event, as on Windows. run moves its batch to a queue on the FSWatcher. deliver_one pops one event, queues a task for the rest, then calls the listener.
• The event loop drains after every task (src/runtime/dispatch.rs:647), so no drain is added. The batch still crosses threads as one post.
• Correct because node makes one MakeCallback per event (fs_event_wrap.cc#L239). One queue per watcher keeps the order when a listener spins the event loop. close() empties it.
• Verified: five new test cases in test/js/node/watch/fs.watch.test.ts fail on main and pass here. Also ran test/js/node/watch/ and node's 42 fs watch tests. Self-reviewed: 8 concerns, 7 addressed, 1 rejected (Notes).

Background

• A drain runs the process.nextTick queue, then the promise jobs.
• Bun drains after each task. run_callback drains only as the outermost entry into the loop, and a task always runs inside tick().
• The watcher thread posts each read of OS events as one task. One rename(2) is two events in one read.
• expect().resolves in bun:test spins the event loop until the promise settles.

Notes

Repro (from the report, runs on node as the reference):

import fs from 'node:fs'; import os from 'node:os'; import path from 'node:path'; import { once } from 'node:events';
const d = fs.mkdtempSync(path.join(os.tmpdir(), 'w-')); const seq = []; let n = 0;
const w = fs.watch(d, () => { const i = n++; seq.push('ev' + i); queueMicrotask(() => seq.push('mt' + i)); process.nextTick(() => seq.push('tick' + i)); });
const w2 = fs.watch(d); let seenByOnceLoop = 0;
(async () => { for (;;) { await once(w2, 'change'); seenByOnceLoop++; } })().catch(() => {});
setTimeout(() => { for (let i = 0; i < 20; i++) fs.writeFileSync(path.join(d, 'f' + i), 'x'); }, 50);
setTimeout(() => {
  w.close(); w2.close();
  let early = 0; for (let k = 0; k + 1 < seq.length; k++) if (seq[k].startsWith('ev') && seq[k + 1].startsWith('ev')) early++;
  console.log('events', n, '| early', early, '| first 12:', seq.slice(0, 12).join(' '), '| once loop saw', seenByOnceLoop, 'of', n);
  fs.rmSync(d, { recursive: true }); process.exit(0);
}, 400);

node v26.3.0     events 40 | early 0  | first 12: ev0 tick0 mt0 ev1 tick1 mt1 ev2 tick2 mt2 ev3 tick3 mt3 | once loop saw 40 of 40
bun 1.4.3 canary events 40 | early 35 | first 12: ev0 ev1 ev2 ev3 ev4 ev5 ev6 ev7 tick0 tick1 tick2 tick3 | once loop saw 5 of 40
this branch      events 40 | early 0  | first 12: ev0 tick0 mt0 ev1 tick1 mt1 ev2 tick2 mt2 ev3 tick3 mt3 | once loop saw 40 of 40  (3 of 3 runs)

The counts on the unfixed build vary with how the watcher thread cut the batches (early 3 to 35 in my runs).

Node reference. libuv calls the fs event callback once per OS event. FSEventWrap::OnEvent makes one MakeCallback for it. InternalCallbackScope::Close then runs the tick queue and the microtasks when the scope is the outermost (callback.cc#L165-L204).

Suites run with the fix (ASAN debug build): all of test/js/node/watch/ (fs.watch, close-exit, deadlock, events-cb-race, rewrite, fs.watchFile) and the 42 test-fs-watch* and test-fs-promises-watch* files of test/js/node/test/parallel/. All pass.

Tests. All five cases fail fast on the release build of main (no hang) and pass on this branch. Under load (four parallel loops) the first four passed 24 of 24 runs. All five passed 10 of 10 runs.

• "nextTicks and microtasks queued by the listener run before the next event" and "a once('change') listener re-armed after an await sees every event". Each burst creates and renames 8 files. A rename is two inotify events from one syscall, so the batch has two or more events even when the watcher thread keeps up. Failed 8 of 8 runs on main, pass 15 of 15 runs here. The same logic as a plain script passes on node v26.3.0. On Windows they pass with and without the fix, because Windows was never affected.
• "an event loop spin in $spinSite until '$until' is seen gets the events in order" (Linux only, three cases, they depend on inotify semantics). The spin is expect().resolves, after the first of the two events of one rename. Expected before, spin, after. Until late (a file written just before the spin), in the listener: main gives before, spin, late, late, after, because the rest of the batch waits in the suspended task. Until late, in a continuation: main gives before, after, spin, late, because no checkpoint runs between the two events. Until after (the second event of the same batch), in a continuation: main gives before, after, spin. This is the hang cell of the pre-merge check: on the second version of this PR it still spun after 15 s, 3 of 3 runs. On this head it returns, 6 of 6 runs with expect().resolves and 6 of 6 with Bun.build() and an async plugin setup().

Earlier versions of this PR, replaced.

1. A drain_microtasks() call inside the loop of FSWatchTaskPosix::run. A continuation that ran in that drain and spun the event loop let a later batch of the same watcher run first, while the suspended frame still held the rest of the current batch. The review caught it: before, late, late, after.
2. A queue on the watcher, delivered by a loop that drained between events. The order held, but the loop held the rest of the queue while a listener or a continuation spun. A spin that waited for one of those events hung when no later batch arrived to deliver them (2 of 10 runs with a re-entrant listener).
3. This version. No frame holds undelivered events while user code runs. If events remain, a task for them is in the event loop queue before the listener is called, so any spin runs it.

Cost. One small task allocation and one same-thread enqueue_task per event after the first of a batch. Windows already pays one task per event. The cross-thread post stays one per batch.

Edge cases checked on the ASAN build, compared with node:

• A microtask closes the watcher in the middle of a batch: 1 event, then 'close', no later events. Same as node. detach() empties the queue, and the queued task finds the watcher closed.
• A microtask throws, with an uncaughtException handler: the sequence is ev0 caught ev1 caught ..., identical to node. The unfixed build gives ev0 caught ev1 ev2 ev3 caught caught caught ....
• A microtask calls process.exit(0): exit after 1 event. Same as node.
• The same in a worker, and worker.terminate() from the parent in the middle of a batch: clean exit. The queued task is released unrun at teardown, and what is left in the queue is freed with the watcher.
• A listener that starts a new spin on every call (re-entrant spins): no hang in 15 of 15 runs, order before, after, late.

Lifetime. Every task, posted or queued by deliver_one, holds one pending-activity unit on the FSWatcher until its run returns, so a close() plus GC cannot finalize the watcher while a task runs. The queue is touched only on the JS thread, in three short closures that do not re-enter (push_back, pop_front, clear). A task that runs after close() does not touch the queue. Its entries are freed by deinit as before.

Alternatives not taken.

• One task per event from the watcher thread: one cross-thread post per event. The batch exists to avoid that.
• Re-enqueue the rest of the batch as a task that owns it: it lands behind later batches of the same watcher and reorders events. The queue on the watcher avoids that.
• A checkpoint inside run_callback at task depth: run_callback has 50+ callers, some beneath JS frames, where a checkpoint is wrong.
• Let the running task queue itself again and skip the allocation: the dispatcher frees the task after run, so this needs a change to the dispatch arm and to the Windows task. Not worth it for this fix.

Self-review. Addressed: the node source link and the reason for the design are in the comments and in this body. The once-loop test keeps its consumer promise handled on a failure path. The nested spin reorder (the queue). The hang of a spin that waits for a queued event (one event per task). The test that passed on main (it now observes the checkpoint). A test of the interleaving of two watchers, which depended on a cross-thread race (removed, see below). The hang cell of the pre-merge check is now a test case. Rejected: pass the dispatcher's &mut EventLoop into run. The code no longer drains, and enqueue_task through event_loop_mut() is what the Windows task already does.

Not changed here.

• A listener that throws, with an uncaughtException handler. Node skips the checkpoint of a callback that threw (InternalCallbackScope::Close returns early for a failed scope): ev0 UNCAUGHT ev1 tick0 tick1 mt0 mt1. Bun runs the checkpoint after every task, also after one whose callback threw: ev0 UNCAUGHT tick0 mt0 ev1 tick1 mt1. Main gives node's line only when both events share a native batch. This is how Bun's event loop treats every callback source: four fs.stat callbacks of which the first throws give cb0 UNCAUGHT tick0 mt0 cb1 ... on main, and cb0 UNCAUGHT cb1 tick0 tick1 ... on node. It needs a change in the event loop, not in fs.watch.
• Two watchers of one directory. Node calls them in turn for each OS event. Bun posts one batch per watcher, so they now get each event in turn only when both batches reach the JS thread in the same drain (a before, b before, a after, b after, the usual case in my runs). When the posts land in different drains, watcher a gets its events first, as on main. The order within each watcher always matches node. No test asserts the interleaving.
• node_fs_watcher: batch events as BoundedArray<Event, 8> #37561 and fs.watch/fs.watchFile: remove unsafe from the watcher modules #40200 rewrite the head of the same loop. They will conflict textually with this change. Neither adds a checkpoint.
• fs.watch(path, listener) does not register listener as a 'change' listener of the returned watcher (node does: order, listenerCount, removeListener). That is a separate defect in src/js/internal/fs/watch.ts.

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no test proof · iteration 0 · 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

[coderabbitai]

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Walkthrough

POSIX filesystem watcher events now use a FIFO queue and one-event-per-task delivery. Detachment clears pending events. Tests cover callback ordering, once consumers, abort handling, and event-loop spins.

Changes

POSIX watcher delivery

Layer / File(s)	Summary
Queue and deliver POSIX events src/runtime/node/node_fs_watcher.rs	POSIX events move into FSWatcher.undelivered, dispatch one event per task, reschedule remaining events, skip closed watchers, and clear pending events during detachment.
Validate event ordering and lifecycle test/js/node/watch/fs.watch.test.ts	Tests cover callback checkpoints, repeated once consumption, AbortError handling, and Linux event-loop spin ordering.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 4bab5

The parameterized test should use the repository’s required describe.each() structure, but no merge-blocking behavioral risk is established.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly summarizes the primary change: POSIX fs.watch now allows nextTicks and microtasks to run between events from one batch.
Description check	✅ Passed	The description explains the problem, implementation, rationale, verification steps, test results, and edge cases. It does not use the template headings exactly, but it provides the required informati…

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[robobun]

Status: ready for a maintainer. The diff is green. CI is red only on two tests that this PR does not touch.

How I reproduced it: the script in the Notes of the PR body, on Linux x64.

• bun 1.4.3 canary (release): early 35, the await once(watcher, 'change') loop saw 5 of 40 events (3 to 35 early across runs).
• node v26.3.0: early 0, the loop saw 40 of 40.
• This branch (ASAN debug build): early 0, the loop saw 40 of 40, 3 of 3 runs.

The five new test cases in test/js/node/watch/fs.watch.test.ts fail on the release build of main. They pass on this branch (10 or more runs each, no failure, no hang).

CI, build #119413 at 4bab562: 179 jobs passed, 2 failed. Nothing under test/js/node/watch/ failed on any lane (Linux, macOS x64 and aarch64, Windows).

• test/js/bun/spawn/spawn.test.ts ("an idle reader stopped at the highwater mark does not keep the process alive") on debian 13 x64-asan. It does not use fs.watch. The same failure is in 12 of the 15 other failed builds I checked, on unrelated PRs, for example #119410, #119394, #119373 and #119366.
• test/bake/deinitialization.test.ts on alpine 3.23 aarch64. It does not use fs.watch. The same failure is in #119405, #119373 and #119372, and it passed only on a retry in #119414 and #119394.

The other entries of the build passed on a retry. I did not push a retrigger, because both tests fail in the same way on the other branches.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding, I also checked two things around the new mid-batch drain and ruled them out: a listener's nextTick/microtask calling watcher.close() mid-batch cannot free the FSWatcher under the loop (the task's ref_task activity unit is held until run returns, and emit bails on a detached js_this), and the Stopped early return drops the rest of the batch the same way the existing emitted? path already does when the VM is tearing down.

Extended reasoning...

The change adds one drain_microtasks() call between the events of a POSIX fs.watch batch in src/runtime/node/node_fs_watcher.rs plus two tests in test/js/node/watch/fs.watch.test.ts; no security-sensitive surface. It alters event-loop checkpoint semantics inside a task, and the confirmed inline finding (batch reordering when a continuation spins a nested tick) is the reason a human should weigh in.

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding, I also checked the entry move in FSWatchTaskPosix::run for a double free: count is zeroed before assume_init_read, so deinit/clean_entries sees nothing, and the only needs_free: false entry is Event::Abort, which owns no heap data, so dropping events out of undelivered (pop or detach's clear) is sound. The closed early return preserves the old behavior since emit_abort/emit_error already returned on a closed watcher, and the shared per-watcher queue in the second commit covers the nested-spin ordering case from the earlier review.

Extended reasoning...

The change moves POSIX fs.watch event delivery from a per-task loop into a per-watcher VecDeque drained by FSWatcher::deliver, which runs EventLoop::drain_microtasks between events; it touches unsafe MaybeUninit moves, task pending-activity balancing, and event-loop re-entrancy from a listener that can close the watcher mid-batch. No security-sensitive surface is involved. Ownership and refcount paths check out on reading, but the mid-task checkpoint and nested-tick interaction is runtime event-loop semantics rather than a mechanical change, so a human should weigh the design.

[claude]

Beyond the inline finding on the two-watchers test, I checked the follow-up task's lifetime in deliver_one and found it balanced: ref_task() is taken only before the alloc, run's guard or release_unrun gives it back on every exit, and a close() between enqueue and run leaves the task a no-op (the deque is cleared in detach(), entries stay owned by deinit via the untouched count). The mem::take(count) before assume_init_read rules out a double free through clean_entries, and interleaved batches of one watcher stay ordered because every task pops from the front of the shared deque rather than its own entries.

Extended reasoning...

The change rewrites POSIX fs.watch batch delivery in src/runtime/node/node_fs_watcher.rs to one event per event-loop task through a per-watcher VecDeque, plus four new tests; it touches no security-sensitive surface. The one confirmed finding concerns a Linux-only test whose interleaving depends on whether two concurrent posts land in the same drain, not the runtime change itself. The ownership and refcount paths of the newly self-enqueued task (run, release_unrun, close/detach mid-batch) were traced and found balanced, but the unsafe MaybeUninit move and the raw ParentRef back-reference are lifetime-critical enough that a human should still look before merge.

[robobun]

^{Updated 3:51 PM PT - Sep 21st, 2026}

❌ @robobun, your commit 4bab562 has 2 failures in Build #119413 (All Failures):

• test/bake/deinitialization.test.ts - code 1: (unnamed) — expect(received).toBe(expected) on 🐧 3.23 aarch64
• test/js/bun/spawn/spawn.test.ts - code 1: stdout reader of an unref'd child and process lifetime > an idle reader stopped at the highwater … on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 43715

That installs a local version of the PR into your bun-43715 executable, so you can run:

bun-43715 --bun

[claude]

I reviewed the current revision and didn't find any bugs. Since this changes how POSIX fs.watch tasks own and hand off events across the event loop (unsafe moves out of MaybeUninit, a per-watcher queue, and re-entrant follow-up tasks), a human look at the lifetime story would still be worthwhile.

What was reviewed:

• Task ownership in run/deliver_one: count is zeroed before assume_init_read, so deinit's clean_entries cannot double-drop; the closed early-return leaves count intact so deinit still frees the batch; the only needs_free=false entry is Event::Abort, which owns nothing.
• Pending-activity balance: every batch and follow-up task holds one ref_task unit released by the scopeguard in run, by release_unrun on teardown, or by enqueue_task's refusal path — all balanced.
• Re-entrancy: undelivered is only touched inside short with_mut/get calls, none held across emit; a listener that closes the watcher clears the queue in detach and the queued follow-up returns on closed.
• The tests exercise the checkpoint between events and the nested-spin ordering; the flaky two-watcher interleaving test flagged last round was removed.

Extended reasoning...

The diff touches src/runtime/node/node_fs_watcher.rs (about 60 lines of POSIX-only Rust) and adds three tests to test/js/node/watch/fs.watch.test.ts; no security-sensitive surface is involved. The Rust change moves batched events into a per-watcher VecDeque and delivers one event per event-loop task, mirroring the existing Windows path and node's one MakeCallback per event. All three earlier inline findings were addressed by subsequent commits (queue-based delivery, a test that observes the checkpoint, removal of the timing-dependent two-watcher test), and no CODEOWNER covers the files. Deferring rather than approving because the change relies on task-ownership and re-entrancy invariants that are easy to get subtly wrong and were not verified by building here.

[coderabbitai]

Actionable comments posted: 1

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In `@test/js/node/watch/fs.watch.test.ts`:
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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[robobun]

#44008 now holds the commits of this branch (merge bec5bed on its branch). It needs what this PR does: with one task for each event, a listener that once puts back in a continuation gets the second event of a batch. Without that, a record that #44008 merges into such an event never reaches a once loop.

If this PR lands first, the diff of #44008 gets smaller. If this branch changes, I merge it into #44008 again.