FileReader: release the event loop while a pipe reader is stopped at its highwater mark by robobun · Pull Request #42038 · oven-sh/bun

robobun · 2026-09-08T16:44:15Z

Problem

A locked, idle reader on Bun.spawn stdout or Bun.stdin.stream() pins the process once the peer goes away. Repro on 1.4.2: read one chunk of a child's stdout, SIGKILL the child, await proc.exited. The process never exits. Node exits.
FileReader::on_read_chunk stops reading at its 16 KB highwater mark when no JS read waits. On POSIX it only returned false: the read loop ends, but the fired one-shot poll stays unarmed yet still counts as active on the loop. Unarmed, it cannot see the peer close, so the count never drops.
The reverse too: child.stdout.pause() then resume() re-armed the poll without its hold, so an unref'd child's parent exited mid-stream.

Fix

on_read_chunk calls reader.pause() on every platform (it was Windows-only). That unregisters the poll and drops its hold. on_pull unpauses.
try_register_poll() re-applies KEEP_ALIVE on every arm (before: first registration only), and update_ref(true) takes the hold only while armed. A ref'd reader holds the loop exactly while it can deliver something, as in Node (readStop() leaves the handle inactive).
A forced FilePoll unregister (teardown) also deletes the kernel entry that a fired, softly unregistered poll leaves behind. Without it, cancelling a stopped stdin reader left fd 0 registered and the next reader got EEXIST.
Verified: new rows in spawn.test.ts and process-stdin.test.ts. Six fail on 1.4.2 and pass now, two guard holds that must survive.

Background

PosixBufferedReader arms its fd with EPOLLONESHOT / EV_DISPATCH: each wakeup disarms the poll until register_poll re-arms it. The poll adds one to the loop's active count while it "keeps the process alive". That count keeps the loop running.
FileReader is the ReadableStream source for pipes, sockets and FIFOs. With no pending JS read it buffers up to highwater_mark, then stops until the next pull.

Notes

Second behavior change, on purpose: pausing at the mark also turns the EAGAIN re-arm in read_loop into a no-op. Before, when the read that crossed the mark ended in EAGAIN, the poll was re-armed despite the stop, so an idle reader kept draining a trickling writer into buffered between pulls (a FIFO trace showed 50+ consecutive 8 KB chunks with no pull, far past 16 KB). Now a stopped POSIX reader reads nothing until the next pull, which is what Windows already did. The "trickling writer" row covers it.
Node ground truth (v26.3.0): process.stdin.once("data", () => process.stdin.pause()) and Readable.toWeb(process.stdin).getReader().read() both exit 1 to 2 ms after the first chunk with readableLength=65536, reading=false while the writer is still alive. uv_ref on an inactive handle does not hold the loop either, which is what the update_ref(true) gate mirrors (process.stdin.ref() while stopped at the mark does not re-pin, the next read does).
Alternative considered: keep HUP/ERR interest while stopped. epoll can wait for EPOLLHUP without EPOLLIN, but kqueue's EVFILT_READ is level-triggered on available bytes and a stopped reader always has unread bytes in the pipe, so there is no portable HUP-only wait. Releasing the loop is uniform across platforms and is what Node does. In a long-lived process the stopped stream stays open (and GC-rooted through waiting_for_on_reader_done) until it is read or cancelled, as before.
PipeReader's contract says returning false from on_read_chunk "ends only the current read loop. To stop the reader, call pause()". The sink-backpressure path and setFlowing(false) already did. The highwater stop now does too. Bun.serve: pause the file reader while the response is backpressured #41244's notes recorded the missing half: "pause() unregisters the poll, and that also releases the poll's hold on the event loop. unpause() does not take it back". Re-applying KEEP_ALIVE on every arm is that fix, and the child_process pause/resume row is its test.
The teardown hole: pause() on a fired poll takes the needs_rearm shortcut in unregister_with_fd_impl (no syscall, Poll* flags cleared, disarmed entry kept so the next arm is a CTL_MOD). PollOrFd::close then called deinit_force_unregister, which saw no Poll* flag and skipped the delete. For an fd that outlives the reader (stdin) the next EPOLL_CTL_ADD returned EEXIST. On 1.4.2 the cancel usually happened while the flags were still set, so the delete ran. Stopping through pause() made the hole reachable. Found by a parallel investigation of the same report (branches robobun/392a1a11/idle-pipe-reader-loop-ref and robobun/e37efcfc/pipe-reader-keepalive-while-armed, whose fd-0 reuse, shell-pipe and pause/resume rows are adapted here).
Windows: on_read_chunk already paused there and read_into is unpause(), so the added unpause() before it is a no-op. No behavior change intended.
Test mechanics: the spawn rows unref the still-running child after taking the reader, so only the stdout reader can hold the parent. Rows that wait inside the child avoid top-level await, since an unsettled entry-module promise keeps a Bun process alive by itself. The fd-0 row waits for getEventLoopStats().loopActive to drop instead of sleeping.
Suites run locally (debug+ASAN): spawn.test.ts, spawn-stdin-readable-stream, spawn-streaming-*, readablestream-helpers, spawn leak/GC/stale-fd tests, process-stdin, child_process, streams.test.js, native-source-onclose-leak, bun-serve-file, serve-body-leak, fetch-backpressure, filesink, bunshell, shell file-io/pipeline/hang, terminal.
Pre-existing in this container with or without the change (debug+ASAN timing or environment): spawn-pipe-leak.test.ts (todo on ASAN CI), shell/leak.test.ts memleak rows, child_process.test.ts "default shell" and "extra stdio pipes are not double-closed on GC", the S3 rows of fetch-backpressure.test.ts. bun-serve-file.test.ts "resumes when the client reads" runs 3.7 to 4.3 s here and occasionally crosses 5 s.
Open PRs in the same functions: child_process: keep the parent alive for un-read stdio pipes after unref() #41632 adds the same unpause() before read_into and force-unregisters in pause(). Both compose with this.
Self-reviewed: 3 concerns raised, 3 addressed (route the stop through pause() instead of a new io-layer state, close the EAGAIN re-arm hole, add the must-still-hold tests).

no test proof · iteration 3 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/process/process-stdin.test.ts, test/js/bun/spawn/spawn.test.ts

…its highwater mark A streaming FileReader with no JS read waiting stops the read loop once it has buffered its highwater mark. On POSIX it only returned false from on_read_chunk, which ends the current loop but leaves the fired one-shot poll allocated, unarmed and still counted as active on the event loop. A locked idle reader on Bun.spawn stdout or Bun.stdin.stream() therefore pinned the process: unarmed, the poll could not even observe the peer closing. When the read that crossed the mark ended in EAGAIN the poll was re-armed instead, so the stop was not honored at all. on_read_chunk now calls pause() on every platform (it was Windows-only), which unregisters the poll and drops its hold on the loop, and on_pull unpauses before it reads again. In PosixBufferedReader, try_register_poll() re-applies KEEP_ALIVE on every arm so an unpaused reader takes the hold back, and update_ref(true) only takes it while the poll is armed. A ref'd reader thus holds the loop exactly while it can deliver something, which is what Node does (readStop() at the highWaterMark leaves the handle inactive).

coderabbitai · 2026-09-08T16:44:25Z

Walkthrough

The change updates pipe poll keep-alive handling, documents reader pause behavior, and adds process-lifetime tests for stdout and stdin backpressure.

Changes

Reader lifecycle

Layer / File(s)	Summary
Pipe reader keep-alive handling `src/io/PipeReader.rs`	`update_ref` leaves inactive polls unchanged. Poll arming reapplies `KEEP_ALIVE` on every arm.
Cross-platform reader pause recovery `src/runtime/webcore/FileReader.rs`	Comments describe how `false`, `pause()`, and `on_pull` control the read loop and event-loop hold.
Process-lifetime validation `test/js/bun/spawn/spawn.test.ts`, `test/js/node/process/process-stdin.test.ts`	Tests cover unref’d child output, pending reads, stdin `ref()` recovery, high-water backpressure, EOF, and process exit.

Suggested reviewers: jarred-sumner, dylan-conway

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to 07d3f

This changes pipe-reader event-loop lifetime behavior and adds nested subprocess coverage. The remaining risk is CI test flakiness under debug/ASAN configurations, rather than a demonstrated runtime behavior failure.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the problem, implementation, verification, test coverage, and known CI limitations. It uses equivalent sections instead of the template headings.
Linked Issues check	✅ Passed	The description references relevant issue and pull request context, including `#41244` and `#41632`, and explains how that context relates to the implementation.
Out of Scope Changes check	✅ Passed	The code and tests remain focused on pipe-reader polling, event-loop lifetime, backpressure, teardown, and related regressions described in the objectives.
Title check	✅ Passed	The title clearly identifies the main change: releasing the event loop when a FileReader pipe reader stops at its high-water mark.

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robobun · 2026-09-08T16:44:30Z

^{Updated 3:05 PM PT - Sep 8th, 2026}

✅ @robobun, your commit 8d2a2b7c082558d768a55495fe04055b3be3c3ac passed in Build #113250! 🎉

🧪 To try this PR locally:

bunx bun-pr 42038

That installs a local version of the PR into your bun-42038 executable, so you can run:

bun-42038 --bun

robobun · 2026-09-08T16:45:00Z

Status

Reproduced on bun 1.4.2 (Linux x64) before the change:

Bun.spawn stdout: getReader(), one read(), then idle while the child keeps writing; SIGKILL the child, await proc.exited. The parent never exits (an EPOLLONESHOT entry with events: 40000000 stays in /proc/<pid>/fdinfo/<epfd>).
Bun.stdin.stream(): producer | bun -e 'await Bun.stdin.stream().getReader().read()', kill the producer. The reader process never exits.
node:child_process: child.stdout.pause() then resume() on an unref'd child. The parent exits mid-stream and the child gets EPIPE.
Node v26.3.0 behaves correctly in all three.

With this branch all three behave like Node, and a reader cancelled while stopped at its highwater mark leaves fd 0 reusable (the soft-unregistered registration is now deleted at teardown). The new "does not keep the process alive" / "keeps the process alive again" / fd 0 rows fail on 1.4.2 and pass here; the two "keeps the process alive" guards pass on both.

PR: #42038

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robobun · 2026-09-08T18:05:27Z

I worked the same report independently and landed on the same invariant (the poll holds the loop only while armed and KEEP_ALIVE is set). My branch is https://github.com/oven-sh/bun/tree/robobun/e37efcfc/pipe-reader-keepalive-while-armed. It has the same update_ref / try_register_poll hunks; instead of pause() at the highwater stop it drops the hold at the !keep_going exit of read_loop and leaves IS_PAUSED alone. I am not opening a PR for it since this one covers the bug.

Two things from that branch that may be worth lifting here:

test/js/bun/spawn/spawn-pipe-reader-keepalive.test.ts has a Bun.stdin.stream() row over a real FIFO (sh -c 'exec bun ... < fifo', the blocking-pipe read path rather than the socketpair one) and a node:child_process row for the inverse direction: stdout paused past its highWaterMark, then resumed, on an unref'd child must keep the parent alive until 'end'. That row fails on 1.4.3 (parent exits mid-stream, child gets EPIPE) and passes with either branch. All four rows also pass unchanged on the Windows canary.
One review note on the shape: pausing at the highwater stop also turns the EAGAIN re-arm into a no-op, so a POSIX reader that stops at the mark no longer drains a trickling writer into buffered between pulls. That is the Windows behavior and probably the intent, but it is a second behavior change next to the liveness fix, so it may deserve its own line in the body.

robobun · 2026-09-08T18:23:03Z

I worked the same bug from the Bun.stdin.stream() / FIFO side and ended up with the same update_ref and try_register_poll changes, so I am not opening a second PR. Branch for reference: robobun/392a1a11/idle-pipe-reader-loop-ref (a66a292). It unregisters in read_loop instead of calling pause() from FileReader; either works.

One thing this PR needs before it lands: stopping through pause() leaves a hole at teardown. pause() unregisters a fired one-shot poll without force, which takes the needs_rearm shortcut in FilePoll::unregister_with_fd_impl: no EPOLL_CTL_DEL, the Poll* flags are cleared, and the disarmed entry stays in the kernel. When the stream is then cancelled, PollOrFd::close calls deinit_force_unregister, which returns early because no Poll* flag is set, so fd 0 is never removed from the epoll set. fd 0 is not closed either, so the next poll on it fails:

// flood stdin:  bun writer.js | bun reader.js   (writer keeps the pipe full)
(async () => {
  const reader = Bun.stdin.stream().getReader();
  await reader.read();                 // one chunk
  await Bun.sleep(200);                // reader stops at the highwater mark
  await reader.cancel();
  const second = await Bun.file(0).stream().getReader().read();
  // 1.4.2: data. With the highwater pause and no teardown fix:
  // EEXIST: file already exists, epoll_ctl
})();

On 1.4.2 the cancel happened while the flags were still set, so the delete ran. With the stop going through the soft unregister it does not. The fix on my branch is in src/io/posix_event_loop.rs: a forced unregister also deletes a registration that an unforced unregister of a fired one-shot poll left in the kernel (epoll deletes by fd; kqueue gets a delete for both filters and tolerates the missing one). The test "a stopped reader that is cancelled frees fd 0 for the next one" in test/js/node/process/process-stdin.test.ts on that branch covers it, next to five liveness cases that run the reader at the end of a real pipe (writer.js | reader.js through exec) so EOF can never rescue a reader that wrongly stays alive. Feel free to take both the hunk and the tests from there.

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In `@test/js/bun/spawn/spawn.test.ts`:
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📒 Files selected for processing (4)

src/io/PipeReader.rs
src/runtime/webcore/FileReader.rs
test/js/bun/spawn/spawn.test.ts
test/js/node/process/process-stdin.test.ts

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…ne-shot poll left behind pause() on a fired one-shot poll takes the needs_rearm shortcut: no syscall, the Poll* flags are cleared, and the disarmed registration stays in the kernel so the next arm is an EPOLL_CTL_MOD. When the reader is torn down in that state (a Bun.stdin.stream() reader cancelled while stopped at its highwater mark), deinit_force_unregister() found no Poll* flag and returned early, so the registration outlived the poll. fd 0 stays open, and the next reader on it failed with EEXIST from EPOLL_CTL_ADD. A forced unregister now deletes such a leftover registration (epoll by fd; kqueue gets EV_DELETE for both filters and tolerates the missing one). Tests: fd 0 is reusable after cancelling a stopped reader; the stopped reader over a real shell pipe; a child_process stdout resumed after pause() holds the loop again (fails on 1.4.2: the parent exited and the child got EPIPE); the pending-read guard no longer relies on top-level await, which keeps a process alive by itself.

claude

Code review found no issues

No high-confidence issues detected in this change.

robobun · 2026-09-11T01:54:52Z

The run() rows added here exit right after a read. On macOS the parent's write then sometimes fails with ENOTCONN instead of EPIPE, which ignoreEpipe rethrows (22 pass, 0 fail, 42 errors on the darwin lanes, 25 builds since 113381). #42260 makes the helper accept it.

…epalive FileReader.rs: main moved on_pull into the SourceContext impl and already unpauses before read_into (#42038). The branch's copy of on_pull goes away. PipeReader.rs: take main's pause(). The branch forced the unregister there. Main keeps the soft path, so the next arm is one EPOLL_CTL_MOD, and #42038 deletes the leftover kernel entry at teardown.

robobun assigned Jarred-Sumner Sep 8, 2026

github-actions Bot added the claude label Sep 8, 2026

[autofix.ci] apply automated fixes

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Comment thread src/io/PipeReader.rs Resolved

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Comment thread src/runtime/webcore/FileReader.rs Resolved

Shorten the keep-alive comments

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ci: retrigger

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Comment thread test/js/bun/spawn/spawn.test.ts Resolved

Comment thread test/js/bun/spawn/spawn.test.ts Resolved

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Comment thread src/io/posix_event_loop.rs Resolved

Shorten the unregister comment

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Comment thread src/io/posix_event_loop.rs Resolved

One-line unregister comment

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Jarred-Sumner merged commit 14c6fda into main Sep 8, 2026
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Jarred-Sumner deleted the robobun/d5a59cc0/park-reader-keepalive branch September 8, 2026 23:56

claude Bot mentioned this pull request Sep 9, 2026

Resolve bare undici to the installed package, keep the shim as fallback #36102

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robobun mentioned this pull request Sep 11, 2026

test(process-stdin): join stdin chunks before comparing, ignore ENOTCONN on macOS #42260

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node:stream: deliver child stdio 'data' listener throws as uncaughtException #37171

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robobun mentioned this pull request Oct 27, 2026

child_process: keep the parent alive for un-read stdio pipes after unref() #41632

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FileReader: release the event loop while a pipe reader is stopped at its highwater mark - #42038

FileReader: release the event loop while a pipe reader is stopped at its highwater mark#42038
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