process.stdout/stderr: clear O_NONBLOCK for spawned children, don't drop console output on EAGAIN - #43868
process.stdout/stderr: clear O_NONBLOCK for spawned children, don't drop console output on EAGAIN#43868Jarred-Sumner wants to merge 16 commits into
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Touching process.stdout or process.stderr used to set O_NONBLOCK on fd 1/2. The flag lives on the open file description, so it leaked to children with inherited stdio (plain tools then fail write(2) with EAGAIN) and to the native console writer (console.log dropped output once the pipe filled). Never set O_NONBLOCK on a caller-supplied fd. process.stdout/stderr on a pipe or socket now write through a per-call nonblocking path instead: pwritev2(RWF_NOWAIT) on Linux, a bounded write sized by kqueue's writable byte count on macOS, MSG_NBIO for sockets on macOS. write() still returns false and emits 'drain' on a full pipe. The console writer also waits on EAGAIN instead of dropping output if another process set the flag.
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WalkthroughUnix pipe writes now use bounded fallbacks and wait for writable status after ChangesUnix stdio pipe handling
Suggested reviewers: Priority: ⬇️ Low Merge Risk: 🟡 Moderate · up to Spawning a child with inherited or caller-supplied pipes now switches those pipes to blocking mode for the parent too. A later write to a full pipe can freeze the parent's event loop instead of applying backpressure. A related race in the pipe-write fallback can also block. Resolve both before merging, or explicitly accept them. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@src/io/PipeWriter.rs`:
- Line 232: Replace the blocking sys::write call in the Some(space) branch with
a write operation or mechanism that cannot wait for pipe space; do not use the
reported space value to authorize a potentially blocking write.
In `@src/sys/lib.rs`:
- Line 7400: Add a safety comment immediately before the `unsafe`
zero-initialization of `out` in the `kevent` handling code, documenting why
zero-initializing this `libc::kevent` value is safe.
- Around line 7383-7384: Change the thread-local KQ descriptor used by
pipe_writable_space to an OwnedFd or equivalent drop-owning wrapper, and pass
its raw descriptor to kevent. Preserve the existing lazy initialization behavior
while ensuring the descriptor closes when the thread exits.
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The kqueue probe reported 0 writable bytes for a pty master, so Bun.Terminal writes (which share the blocking-pipe path) never landed. fstat on a macOS pipe gives capacity (st_blksize) and queued bytes (st_size) in one stateless call, and S_ISFIFO limits it to real pipes. Elsewhere the fallback writes at most PIPE_BUF after POLLOUT, which a pipe always accepts. Also flag the Terminal writer's poll Nonblocking, since its fd is, so it takes the plain write path.
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Inline comments:
In `@src/sys/lib.rs`:
- Line 7376: Update the pipe-capacity calculation that returns `st_blksize -
st_size` so it does not treat `st_blksize` as guaranteed writable space; use a
write path that cannot block when the actual pipe buffer is smaller.
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- write_nonblocking's no-RWF_NOWAIT fallback wrote the whole buffer after POLLOUT; it now goes through write_bounded like the steady-state path. - Linux bound: free page slots from F_GETPIPE_SZ/FIONREAD (a part-read head and part-filled tail each cost a slot), else one page after POLLOUT. - macOS: only anonymous pipes (st_dev 0) report capacity via fstat; named FIFOs are socket-backed, so they get PIPE_BUF after POLLOUT. - on_poll capped a drain at the kqueue byte hint and then reported the capped write as Drained, stranding the tail until exit. Report Pending. - Tests: run the 1 MiB and false+'drain' cases through sh so fd 1 is a real pipe(2), not Bun.spawn's socketpair; rename the probe to -fixture; make the block concurrent.
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Inline comments:
In `@src/sys/lib.rs`:
- Around line 7393-7394: Update the pipe-capacity handling in write_bounded so
an F_GETPIPE_SZ failure cannot classify a pipe as a non-pipe and allow an
unbounded blocking write; preserve pipe classification or return an error on
query failure.
In `@test/js/node/process/process-stdio.test.ts`:
- Line 299: Update the probe’s stdio configuration so fd 1 is inherited and
remains the descriptor whose blocking state is checked; report the probe result
through a separate captured descriptor, and read it from that descriptor in the
test.
- Around line 293-294: In the reader script, register the SIGUSR1 handler before
writing the reader PID to stderr so the parent cannot signal the process before
the handler is active. Keep the existing signal-handler behavior and PID
announcement unchanged.
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Still open from earlier reviews (5):
- Unresolved: 3 minor or pre-existing, 2 blocking on lines changed since (possibly already fixed).
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| self.try_write_with_write_fn(buf, sys::write) | ||
| } | ||
| FileType::Pipe => self.try_write_with_write_fn(buf, write_to_blocking_pipe), | ||
| FileType::Pipe => self.try_write_with_write_fn(buf, sys::write_nonblocking), |
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🟣 pre-existing, not blocking: pre-existing: on macOS a Bun shell builtin (echo, cat) whose stdout is a socket still parks the JS thread in one full blocking write(2) until the reader drains. The shell's stdout IOWriter never sets is_socket (src/runtime/shell/interpreter.rs:1211-1218) and src/runtime/shell/IOWriter.rs:434-435 refuses the Socket flag on macOS, so the write lands in the FileType::Pipe arm at src/io/PipeWriter.rs:79 and write_bounded issues an unbounded write on a blocking socket. Fix: route every blocking-socket stdio writer through write_to_socket now that SEND_FLAGS_NONBLOCK carries MSG_NBIO: derive is_socket from fstat in interpreter.rs and drop the macOS guard at IOWriter.rs:432-435.
A small fix can ride a push you are already making; otherwise a short reply is enough.
Why this was flagged
A Bun process on macOS whose fd 1 is a socket (any child of Bun.spawn with stdout: "pipe", which is a socketpair per src/spawn_sys/spawn_process.rs) runs Bun.$cat bigfile without .quiet(), so the builtin's output goes to the interpreter's stdout IOWriter, a dup of fd 1 (src/runtime/shell/interpreter.rs:1200-1218) created with Flags { pollable, ..Default::default() } so is_socket and nonblock are false. In __start the writer therefore gets FilePollFlag::Fifo, not Socket (src/runtime/shell/IOWriter.rs:434-438; the comment there says MSG_DONTWAIT can still block on macOS). try_write then takes the FileType::Pipe arm at src/io/PipeWriter.rs:79, sys::write_nonblocking -> write_bounded (src/sys/lib.rs:7355-7376): pipe_writable_space does fstat, sees S_IFSOCK, returns None (src/sys/lib.rs:7413-7415), is_writable reports POLLOUT once 2048 bytes of space exist, and write(fd, buf.len()) is issued on a blocking socket, which XNU completes only after every byte is sent. The JS thread is parked until the parent reads. The base branch fails the same way through the deleted write_to_blocking_pipe,…
Verification: pre-existing — triggers on macOS when a Bun process whose fd 1 is a blocking SOCK_STREAM socket (e.g. any Bun.spawn child with stdout: "pipe": src/spawn_sys/spawn_process.rs:842-844 creates an AF_UNIX socketpair and :909 only calls set_nonblocking on the parent end fds[0], so the child's fd 1 stays blocking) runs a non-quiet shell builtin that writes more than the socket's free send space.…
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Acknowledged, pre-existing; shell IOWriter is out of scope for this PR.
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Checked this one properly and it does not reproduce. The shell IOWriter never does a direct first write: it registers the writable poll and writes only from on_poll, which already caps each write to kqueue's EVFILT_WRITE byte count for blocking fds (PipeWriter.rs max_write = size_hint). Trace of main's debug build running echo <1 MiB> into a Bun.spawn socket stdout that nobody is reading yet:
register writable → onPoll(1048577) → wrote 524288
register writable → onPoll(524289) → wrote 458752
register writable → onPoll(65537) → wrote 65537, unregister
No full-buffer write(2), and the child answers a stdin ping while the echo is still pending, on 1.4.3 and on this branch alike. Leaving the shell path alone.
Linux packs a write's page-multiple part into fresh slots, so queued bytes undercount used slots and a byte-derived bound could still block. Use the one guarantee POLLOUT gives (a free page; PIPE_BUF on BSD). On macOS let an empty anonymous pipe take 64K so XNU grows it. In the real-pipe drain test the probe now inspects the inherited fd 1 and reports on fd 2, and the reader installs SIGUSR1 before announcing.
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Updated 6:41 PM PT - Sep 25th, 2026
✅ @Jarred-Sumner, your commit da3ab6e0e8ff653024d7e0d14894975a628ab2e9 passed in 🧪 To try this PR locally: bunx bun-pr 43868That installs a local version of the PR into your bun-43868 --bun |
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Inline comments:
In `@src/sys/lib.rs`:
- Line 7401: Update the `write_bounded` fallback so it cannot call a potentially
blocking `write` on the event-loop thread after checking `POLLOUT`; keep the
fallback off that thread or use a write operation that remains nonblocking if
the pipe fills.
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The last 3 reviews of this pull request each found new blocking issues, repeatedly in src/sys/lib.rs. Where they share a root cause, one fix may close them together.
Still open from earlier reviews (4):
- Unresolved: 3 minor or pre-existing, 1 blocking on lines changed since (possibly already fixed).
If you have decided not to act on one of these findings, resolve its thread (a reply alone leaves it open) and the next review stops counting it. To review this commit again now, use Re-run on its "Claude Code Review" check.
…dren Drop the per-call nonblocking write machinery. process.stdout/stderr on a pipe or socket set O_NONBLOCK on fd 1/2 as before (libuv's uv_pipe_open does the same). Two fixes for where that flag leaks: - spawn clears O_NONBLOCK on the fds that become the child's 0-2, like libuv's uv__process_child_init, so plain tools with inherited stdio do not get EAGAIN. - the native console writer waits for POLLOUT on EAGAIN instead of dropping the rest of the line.
Jarred-Sumner
changed the title
… really doesn't wait
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Inline comments:
In `@src/spawn_sys/spawn_process.rs`:
- Line 801: Change the `update_nonblocking` setup for `PosixStdio::Pipe(fd)` so
making the child descriptor blocking does not clear `O_NONBLOCK` on the
caller-owned descriptor shared with the parent. Use child-specific descriptor
setup in both spawn paths, preserving the parent’s nonblocking mode and the
child’s required blocking behavior.
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Beyond the inline finding, I also checked the new EAGAIN branch in fd_write_all_quiet for hangs: posix::poll retries EINTR itself, and a POLLHUP/POLLERR wakeup falls through to a retried write(2) that returns EPIPE and exits the loop, so an infinite-timeout poll cannot park the caller once the reader is gone.
Extended reasoning...
This push drops the earlier PipeWriter/pipe_writable_space rework and keeps three native changes: clearing O_NONBLOCK on inherited and piped child stdio fds in spawn_process_posix, adding the private MSG_NBIO flag to SEND_FLAGS_NONBLOCK on macOS, and polling for POLLOUT on EAGAIN in the console writer. No security-sensitive surface is touched. The remaining inline finding concerns the parent's own stdout pipe becoming blocking after any inherit spawn, which is a behavior change from the base branch that a human should weigh against the stated goal of matching libuv.
The last 4 reviews of this pull request each found new blocking issues, repeatedly in src/sys/lib.rs. Where they share a root cause, one fix may close them together.
The pipe write path in src/sys/lib.rs sizes blocking writes from guessed free space (fstat fields, slot arithmetic, EOPNOTSUPP fallback); any wrong guess parks the JS thread.
Bounding each pipe write to what the kernel guarantees after POLLOUT, never to an estimated free-space count, should close the reported cases that share this cause.
Still open from earlier reviews (1):
- Unresolved: 1 minor or pre-existing.
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The case in #33827 still fails on this branch (6bd759f). This PR clears { bun -e 'void process.stdout.isTTY'; head -c 300000 /dev/zero; } | { sleep 0.4; wc -c; }
The I left #33827 open for that reason. Fold the change in here or land it separately, as you prefer. |
…free A stdout pipe on Linux now always writes with pwritev2(RWF_NOWAIT), and a stdout socket on any Unix with send(MSG_DONTWAIT|MSG_NBIO), instead of a plain write that relied on the fd's O_NONBLOCK. Both are the same single syscall, so the parent stays asynchronous even after an inherit spawn cleared the flag on the shared description. TTYs get their own poll flag so they keep the plain blocking write. macOS pipes have no per-call option and behave like Node there.
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The last 5 reviews of this pull request each found new blocking issues, repeatedly in src/sys/lib.rs and test/js/node/process/process-stdio.test.ts. Where they share a root cause, one fix may close them together.
The pipe write path in src/io/PipeWriter.rs sizes plain write(2) calls on the now-blocking fd 1/2 from a guess (fstat snapshot, slot estimate, global RWF flag) and parks when wrong.
Routing every stdout/stderr write through one path bounded by what the kernel guarantees (per-call nonblocking flag, else POLLOUT then PIPE_BUF) should close the reported cases that share this cause.
Still open from earlier reviews (2):
- 🔴
src/spawn_sys/spawn_process.rs:801—After spawning any child with an inherited fd 1/2, a parent whose stdout is a pipe can park its JS thread inside write(… - Also unresolved: 1 minor or pre-existing.
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- A tty or pty answering EOPNOTSUPP to preadv2/pwritev2(RWF_NOWAIT) no longer turns the flag off for every pipe in the process. Only ENOSYS does; each reader/writer remembers "not on this fd" and stops asking. - is_tty is unix-only (dead_code on Windows broke the build). - Test: gate the RWF_NOWAIT case on Linux >= 6.4 (pipes lack FMODE_NOWAIT before that); assert exact bytes rather than lengths.
…led nonblocking; dedupe stdio drain tests
| env: bunEnv, | ||
| stdout: "pipe", |
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🟡 nit (optional): when this test fails before the kill, the gated reader child is orphaned and idles forever on the CI runner. process.kill(readerPid!, "SIGUSR1") at test/js/node/process/process-stdio.test.ts:251 runs only after expect(ret).toBe(false) passes, and await using proc only terminates sh, not the pipeline's two bun children. Fix: signal or kill the reader in a try/finally (or once its pid is known, register it for cleanup) so it exits on every failure path, including the "stderr closed early" throw from next().
Why this was flagged
expectDrainOverPipe at test/js/node/process/process-stdio.test.ts:222-245 spawns sh -c '"$0" -e "$1" | "$0" -e "$2"'; the reader child (gatedReader, line 208-210) installs a SIGUSR1 handler and a setInterval of 1<<30 ms, so it never exits on its own. The only thing that ends it is process.kill(readerPid!, "SIGUSR1") at line 240, which is reached only after expect(ret).toBe(false) at line 239 succeeds and after every events.next() call returned without throwing. If ret is true, or next() throws "stderr closed early", the test fails and the async dispose of proc kills only the sh process; Bun.spawn does not put the pipeline in its own process group, so the reader bun process (and the writer, if it is still blocked in write) keep running as orphans. On a persistent CI runner that is a leaked process per failed run; the base branch has no such test.
Verification: nit — triggers whenever expectDrainOverPipe fails before its kill line (writer reports ret:true, or the writer prints a non-JSON line such as a crash stack trace so JSON.parse at line 183 throws). In /home/claude/bun/test/js/node/process/process-stdio.test.ts the gated reader (lines 209-211) installs process.on("SIGUSR1", ...) and setInterval(() => {}, 1 << 30) and only reads stdin inside…
| #if defined(__APPLE__) | ||
| extern int Bun__doesMacOSVersionSupportSendRecvMsgX(); | ||
| /* XNU's sosend() only skips waiting for buffer space for MSG_NBIO (private, 0x20000); MSG_DONTWAIT alone still blocks there. */ | ||
| #define LIBUS_SEND_DONTWAIT (MSG_DONTWAIT | 0x20000) |
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🟡 nit (optional): maintainers now have the private XNU MSG_NBIO value written twice as a bare literal, so a future correction to one copy silently misses the other. bsd.c:66 spells it as 0x20000 inside LIBUS_SEND_DONTWAIT while src/sys/lib.rs:3104 defines MSG_NBIO: i32 = 0x20000 for the same purpose. Fix: give the C side a named MSG_NBIO-style define (with the XNU source line it comes from) that LIBUS_SEND_DONTWAIT ORs in, so both send paths (bsd_send, bsd_sendmsg) and the Rust SEND_FLAGS_NONBLOCK name the same constant instead of repeating the magic number.
Why this was flagged
The diff adds the Darwin-private send flag in two unrelated modules: packages/bun-usockets/src/bsd.c:66 #define LIBUS_SEND_DONTWAIT (MSG_DONTWAIT | 0x20000) and src/sys/lib.rs:3104 const MSG_NBIO: i32 = 0x20000;. Neither references the other, and the C copy is an unnamed literal inside a macro. If the value or its applicability is ever revised (for example after the private-flag concern already discussed on this PR), whoever edits src/sys/lib.rs will not find the bsd.c copy by name, and usockets TCP/unix socket sends on macOS keep the stale flag while FileSink/spawn stdio sends use the new one. No runtime failure today; the base branch had neither copy. Repository review guidance asks that a fact live in one place and that magic numbers be named constants.
Verification: nit. Both citations check out: packages/bun-usockets/src/bsd.c:66 adds #define LIBUS_SEND_DONTWAIT (MSG_DONTWAIT | 0x20000) with the value as an anonymous literal inside the macro (only the comment on line 65 names it as MSG_NBIO), and src/sys/lib.rs:3104 adds const MSG_NBIO: i32 = 0x20000; (macOS-gated, folded into SEND_FLAGS_NONBLOCK at :3108). The two definitions are in separate modules…
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Leaving the literal in both places: the C side shares no header with bun_sys, and each site carries the one-line XNU note.
…(macOS poll(2) is unreliable on FIFOs)
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What does this PR do?
Same model as Node/libuv for
process.stdout/process.stderron POSIX: a pipe or socket behind fd 1/2 isO_NONBLOCK(libuv'suv_pipe_opendoes this too) and written asynchronously; TTYs and files are synchronous.O_NONBLOCKlives on the shared open file description, so it leaked in two places that Node covers and Bun did not:stdio: "inherit"child runshead -c 1000000after parent usedprocess.stdoutEAGAIN, short output, exit 1O_NONBLOCKconsole.log(1 MiB line)afterprocess.stdout.isTTY, pipedprocess.stdout.write(1 MiB)to a full pipefalse+'drain'false+'drain'(unchanged)false+'drain'spawnclearsO_NONBLOCKon the fds that become the child's 0–2, as libuv'suv__process_child_initdoes. Done parent-side beforeposix_spawn(same effect, since the description is shared, and macOS file actions cannotfcntl).POLLOUTonEAGAINinstead of dropping the rest of the line. Node does not need this becauseconsole.*goes through the stdout stream there.Supersedes #33560 and #33827.
How did you verify your code works?
test/js/node/process/process-stdio.test.tsgains 9 tests (fd-flag probe in an inherit child,head -cthrough inherited stdout, 1 MiBconsole.log/console.error, 1 MiB through a realpipe(2)viash, and thefalse+'drain'contract over both a socketpair and a real pipe). 6 fail on 1.4.3 (the post-spawn ones guard this PR's own spawn change), all pass here; also ranprocess-stdin,spawn,child-process-stdio,console-loglocally on macOS.