io: leave the fd with the caller when a POSIX pipe writer fails to start - #38354
io: leave the fd with the caller when a POSIX pipe writer fails to start#38354robobun wants to merge 3 commits into
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PosixStreamingWriter::start and PosixBufferedWriter::start stored the fd in a freshly created poll before registering it; when registration failed they returned Err with the poll still holding the fd. FileSink::setup closes the fd itself on that Err, and the sink's teardown then dropped the writer, which queued a second close of the same fd number on the thread pool. By the time it ran the number usually belonged to an unrelated fd; debug builds trip the EBADF assertion in Closer instead. A failed start() now releases the poll it created and leaves the fd to the caller, on both writers. The callers that relied on the writer closing it on failure close it themselves: Bun.Terminal's pty write fd (now the same on every platform), the subprocess stdin FileSink in Writable::init, and StaticPipeWriter::start. The shell IOWriter's EINVAL/EPERM retry no longer has to tear the poll down by hand. Tests: filesink.test.ts provokes the registration failure (EEXIST on a reused dup number) and checks the fd reused afterwards survives; the spawn and Terminal callers are checked with an LD_PRELOAD shim failing writable epoll_ctl registrations, asserting no fd leaks and no double close.
WalkthroughChangesPipe writer startup now preserves caller-owned descriptors when poll registration fails. Poll handles, sinks, terminals, subprocess stdin state, and static pipe descriptors receive explicit failure cleanup. Linux regression tests force registration errors and verify descriptor and wrapper cleanup. Pipe startup failure cleanup
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Status: reproduced locally with the exact crash stack from the ASAN lanes ( |
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…e_fd on Windows With the writer holding nothing after a failed start(), the close() issued at process exit no longer reports on_close, which is what used to swap the Writable::Buffer slot to Ignore. The slot then counted as pending activity and kept the Subprocess wrapper alive for the rest of the process. Retire it on the error path in the spawn bindings (POSIX only: Windows adopts the pipe before start(), which cannot fail there). PollOrFd::close_impl ignored close_fd on Windows; honor it so close_without_closing_fd means the same thing on every platform. The spawn-pipe-start-error fixtures now also count live Subprocess/Terminal wrappers against a baseline; the buffer stdin fixture reports one leaked wrapper without the bindings change.
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In `@test/js/bun/util/filesink.test.ts`:
- Around line 672-674: Bound the FIFO readiness loop around fifoWriter and the
parked readFile calls with a deadline, and throw a clear setup error if
fdsOf(fifoPath).length remains below three before it expires. Keep awaiting the
observable descriptor condition, while retaining the existing Bun.sleep polling
interval.
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📒 Files selected for processing (9)
src/io/PipeWriter.rssrc/io/pipes.rssrc/runtime/api/bun/Terminal.rssrc/runtime/api/bun/js_bun_spawn_bindings.rssrc/runtime/api/bun/subprocess/Writable.rssrc/runtime/shell/IOWriter.rssrc/spawn/static_pipe_writer.rstest/js/bun/spawn/spawn-pipe-start-error.test.tstest/js/bun/util/filesink.test.ts
💤 Files with no reviewable changes (1)
- src/runtime/shell/IOWriter.rs
…t in the filesink test
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I re-reviewed after 02273aa and 32855e5 — both of my earlier findings (the Writable::Buffer wrapper leak and the Windows close_fd semantics) are addressed, and this run found nothing new. Given the fd-ownership convention change ripples across four callers' error paths (FileSink, Terminal, StaticPipeWriter, shell IOWriter) plus the on_close_io retirement, a maintainer sign-off would still be worthwhile.
Checked this pass: on_close_io(Stdin) on the Buffer arm swaps to Ignore, detaches the source, and derefs the RefPtr, so on_process_exit later sees Ignore and update_has_pending_activity can downgrade; WindowsBufferedWriter::start_with_current_pipe unconditionally returns Ok(()), so the #[cfg(not(windows))] gate on that call is sound; the shell's removed close_impl/= Closed was made redundant by close_without_closing_fd leaving handle == Closed, and the EINVAL/EPERM retry re-enters the !pollable branch which reassigns handle = Fd(fd).
Extended reasoning...
Overview
Fixes a double-close on the POSIX pipe-writer error path: when epoll_ctl registration fails, Posix{Buffered,Streaming}Writer::start previously left the just-created poll holding the fd, so the caller (which closes the fd on Err) and the writer's later teardown both closed the same fd number. The fix makes a failed start() release the fresh poll without closing the fd, and updates the four callers that had relied on the opposite convention (Terminal, Writable::init, StaticPipeWriter::start, shell IOWriter). A follow-up commit added on_close_io(Stdin) in the spawn bindings so a failed buffer-stdin start() retires the Writable::Buffer slot (my earlier finding), and made close_impl honor close_fd on Windows so close_without_closing_fd is truthful everywhere.
Security risks
None. This is internal fd-lifecycle bookkeeping on error paths; no user input parsing, auth, or trust-boundary changes.
Level of scrutiny
High. This is exactly the memory-safety category REVIEW.md flags hardest: fd ownership on error paths across several subsystems, where a mistake is a silent close of an unrelated fd in release builds. The PR already needed one review round to catch a Strong-ref leak the ownership flip introduced. The fix is well-reasoned and the LD_PRELOAD + heapStats tests pin down both the fd count and the wrapper count for each caller, but the number of distinct error paths whose ownership contract changed makes this a maintainer-level call rather than an auto-approve.
Other factors
All prior thread feedback is resolved (my two findings, the comment-cop trims, CodeRabbit's bounded-poll suggestion). I verified this pass that on_close_io(Stdin) on the Buffer arm fully retires the slot (swap to Ignore + source.detach() + buffer.deref()) so on_process_exit and update_has_pending_activity behave correctly afterward; that the #[cfg(not(windows))] on the new on_close_io call is sound because WindowsBufferedWriter::start_with_current_pipe returns Ok(()) unconditionally; and that the shell IOWriter retry still works now that start() itself leaves handle == Closed on failure. CI (#95479) is still running.
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Agreed that this wants a maintainer's eyes on the ownership direction (the "why this direction" bullet in the description is the judgment call; the rest follows from it). Nothing further planned from my side unless CI turns something up: build 95479 is at 177 passed with 2 jobs left. |
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Problem
bun test --test-worker --isolate) die withpanic: assertion failed: err.is_none()in<bun_core::util::Fd as bun_sys::fd::FdExt>::close(src/sys/fd.rs:73), called frombun_io::closer::Closeron a thread pool thread: the deferredclose(2)got EBADF, so the fd number had already been closed by someone else. Seen three times across seven PR builds (for example build 94781); the worker's in-flight file is reported as the crash and passes on retry, so it never fails a build.PosixStreamingWriter::start(src/io/PipeWriter.rs) stores the fd in a freshly created poll and then registers it. Whenepoll_ctlfails it returnsErrwith the poll still holding the fd.FileSink::setup(src/runtime/webcore/FileSink.rs:679) closes the fd on thatErr, andBlob.writer()then releases the sink, whose writerDrophands the same fd number toCloser. Debug builds trip the assertion; release builds close whatever fd has been given that number in the meantime (reproduced: a/dev/nullfd opened right after came back EBADF).EEXISTfrom bun test --isolate (Linux): intermittent unhandledEEXIST: file already exists, epoll_ctlwhile initializing process.stderr fails test files with no named tests #37968: under--isolateeach file'sprocess.stdout/stderrsink leaves a stale epoll entry keyed on a dup number that a later dup lands on. Sinceinternal/fs/streamswrapsBun.file(fd).writer()in a try/catch, that failure is silent and only the double close is visible. test --isolate: end the outgoing file's process stdio sinks at the global swap #38008 removes that trigger; this PR fixes the double close, which follows any registration failure (ENOSPC whenmax_user_watchesis exhausted, for instance).PosixBufferedWriter::starthas the same shape. Its callers happened to rely on the opposite convention (the writer closes the fd on failure), as didBun.Terminaland the subprocess stdinFileSink, so the two conventions coexisted and the callers disagreed about who owns the fd after a failedstart().Fix
start()on either POSIX writer releases the poll it created (PollOrFd::close_without_closing_fd, new) and leaves the fd with the caller. A poll left over from an earlierstart()is not touched: it still holds that earlier fd, which the writer does own (the shell re-arms its writer this way).start()have no effect on failure, matches the Windows writer (a failed open never adopts the handle), and makes the fd's owner the same on every path: whoever passed it in closes it onErr, the writer closes it afterOk.FileSink::setupis already written this way and is now correct without changes.Bun.Terminalcloses its pty write fd on every platform (the Windows branch already did),Writable::initcloses the stdin pipe after releasing the sink,StaticPipeWriter::startcloses itsstdio_result. The shellIOWriter's EINVAL/EPERM retry no longer needs to tear the poll down itself; it owns its fd separately and is otherwise unchanged.close()reportingon_close, which an empty writer no longer does. ForBun.spawnwith a buffer stdin that report was what retired theWritable::Bufferslot; left in place it counts as pending activity and pins theSubprocesswrapper forever (caught in review). The spawn bindings now retire the slot on that error path themselves (on_close_io(Stdin)), on POSIX only since Windows adopts the pipe beforestart()and cannot fail there. The shell drops its subprocess outright on this path andBun.Terminaldestroys itself, so they needed nothing.close_implon Windows ignored itsclose_fdargument; it now honors it so the new helper means the same thing on every platform (no live Windows caller passesfalse;PollOrFdonly backs the POSIX reader and writers).test/js/bun/util/filesink.test.ts("whose registration fails closes the dup exactly once"): recreates the CI condition on a socketpair fd (close a sink's dup from under it, the next dup gets EEXIST), with the thread pool parked so the stray close has to land on the fd that reused the number. Unfixed debug and release builds reportreusedStillOpen: false; with the fix it stays open. Linux only, as kqueue drops registrations with the fd.test/js/bun/spawn/spawn-pipe-start-error.test.ts: an LD_PRELOAD shim fails every writableepoll_ctlregistration (Bun issues them throughsyscall(2)), and fixtures forstdin: "pipe", a buffer stdin (memfd disabled so the pipe writer path is taken) andnew Bun.Terminal()assert that the fd count and the number of liveSubprocess/Terminalwrappers return to baseline with an empty stderr. All three pass on bun 1.4.0 and with this PR; with the three caller-side closes removed each fixture reportsleakedFds: 1, and without the bindings change the buffer fixture reportsleakedWrappers: 1. 30/30 runs stable on the debug build.cargo fmt --checkandcargo clippyonbun_io/bun_spawn;cargo checkofbun_io,bun_spawnandbun_runtimeforx86_64-pc-windows-msvc.Background
PollOrFdis the handle inside the POSIX pipe readers and writers: either a bare fd or aFilePoll, the event loop's epoll/kqueue registration, which records the fd it watches. Closing the handle unregisters the poll and then closes the fd, normally throughCloser, which performs theclose(2)on the thread pool so the event loop does not block on it. That delay is why the second close lands after the fd number has been reused.Fd::closeasserts thatclose(2)did not return EBADF in debug and ASAN builds, because for an internally owned fd EBADF means a double close and therefore a possible close of an unrelated fd. That assertion is the crash in the report and is intentionally left in place.FileSinkis the native writer behindBun.file(...).writer(),process.stdout/stderrand a piped subprocess stdin;StaticPipeWriterwrites a Buffer/Blob stdin into a child; both own aPosixStreamingWriter/PosixBufferedWriterfromsrc/io/PipeWriter.rs.Subprocessholds a strong reference to its own JS wrapper while it has pending activity: a child that has not exited, or a stdio slot still in use. Each slot is retired throughon_close_io, which the stdio object reports when it closes; once the slots are retired and the child has exited, the reference is downgraded and the wrapper becomes collectable.fs.epoll.max_user_watches) or, as in bun test --isolate (Linux): intermittent unhandledEEXIST: file already exists, epoll_ctlwhile initializing process.stderr fails test files with no named tests #37968, EEXIST when epoll still holds an entry for the same open file under the same fd number; epoll keys entries on (open file, fd number), so an entry outlives an fd number that was closed while another fd kept the file open.Bugs noticed on the way, tracked separately
ShellSubprocess::abort_after_failed_startleaks the not-yet-started stdout/stderr pipe fds when the buffered stdin writer fails to start (visible with the shim from the new test). Handed off.StaticPipeWriter::startreleases one ref too many; spawn(windows): free the buffer-stdin writer when its uv_write fails inside start() #37898 already changes that site.