io: leave the fd with the caller when a POSIX pipe writer fails to start by robobun · Pull Request #38354 · oven-sh/bun

robobun · 2026-08-14T03:42:31Z

Problem

ASAN test workers (bun test --test-worker --isolate) die with panic: assertion failed: err.is_none() in <bun_core::util::Fd as bun_sys::fd::FdExt>::close (src/sys/fd.rs:73), called from bun_io::closer::Closer on a thread pool thread: the deferred close(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.
Cause: PosixStreamingWriter::start (src/io/PipeWriter.rs) stores the fd in a freshly created poll and then registers it. When epoll_ctl fails it returns Err with the poll still holding the fd. FileSink::setup (src/runtime/webcore/FileSink.rs:679) closes the fd on that Err, and Blob.writer() then releases the sink, whose writer Drop hands the same fd number to Closer. Debug builds trip the assertion; release builds close whatever fd has been given that number in the meantime (reproduced: a /dev/null fd opened right after came back EBADF).
What makes the registration fail in CI is the EEXIST from bun test --isolate (Linux): intermittent unhandled EEXIST: file already exists, epoll_ctl while initializing process.stderr fails test files with no named tests #37968: under --isolate each file's process.stdout/stderr sink leaves a stale epoll entry keyed on a dup number that a later dup lands on. Since internal/fs/streams wraps Bun.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 when max_user_watches is exhausted, for instance).
PosixBufferedWriter::start has the same shape. Its callers happened to rely on the opposite convention (the writer closes the fd on failure), as did Bun.Terminal and the subprocess stdin FileSink, so the two conventions coexisted and the callers disagreed about who owns the fd after a failed start().

Fix

A failed 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 earlier start() is not touched: it still holds that earlier fd, which the writer does own (the shell re-arms its writer this way).
Why this direction: it makes 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 on Err, the writer closes it after Ok. FileSink::setup is already written this way and is now correct without changes.
Callers that relied on the writer closing the fd on failure now close it themselves: Bun.Terminal closes its pty write fd on every platform (the Windows branch already did), Writable::init closes the stdin pipe after releasing the sink, StaticPipeWriter::start closes its stdio_result. The shell IOWriter's EINVAL/EPERM retry no longer needs to tear the poll down itself; it owns its fd separately and is otherwise unchanged.
The same callers also relied on the writer's later close() reporting on_close, which an empty writer no longer does. For Bun.spawn with a buffer stdin that report was what retired the Writable::Buffer slot; left in place it counts as pending activity and pins the Subprocess wrapper 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 before start() and cannot fail there. The shell drops its subprocess outright on this path and Bun.Terminal destroys itself, so they needed nothing.
close_impl on Windows ignored its close_fd argument; it now honors it so the new helper means the same thing on every platform (no live Windows caller passes false; PollOrFd only backs the POSIX reader and writers).
Verification:
- 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 report reusedStillOpen: 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 writable epoll_ctl registration (Bun issues them through syscall(2)), and fixtures for stdin: "pipe", a buffer stdin (memfd disabled so the pipe writer path is taken) and new Bun.Terminal() assert that the fd count and the number of live Subprocess/Terminal wrappers 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 reports leakedFds: 1, and without the bindings change the buffer fixture reports leakedWrappers: 1. 30/30 runs stable on the debug build.
- Also run: filesink, bunshell, shell file-io/epipe/output, spawn, spawnSync, terminal, spawn stdin stream suites; cargo fmt --check and cargo clippy on bun_io/bun_spawn; cargo check of bun_io, bun_spawn and bun_runtime for x86_64-pc-windows-msvc.

Background

PollOrFd is the handle inside the POSIX pipe readers and writers: either a bare fd or a FilePoll, 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 through Closer, which performs the close(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::close asserts that close(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.
FileSink is the native writer behind Bun.file(...).writer(), process.stdout/stderr and a piped subprocess stdin; StaticPipeWriter writes a Buffer/Blob stdin into a child; both own a PosixStreamingWriter/PosixBufferedWriter from src/io/PipeWriter.rs.
A Subprocess holds 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 through on_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.
Registering a pollable fd can fail in production with ENOSPC (fs.epoll.max_user_watches) or, as in bun test --isolate (Linux): intermittent unhandled EEXIST: file already exists, epoll_ctl while 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_start leaks 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.
The security scanner's handling of a failed StaticPipeWriter::start releases one ref too many; spawn(windows): free the buffer-stdin writer when its uv_write fails inside start() #37898 already changes that site.

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/bun/util/filesink.test.ts, test/js/bun/spawn/spawn-pipe-start-error.test.ts

coderabbitai · 2026-08-14T03:42:39Z

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Walkthrough

Changes

Pipe 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

Layer / File(s)	Summary
Poll ownership and retry handling `src/io/PipeWriter.rs`, `src/io/pipes.rs`, `src/runtime/shell/IOWriter.rs`	Writer startup distinguishes new poll handles from existing handles. Failed registration releases new handles without closing caller-owned descriptors. Poll retry paths no longer duplicate handle cleanup.
Writer startup caller cleanup `src/runtime/api/bun/Terminal.rs`, `src/runtime/api/bun/js_bun_spawn_bindings.rs`, `src/runtime/api/bun/subprocess/Writable.rs`, `src/spawn/static_pipe_writer.rs`	Startup callers explicitly close or invalidate descriptors and clear failed stdin state when writers do not take ownership.
Failure and descriptor regression coverage `test/js/bun/spawn/spawn-pipe-start-error.test.ts`, `test/js/bun/util/filesink.test.ts`	Linux tests force registration failures and verify errors, descriptor counts, wrapper release, stderr, and process status.

Merge Risk: ⚪ Minimal · up to 8f986

The change adds regression coverage for failed pipe-writer registration and descriptor cleanup paths. No current merge-blocking risk is identified.

🚥 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 main change: POSIX pipe writers leave the file descriptor with the caller when startup fails.
Description check	✅ Passed	The description provides detailed problem, cause, fix, affected callers, background, verification steps, and known separate issues. It uses equivalent headings instead of the template headings, but it…

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robobun · 2026-08-14T03:43:02Z

^{Updated 8:27 AM PT - Sep 4th, 2026}

✅ @robobun, your commit 8f9864d2ebd5c7d598b9b7f76e7479e06d06b37c passed in Build #110167! 🎉

🧪 To try this PR locally:

bunx bun-pr 38354

That installs a local version of the PR into your bun-38354 executable, so you can run:

bun-38354 --bun

robobun · 2026-08-14T03:43:15Z

Status: reproduced locally with the exact crash stack from the ASAN lanes (Closer::run -> Fd::close, src/sys/fd.rs:73) by making Bun.file(fd).writer() fail its epoll registration; on the released build the same scenario closes an unrelated fd instead. The filesink test fails on an unfixed debug build and on bun 1.4.0 and passes with the fix; the spawn/Terminal fixtures guard the caller-side cleanup (each reports leakedFds: 1 with its close removed, and the buffer-stdin one reported leakedWrappers: 1 before 02273aa). Review so far: the buffer-stdin wrapper leak found in review is fixed in 02273aa, close_fd is honored on Windows too, comments trimmed and the FIFO wait bounded in 32855e5. Waiting on CI.

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Inline comments:
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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src/io/PipeWriter.rs
src/io/pipes.rs
src/runtime/api/bun/Terminal.rs
src/runtime/api/bun/js_bun_spawn_bindings.rs
src/runtime/api/bun/subprocess/Writable.rs
src/runtime/shell/IOWriter.rs
src/spawn/static_pipe_writer.rs
test/js/bun/spawn/spawn-pipe-start-error.test.ts
test/js/bun/util/filesink.test.ts

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src/runtime/shell/IOWriter.rs

claude

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.

robobun · 2026-08-14T05:36:33Z

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.

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.

…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.

…t in the filesink test

robobun · 2026-09-04T14:16:54Z

Rebased onto main (4661e49). The two conflicts come from API changes on main:

src/runtime/api/bun/subprocess/Writable.rs: FileSink::create now returns a RefPtr, so the explicit pipe_release is gone. The error arm only closes fd. The sink drops at the return, and its writer holds nothing by then.
src/runtime/api/bun/js_bun_spawn_bindings.rs: throw_value now returns the JsError.

Verified on a debug build of the rebased branch:

test/js/bun/util/filesink.test.ts and test/js/bun/spawn/spawn-pipe-start-error.test.ts pass. The new filesink test still fails on bun 1.4.1 (reusedStillOpen: false).
A second trigger for the same double close: Bun.file(fifo).writer() where epoll_ctl(EPOLL_CTL_ADD) fails with ENOMEM (an LD_PRELOAD shim on syscall(2)). On bun 1.4.1 the fd number is closed on the JS thread and again on a thread pool thread. An fd opened between the two closes comes back EBADF (5 of 5 runs). With this branch the fd is closed once and the other fd stays open.
Also run: test/js/bun/terminal/terminal.test.ts, spawn-stdin-pipe-fd-leak, spawn-pipe-stale-fd-unregister, spawn-stdin-readable-stream-edge-cases, and the shell file-io, epipe, shell-write-fault and shell-pipe-read-fault suites.

coderabbitai · 2026-09-04T14:16:54Z

Note

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robobun · 2026-09-04T14:17:42Z

State after the rebase: no open review threads, CI for 3324d91 is running as build 110154. I will act on the CodeRabbit review when it lands if it raises something new.

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Inline comments:
In `@test/js/bun/spawn/spawn-pipe-start-error.test.ts`:
- Line 120: Replace both require calls in the generated fixture with
module-scope imports while keeping fixture.js unchanged, preserving the
pipe-cleanup test behavior.

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📒 Files selected for processing (9)

src/io/PipeWriter.rs
src/io/pipes.rs
src/runtime/api/bun/Terminal.rs
src/runtime/api/bun/js_bun_spawn_bindings.rs
src/runtime/api/bun/subprocess/Writable.rs
src/runtime/shell/IOWriter.rs
src/spawn/static_pipe_writer.rs
test/js/bun/spawn/spawn-pipe-start-error.test.ts
test/js/bun/util/filesink.test.ts

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src/runtime/shell/IOWriter.rs

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robobun · 2026-09-04T15:04:50Z

The same double close also causes a use-after-poison on the main thread. This branch removes that too. I checked it with the sequence below. The fd numbers are from one run.

Bun.file(A).writer() on a FIFO. epoll_ctl(EPOLL_CTL_ADD) fails with ENOMEM. FileSink::setup closes fd 10, and the sink teardown queues a second close of fd 10 on the thread pool.
Bun.file(B).writer() (W2) on a second FIFO gets fd 10. A 512 KiB write fills the pipe and arms EPOLLOUT.
Bun.file(10).writer() (W3) dups fd 10 to fd 11. The file description of B stays open through fd 11.
The queued close runs and closes W2's fd 10. epoll keeps the registration, because fd 11 still refers to the same file.
W2's next write fails with EBADF. Its teardown cannot remove the registration (EPOLL_CTL_DEL on fd 10 fails), and its FilePoll slot goes back to the hive, poisoned.
A read drains B. epoll reports EPOLLOUT with W2's freed FilePoll as the event data.

An LD_PRELOAD shim on syscall(2) injects the ENOMEM. The shim also holds the pool close until step 3 is done, so every run takes the same order.

Debug ASAN build of main (4661e49): AddressSanitizer: use-after-poison, READ of size 4 on the main thread, in Bun__internal_dispatch_ready_poll (src/io/posix_event_loop.rs:1484) from us_internal_dispatch_ready_polls. 3 of 3 runs.
bun 1.4.1 release: no crash. W2.end() rejects with EBADF, and only 64 KiB of the 512 KiB arrive. 3 of 3 runs.
Debug ASAN build of this branch: fd 10 is closed once, nothing is closed on the pool, and all 512 KiB arrive. 5 of 5 runs.

W3 must stay reachable until the drain. If the GC collects it, fd 11 closes, the kernel drops the stale registration, and the read in step 6 does not crash.

claude

I re-reviewed after the latest push and both of my earlier findings are addressed — close_impl now honors close_fd on Windows so close_without_closing_fd() is honest everywhere, and the buffer-stdin error path in js_bun_spawn_bindings.rs retires the slot via on_close_io(Stdin) so the wrapper is no longer pinned. No new issues found. Given this reworks fd ownership on error paths across six call sites, a human pass on the ownership convention is still worth having.

Checked: Writable::init's new fd.close() against the dropped pipe_ref — the writer's handle is Closed after a failed start() and FileSink's Drop doesn't touch self.fd, so no double-close there.
Checked: the IOWriter retry path — start() now leaves handle == Closed on Err, so the removed manual teardown was redundant and the recursive __start() re-entry sees the same state as before.
Checked: the existing_poll.is_none() gate in both writers — a poll from a prior successful start() (shell re-arm) is left intact, only the freshly-created poll is torn down.

Extended reasoning...

Overview

This PR changes the ownership contract of PosixBufferedWriter::start() and PosixStreamingWriter::start() so that on Err the writer holds nothing and the caller retains ownership of the fd it passed in. A new PollOrFd::close_without_closing_fd() helper tears down a freshly-created poll without closing its fd; the Windows arm of close_impl is fixed to honor its close_fd flag so the helper is honest cross-platform. Six call sites are updated to the new convention: Writable::init and StaticPipeWriter::start now close the fd themselves on failure, js_bun_spawn_bindings retires the buffer-stdin slot via on_close_io(Stdin) so the Subprocess wrapper isn't pinned as pending activity, Terminal closes write_fd unconditionally, and the shell IOWriter drops its now-redundant manual poll teardown. Two Linux-only tests are added: an LD_PRELOAD shim that fails writable epoll_ctl adds, and an EEXIST provocation via a stale epoll registration on a reused dup number.

Security risks

No direct security surface. The bug being fixed is a double-close of a reused fd number, which in release builds silently closes an unrelated fd — a correctness/reliability issue rather than a security boundary. The fix does not introduce new untrusted-input parsing, allocation, or privilege paths.

Level of scrutiny

High. This is native-code resource ownership on error paths — the category REVIEW.md calls out as most-blocked. The change flips an ownership convention that six independent callers relied on in different directions, and the PR itself notes two more sites (shell subprocess abort, security scanner) with adjacent bugs handed off separately. Error paths like epoll_ctl failure are rarely exercised, so a missed caller would ship silently. The tests are Linux-only and rely on LD_PRELOAD and epoll implementation detail (EEXIST on reused dup numbers), so macOS coverage is by inspection only.

Other factors

Both concerns I raised on the earlier push are now addressed in the diff, and I traced the fixes through: the on_close_io(Stdin) call is placed after the borrow of subprocess.stdin is released (the match result is captured to a local first), and it's cfg-gated to POSIX where the failure can occur. The Windows close_impl fix has no live Windows caller of close_without_closing_fd() today, so it's belt-and-suspenders. There are several github-actions[bot] inline threads on this PR that were resolved by the author; I cannot see their content to judge whether they were substantive, which is another reason to leave the final call to a human. The tests follow harness conventions (tempDir, bunEnv/bunExe, test.concurrent, bounded polling instead of sleep, stdout asserted before exitCode) and the fixture asserts both fd count and wrapper count via heapStats, which covers the pending-activity leak I flagged earlier.

### Problem - `new Bun.Terminal()` and `Bun.spawn({ terminal: {...} })` use a freed `Terminal` when the pty reader's `epoll_ctl(EPOLL_CTL_ADD)` fails. ASAN: `heap-use-after-free` in `RefCount<Terminal>::ref_` from `Terminal::init_terminal` (`Terminal.rs:492`), freed by `Terminal::on_reader_finished` (`Terminal.rs:1829`). JS receives the dangling pointer. - `PosixBufferedReader::start()` (`src/io/PipeReader.rs:547`) reports a failed poll registration by calling `on_reader_error`, and still returns `Ok`. `Terminal::on_reader_error` releases the reader's ref. `init_terminal` took that ref only after `start()` returned, so the release dropped the last ref. ### Fix - `init_terminal` takes the reader's ref before `start()`, as `SubprocessPipeReader::start` does for the same contract. - If the reader is done after `start()`, `init_terminal` releases the pty and the constructor throws `Failed to start terminal reader`, as the writer arm does. - The same check runs after the constructor's first `read()`. On Linux only fault injection reaches it (see Notes). - Verified: `test/js/bun/spawn/spawn-pipe-start-error.test.ts` (4 new cases, main fails all 4: 2 ASAN reports, 2 leaked wrappers). Also `test/js/bun/terminal/`. Self-reviewed: 3 concerns raised, 3 addressed (all on the overlap with #40204, see Notes). ### Background - `Terminal` is intrusively refcounted. The JS wrapper, the pty writer and the pty reader hold one ref each. The reader and the writer release theirs in their close or error callback. - `PosixBufferedReader` (`src/io`) is the poll-driven pipe reader. `start()` registers the fd with epoll or kqueue. - Reach: the writer's ADD comes first and already throws (#38354). The reader's ADD fails alone only with exactly one epoll watch left (`fs.epoll.max_user_watches`), or on a transient `ENOMEM`. <details><summary>Notes</summary> - Reported via a fuzz ledger (entry 50336): one injected `epoll_ctl` failure under ASAN on main 09bb546, reproduced with `strace -f -qq -o /dev/null -e trace=epoll_ctl -e inject=epoll_ctl:error=ENOMEM:when=4 bun t.mjs`. Repro without strace: compile the shim from the test file, then run `FAIL_EPOLL_CTL=pty-reader-add LD_PRELOAD=./shim.so bun -e 'new Bun.Terminal({})'`. - Overlap with #40204 (open, conflicting with main): that refactor also moves the reader's ref before `reader.start()` and lists it under "pre-existing bugs fixed in passing". It has only the reorder: no `READER_DONE` check, no throw, no test. With the reorder alone the constructor returns a `Terminal` that is already dead (no `exit` callback, wrapper never collected) where main frees it, and the 4 new cases still fail. This PR adds the checks, the teardown and the tests. When #40204 rebases over this, `init_terminal` must keep both `READER_DONE` checks and the `ReaderStartFailed` teardown. - The post-read check is a guard for the same contract, not a second observed failure. Without it, a reader that ends during the constructor's first `read()` leaves that same dead `Terminal`. The re-arm after the first read is an `EPOLL_CTL_MOD`. The kernel's `ep_modify` does not return `ENOSPC` or `ENOMEM`, and a fresh pty master with its slave open reads `EAGAIN`, not EOF or an error. The `pty-reader-mod` shim mode injects a failure there to exercise the guard. - The release canary `1.4.3-canary.1+b99371011` also misbehaves under the ADD injection: it aborts with `panic: usockets_loop: uws_loop not initialized (call ensure_waker first)`, which is a read of the freed `Terminal`'s event loop handle. - State after `on_reader_error` runs inside `start()` on POSIX, with the fix: `finish_io_after_eof` has closed the reader and ended the writer, so both of their refs are released and only the initial ref is left. `fail_reader_start` closes the master and slave fds through `close_internal` and drops the initial ref. The `Err` arm of `start()` (reachable on Windows only) returns the reader's ref itself, because no callback ran. - Windows: the `Err` arm of `start()` is not reachable through the Linux shim. I ran it by hand on a Windows x64 debug build with the debug-only `BUN_INTERNAL_FAIL_PIPE_READER_START=1` injection: the constructor throws `Failed to start terminal reader`, the debug `RefCount` assertions stay quiet, and no wrapper is left after GC. - I did not change `PosixBufferedReader::start()` to return `Err` for a failed registration. Every other POSIX caller depends on the current contract, and `SubprocessPipeReader::start` documents why it does not want `Err` there. - Other callers of `start()`: `filter_run`, `multi_run`, `cron`, shell `IOReader`, `git_runner`, `lifecycle_script_runner`, `security_scanner` and `FileReader` take their ref or counter before `start()`, or are not refcounted. `FileResponseStream::start` (`src/runtime/server/FileResponseStream.rs:276`) is the exception in shape: it takes its read ref after `start()` returns `Ok` and does not check for a finished stream. Its release is gated on a flag, so it cannot free early. I served a FIFO through `Bun.serve` with the same injection and saw no leaked fd: the second failed registration releases the stream. That site is not part of this PR. - The test extends the `LD_PRELOAD` shim in that file. The shim interposes `syscall()` because Bun registers file polls through `syscall(SYS_epoll_ctl, ...)`. The new modes fail only a readable registration of a pty master (`TIOCGPTN` succeeds on that fd only). - The test file change moves the shim build and `runFixture` from the first `describe` to module scope so that the new `describe` can share them. The three existing tests are unchanged. </details>  --- **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/bun/spawn/spawn-pipe-start-error.test.ts

### Problem - `new Bun.Terminal()` and `Bun.spawn({ terminal: {...} })` use a freed `Terminal` when the pty reader's `epoll_ctl(EPOLL_CTL_ADD)` fails. ASAN: `heap-use-after-free` in `RefCount<Terminal>::ref_` from `Terminal::init_terminal` (`Terminal.rs:492`), freed by `Terminal::on_reader_finished` (`Terminal.rs:1829`). JS receives the dangling pointer. - `PosixBufferedReader::start()` (`src/io/PipeReader.rs:547`) reports a failed poll registration by calling `on_reader_error`, and still returns `Ok`. `Terminal::on_reader_error` releases the reader's ref. `init_terminal` took that ref only after `start()` returned, so the release dropped the last ref. ### Fix - `init_terminal` takes the reader's ref before `start()`, as `SubprocessPipeReader::start` does for the same contract. - If the reader is done after `start()`, `init_terminal` releases the pty and the constructor throws `Failed to start terminal reader`, as the writer arm does. - The same check runs after the constructor's first `read()`. On Linux only fault injection reaches it (see Notes). - Verified: `test/js/bun/spawn/spawn-pipe-start-error.test.ts` (4 new cases, main fails all 4: 2 ASAN reports, 2 leaked wrappers). Also `test/js/bun/terminal/`. Self-reviewed: 3 concerns raised, 3 addressed (all on the overlap with oven-sh#40204, see Notes). ### Background - `Terminal` is intrusively refcounted. The JS wrapper, the pty writer and the pty reader hold one ref each. The reader and the writer release theirs in their close or error callback. - `PosixBufferedReader` (`src/io`) is the poll-driven pipe reader. `start()` registers the fd with epoll or kqueue. - Reach: the writer's ADD comes first and already throws (oven-sh#38354). The reader's ADD fails alone only with exactly one epoll watch left (`fs.epoll.max_user_watches`), or on a transient `ENOMEM`. <details><summary>Notes</summary> - Reported via a fuzz ledger (entry 50336): one injected `epoll_ctl` failure under ASAN on main 09bb546, reproduced with `strace -f -qq -o /dev/null -e trace=epoll_ctl -e inject=epoll_ctl:error=ENOMEM:when=4 bun t.mjs`. Repro without strace: compile the shim from the test file, then run `FAIL_EPOLL_CTL=pty-reader-add LD_PRELOAD=./shim.so bun -e 'new Bun.Terminal({})'`. - Overlap with oven-sh#40204 (open, conflicting with main): that refactor also moves the reader's ref before `reader.start()` and lists it under "pre-existing bugs fixed in passing". It has only the reorder: no `READER_DONE` check, no throw, no test. With the reorder alone the constructor returns a `Terminal` that is already dead (no `exit` callback, wrapper never collected) where main frees it, and the 4 new cases still fail. This PR adds the checks, the teardown and the tests. When oven-sh#40204 rebases over this, `init_terminal` must keep both `READER_DONE` checks and the `ReaderStartFailed` teardown. - The post-read check is a guard for the same contract, not a second observed failure. Without it, a reader that ends during the constructor's first `read()` leaves that same dead `Terminal`. The re-arm after the first read is an `EPOLL_CTL_MOD`. The kernel's `ep_modify` does not return `ENOSPC` or `ENOMEM`, and a fresh pty master with its slave open reads `EAGAIN`, not EOF or an error. The `pty-reader-mod` shim mode injects a failure there to exercise the guard. - The release canary `1.4.3-canary.1+b99371011` also misbehaves under the ADD injection: it aborts with `panic: usockets_loop: uws_loop not initialized (call ensure_waker first)`, which is a read of the freed `Terminal`'s event loop handle. - State after `on_reader_error` runs inside `start()` on POSIX, with the fix: `finish_io_after_eof` has closed the reader and ended the writer, so both of their refs are released and only the initial ref is left. `fail_reader_start` closes the master and slave fds through `close_internal` and drops the initial ref. The `Err` arm of `start()` (reachable on Windows only) returns the reader's ref itself, because no callback ran. - Windows: the `Err` arm of `start()` is not reachable through the Linux shim. I ran it by hand on a Windows x64 debug build with the debug-only `BUN_INTERNAL_FAIL_PIPE_READER_START=1` injection: the constructor throws `Failed to start terminal reader`, the debug `RefCount` assertions stay quiet, and no wrapper is left after GC. - I did not change `PosixBufferedReader::start()` to return `Err` for a failed registration. Every other POSIX caller depends on the current contract, and `SubprocessPipeReader::start` documents why it does not want `Err` there. - Other callers of `start()`: `filter_run`, `multi_run`, `cron`, shell `IOReader`, `git_runner`, `lifecycle_script_runner`, `security_scanner` and `FileReader` take their ref or counter before `start()`, or are not refcounted. `FileResponseStream::start` (`src/runtime/server/FileResponseStream.rs:276`) is the exception in shape: it takes its read ref after `start()` returns `Ok` and does not check for a finished stream. Its release is gated on a flag, so it cannot free early. I served a FIFO through `Bun.serve` with the same injection and saw no leaked fd: the second failed registration releases the stream. That site is not part of this PR. - The test extends the `LD_PRELOAD` shim in that file. The shim interposes `syscall()` because Bun registers file polls through `syscall(SYS_epoll_ctl, ...)`. The new modes fail only a readable registration of a pty master (`TIOCGPTN` succeeds on that fd only). - The test file change moves the shim build and `runFixture` from the first `describe` to module scope so that the new `describe` can share them. The three existing tests are unchanged. </details>  --- **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/bun/spawn/spawn-pipe-start-error.test.ts

main's #38354 made a failed POSIX `start()` leave the fd with the caller. Take that contract here: - `FileSink::setup` and `PosixStreamingWriter::start` keep main's bodies. This branch's fd-ownership change is gone. `start()` still passes the `EventLoopCtx` to `register_with_fd`. - Drop this branch's filesink.test.ts case. main's "whose registration fails closes the dup exactly once" covers the double close. - `unregister_with_fd_impl` takes main's `disarmed_only` logic and resolves the watcher fd through `loop_mut(vm)`.

github-actions Bot added the claude label Aug 14, 2026

claude Bot reviewed Aug 14, 2026

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Comment thread src/spawn/static_pipe_writer.rs Resolved

Comment thread src/io/pipes.rs Resolved