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When a write fails (EPIPE because the reader of stdout went away, for example), IOWriter::on_error and the EndOfFile arm of on_write_pollable delivered one error completion per distinct child and dropped the rest of that child's queued chunks. rm and the OutputTask builtins (ls, mkdir, touch, cp) queue one chunk per argument under the same ChildPtr and finish once every chunk has completed, so with two or more chunks queued they waited forever and the awaited `$` never settled. fail_pending_writers now hands every still-queued chunk its own error completion, walking the queue in place so that a child which stops at its first error can cancel_chunks() the rest from inside the callback. The two children that do stop early, cat and the subprocess CapturedWriter, now do that; the CapturedWriter in particular can be freed by its first error completion, so a second one would be a use-after-free. The EndOfFile path shares the same function instead of its own per-child copy, and on_sync_error withholds the enqueuing child's own chunk as before.
`ls -R dir | true` with many subdirectories queues one chunk per directory on the pipe's IOWriter. When `true` exits, the writer fails with several chunks still queued. With one error completion per child, ls counted one of them and waited for the others forever.
A pipe between two pipeline stages is an IOWriter too. The stage on its left loses its reader when the next stage exits without reading its stdin. Cover the writers that queue one chunk per directory or operand (ls -R, ls -d, mkdir -v, rm -v), a builtin reader, an external reader that exits at once, head -n 1, and a reader in the middle of the pipeline. The builtin-only shapes run on Windows too.
WalkthroughChangesThe runtime now uses shared pending-writer failure handling for EPIPE and other write errors. Builtin EPIPE handling and pipeline completion
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🟠 High · up to A broken pipe with multiple captured-output chunks can trigger a use-after-free and crash the shell process. Fix the queue-entry withholding before merging. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 3:08 PM PT - Sep 18th, 2026
✅ @robobun, your commit 6ddd1a6d6d5f437bc134613506601b8f0b9cdcdc passed in 🧪 To try this PR locally: bunx bun-pr 43183That installs a local version of the PR into your bun-43183 --bun |
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Status Reproduced on bun 1.4.3-canary.1+c6b7fcb5b (Linux x64, release), on a debug ASAN build of main, and on 1.4.3-canary.1+b52d51348 (Windows x64): The fix and the tests are in this PR. On main, 11 of the 15 tests that run on Linux in Since 4bc407c the PR also changes the builtin |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked two things in fail_pending_writers (src/runtime/shell/IOWriter.rs): a child that re-enqueues from its error callback cannot append to the queue being walked, because s.err/broken_pipe are set before the first callback and enqueue goes through handle_dead_writer first; and the EPIPE branch of the write loop is now covered by the keepalive, which the deleted broken_pipe_for_writers path did not take.
Extended reasoning...
Two confirmed findings are posted inline (the untested cat cancel-on-write-error path, and the pre-existing cancel-then-Suspend hang in on_io_reader_done), so human review is already signalled. This note only records what else was examined and ruled out from reading the diff: re-entrant enqueue during the failure loop is rejected synchronously by handle_dead_writer (IOWriter.rs:927-945) since s.err is assigned before any run_yield, and the keepalive Arc is now acquired at the top of the shared fail_pending_writers, so the EPIPE branch in on_write_pollable (which previously called broken_pipe_for_writers with no keepalive) can no longer drop the last Arc mid-callback. Not a guarantee of correctness; the per-chunk-vs-per-child completion semantics and the CapturedWriter cancel interaction still deserve a maintainer's eye.
On Windows cat is a builtin. It finishes at its first failed chunk, so it cancels the chunks queued behind it. Without that cancel a second error completion reaches the finished cat and panics with "expected Node::Cmd at Node#3, got Free".
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline note on the fixture's fixed pause, I also checked the re-entrancy of the new fail_pending_writers loop: cancel_chunks only marks entries dead in place (never removes them), so the index stays valid when a completion cancels the chunks behind it; WriteOutcome::Failed can only surface from write() with no write or poll in flight, so the withhold chunk in on_sync_error is the lone live entry and the None/Yield::done fallback only fires when that same child cancelled itself inline (cat and CapturedWriter return the enqueue Yield without touching their own state afterwards, so that is safe). The EOF path now also sets s.err, but handle_dead_writer checks broken_pipe first, so rejection behavior there is unchanged. The Windows cat multi-chunk test from 74a0fb6 covers the earlier open point on the cat cancel path.
Extended reasoning...
This is a findings-present ruled-out note. The IOWriter change replaces per-child deduplicated error delivery with per-chunk delivery inside a loop that re-derives state() on every iteration and runs child callbacks inline; the risky questions were index validity across cancel_chunks, whether a synchronous failure can leave several live chunks of the withholding child (it cannot: write() returns Suspended whenever a uv write or poll is in flight, so Failed only surfaces on a drained queue), whether returning Yield::done from on_sync_error can strand a child (only when that child's own inline completion already finished it, and both such children return the enqueue Yield immediately), and whether the EOF path newly setting s.err alters handle_dead_writer (it does not, broken_pipe is checked first). Since the prior review, commit 74a0fb6 added the Windows-gated multi-chunk cat pipeline test that the earlier inline nit asked for. The change is still re-entrant callback-dispatch code in src/runtime/shell/IOWriter.rs, src/runtime/shell/builtin/cat.rs and src/runtime/shell/subproc.rs, so a human maintainer's look is worthwhile alongside the inline finding rather than an outright approval.
…sing The fixture blocked the main thread for 100 ms so that every chunk was queued before the first write failed. Only a run with several chunks queued takes the path under test, so the fixture now runs the command ten times and has no timed wait. It is a file now (epipe-fixture.ts).
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I re-reviewed the latest push (the fixture now loops ten runs instead of sleepSync) and found no bugs; since this reworks the IOWriter failure path that dispatches raw ChildPtr completions re-entrantly, a human look is still worthwhile.
What was reviewed:
fail_pending_writersindex loop:cancel_chunksonly marks entries dead (never shrinkswriters), ands.erris set before any completion so re-enqueues from callbacks are rejected byhandle_dead_writerrather than pushed, which is why resetting the queue after the loop is safe.on_sync_errorwithhold path: an inline completion for another chunk of the same child (e.g.CapturedWriterrecording its error) cancels the withheld chunk, and the!is_dead()filter then falls back toYield::doneinstead of a second completion into a finished relay.- Every multi-chunk
IOWriterchild (ls/mkdir/touch/cpOutputTasks,rm) counts completions and ignores the error value;catandCapturedWritercancel at their first error;yes,echo,seq,Pipeline,CondExprandBuiltin::write_failing_errorqueue one chunk at a time. - Keepalive: the old EPIPE-from-poll path held no
Arc; the shared helper now takes one and it is dropped on function exit.
Extended reasoning...
Overview
The PR replaces two separate IOWriter failure paths (broken_pipe_for_writers for EPIPE from the poll callback, and fail_pending_writers used by on_error/on_sync_error) with a single fail_pending_writers(err, withhold) that walks writers[writer_idx..] by index, re-fetching self.state().writers each iteration, and runs one Yield::OnIoWriterChunk error completion per live queued chunk instead of one per ChildPtr. on_sync_error gets its inline completion for the enqueuing child returned rather than dispatched, falling back to Yield::done when that chunk was cancelled from another completion. Two consumers that finish at their first error (cat, CapturedWriter) now call cancel_chunks so their remaining queued chunks never call back into freed state. Tests add in-process pipeline cases (ls -d/mkdir -v/rm -v with 64 operands, ls -R over 300 dirs into true, true | cat, sh -c 'exit 0', head -n 1), a Windows-only spawned cat big.txt | true guard for the cat cancel, and a spawned fixture that waits for its own stdout to hit EPIPE and then runs each builtin ten times.
Security risks
None that I can see: no user-controlled parsing, no path handling, no credentials. The change is confined to the shell's internal write-queue error dispatch and to test files.
Level of scrutiny
High. This is native code with raw ChildPtr dispatch, re-entrant Yield::run calls from inside a loop over a Vec that callbacks can mutate, and node-arena slot reuse after Builtin::done. I traced: (1) cancel_chunks only sets a dead flag, so index-based iteration stays valid across callbacks; (2) s.err is set before any completion runs, so handle_dead_writer rejects re-enqueues without pushing to writers, which makes the move of the queue reset from before to after the completions behavior-preserving (the old doc comment's stated rationale did not apply even on main because s.err was already set first); (3) the withheld chunk is re-validated for liveness after the loop, covering the case where an inline completion for another chunk of the same child cancels it; (4) every consumer that can have more than one chunk queued either counts completions (OutputTask builtins, rm) or cancels at first error (cat, CapturedWriter), and the remaining children queue one chunk at a time. The prior-run nit about the sleepSync(100) fixture was addressed in the latest commit. What I could not verify locally is the Windows-only cat test and the ASAN run the author reports, and the same-child inline-completion ordering under on_sync_error with multiple chunks pending is only reachable via a poll re-registration failure, which no test exercises; a maintainer familiar with the shell's trampoline depth guard should weigh that.
Other factors
The bug hunt exited on dry_streak with no findings. Both of my earlier inline threads were resolved by the author; the cat-cancel test nit was addressed by the Windows-gated spawned test, and the cancel-then-suspend thread was filed as pre-existing. The new tests exercise the exact hang the PR describes and are hermetic (tempDir, no network), with signalCode === null asserted on the spawned cases. Not approving outright because the change is not simple and touches memory-safety-sensitive re-entrant native code with a Windows-only guard test that cannot be run here.
The file form of cat kept an out_done flag that stayed set once its output queue had drained. When more input arrived afterwards, cat still finished at the end of its input with chunks queued. The node is reused by the next command, which then received the completions of those chunks: output was lost, and with one error completion per chunk the second one reached a freed node (panic: expected Node::Cmd at Node#2, got Free). cat now compares chunks_done with chunks_queued, as the stdin form already does.
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Actionable comments posted: 1
- 🪄 Fix CodeRabbit comments on this PR
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@src/runtime/shell/IOWriter.rs`:
- Around line 819-835: Update the enqueue and on_sync_error flow to pass a
stable identity for the initiating queue entry into fail_pending_writers, and
change the withholding logic in the writer-processing loop to match that
identity rather than only ChildPtr. Preserve withholding the exact initiating
entry while continuing to dispatch other live entries normally.
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src/runtime/shell/IOWriter.rssrc/runtime/shell/builtin/cat.rssrc/runtime/shell/subproc.rstest/js/bun/shell/epipe-fixture.tstest/js/bun/shell/epipe.test.ts
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I re-reviewed the two commits pushed since my last pass (the cat out_done removal and the new Windows/fifo tests) and found no bugs; my earlier notes on the missing cat cancel test and the fixture's fixed sleep are addressed by the Windows cat big.txt | true case and the ten-run fixture loop. Because the core change reworks completion dispatch inside IOWriter (raw-pointer children, callbacks that can free the callee mid-loop), a human look at fail_pending_writers is still worthwhile before merge.
What was reviewed:
fail_pending_writersre-entrancy:s.erris set before the loop, so an enqueue from a completion is rejected byhandle_dead_writerwithout pushing, andcancel_chunksonly marks entries dead, so the index-based walk stays valid.- The
withholdpath: the withheld child's later chunks are dispatched inline before its first; this needs a syncwrite()failure with older same-child chunks queued, which theis_writing/poll.is_watching()guards inwrite()rule out on both platforms. catExecFilepathArgs: counters reset per file andStep::Nextnow requiresin_done && chunks_done >= chunks_queued, matching the stdin form;CapturedWriter::cancel_chunksuses the samechild_ptr()identity asenqueue.
Extended reasoning...
Overview
The diff touches src/runtime/shell/IOWriter.rs (deletes broken_pipe_for_writers, reshapes fail_pending_writers to fire one error completion per queued chunk with an optional withheld child for the sync-error path), src/runtime/shell/builtin/cat.rs (drops the sticky out_done flag, adds cancel_stdout_chunks and calls it on the write-error path), src/runtime/shell/subproc.rs (CapturedWriter cancels its remaining chunks on first error via a shared child_ptr() helper), and adds test/js/bun/shell/epipe.test.ts cases plus epipe-fixture.ts. Since my previous review, two commits landed: the cat out_done fix with fifo/Windows tests, and a Windows-only multi-chunk file test.
Security risks
None specific to this change. The code handles EPIPE/write errors on shell pipes; no user-controlled parsing, paths, or credentials are introduced. The risk class is memory safety (use-after-free through raw ChildPtr dispatch) rather than security exposure.
Level of scrutiny
High. fail_pending_writers runs user-adjacent completion callbacks in a loop where each callback may mutate the writer queue, drop the last external Arc, or finish and free the child node. I checked the load-bearing invariants: the keepalive is taken at the top; s.err is set before any callback so re-entrant enqueues cannot push onto writers; cancel_chunks never removes entries, so the index walk over self.state().writers.get(idx) remains valid; the withheld entry is re-validated for liveness before being returned. The withhold ordering change (a child's later chunks may complete inline before its first) is theoretically new behavior, but reaching it requires a synchronous write() failure from enqueue while older chunks of the same child are queued, and write() returns Suspended under is_writing (Windows) or poll.is_watching() (POSIX) in exactly that situation. I could not construct a reachable case. The automated hunt ran to a dry streak with no findings.
Other factors
Prior inline notes from this bot were addressed by later commits (Windows cat | true test; fixture loops ten times instead of sleepSync). The pre-existing cancel-then-suspend shape in cat::on_io_reader_done remains on the base branch and is out of scope for blocking. Test coverage is substantial (in-process pipeline rows, a spawned fixture for the process-stdout EPIPE case, a fifo-driven cat case, and Windows-gated builtin cat cases), and the PR reports the tests time out on main. Given the complexity and the lifetime reasoning involved, a human maintainer familiar with the shell interpreter should still confirm the fail_pending_writers dispatch model before merge.
On macOS both tests time out in CI: the handshake that reads the first line back before it writes more input never completes there.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
-
🟡
src/runtime/shell/IOWriter.rs— nit: maintainers reading theWriterTag::Subprocdispatch get a SAFETY justification that this PR makes false. IOWriter.rs:1202-1205 says the PipeReader stays alive "untilon_close_ioruns, which only happens after the writer has finished draining"; after this change the first errored chunk runson_close_iowhile later chunks are still queued, and the deref is safe only becauseCapturedWriter::on_iowriter_chunkcancels them (subproc.rs:1640-1643). Fix: reword the SAFETY comment to name the real invariant, that every queued chunk of a CapturedWriter is cancelled before the PipeReader can be freed, so REVIEW.md's "SAFETY comments must be accurate" holds.Extended reasoning...
The unsafe block at IOWriter.rs:1206 dereferences
child.rawas&mut CapturedWriter. Its SAFETY text (lines 1202-1205) claims the owning ShellSubprocess keeps the PipeReader alive until the writer has finished draining. On the base branch that was the guarantee: one error completion per child, thencancel_chunks, thenon_close_io. After this difffail_pending_writers(IOWriter.rs:819-836) delivers one completion per queued chunk. The first one reachesCapturedWriter::on_iowriter_chunk, which setserr, callswriter.cancel_chunks(child)(subproc.rs:1640-1643), thentry_signal_done_to_cmd→Cmd::buffered_output_close→child.close_io(Cmd.rs:1061), which drops theReadable::PipeArc and can free the PipeReader while the writer queue still holds that child's later chunks. Those chunks are skipped only because they were marked dead; the comment does not mention that, and the PR description itself says removing that cancel produces a heap-use-after-free in this very deref. No runtime failure today; the cost is a SAFETY comment that misleads the next person editing…Verification: nit. Triggering condition: any EPIPE/write error on an IOWriter with two or more CapturedWriter chunks queued (the relay case this PR's own test covers). The SAFETY comment at /home/claude/bun/src/runtime/shell/IOWriter.rs:1201-1205 is untouched by the diff and still says the PipeReader "is kept alive by the
Readable::PipeArc on the owning ShellSubprocess untilon_close_ioruns, which only…
The PipeReader can now be released at the first failed chunk while later chunks of its CapturedWriter are still in the queue. They are cancelled first, so say that.
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Problem
await $`ls -R dir | true`never settles. So dols -d a b,mkdir -v a bandrm -v a b. Linux and Windows.fail_pending_writersandbroken_pipe_for_writers(src/runtime/shell/IOWriter.rs:831,:797) send one error completion per child and drop its other queued chunks. These builtins wait for one completion per chunk.catcan finish with chunks still queued (cat.rs:305). The next command reuses its node and gets their completions: lost output, orpanic: expected Node::Cmd at Node#3, got Free.Fix
fail_pending_writersgives every queued chunk its own error completion. The three failure paths share it.catand the subprocess output relay (CapturedWriter) stop at their first error, so they callcancel_chunksthere. Without it the relay's second completion is a heap-use-after-free.catfinishes only whenchunks_done >= chunks_queued, as its stdin form does. Required: per-chunk completions make its leftover chunks panic on EPIPE.test/js/bun/shell/epipe.test.ts. 11 of its 15 Linux tests fail on main. All pass on Linux (debug ASAN) and Windows (debug).Background
IOWriteris the shell's write queue for one file descriptor (the process's stdout, or a pipe between pipeline stages). A child queues a chunk and gets one completion callback.cancel_chunksmarks a child's queued chunks dead. A dead chunk gets no callback.Supersedes #37719. #40228 and #42819 also fix the hang: see Notes.
Notes
The pipeline case. The pipe between two stages is a socketpair with an
IOWriteron the write end (src/runtime/shell/states/Pipeline.rs).ls -Rstarts one thread pool task per directory. Each finished task queues its listing as one chunk under the builtin'sChildPtr. The next stage (true,pwd, a secondls, an external command that exits early) closes the read end without reading. The first write fails with EPIPE while several chunks are queued. A debug log of the unfixed build shows fourOutputTaskstarts and oneonIOWriterChunk. The hang is racy because it needs two or more chunks in the queue when the write fails. A chunk that is queued after the failure is rejected byhandle_dead_writerwith its own completion, which is why a single operand never hangs.Measured, never settled of 10 runs each (
ls -R d | ls -R d,ls -R d | pwd,ls -R d | true; the tree has 4 directories and 80 files):ls -RpwdtrueThe controls (
ls -R d | cat,ls d | ls d,seq 400 | ls -R d) settle in every run on every build.Why the fix is in the queue and not in the builtins. The
OutputTaskbuiltins keep one FIFO for the chunks they queued on stdout and on stderr, and both can be different writers. One notification per child cannot tell them how many of their chunks failed. The Zig shell registered everyOutputTaskas its own writer child, so one error per child was one error per chunk there. The Rust port put all chunks of a builtin under the builtin'sChildPtrand kept the per-child error path.rmcounted chunks under one child in Zig too.Children of
IOWriter, checked one by one. Onlycat,rm,ls,mkdir,touch,cpandCapturedWritercan have two or more chunks queued.rmand theOutputTaskbuiltins count completions and ignore the error value.catandCapturedWriterfinish at their first error, so they cancel. Every other child (echo,pwd,which,seq,yes,basename,dirname,cd,exit,export,mv,Cmd,CondExpr,Pipeline) waits for its one chunk before it queues the next.on_sync_error. The chunk of the child whoseenqueueis on the stack is still returned, not dispatched, so the trampoline delivers it afterenqueueunwinds. Other queued chunks are dispatched inline, as before.CapturedWriter::do_writeonly runs while its reader is pending, so an inline completion there cannot free the relay.The
catchange. The file form of the builtincatsetout_donewhen its output queue drained and never cleared it. When more input arrived later,catstill finished at the end of its input, with the new chunks queued.Builtin::donefrees the node, the next command reuses the id, and the completions of the leftover chunks go to that command (or to a freed node). The builtin is the defaultcaton Windows and is behindBUN_ENABLE_EXPERIMENTAL_SHELL_BUILTINSelsewhere. Measured withcat fifo; echo tail, where the test readsfirst\nback before it writes 1 MB more to the FIFO:echo)catchange (debug ASAN)panic: expected Node::Cmd at Node#2, got Free, 10 of 10On Windows,
cat big.txt | cat > out.txt(1 MB) five times in one process panics withexpected Node::Cmd at Node#3, got Freein 6 of 6 processes on the canary, and in none on this branch. The read-error path ofcat(cancel, then suspend) is not changed here. #37743 is open for it.Tests.
pipeline stage whose reader exits without reading(new, in-process, runs on Windows too except the two rows with external commands):ls -d,mkdir -vandrm -vwith 64 operands intotrue,ls -Rof 300 directories intotrue, intotrue | cat, intosh -c 'exit 0', and intohead -n 1.command output after the stdout reader went away(from shell: fail every queued chunk when an IOWriter dies, not one per child #37719, reworked):epipe-fixture.tswaits until its own stdout fails with EPIPE, then runsls -d,mkdir -v,rm -vor a relayedhead -c 1048576 /dev/zeroten times. It has no timed wait. A run takes the path under test only when several chunks are queued before the first write fails. Ten runs replace the 100 ms pause that shell: fail every queued chunk when an IOWriter dies, not one per child #37719 used for that.CapturedWritercancel. With that call removed, it fails in 7 of 8 runs on a debug ASAN build (heap-use-after-free inCapturedWriter::on_iowriter_chunkfromfail_pending_writers, or a timeout).cat whose input ends while its output is still queued: the two FIFO shapes from the table above (Linux, with the experimental builtins flag in a child process), and on Windowscat big.txt | cat > out.txtfive times in one child process. The FIFO tests do not run on macOS: both timed out there in CI because the handshake never completed, and I could not find out why without a macOS machine. Thecatlogic they cover is the same on every platform. The read-everything test fails on main in 3 of 3 runs, the Windows test in 4 of 4.cat with several chunks queued(Windows only, wherecatis a builtin by default) runscat big.txt | truewith a 1 MB file in a child process. It guards thecatcancel. With that call removed, the child panics withexpected Node::Cmd at Node#3, got Freein every run on a Windows debug build. It passes on the canary, which has no per-chunk completions.Related open PRs. #40228 fixes the same hang in another way: it keeps the one-completion-per-child path (
IOWriter.rs:792and:830on its head) and gives everyOutputTaskchunk its own identity (ChildPtr::builtin_task(node, seq)), with closed-pipe tests forls,mkdirandrm. #42819 rewrites the failure path around a per-chunkfailedqueue, which is the contract of this PR, and it also callscancel_chunksinCapturedWriter::on_iowriter_chunk. Both are large refactors (68 and 419 files). This PR is the small version of the fix against main. If it lands first, #40228 no longer needs the dedup or the per-chunkseqfor this hang, but the twocancel_chunkscalls and thecatcount have to carry over: itscat.rsstill hasout_done. The open design question is which contract the queue keeps: one completion per queued chunk (this PR, #42819) or one per child identity (#40228).Not changed. After the EPIPE,
ls -Rstill walks the rest of the tree before it finishes. It settles, but a large tree takes as long as a full listing. Exit codes on EPIPE are not changed and not asserted for the failing stage (#40033 changes them).no test proof · iteration 2 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/shell/epipe.test.ts