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…stderr fails When the shell's stdout or stderr writer fails (EPIPE, the reader of the process's stdout is gone), CapturedWriter recorded the error but the PipeReader kept reading the child's pipe into buffered_output for as long as the child kept writing. The command never finished, so the awaited `$` never settled, and the child was never told that its output goes nowhere. PipeReader::on_capture_failed closes the shell's end of the pipe while the child still has its end open and settles the reader's state, so the existing try_signal_done_to_cmd path reports the relay error to the Cmd right away and the child's next write fails, as it would under a real shell. The rejection can also arrive synchronously, from inside the reader's own on_read_chunk (the writer is already dead when the chunk is enqueued). The Cmd then drops its Arc<PipeReader> while the bun_io read loop is still on the stack, so the reader now gives the loop a ref_/deref keepalive, like IOReader does, and on_read_chunk / do_write no longer hold a borrow of the PipeReader across the trampoline.
WalkthroughChangesThe shell subprocess pipe path now uses raw-pointer callbacks, interpreter snapshots, reader keepalives, and explicit capture-failure completion. POSIX tests cover EPIPE relay scenarios. Shell pipe re-entrancy and EPIPE handling
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Updated 2:54 AM PT - Aug 22nd, 2026
✅ @robobun, your commit a6d4b2cbd19b0261e9c43ee88a6e9c510fc0f55f passed in 🧪 To try this PR locally: bunx bun-pr 40035That installs a local version of the PR into your bun-40035 --bun |
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Status: the fix is pushed and the PR is ready for review (latest commit 1a640be). Reproduced on bun 1.4.0 with the script in the Notes of the description: Test: A self-review led to three changes (see the comment below the description): the command now waits for the child instead of killing it when stdout is the only piped stream, CI (build 103237, commit 8431361, before the self-review changes): every lane passed the new tests. The only test that was red on every lane is |
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The fixture's stdout is a socket pair. If the test closes its end while a line is still unread, the next write fails with ECONNRESET, not EPIPE. The producer already treats both as the reader being gone.
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In `@test/js/bun/shell/epipe-relay-fixture.ts`:
- Around line 37-45: Bound the infinite wait loop around writeSync with a
reasonable deadline, and fail explicitly with a clear timeout message if the
reader is not dropped in time. Preserve the existing EPIPE and ECONNRESET exit
behavior, while continuing to retry EAGAIN until the deadline.
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src/runtime/shell/states/Cmd.rssrc/runtime/shell/subproc.rstest/js/bun/shell/epipe-relay-fixture.tstest/js/bun/shell/epipe.test.ts
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I re-reviewed after e2aec7b/5fa8142/669e7ed and didn't find any bugs. Because this reworks unsafe raw-pointer dispatch and Arc refcounting on a re-entrancy path in the shell subprocess reader, a human look would still be worthwhile.
What was reviewed:
on_capture_failed→reader.deinit()setsCLOSED_WITHOUT_REPORTING, so the read loop's post-callbackis_done()check exits cleanly andPipeReader::drop's debug assert holds.- The new
ref_/derefhooks are picked up byvtable.ref_parent()inPosixBufferedReader::read/on_poll(and the Windows callbacks), so the Arc keepalive spans the whole read-loop entry. do_writesnapshotsinterpbeforeenqueue;handle_dead_writerreturnsYield::OnIoWriterChunk, so the Cmd re-entry that can free thePipeReaderruns insiderun_yield_with, not while a&PipeReaderis held.- Fixture wait loop now handles
ECONNRESET(my earlier comment); thebun-audit.test.tsCI failure is unrelated.
Extended reasoning...
Overview
The PR fixes a hang where await $yes`` never settles once the process's own stdout has no reader (e.g. bun script.ts | head -c1). The shell's `CapturedWriter` recorded the EPIPE from the relay but nothing stopped the `PipeReader`, so it kept reading the child's output into the capture buffer indefinitely. The fix adds `PipeReader::on_capture_failed`, which closes the shell's end of the pipe (`reader.deinit()`) and settles the state so `try_signal_done_to_cmd` reports the error to the `Cmd` at once. Because that report can now run from inside the reader's own `on_read_chunk` (the dead-writer synchronous-rejection path), the `Cmd` may drop its `Readable::Pipe` `Arc` while the `bun_io` read loop is still on the stack. To cover that, the reader now supplies `ref_`/`deref` hooks (Arc strong-count inc/dec) to `impl_buffered_reader_parent!`, `on_read_chunk` is converted to the raw-`*mut Self` form, and `do_write` snapshots `interp` before the enqueue so it holds no `&PipeReader` across the trampoline. `Cmd.rs` only has doc-comment updates for the renamed `run_yield_with`. Five new test cases (POSIX-only) and a fixture cover the async-poll, sync-dead-writer, only-stdout, and already-worked paths.
Security risks
None identified. The change is confined to lifecycle/refcounting of the shell's own subprocess pipe reader; no user input parsing, path handling, or trust-boundary crossing is touched.
Level of scrutiny
High. This is unsafe Rust in a memory-safety-critical path: it introduces a new re-entrancy edge (on_read_chunk → Cmd teardown), converts a &mut self callback to a raw-pointer form, and adds Arc refcounting to keep an allocation alive under that re-entrancy. The author's ASAN trace demonstrates the UAF without the keepalive, and I traced that ref_parent() in PosixBufferedReader::read/on_poll and the Windows on_stream_read/on_file_read callbacks does bracket the entire read-loop entry, that deinit() sets CLOSED_WITHOUT_REPORTING (making the loop's post-callback is_done() return true and satisfying PipeReader::drop's POSIX debug assert), and that handle_dead_writer returns a Yield::OnIoWriterChunk (so the freeing re-entry happens inside run_yield_with, after do_write has released every &self borrow). Still, the shell's PipeReader/Cmd/IOWriter ownership graph is intricate enough that a maintainer familiar with it should confirm the new state transition (Pending → Done(empty) under a live child) composes correctly with every existing consumer — e.g. that no other path reads state between on_capture_failed and the Cmd's close_io, and that the Windows reader.deinit() (which hands the source to uv_close) is safe from inside the read callback there.
Other factors
All prior review threads are resolved: my ECONNRESET note on the fixture's wait loop was applied in e2aec7b, and the comment-cop length nits were addressed in 5fa8142/669e7ed (the remaining doc comments are the required # Safety sections on new unsafe fns, matching the sibling on_reader_done/try_signal_done_to_cmd shape). The author reports 10× ASAN runs of the new test plus the full listed shell suite passing; the PR description's reasoning about Done(empty) vs Err (so take_captured_error keeps the relay error) and about not nesting the error Yield checks out against the code. The one CI failure so far (bun-audit.test.ts) is unrelated to this change.
The fixture and all five of its modes pass on a Windows debug build, and the unfixed canary hangs there the same way (the relay error maps to 4047).
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On the two points left for a human look:
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I reviewed this PR and didn't find any bugs. My earlier note on ECONNRESET in the fixture's wait loop was addressed in e2aec7b, and the later commits are comment trims plus enabling the tests on Windows. Because the fix reworks PipeReader's re-entrancy and Arc-lifetime contract in unsafe code (on_read_chunk moving to *mut Self, the new ref_/deref read-loop keepalive, and on_capture_failed deiniting the reader from inside the read callback), a human look from someone who owns the shell subprocess path would still be worthwhile.
What was reviewed:
- Confirmed the read loop brackets
on_read_chunkwithvtable.ref_parent()on both POSIX (read,on_poll,read_into) and Windows (on_stream_read,on_file_read), so the newArc::increment/decrement_strong_counthooks keep thePipeReaderalive across the Cmd re-entry. - Confirmed
reader.deinit()fires no callback and leaves the reader in a state where the loop's post-callbackis_done()check (POSIXCLOSED_WITHOUT_REPORTING; Windowssource = None) exits cleanly, andPipeReader::drop's debug asserts pass on both platforms. - Traced the async IOWriter-error path:
on_iowriter_chunk→on_capture_failedmay free thePipeReaderinside the&mut selfcall, but nothing touchesselfafterward — the same shape as the pre-existingtry_signal_done_to_cmdtail on the all-written branch (noted in the description as tracked with #37719).
Extended reasoning...
Overview
The PR fixes a hang where await $yes`` never settles once the process's own stdout has no reader. It adds PipeReader::on_capture_failed (closes the shell's pipe end and forces the reader to `Done` so `try_signal_done_to_cmd` reports the relay error to the `Cmd`), converts `on_read_chunk` to a raw-`*mut Self` receiver so `do_write` can re-enter the `Cmd` without aliasing a live `&mut PipeReader`, wires `ref_`/`deref` on the `BufferedReaderParent` impl so the read loop's existing `ref_parent()` keepalive holds the `Arc` across that re-entry, and folds `run_yield` into the free-function `run_yield_with` so no `&PipeReader` spans the trampoline. `Cmd.rs` changes are comment-only. Five new subprocess-based tests in `epipe.test.ts` cover the async-poll and dead-writer synchronous paths, with and without stderr piped, plus a guard for the path that already worked.
Security risks
None. This is subprocess-output plumbing inside the shell; no parsing of untrusted input, no auth/crypto/permissions surface.
Level of scrutiny
High. Per REVIEW.md this sits squarely in "Native code: memory safety (the most-blocked category)": Arc strong-count manipulation, raw-pointer receivers on callbacks that can free their own allocation, and a new terminal-state transition reachable from inside the read loop. The description is unusually thorough (ASAN trace with the keepalive removed, 10× ASAN-debug runs of the shell suites, cross-check against Windows), and I traced the read-loop keepalive on both platforms in src/io/PipeReader.rs to confirm the new hooks are actually bracketed around every on_read_chunk dispatch. That all checks out. Still, the change hand-tunes an ownership handoff whose correctness rests on "nothing touches self after this line" in three places, and it deliberately extends the pre-existing free-inside-&mut self shape at the on_iowriter_chunk boundary — a maintainer who owns this subsystem should confirm they're comfortable with that and with the interaction with the open work in #37719/#38380 the description flags.
Other factors
- All prior review threads (my ECONNRESET note, comment-cop's long-comment flags, CodeRabbit's deadline suggestion) are resolved; the follow-up commits since my last look are comment shortening and enabling the new tests on Windows — no logic change.
- CI on the last-reported build passed the new tests on every lane; the one unrelated red file (
bun-audit.test.ts) also fails on main. take_captured_errorkeeping an existingErris why the new state is set toDone(empty)rather thanErr; I checked that reasoning holds against the currenttake_captured_errorbody.- The
do_writechange (snapshotinterpbeforeenqueue, thenrun_yield_with(interp, y)) is what makes the dead-writer path safe: previouslyself.parent().run_yield(y)would deref thePipeReaderafterenqueuehad potentially freed it via the synchronous error dispatch. That's a real fix, not just a rename.
… dispatch A relay failure used to set the command's exit status at once. With stdout as the only piped stream (`2>&1`, `2>/dev/null`) that finished the command while the child was still running, and Cmd::deinit killed it with SIGKILL, while the default shape let the same child see EPIPE and exit by itself. The relay error is now a CaptureError::Relay: the stream is closed, the status is kept on SubprocExec::relay_status, and Cmd::on_exit applies it once the child has exited, so the command always waits for the child like a real shell does. Read errors (CaptureError::Read) keep finishing the command at once. CapturedWriter::on_iowriter_chunk takes *mut Self and the IOWriter dispatch forms no &mut: the call can drop the last Arc<PipeReader>, which frees the CapturedWriter, the same way on_read_chunk already documents. WindowsBufferedReader::deinit released `_buffer` before close_impl could move it to a File with a read in flight (the `2>&1` dup shape reads through a uv_fs_read). The buffer is now released after close_impl. The fixture's producer records its failed write in a file, so every mode can check that the child saw the closed pipe and exited on its own, and `2>&1` is covered.
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A self-review of the first revision turned up three things, fixed in 95c7d5e and 1a640be:
The fixture's producer now records its failed write in a file, so every mode checks that the child saw the closed pipe and exited by itself, and |
…elay-failure-stops-reading # Conflicts: # src/io/PipeReader.rs
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I reviewed this PR and didn't find any bugs; the earlier ECONNRESET note on the fixture's wait loop was addressed in e2aec7b. Because the change reworks raw-pointer callback dispatch and Arc keepalive semantics in the shell subprocess pipe path (re-entrancy where on_read_chunk/on_iowriter_chunk can free their own owner), a human look at the ownership reasoning is still worthwhile.
What was reviewed:
on_capture_failedclosing the reader mid-Pendingand theref_/derefkeepalive covering the read loop after theCmddrops itsReadable::PipeArc.CaptureError::Relaydeferral intoSubprocExec::relay_statusso2>&1/2>/dev/nullshapes wait for the child instead of SIGKILLing it;CaptureError::Readstill ends the command immediately.- The
*mut Selfconversion ofon_iowriter_chunkand its dispatch site inIOWriter.rs— no&mutis formed across the freeing tail call. - The six fixture modes cover async relay failure, dead-writer synchronous rejection, and the only-stdout /
2>&1variants.
Extended reasoning...
Overview
This PR fixes a hang where await $yes`` never settles once the process's stdout has no reader (e.g. bun script.ts | head -c1). The fix spans three shell source files and adds a fixture-driven test with six modes:
src/runtime/shell/subproc.rs: newCaptureErrorenum (Read vs Relay), newPipeReader::on_capture_failedthat closes the shell's end of the pipe and settles the reader state when relaying fails, conversion ofon_read_chunkandon_iowriter_chunkto raw*mut Selfreceivers, andref_/derefhooks on theBufferedReaderParentimpl so the read loop's keepalive covers the Arc theCmdmay drop mid-callback.src/runtime/shell/states/Cmd.rs: newSubprocExec::relay_statusfield,record_capture_errorthat distinguishes read errors (end command now) from relay errors (defer status until child exits), andon_exitapplying the deferred status.src/runtime/shell/IOWriter.rs: theWriterTag::Subprocdispatch now callson_iowriter_chunkby raw pointer instead of forming&mut CapturedWriter.- New
epipe-relay-fixture.tsand sixtest.concurrentcases inepipe.test.ts.
Security risks
None identified. This is process-lifecycle and pipe-close ordering inside the shell; no untrusted input parsing, no auth/crypto/permissions surface.
Level of scrutiny
High. This is native memory-safety-sensitive code in the most-blocked review category: raw-pointer callback receivers, Arc refcount manipulation, and re-entrancy where a callback can synchronously free its own owner. The PR description explicitly documents an ASAN heap-use-after-free that the ref_/deref keepalive fixes, and the author went through two self-review iterations that materially changed the design (deferring the relay status to on_exit instead of setting it immediately, and converting on_iowriter_chunk to raw-pointer dispatch). The reasoning in the PR notes is thorough and the test coverage is good (5 of 6 modes hang on the released bun, verified on Linux ASAN and Windows debug), but the correctness of the Stacked-Borrows / Arc-lifetime argument at each raw-pointer site is exactly what a maintainer familiar with the shell's ownership model should confirm.
Other factors
- All prior review threads are resolved: my ECONNRESET note on the fixture wait loop was fixed in e2aec7b, the comment-cop flags were addressed by shortening comments in 5fa8142/669e7ed/1a640be, and CodeRabbit withdrew its deadline suggestion.
- The PR merges main to pick up #31940 (
WindowsBufferedReader::deinitbuffer ordering), so the diff no longer touchessrc/io. - CI on the pre-self-review commit passed the new tests on every lane; the only unrelated failure (
bun-audit.test.ts) also fails on main. - The change is not simple or mechanical — it is a carefully-argued fix with cross-platform and re-entrancy implications that warrant a human maintainer's sign-off.
Problem
await $`yes`never settles once nothing reads the process's stdout (bun script.ts | head -c1). Bun reads the child's output into the capture buffer without end and the child keeps running.CapturedWriter::on_iowriter_chunk(src/runtime/shell/subproc.rs) records the relay error, buttry_signal_done_to_cmddoes nothing while the child still holds the pipe. Nothing stops the reader.Fix
PipeReader::on_capture_failed: while the reader is stillPending, close the shell's end of the pipe and set the EOF state, so the error reaches theCmdnow as aCaptureError::Relay. The stream closes, but the status waits inSubprocExec::relay_statusuntil the child exits. A read error (CaptureError::Read) still ends the command at once.on_read_chunk(dead writer), where theCmddrops itsArc<PipeReader>under the read loop. The reader gives the loop aref_/derefkeepalive likeIOReader(ASAN use-after-free without it, Notes).on_read_chunkandon_iowriter_chunktake*mut Selflike the other callbacks that can free their owner.test/js/bun/shell/epipe.test.ts, 6 new cases, 5 time out on the released bun, on Linux and Windows. Also the shell suites in Notes.Background
PipeReadereach. A chunk goes to the$result and, throughCapturedWriter, to theIOWriterof the process's own stdout or stderr.IOWriterreports a failed write once, then rejects new chunks synchronously.Cmdfinishes when it has a status and all piped streams are closed.Cmd::deinitkills a child still running then.2>&1or2>/dev/nullleaves stdout as the only piped stream.ref_/derefon aBufferedReaderParentkeep the owner alive for one read-loop entry. On Windows a2>&1stdout is read withuv_fs_read;WindowsBufferedReader::deinitreleasing its buffer under that read was fixed on main by Fix Windows use-after-free of the pipe reader buffer retained by in-flight libuv reads #31940, which this branch merges.Notes
Repro (bun 1.4.0, Linux),
relay.mjs:timeout 20 bun relay.mjs | head -c1: before, "settled" is never printed, the timeout kills bun,yesis in state R and bun's RSS is 2 GB after 3 s. After:settled 32after 277 ms,yesreports that its stdout is gone and exits, no process is left. A command that exits by itself (/bin/echo hi) already settled with 32 before this change.Paths and test cases. The fixture is
epipe-relay-fixture.ts. The producer is bun itself: it exits 0 after its first failed write and records that write in a file, so every case can check that the child saw the closed pipe and exited by itself, and that the failed relay is what fails the command.relay: the error arrives from the IOWriter's poll. stderr is still relayed, so the command finishes when the child exits after its write fails. The test also checks the relayed stderr line.relay-only-stdout(2> /dev/null) andrelay-stderr-to-stdout(2>&1): stdout is the only piped stream. The command still waits for the child's exit.dead-writer(echo hello; ...):echoalready broke the writer, so the child's first chunk is rejected from insideon_read_chunk. This is the synchronous path that needs the keepalive.dead-writer-only-stdout: the same, with stdout as the only piped stream.exits-by-itself: the path that already worked, kept as a guard.The producer can see EPIPE or ECONNRESET (Linux reports ECONNRESET when the closed socket still held unread data), so the test only checks that a write failed. The exit status is not pinned: other open work changes what a write error maps to.
ASAN output with the keepalive removed, both
dead-writercases:on_read_chunkanddo_writeno longer borrow thePipeReaderacross the trampoline. The error Yield is still returned to the trampoline that delivered the chunk, not run nested:Interpreter::finishruns JS continuations synchronously (the ASAN run below printed the fixture's result line before it crashed), so a nested run would use one more level of the trampoline's depth budget.In the asynchronous case the
PipeReaderis freed insideon_iowriter_chunk. That is the same shape as the existing path for an error that arrives after EOF (see the ASAN note in #37719). Nothing touches the writer after the call.The state is set to
Done(empty)and not toErr:take_capture_errorkeeps an existing read error over the relay error.Doneis whaton_reader_donesets too, andreader.deinit()has just emptied the buffer it would take.Why the status waits for the exit (self-review finding): an earlier revision set the command's status from the relay error at once. With stdout as the only piped stream (
2>&1,2>/dev/null) that finished the command while the child was still running andCmd::deinitsent it SIGKILL, while the default shape let the same child see EPIPE and exit by itself. bash reports the child's own fate in both shapes. Now the relay error only closes the stream;relay_statusis applied byon_exit, so the command ends when the child does, in every shape, and the child is reaped by the normal exit path (no zombie, no kill). Read errors keep the old behavior:shell-pipe-read-fault.test.tspins that a read error on asleep 5child ends the command at once with the errno.The
on_iowriter_chunkreceiver (self-review finding): the async relay error frees thePipeReader, and with it theCapturedWriter, inside this call. It now takes*mut Selfand theIOWriterdispatch forms no&mut, the same shape ason_read_chunk,on_reader_doneandon_reader_error.WindowsBufferedReader::deinit(self-review finding):2>&1on Windows hands the shell aBufferFd, read throughuv_fs_read, so a read is in flight between callbacks.deinitreleased_bufferbeforeclose_implcould move it to the File (orphaned_read_buf). #31940 fixed this on main while this PR was open (the first revision here carried the same change); the branch merges main, so the PR's own diff no longer touchessrc/io. This PR is the first caller ofdeiniton a reading WindowsPipeReader; the other callers are POSIX only.Before the error is delivered, one event-loop turn of output is still read and captured. With
yeson a debug build that was 39 MB. That is the existing greedy socket read loop, not changed here.Related open work: #37719 (one error per queued chunk; it also edits
on_iowriter_chunkand this test file), #38380 (reaping a child that the shell kills or that exits after the command finished; this PR adds one more path that reaches it, as the existing error paths do).Suites run on the ASAN debug build:
epipe(10 runs),bunshell(424 pass),shell/commands,shell-pipe-read-fault,shell-write-fault,shell-hang,exec,shelloutput,shell-blocking-pipe,pipeline_stack,yield,shell-worker-terminate-leak,throw,bunshell-instance,bunshell-default,shell-cmdsub-crash,bunshell-file,file-io,lazy,shell-load. The failures seen are environmental and also fail with the released bun here: thels/rmnode_modulescases (no registry access), thelspermission cases (root),shell-load(512 pid limit),shell-blocking-pipe(the heap snapshot takes over 5 s on this build, passes alone).cargo check -p bun_runtime --target x86_64-pc-windows-msvcpasses.Windows (Server 2019 x64, debug build of this branch at 1a640be): all six fixture modes settle, the producer reports
EPIPEin every mode that has one, including2>&1, and the command's status is 4047 (libuv's EPIPE). The unfixed canary hangs inrelaymode there too, so the new cases run on Windows as well.reader.deinit()hands the stdout pipe touv_closefrom inside its own read callback, which libuv allows; the read loop's keepalive still covers theArc.[review] gate passed · iteration 0 · 5 files touched
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