`bun lint`, `bun fmt`: write a child's stdin from the event loop by Jarred-Sumner · Pull Request #45424 · oven-sh/bun

Jarred-Sumner · 2026-10-15T22:55:58Z

What does this PR do?

bun lint and bun fmt start two kinds of child process: git, and a bun that evaluates a config file that is a program (eslint.config.js, prettier.config.mjs, ..). Both can get input on stdin. run_to_end in src/runtime/cli/lint_command.rs wrote it with a blocking write_all on a thread it started for that, next to sync::spawn.

Now the child runs on an event loop, in the new CapturedChild (src/runtime/cli/captured_child.rs):

	Before	Now
stdin	Blocking `write_all` on a `std::thread`	`StaticPipeWriter`
stdout, stderr	The `poll()` loop of `sync::spawn`	`BufferedReader`
Exit	`wait4`	The exit handler of `Process`
No input	A pipe that is closed at once	No pipe

It is modeled on GitSubprocess in src/install/git_runner.rs, which does the same for bun install without the stdin.

Which loop. A lint thread can have a JS VM (for plugins), and then the thread's own loop is the VM's. A tick of it would run the VM's timers and callbacks in the middle of native code.

Thread	Loop
Has a VM	The VM's `spawnSync` loop, which `Bun.spawnSync` keeps for the same reason
Has none	The thread's mini event loop

This happens with a JS plugin, the rule bun/format and a prettier.config.mjs: the thread that lints the first file is the first to need the Prettier config. On Windows the old code called MiniEventLoop::init_global there, which puts a mini event loop on the VM's native loop. That is gone too. I have not reproduced a failure from it.

Who closes stdin. On Linux a pipe writer leaves its close to the thread pool (Closer), and the child sees the end of its input only then. bun fmt works on that pool. With two threads, one waited for the child and the other for its turn to start one, so nothing closed the descriptor and bun fmt hung. The first commit of this PR did that on every Linux lane. So CapturedChild keeps the descriptor and closes it itself as soon as the writer is done. StaticPipeWriter::leave_fd_open is new for that, and sets the close_fd that the writer under it already had.

When the wait ends early

A pipe that fails ends the child (SIGTERM), which could otherwise wait for that pipe forever.
Once the child has exited, its stdin is closed and no longer waited for. A process that it started can hold the other end open. The old code hung there.

What these children no longer get. sync::spawn forwards signals such as SIGTERM to the child, and has its own process group and job control under --no-orphans.

On Linux the kernel now sends the child SIGTERM if bun dies, however it dies (linux_pdeathsig). Git catches that to remove .git/index.lock.
The bun that evaluates a config is started with --no-orphans, so it ends with its parent on every platform.
On macOS and Windows a git command can outlive a bun lint that is killed. They are short.
Ctrl-C in a terminal reaches both processes, as before.

Also

New entries for CapturedChild: PollTag, its arm in dispatch.rs, BufferedReaderParentLink and ProcessExit.
SyncStdio::Pipe had no other user and is deleted.

Nothing that a user sees should change.

How did you verify your code works?

On a Linux x64 debug build, rebased on main at 0793250:

Two new tests:
- format.test.ts: two config files that are programs, with GOMAXPROCS=2. It times out on the first version of this PR (the hang above), on any machine. It passes on main.
- js-plugins.test.ts: a config file that is a program is read on a thread that has a VM. It passes on main too (on POSIX the old code was safe there). A first version, which always used the mini event loop, aborted on it.
With GOMAXPROCS=2 these pass: js-plugins (147), git (71), doctor (28), prettier-rule (18), config-files (842) in test/cli/lint, and format (1,628), format-doctor (34) in test/cli/format. Without it, format, js-plugins and git pass. git now has the --staged tests that send blob IDs to git cat-file --batch on stdin. "a command line of a megabyte" in config-files sends about 1 MB through stdin.
By hand, before the rebase, with GOMAXPROCS=2 and without, with 6,000 staged files, so that bun fmt --staged writes 486 KB to git update-index --stdin (the socket buffer is 208 KB):
- With git: every file is formatted and staged again, exit code 0.
- With a git that exits without reading its stdin: no hang, its stderr is shown, exit code 2.
- With a git that starts a background process on its stdin and exits: no hang.
By hand, before the rebase: kill -KILL of bun while a stand-in git waits. Its trap for TERM runs, and it is gone.

On CI, before the rebase, format, config-files, git and js-plugins passed on every lane: Linux (7), Windows x64 and aarch64, macOS x64 and aarch64.

The Windows code follows GitSubprocess and Bun.spawn. It has only been compiled and run on CI.

robobun · 2026-10-15T22:56:11Z

^{Updated 10:05 PM PT - Oct 21st, 2026}

❌ @Jarred-Sumner, your commit 0209c51 has some failures in Build #130505 (All Failures)

🧪 To try this PR locally:

bunx bun-pr 45424

That installs a local version of the PR into your bun-45424 executable, so you can run:

bun-45424 --bun

coderabbitai · 2026-10-15T22:59:44Z

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 095a6bb and 0209c51.

📒 Files selected for processing: 5

src/runtime/cli/lint_command.rs
src/runtime/dispatch.rs
src/spawn/process.rs
test/cli/format/format.test.ts
test/cli/lint/js-plugins.test.ts

💤 Files with no reviewable changes: 1

src/spawn/process.rs

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Walkthrough

The PR adds CapturedChild to spawn child processes, write optional stdin, and capture stdout and stderr. Lint subprocess execution now uses this runner. Poll dispatch and process interfaces support it. Concurrent lint and format tests cover diagnostics and exit codes.

Changes

Captured child process runner

Layer / File(s)	Summary
Capture and process contracts `src/io/lib.rs`, `src/io/posix_event_loop.rs`, `src/spawn/lib.rs`, `src/spawn/process.rs`, `src/spawn/static_pipe_writer.rs`	Adds captured-child parent and poll variants, adds a `ProcessExit` handler variant, removes `SyncStdio::Pipe(Fd)`, and adds Unix descriptor handling for `StaticPipeWriter`.
Captured-child lifecycle `src/runtime/cli/captured_child.rs`, `src/runtime/cli/mod.rs`, `src/runtime/dispatch.rs`	Adds child spawning, pipe setup, output capture, and process and pipe completion handling. Registers the module and routes captured-child poll events.
Lint subprocess integration `src/runtime/cli/lint_command.rs`, `test/cli/lint/js-plugins.test.ts`, `test/cli/format/format.test.ts`	Changes lint subprocess execution to use `CapturedChild`. Adds concurrent lint and format tests that check diagnostics and exit codes.

Suggested reviewers: dylan-conway

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 0209c

No actionable merge-blocking risk remains in the reviewed changes after normal checks.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly identifies the main change: `bun lint` and `bun fmt` write child stdin from the event loop.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The pull request description includes both required sections. It clearly explains the implementation, design decisions, behavior changes, edge cases, and verification results, including platform cover…

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Actionable comments posted: 1

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Inline comments:
Review comments at @test/cli/lint/js-plugins.test.ts:
- Line 88: Remove the third, per-test timeout argument from the new
test.concurrent call; rely on Bun and CI timeout enforcement instead.

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Reviewing files that changed from the base of the PR and between 116dd64 and 89bc083.

📒 Files selected for processing: 9

src/io/lib.rs
src/io/posix_event_loop.rs
src/runtime/cli/captured_child.rs
src/runtime/cli/lint_command.rs
src/runtime/cli/mod.rs
src/runtime/dispatch.rs
src/spawn/lib.rs
src/spawn/process.rs
test/cli/lint/js-plugins.test.ts

💤 Files with no reviewable changes: 1

src/spawn/process.rs

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claude

Beyond the inline findings, I also checked: the new CapturedChildStaticPipeWriter tag has its arm in src/runtime/dispatch.rs; the Windows uv::Pipe allocations are released on the spawn-failure path in captured_child.rs; SyncStdio::Pipe has no remaining users after its deletion; and the mini-loop/VM-loop choice in run does not leave a thread that later gains a VM with the stale mini loop dispatching that VM's polls.

Extended reasoning...

The change replaces the blocking stdin-writer thread plus sync::spawn in bun lint/bun fmt with a new event-loop-driven CapturedChild (new file, ~300 lines, with cfg(unix)/cfg(windows) branches, a new PollTag, and raw-pointer callbacks), and deletes SyncStdio::Pipe. It touches no auth, crypto or injection surface, though it spawns child processes with caller-built argv (NUL bytes are now rejected). Three confirmed findings (stale .git/index.lock on SIGTERM, a hang after a reader error, and a hang when a grandchild inherits stdin) are posted inline, so this is not approvable; the note above records what else was examined.

claude

I re-reviewed after the follow-up commit (CapturedChild: close stdin itself, and don't wait for what can't come) and found no new bugs; that commit addresses the three points I raised earlier (SIGTERM as the pdeathsig, terminating the child on a pipe error, closing stdin once the child has exited). Since this is ~350 lines of new unsafe, cross-platform event-loop plumbing whose Windows arm has not been run outside CI, a human look is still worthwhile.

What was reviewed:

on_process_exit -> StaticPipeWriter::close(): the local clone keeps the writer alive while on_close takes the slot and start's ref, so the count reaches zero only after the call returns.
leave_fd_open on the EPIPE/EndOfFile path: close_without_reporting now leaves the poll registered on a descriptor the owner closes, until the writer's Drop unregisters it and re-enters on_close_io — benign because it all runs synchronously in one call chain and on_stdin_closed finds the slot empty.
Reader error before this.process is set: the watch_or_reap Ok(false) arm checks pipe_error and terminates, so the early-failure window is covered.
The new format test's GOMAXPROCS: "2" is read by bun_core::util::get_thread_count (min 2), so the two-thread deadlock it targets is genuinely exercised.

Extended reasoning...

The PR replaces the sync-spawn-plus-writer-thread path in src/runtime/cli/lint_command.rs with a new src/runtime/cli/captured_child.rs that drives a child's stdin/stdout/stderr on a mini event loop or the VM's spawnSync loop, plus PollTag/dispatch/link_interface plumbing, a unix-only StaticPipeWriter::leave_fd_open, removal of SyncStdio::Pipe, and two new tests. It touches no auth, crypto, or injection surface; the children are git and bun with argv built from CString (NUL rejected). The second commit resolves all three findings from the prior review and this hunt ran dry with no new reports, but the change is large, relies on raw-pointer re-entrancy invariants across several callback paths, and its Windows branch is untested outside CI, so it is not a candidate for approval without a human reviewer.

`bun lint` and `bun fmt` start two kinds of child process: git, and a `bun` that evaluates a config file that is a program. Both can get input on stdin. It was written by a blocking `write_all` on a thread started for that, next to `sync::spawn`. Now `CapturedChild` runs the child on an event loop. `StaticPipeWriter` writes stdin, two `BufferedReader`s read stdout and stderr, and the usual exit handler reports the exit. No thread is started and nothing blocks in a write. A child with no input gets no pipe for stdin. Which loop: a lint thread can have a JS VM, and then the thread's own loop is the VM's. There the child is waited for on the VM's `spawnSync` loop, so none of the VM's timers or callbacks run meanwhile. Other threads use their mini event loop. Who closes stdin: on Linux a pipe writer leaves its `close` to the thread pool, and the child sees the end of its input only then. `bun fmt` works on that pool, so with two threads nothing closed the descriptor and `bun fmt` hung. `CapturedChild` keeps the descriptor (`StaticPipeWriter::leave_fd_open`) and closes it as soon as the writer is done. When the wait ends early: a pipe that fails ends the child, and once the child has exited its stdin is no longer waited for. On Linux the kernel sends the child SIGTERM if bun dies, which git catches to remove `.git/index.lock`. `sync::spawn` forwarded signals instead, which these children no longer get. `SyncStdio::Pipe` had no other user and is gone.

claude

I reviewed the pushed version and found no bugs; the three points from my earlier review are now handled in src/runtime/cli/captured_child.rs (linux_pdeathsig is SIGTERM at :151, a failed pipe SIGTERMs the child at :258/:295, and on_process_exit closes the stdin writer at :309-312). A human look is still worthwhile: it is ~350 lines of new unsafe event-loop and process-lifecycle code, and the Windows branches have only been compiled and run on CI.

What was reviewed:

StaticPipeWriter refcount across create/start/on_close_io/the close() from on_process_exit: balanced on each path (slot ref taken in on_stdin_closed, start ref released in on_close, local clone dropped after).
leave_fd_open: close_impl still unregisters the poll before on_close_io closes the fd, and a failed start() closes it itself with stdin_fd cleared, so no double close.
The VM path matches Bun.spawnSync (spawn_sync_event_loop + prepare + EventLoopHandle::init on the isolated loop); the no-VM path uses the thread-local mini loop, so pool threads do not share one.
The no-pidfd busy-tick candidate (uws returns at once with num_polls == 0 while the waiter thread owns the exit): shared with GitSubprocess and ends when the child exits, so not a hang.

Extended reasoning...

The change replaces the sync spawn path in bun lint/bun fmt with a new CapturedChild that drives a child's stdin writer, stdout/stderr readers and exit handler on either the thread's mini event loop or the VM's spawn-sync loop, plus small wiring edits (PollTag, dispatch arm, link_interface lists, StaticPipeWriter::leave_fd_open, removal of SyncStdio::Pipe) and two tests. It touches no auth, crypto or user-input parsing surface beyond rejecting interior NULs in argv. The current code addresses all three findings from the prior review and the hunt ran dry with no new findings, but the file is large, unsafe-heavy and has Windows-only branches the author has only exercised on CI, so a human should still look rather than auto-approving.

robobun · 2026-10-24T19:58:40Z

A question about StaticPipeWriter::leave_fd_open, because #46800 changes the default that its doc comment describes ("By default the thread pool closes it").

#46800 moves that close into the shared helper. On POSIX, PollOrFd closes a pipe, a socket or the pty of Bun.Terminal on the calling thread, before it reports closed. Files and devices stay on the pool. The stall from "Who closes stdin" also exists for child.stdin, for proc.stdout on cancel(), for Bun.Terminal and for a FIFO body of Bun.serve. On two CPUs with two idle fs streams, cat never exits after child.stdin.end().

Which one do you want?

In PollOrFd: io: close a pipe in PollOrFd before it reports closed (POSIX) #46800 merges. leave_fd_open is then a second way to the same close, so it goes or its comment changes.
Per owner, as here: io: close a pipe in PollOrFd before it reports closed (POSIX) #46800 closes, and its test rows move to the owners that have a waiting peer.

coderabbitai Bot reviewed Oct 15, 2026

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Comment thread test/cli/lint/js-plugins.test.ts Resolved

claude Bot reviewed Oct 15, 2026

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Comment thread src/runtime/cli/captured_child.rs Resolved

Comment thread src/runtime/cli/captured_child.rs Resolved

Comment thread src/runtime/cli/captured_child.rs Resolved

claude Bot reviewed Oct 16, 2026

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robobun mentioned this pull request Oct 19, 2026

FileSink: close a file before end() or Bun.write() reports it closed #46088

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Jarred-Sumner force-pushed the claude/lint-child-stdin-on-the-event-loop branch from 095a6bb to 0209c51 Compare October 22, 2026 00:06

claude Bot reviewed Oct 22, 2026

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This was referenced Oct 24, 2026

Bun.serve: stop a file response whose reader failed to start #46684

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io: close a pipe in PollOrFd before it reports closed (POSIX) #46800

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`bun lint`, `bun fmt`: write a child's stdin from the event loop - #45424

`bun lint`, `bun fmt`: write a child's stdin from the event loop#45424
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