spawnSync: don't run the bun:test timeout callback while the isolated loop is active

[Jarred-Sumner]

What does this PR do?

test/regression/issue/07261.test.ts has been hanging in the bun test --parallel batch on the glibc Linux lanes (17 of the last ~110 PR builds; passes when retried alone). In the shard log the batch goes quiet with 07261 as the only file left, the 90s per-test deadline passes with no (fail) … timed out line ever printed, and the runner's idle timeout kills the batch 4 minutes later. A live capture of a hung batch shows the worker running the file spinning at 100% CPU with no syscalls, its spawnSync child a zombie, the pidfd still open.

When the per-test deadline passed while Bun.spawnSync was blocking, the sync wait loop called BunTest::bun_test_timeout_callback in place — re-entering the whole test runner (result queue, handle_test_completed, junit, worker IPC, timer heap) while vm.event_loop_handle still pointed at the isolated loop and before the child had been reaped. Nothing outside the sync child's own I/O should run there.

Now the loop only calls Execution::handle_timeout (kill the dangling processes so the child dies and the wait drains, killed N dangling process is still printed at the deadline) and hands the deadline back to spawn_sync, which fires the runner's callback once spawn_maybe_sync has torn the isolated loop down. If an exception is pending by then (spawn failure, termination), the file timer is left armed and reports the timeout from the main loop; an exception raised by the deferred callback is propagated (Ok(JSValue::ZERO) with the exception on the global, same as the existing post-loop path). Only src/runtime/api/bun/js_bun_spawn_bindings.rs changes.

How did you verify your code works?

• test/cli/test/test-timeout-behavior.test.ts (sync fixture: spawnSync outliving the per-test timeout still prints killed 1 dangling process, (fail) …, no JavaScript execution terminated, and the next test runs), test/js/bun/spawn/spawnSync.test.ts, spawnsync-no-microtask-drain, spawn-signal, spawn-maxbuf, bun-test.test.ts -t timeout, parallel.test.ts (33/35; the unique JEST_WORKER_ID failure is the known debug-build timing flake noted in bun test: only run process.on('exit') listeners when node:test APIs were used #38442) — on the debug build of this branch.
• The CI hang itself only reproduced under host load in a Linux container with the CI binary (~1/100 batches); there is no deterministic test for it in this PR. I'll re-run that stress against this branch's Linux CI build and post the counts here.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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• src/runtime/api/bun/js_bun_spawn_bindings.rs
• src/runtime/test_runner/Execution.rs

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Walkthrough

spawn and spawnSync now pass bun:test deadline state through spawn_maybe_sync. Timeout handling kills processes during isolated execution and defers the timeout callback until after child reaping and event-loop teardown.

Changes

Spawn timeout lifecycle

Layer / File(s)	Summary
Timeout process cleanup src/runtime/test_runner/Execution.rs	handle_timeout delegates dangling-process termination to kill_dangling_processes_on_timeout before recording the timeout result.
Deferred spawnSync timeout handling src/runtime/api/bun/js_bun_spawn_bindings.rs	spawnSync records the deadline, removes the active timeout after isolated execution, and invokes the callback afterward. Timeout handling kills dangling processes and defers callback re-entry. spawn passes an empty deadline slot.

Possibly related issues

• Bug: spawnSync never returns: child exit lost (child stays zombie), wait loop busy-spins at 100% CPU re-registering a finished pipe reader (macOS ARM64) #34069 — The changes address spawnSync timeout and nested test-timeout handling.
• Bun.spawnSync({ timeout }) returns exitCode: null with empty stdout/stderr for a real ssh.exe child on Windows — not reproduced with a synthetic closed-port case #33932 — The changes address dangling-process cleanup during Bun.spawnSync({ timeout }).

Possibly related PRs

• oven-sh/bun#38774 — Directly extends timeout cleanup behavior in Execution.rs.
• oven-sh/bun#37604 — Modifies spawn timeout handling in the same bindings file.
• oven-sh/bun#37609 — Modifies spawn and spawnSync lifecycle handling around timeouts and child processes.

Suggested reviewers: robobun

Merge Risk: 🟡 Moderate · up to 0ad1d

The PR changes timeout handling during synchronous process execution, but an unresolved native event-loop ownership issue could cause incorrect runtime behavior or instability. Merge should wait for that concern to be fixed or explicitly accepted by the owner.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the spawnSync timeout behavior change and its purpose.
Description check	✅ Passed	The description includes the required sections and provides detailed context, implementation behavior, and verification results.
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.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/io/posix_event_loop.rs`:
- Around line 493-507: Update resolve_loop and the keep-alive paths to compare
self.loop_ and the caller’s loop_ as raw pointers before creating any mutable
references; only dereference self.loop_ when the pointers differ, while
preserving the existing loop selection behavior.

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📒 Files selected for processing (1)

• src/io/posix_event_loop.rs

[github-actions]

This PR may be a duplicate of:

1. spawnSync: keep event-loop ref counts on the loop they were taken on #37754 - Fixes the same root cause in the same file: adds a loop pointer field to FilePoll in src/io/posix_event_loop.rs so activate/deactivate/register/unregister and the active count land on the loop the poll was taken on across the spawnSync vm.event_loop_handle swap (it additionally covers KeepAlive, timers, the jsc EventLoop concurrent ref, and SpawnSyncEventLoop, and adds a regression test).

🤖 Generated with Claude Code

[claude]

The Stacked Borrows concern from the previous pass is addressed — resolve_loop now compares against Loop::get() (a raw *mut) before materializing a second &mut Loop, and only does so when the pointers differ. No new issues found on this revision.

Given this touches core event-loop accounting with a new invariant (the debug_assert that spawnSync-loop polls are only touched while the swap is active), ships without a deterministic test, and overlaps with #37754 which takes a broader approach and includes a regression test, a human should decide which fix to land.

What was reviewed:

• resolve_loop aliasing: raw-pointer compare precedes the unsafe { &mut *default }, and the two loops are distinct at that point.
• activate/deactivate flag lifecycle: OnSpawnSyncLoop is set only on first activation and cleared in deactivate; re-registration (CTL_MOD) routes through resolve_loop first so the flag is not flipped mid-swap.
• #[cfg(windows)] gating of loop_add_active/loop_sub_active: remaining POSIX callers were removed; the keep_alive.rs and windows_event_loop.rs uses are already Windows-gated.

Extended reasoning...

Overview

This PR fixes a CI hang where spawnSync's isolated event loop spins forever because a FilePoll activated on one loop is deactivated while vm.event_loop_handle points at the other, decrementing the wrong num_polls and issuing EPOLL_CTL_DEL on the wrong epoll fd. The fix records an OnSpawnSyncLoop flag at activation time and has resolve_loop fall back to the thread's default loop (Loop::get()) for polls that were activated there, regardless of what the ctx currently names. It also gates generation_number to macOS-debug (its only reader) and moves the POSIX add_active/sub_active inline helpers onto the uws Loop type directly.

Prior review

My previous pass flagged that the earlier revision stored self.loop_: *mut Loop and materialized &'static mut Loop from it via bound_loop_mut() while the caller's &mut Loop argument was still live — two aliasing &mut in the common (non-swapped) case. Commit 7ace315 rewrote the approach: no stored pointer, a flag bit instead, and resolve_loop now compares the incoming &mut Loop against the raw Loop::get() pointer via ptr::eq before the unsafe { &mut *default }, which is only reached when they name different objects. That resolves the SB concern and the SAFETY comment is now accurate for the branch it guards.

Security risks

None. This is internal event-loop bookkeeping with no user-controlled input reaching the changed paths.

Level of scrutiny

High. This is core POSIX event-loop code (num_polls/active accounting, epoll/kqueue watcher-fd selection) with an unsafe block whose soundness depends on a non-local invariant — that the spawnSync isolated loop is never the same object as Loop::get(), and that spawnSync-loop polls are never touched after the swap unwinds (encoded only as a debug_assert). Getting either wrong turns into wrong-loop syscalls or aliasing UB. The author explicitly could not construct a deterministic test, so verification rests on the container stress run they promised to report on the PR.

Other factors

• No test. The repo review rules require an automated test for behavioral changes; the author explains why one isn't feasible here, but that's a maintainer call.
• Competing PR #37754 reportedly fixes the same root cause more broadly (also covering KeepAlive, timers, the concurrent ref, and SpawnSyncEventLoop) and ships a regression test. A maintainer should decide whether to land this narrower fix or the broader one.
• Visibility widening of EventLoopCtx::loop_mut from impl-private to pub(crate) is fine (still crate-internal), but the doc comment's "every crate-internal caller is a leaf op that drops the borrow before returning" now has two more call sites in enable/disable_keeping_process_alive — both are leaf ops, so the invariant still holds.

Not a candidate for auto-approval.

[claude]

The flag-based resolve_loop rewrite addresses the earlier aliasing concern — the common path (on_sync_loop == loop_.is_spawn_sync_loop) now returns loop_ directly without conjuring a second &mut Loop. Beyond the inline nits, I also checked that the new is_spawn_sync_loop field is zero on the default loop — us_internal_loop_data_init relies on calloc (loop.c:126), so activate() correctly leaves OnSpawnSyncLoop unset there; and that Loop::add_active/sub_active now exist on bun_uws_sys::Loop, so dropping the local saturating helpers is a straight substitution.

Extended reasoning...

This run's inline findings are both nits (KeepAlive same-class coverage, SAFETY comment wording / release fallthrough of the debug_assert). The prior blocking finding — bound_loop_mut() materializing an aliasing &mut Loop on the common path — was addressed by dropping the stored-pointer design in favor of the OnSpawnSyncLoop flag + internal_loop_data.is_spawn_sync_loop check; that thread is resolved. I additionally verified the new C-side char is_spawn_sync_loop and Rust bool mirror sit at the same trailing offset after tick_depth, and that the loop struct is calloc'd so the default loop reads it as false. Not approving: this touches core event-loop/unsafe code, ships without a test by the author's own account, and overlaps #37754 which covers more sites — those are maintainer calls.

[claude]

The PR was reworked to defer the runner callback in js_bun_spawn_bindings.rs instead of the earlier FilePoll loop-binding approach — my prior inline comments on src/io/posix_event_loop.rs (resolve_loop aliasing / SAFETY wording) and src/io/lib.rs (KeepAlive sibling) no longer apply to this diff. Beyond the inline nit on the double handle_timeout, I also checked that the deferred path re-fetches active_file after the isolated loop is torn down (rather than holding a clone across it) and that an exception raised by the deferred callback returns Ok(JSValue::ZERO) matching the existing post-loop propagation shape.

Extended reasoning...

This run reviewed the reworked single-file change in src/runtime/api/bun/js_bun_spawn_bindings.rs. The earlier revision touched src/io/posix_event_loop.rs and src/io/lib.rs with a FilePoll::resolve_loop / OnSpawnSyncLoop approach; my prior inline findings (aliasing &mut Loop in resolve_loop, inaccurate SAFETY comment, uncovered KeepAlive sibling) targeted that code, which is no longer in the diff — recording that here so they aren't re-investigated. On the current approach the bug-hunting pass surfaced one nit (in-loop handle_timeout also enqueues RefDataValue::Start, so it fires twice); two independent verifiers concluded the duplicate enqueue is idempotent in step_group and the second auto_killer.kill() finds an empty map, so no observable failure. Given this touches event-loop re-entrancy and the PR itself notes there is no deterministic test for the CI hang, leaving the merge call to a human.

[robobun]

^{Updated 1:12 PM PT - Aug 15th, 2026}

@Jarred-Sumner, your commit 0ad1d1e is building: #98662