event loop: report immediates' rejections before polling, and stop polling once a fatal error ended the run

[robobun]

Problem

• A promise rejected inside a setImmediate callback, while something else keeps the event loop alive, is not reported when the immediate returns. It is reported when the loop next wakes up for an unrelated reason: on 1.4.0 that is the idle GC timer, about 1 to 1.7s later; with BUN_GC_TIMER_DISABLE=1 (or once that timer has backed off to its 30s mode) never, and the process just sits there. Node reports it when the callback returns. Main thread and worker_threads Workers alike (the parent's 'error' event is what arrives late or never).

  setInterval(() => {}, 100000);
  setTimeout(() => setImmediate(() => Promise.reject(new Error("x"))), 300);

  bun x.js: error: x after ~1.7s, exit 1. BUN_GC_TIMER_DISABLE=1 bun x.js: nothing, runs until killed. node x.js: error, exit 1, immediately.
• Cause 1: rejections are only examined by handle_rejected_promises() at the end of EventLoop::tick() (src/jsc/event_loop.rs) and at the tail of auto_tick() (src/runtime/jsc_hooks.rs), both after the poll. Immediates run at the top of auto_tick() / auto_tick_active(), and nothing looks at the list between them and tick_with_timeout(), which parks until the next timer or I/O event.
• Cause 2 (found while fixing 1, same shape): once an error nothing handled has been counted (unhandled_error_counter, which is what makes every run loop's while is_event_loop_alive() stop), the auto_tick_active() of that same turn still goes on to park in the poll. So even a rejection that is reported on time (from a timer or I/O callback) only ends the process at the next unrelated wakeup, and whatever that wakeup brings runs first: setTimeout(() => Promise.reject(e), 1); setTimeout(() => console.log("later"), 20) prints later after the error on 1.4.0 (node prints nothing), and with a long interval instead of the second timer the exit waits for the GC timer, or forever. A throw inside an immediate has the same second half: reported at once, exit delayed the same way.

Fix

• EventLoop::tick_immediate_tasks() ends its batch by calling handle_rejected_promises(), the same call tick() makes for its tasks, so the immediates phase reports what it left unhandled before the caller computes the poll. Both auto_tick() (used by bun test and the sync waits) and auto_tick_active() (the run loops) go through it. Skipped when the batch ran nothing (nothing can have been rejected since tick() looked).
• It only reports when the batch's callbacks got their microtask checkpoint in this loop (checkpoints_here(): depth 0, not inside a deferred task, draining not suppressed), which is the same condition under which each callback's exit() drained. Otherwise a rejection whose .catch() is still sitting in the microtask queue would be reported early; in those nested cases the enclosing frame's tick() drains and reports as before. This is also why the existing exception_thrown tracking becomes |=: a throwing immediate deliberately skips its own checkpoint (node defers a failed immediate's ticks), the batch tail drains for it, and a later task that returned false (it had been cleared) used to erase that. The returns / throws, after clearing the immediate queued behind it tests pin both. maybe_drain_microtasks() (since One termination signal, one fold: Err(Thrown) always means an exception is pending, and each event-loop dispatcher takes it in one place #37275 also used by the fold in Task.rs) is unchanged in behaviour and now spells its condition as checkpoints_here() too. As in tick(), the report is skipped once the VM has been stopped or an exception is still pending at the batch tail.
• auto_tick_active() returns after the immediates when an error has been counted and is_event_loop_alive() is therefore false. Its callers are the run loops, while is_event_loop_alive() { tick(); auto_tick_active(); } (Run::start, on_before_exit(), WebWorker::spin, the REPL, the debugger thread's own loop), which are about to stop, so the poll could only delay that; this is now stated as the function's contract in its doc comment. The is_event_loop_alive() half keeps the one case where the counter does not stop those loops today, queued immediates, behaving exactly as before (the loop comes back for them and the poll does not block while they are queued); making the counter end the run in that case too is event loop: a fatal error ends the run even while immediates are queued #38522, which composes with this (its change makes this return fire there as well). Not added to auto_tick(): under bun test the counter is a tally and the runner keeps going.
• The one caller that was not such a loop, the debugger's attach wait (wait_for_debugger_if_necessary, reached by --inspect-wait/--inspect-brk and inspector.waitForDebugger()), loops on the connection arriving, so with the counter nonzero (under bun test after any unhandled error, since the counter is a tally there) the early return would have made it spin until the frontend attached instead of parking: measured at 51 of the 50 clock ticks in a 500ms window. It now ticks with auto_tick(), like wait_for_promise and the tree's other condition waits, which parks regardless of the counter (0 to 1 ticks in the same window); the connection still arrives through the tick() in that loop. AnyEventLoop::tick_once also calls auto_tick_active(), for one pump between batches of install work; a skipped poll there changes nothing (the waits after it use auto_tick()), noted in the doc comment.
• Matches node in what is observable: the rejection is reported when the immediate returns, and after a fatal error nothing else runs. Not node's per-callback granularity (a sibling immediate queued behind the rejecting one still runs before the report); that is Emit unhandledRejection at the end of the tick that created it #33354's subject.
• Overlap with the open event-loop PRs, stated precisely since they edit the same functions: Emit unhandledRejection at the end of the tick that created it #33354 (per-callback reports after every timer and immediate callback, with a C++ signature change; open since July and conflicting with main) contains this same |= line and a batch-tail report in the same tick_immediate_tasks hunk, so whichever of the two lands second drops those lines from its diff; with Emit unhandledRejection at the end of the tick that created it #33354 in, this PR's batch-tail report would be reached with an already empty list. event loop: report promise rejections left by the turn that let the loop go idle #37981 and Drain the microtask checkpoint after unhandledRejection handlers run #33356 add reports at the two exits of auto_tick_active(), the function whose top this PR edits (separate hunks; a merge with event loop: report promise rejections left by the turn that let the loop go idle #37981's branch was checked clean); neither covers the immediates-to-poll gap or the park, and this PR does not cover their turn-went-idle case. Deduplicate auto_tick, IPC drain loops, async module dispatch, and entry-point waits #37365 folds auto_tick_active into a const-generic auto_tick; whichever lands second re-expresses the two-line return as its ACTIVE-only arm. event loop: a fatal error ends the run even while immediates are queued #38522 makes the counter end the run with immediates queued; composes (see above). hot/watch: keep driving timers and surviving errors after an unhandled error #38206 stops counting errors in watcher mode; unaffected. worker: wake the loop when a worker stops itself from an immediate (process.exit() / uncaught error) #38483 (merged, now in this PR's base) fixed the worker's process.exit() / throw-from-an-immediate shapes by waking its poll; the Promise.reject shape is this PR, and its tests and this PR's pass together on this base.
• Verified on the debug build; each new test fails on a build without the src/ change (checked with the 1.4.0 release binary: 4 of the 6 process tests, both worker tests and the runner test fail; the two is not reported controls pass both ways, as intended):
  • test/js/node/process/process.test.js, new block errors nothing handled are acted on before the event loop polls again: listener order, fatal rejection from an immediate, fatal throw from an immediate, fatal rejection from a task callback (cause 2 on its own), and the two controls. Each arms a 0ms timer and blocks past its deadline in the callback under test, so the timer fires in the very next poll if there is one; that makes both directions deterministic and platform independent (on Windows timers fire inside the poll call, so a marker made due by the callback itself cannot distinguish anything there; these markers are due before the poll is entered).
  • test/js/node/worker_threads/worker_threads.test.ts, new block a rejection left by a setImmediate callback is reported before the worker's loop polls (subprocess: under bun test a worker's rejections go to the runner, not its listeners): listener order, and the 'error' event plus the worker's 'exit' handler observing that the due timer never fired.
  • test/js/bun/test/test-test.test.ts: under the runner's own loop the error is printed before the test's due timer fires.
  • test/js/node/inspector/inspector.test.ts (Linux only): a bun test child tallies an unhandled rejection, then calls waitForDebugger(); the test reads the child's main-thread utime+stime from /proc/<pid>/task/<pid>/stat across a 500ms window before any client connects, then attaches and resumes it. This one cannot fail on main (main's attach wait parked because there was no early return); it fails with this PR's early return minus the Debugger.rs change (51 ticks, checked by reverting that file alone) and passes with it (0 to 1 ticks). The four existing waitForDebugger / --inspect-wait tests in inspector.test.ts and test/cli/inspect/inspect.test.ts still pass (the --inspect=<url> websocket cases in the latter fail identically on the release binary in this container, a networking matter).
  • Rebased onto d4ccab4 (main minus its current tip, 2f5c180, which does not compile: PostgresSQLConnection.rs uses a helper One termination signal, one fold: Err(Thrown) always means an exception is pending, and each event-loop dispatcher takes it in one place #37275 removed; tracked separately and unrelated to these files). One termination signal, one fold: Err(Thrown) always means an exception is pending, and each event-loop dispatcher takes it in one place #37275 renamed JsTerminated to Stopped and gave maybe_drain_microtasks() a second caller, which is where the two event_loop.rs details above come from; all of the new tests, the probe matrix below, the 54 upstream files, the timers suites, the runner suites and the worker suites were re-run on the rebased build with the same results.
  • Also run, no new failures: whole process.test.js (only process.env.USER unset here), whole worker_threads.test.ts (one terminate-ordering test failed once under the full-file debug load and passes 4/4 alone), test/js/node/timers + test/js/web/timers (the 4 failures are the bun-asan execPath detection in the leak fixtures, pre-existing), 11 bun test runner suites, test/js/web/workers/worker.test.ts (4 known debug-speed failures), shell/commands/exit + false, spawn/exit-code, hot.test.ts, watch.test.ts, repl.test.ts, and 54 upstream files (test-promise*, test-promises-*, test-timers-immediate* / clearImmediate* / setimmediate* / unref* / ordering, test-microtask-*, test-next-tick-*, test-process-beforeexit*, test-beforeexit-event-exit, test-process-exit-code*, test-process-exception-capture*, test-worker-uncaught-*, test-unhandled-exception-with-worker-inuse).

Background

• Rejected-promise list: JSC tells bun when a promise is rejected with no handler attached; bun appends it to a per-global list. handle_rejected_promises() walks the list and, for each promise still without a handler, emits process's 'unhandledRejection' or, with no listener, calls the VM's fatal hook (main thread: print, count the error; worker: post 'error' to the parent, run its 'exit' handlers, request its stop). A handler attached before the walk takes the promise off the list, which is why the walk must come after the callback's microtask checkpoint.
• Microtask checkpoint: EventLoop::enter() / exit() count nesting; the exit() that brings the depth to 0 drains process.nextTick callbacks and microtasks. An immediate run from the top of the loop therefore drains at its own exit; one run while the loop is being ticked from inside a JS frame (a synchronous wait_for_promise) does not, the enclosing tick() does later.
• Loop turn: every run loop alternates tick() (queued tasks, microtasks, then the rejection walk) with auto_tick_active() (immediates, then a poll whose timeout comes from the timer heap, then due timers). auto_tick() is the same with a rejection walk at its tail, used where a caller waits for something specific. With a keep-alive and no timer due, the poll's timeout is unbounded except for the idle GC timer, a 1s (later 30s) per-VM repeating timer that BUN_GC_TIMER_DISABLE=1 removes.
• unhandled_error_counter: bumped when an error reaches the top with no listener. is_event_loop_alive() is false while it is nonzero (unless immediates are queued, see event loop: a fatal error ends the run even while immediates are queued #38522), which is how bun run and workers exit 1 after such an error; bun test uses it as a tally instead.

Before / after on the shapes above (linux x64; before = 1.4.0 release, after = this branch's debug build)

shape	before	after	node
interval keep-alive, Promise.reject in an immediate	reported + exit after ~1.7s; never with BUN_GC_TIMER_DISABLE=1	reported + exit 1 at once	same
interval keep-alive, Promise.reject in a 300ms timer	reported at once, exit after ~0.7s; never with the GC timer off	exit 1 at once	same
interval keep-alive, throw in an immediate	reported at once, exit after ~0.7s; never with the GC timer off	exit 1 at once	same
Promise.reject in a timer, 20ms sibling timer	error, then later	error only	error only
listener + due timer, reject in an immediate	timer, unhandledRejection	unhandledRejection, timer	same as after
worker: keep-alive, reject in an immediate	parent 'error' after ~0.7s, never with the GC timer off	'error' ~14ms after the immediate (debug build), worker exits	same (worker exit code 0 vs node's 1 is #34193, unchanged)
reject handled by a queued microtask, immediate returns / throws	not reported	not reported	same
fatal in an immediate that queued another immediate	second immediate runs, then exit	unchanged (#38522)	nothing runs

---

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/test/test-test.test.ts test/js/node/inspector/inspector.test.ts test/js/node/process/process.test.js

[coderabbitai]

Walkthrough

The event loop now reports unhandled errors after immediate-task batches and before polling. Debugger waits use the standard tick path. Tests cover timers, workers, subprocesses, and inspector waiting.

Changes

Event-loop error ordering

Layer / File(s)	Summary
Immediate-task checkpoint and error handling src/jsc/event_loop.rs	Checkpoint selection is private. Immediate-task exceptions accumulate across callbacks. Microtasks and unhandled rejections are processed before further polling.
Debugger wait and shutdown control src/jsc/Debugger.rs, src/runtime/jsc_hooks.rs	Wait::Forever uses auto_tick(). auto_tick_active() stops polling after unhandled errors terminate the event loop.
Error-ordering and debugger regression coverage test/js/bun/test/test-test.test.ts, test/js/node/inspector/inspector.test.ts, test/js/node/process/process.test.js, test/js/node/worker_threads/worker_threads.test.ts	Tests verify error reporting before due timers, worker error behavior, and low CPU activity during debugger waiting.

🚥 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 both primary event-loop changes: reporting immediate rejections before polling and stopping polling after fatal errors.
Description check	✅ Passed	The description explains the problem, implementation, behavior changes, affected cases, and extensive verification results, despite using different section headings.

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[robobun]

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[robobun]

Status: reproduced on the 1.4.0 release binary (linux x64) with the repro in the description: setInterval(() => {}, 100000); setTimeout(() => setImmediate(() => Promise.reject(new Error("x"))), 300) reports and exits after ~1.7s, and never with BUN_GC_TIMER_DISABLE=1; the setTimeout and throw variants report at once but also only exit on the next wakeup. With this branch all of them exit 1 within a few ms of the callback (debug build), and the worker variant delivers 'error' ~14ms after the immediate.

Fix: #38524 (this PR). New tests in process.test.js, worker_threads.test.ts and test-test.test.ts, each failing on a build without the src/ change, plus one in inspector.test.ts guarding the debugger attach wait against the new early return (see the Fix section). Rebased onto d4ccab4 after #37275 and #38483 landed; main's current tip (2f5c180) does not compile for an unrelated reason, which is tracked separately.

[robobun]

^{Updated 11:52 AM PT - Aug 14th, 2026}

@robobun, your commit cb2850a is building: #96435

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Inline comments:
In `@test/js/node/process/process.test.js`:
- Around line 1526-1612: Change the outer describe block in
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Reviewing files that changed from the base of the PR and between 7cf6296 and f281ddc.

📒 Files selected for processing (7)

• src/jsc/Debugger.rs
• src/jsc/event_loop.rs
• src/runtime/jsc_hooks.rs
• test/js/bun/test/test-test.test.ts
• test/js/node/inspector/inspector.test.ts
• test/js/node/process/process.test.js
• test/js/node/worker_threads/worker_threads.test.ts

[claude]

The debugger busy-spin concern from the earlier inline comment is resolved: f281ddc switches the Wait::Forever arm to auto_tick() and adds a Linux /proc-based CPU-tick regression test in inspector.test.ts. No further issues found this run. Given the scope — event-loop tick ordering, a new contract on auto_tick_active callers, and rejection-reporting timing — a maintainer look is still worthwhile.

Checked: the removed maybe_drain_microtasks had no other callers (the only remaining match is the unrelated exit_maybe_drain_microtasks); every current auto_tick_active call site — Run::start, on_before_exit, WebWorker::spin, the REPL, Debugger::start, run_event_loop_for_watch, AnyEventLoop::tick_once — either sits inside a while is_event_loop_alive() loop or is a single non-blocking pump, matching the new contract; and the checkpoints_here() predicate matches the condition under which each immediate's own exit() would have drained.

Extended reasoning...

Overview

This PR changes when unhandled promise rejections from setImmediate callbacks are reported, and stops auto_tick_active from parking in the poll after a fatal error has already ended the run. Three source files: src/jsc/event_loop.rs (tick_immediate_tasks now calls handle_rejected_promises() after the batch, gated on a new checkpoints_here() predicate; exception_thrown becomes |=; maybe_drain_microtasks removed), src/runtime/jsc_hooks.rs (auto_tick_active early-returns when unhandled_error_counter > 0 && !is_event_loop_alive(), plus a doc-comment contract on callers), and src/jsc/Debugger.rs (the Wait::Forever attach wait switched to auto_tick()). Four test files gain ~10 new subprocess-based tests.

Prior feedback addressed

My earlier inline comment identified that the auto_tick_active early return would make Debugger::wait_for_debugger_if_necessary's Wait::Forever loop busy-spin under bun test after a tallied error. Commit f281ddc fixes this by switching that arm to auto_tick() (which always polls, like wait_for_promise), updating the auto_tick_active contract comment to name the debugger's attach wait, and adding a regression test that measures the child's main-thread CPU ticks via /proc/<pid>/task/<pid>/stat across a 500ms window while it waits. I verified all remaining auto_tick_active call sites against the new contract.

Security risks

None. This is event-loop scheduling / error-reporting timing; no untrusted input parsing, auth, or crypto.

Level of scrutiny

High. tick_immediate_tasks and auto_tick_active run on every loop turn of every Bun process; the new checkpoints_here() gate and the early-return contract are subtle correctness invariants (a wrong checkpoints_here() reports a rejection whose .catch() is still queued; a wrong early return either delays exit or busy-spins a condition wait). The PR is unusually well-documented and thoroughly tested — including negative controls and the |= fix's own test — but this is exactly the class of change where a maintainer who owns the event loop should confirm the design choices (in particular, that auto_tick_active's new caller contract is the right layering vs. gating on loop_.is_active()).

Other factors

The maybe_drain_microtasks removal is safe — its only caller was inlined into the new logic, and the grep hit in timer_object_internals.rs is the unrelated exit_maybe_drain_microtasks. The auto_tick() substitution in the debugger wait adds run_imminent_gc_timer and a tail handle_rejected_promises() relative to auto_tick_active, both of which are already what wait_for_promise (the analogous condition wait) runs, so no behavior concern there.

[claude]

I re-reviewed after f281ddc/3ab910a and found no further issues — the debugger attach-wait spin I flagged earlier is fixed and covered by the new /proc ticks test. Because this changes core event-loop ordering (when rejections are reported, when auto_tick_active stops polling) and introduces a new caller contract that every run loop must satisfy, a human look is still worthwhile.

Checked: the removed maybe_drain_microtasks had no other callers (exit_maybe_drain_microtasks is a different fn); every remaining auto_tick_active call site is inside a while is_event_loop_alive() loop or is AnyEventLoop::tick_once as the doc comment states; checkpoints_here() mirrors exit()'s drain condition plus suppress_microtask_drain, so a rejection whose .catch() is queued is not reported early; the |= on exception_thrown is pinned by the "throws, after clearing the immediate queued behind it" test.

Extended reasoning...

Overview

The PR changes three source files in the core event loop: src/jsc/event_loop.rs (tick_immediate_tasks now reports unhandled rejections after the batch and accumulates exception_thrown with |=; maybe_drain_microtasks becomes a private checkpoints_here predicate), src/runtime/jsc_hooks.rs (auto_tick_active returns early once unhandled_error_counter > 0 && !is_event_loop_alive(), with a new caller contract in the doc comment), and src/jsc/Debugger.rs (the Wait::Forever attach-wait switches from auto_tick_active to auto_tick). Four test files add coverage for the main thread, workers, the bun test runner, and the debugger wait.

Security risks

None identified. The change is about ordering of error reporting and loop termination; no auth, crypto, or untrusted-input parsing is touched.

Level of scrutiny

High. tick_immediate_tasks and auto_tick_active sit in every turn of every run loop (main thread, workers, REPL, watch mode, the debugger thread). The new early return in auto_tick_active establishes a caller contract ("callers loop on is_event_loop_alive()") that must hold for every current and future caller; my earlier review found one caller (the debugger attach wait) that did not, which was fixed. I re-checked every remaining call site in run_command.rs, test_command.rs, VirtualMachine.rs::on_before_exit, web_worker.rs, repl.rs, Debugger.rs::start, and AnyEventLoop::tick_once — all satisfy the contract or are the documented single-pump exception. The checkpoints_here() gate on the new handle_rejected_promises() call is the same condition under which each callback's exit() would have drained (depth 0, not in a deferred task) plus suppress_microtask_drain, so it does not report a rejection whose handler is still queued when the loop is being ticked from inside JS.

Other factors

The prior review round produced one real bug (debugger spin), which was fixed with a Linux-only /proc/<pid>/task/<pid>/stat test that measures the wait's CPU time before any client connects. The |= change to exception_thrown is a real behavior change (a later cleared/unref'd immediate returning false used to erase an earlier throw's need for a checkpoint) and is pinned by one of the two "is not reported" control tests. The removed public maybe_drain_microtasks had only the one caller inside tick_immediate_tasks itself. Test coverage is thorough and each new test's failure mode on an unfixed build is documented in the PR body. Given that a maintainer should confirm the caller-contract approach (vs. e.g. gating the early return differently) and the observable-timing changes for unhandledRejection ordering, deferring rather than auto-approving.

[claude]

I re-reviewed after f281ddc/cb2850a: the debugger attach-wait spin I flagged earlier is fixed (now ticks with auto_tick, guarded by the new /proc-tick inspector test), and no further issues turned up. Given this changes when unhandled rejections are reported and when the run loops stop polling — observable across bun run, workers, bun test, and the REPL — a human sign-off would still be worthwhile.

Checked: the remaining auto_tick_active callers all sit inside while is_event_loop_alive() loops (Run::start, on_before_exit, WebWorker::spin, REPL, test watch, debugger thread) or are the one-pump AnyEventLoop::tick_once the doc comment names; the eval-and-print one-off at run_command.rs:1514 is reached only after the main loop already exited on the same predicate. The added suppress_microtask_drain term in checkpoints_here() is a no-op for the pre-existing maybe_drain_microtasks path (drain already early-returns on it) and only gates the new handle_rejected_promises call. The |= on exception_thrown and its "throws, after clearing the immediate queued behind it" test look right.

Extended reasoning...

Overview

Three source changes: (1) tick_immediate_tasks now reports the batch's unhandled rejections before returning (gated on checkpoints_here(), skipping when nested or an exception is pending), and exception_thrown accumulates with |= so a later cleared/skipped immediate cannot erase an earlier throw; (2) auto_tick_active returns before polling when unhandled_error_counter > 0 && !is_event_loop_alive(), with the contract stated in its doc comment; (3) wait_for_debugger_if_necessary now ticks with auto_tick instead of auto_tick_active, so it parks when the counter is nonzero under bun test. Four test files add coverage for main-thread, worker, test-runner, and inspector paths.

Security risks

None identified. This is event-loop scheduling and error-reporting timing; no auth, crypto, parsing of untrusted input, or permission checks are touched.

Level of scrutiny

High. The event loop's tick/poll boundary and the run loops' exit condition are load-bearing for every program's lifetime and for how errors surface. The change is small in line count but affects observable behavior (what runs after a fatal error, when unhandledRejection fires) across bun run, workers, the test runner, the REPL and the debugger. The PR description enumerates the interactions with #38522/#33354/#37981/#38483 carefully, and the tests are constructed to be deterministic (arm a 0ms timer, block past its deadline, so the next poll — if any — fires it), but this is exactly the class of change that benefits from a maintainer's read.

Other factors

My earlier finding (the debugger attach wait would busy-spin under bun test after a tallied error) was addressed with both the auto_tick switch and a Linux-only guard test measuring the wait thread's utime+stime across a 500ms window. I re-checked every auto_tick_active caller against the new contract; the one non-loop caller (AnyEventLoop::tick_once) is named in the doc comment and a skipped poll there is harmless. The checkpoints_here() refactor adds a suppress_microtask_drain guard that is redundant for maybe_drain_microtasks (drain checks it internally) but load-bearing for the new rejection-reporting site. CodeRabbit's describe.concurrent nit was applied. The bug-hunting system found nothing on the current revision.