Emit unhandledRejection at the end of the tick that created it

[robobun]

Repro

setImmediate(() => {
  Promise.reject(new Error("x")); // never handled
  setTimeout(() => console.log("later timer"), 0);
});
process.on("unhandledRejection", () => console.log("unhandledRejection"));

node: unhandledRejection, later timer
bun:  later timer, unhandledRejection

Without an unhandledRejection handler, node prints the error and exits before later timer runs; Bun runs the timer first.

Cause

Node reports unhandled rejections from processTicksAndRejections, which its timer and immediate queues run after every callback. So a rejection is always delivered before anything that callback scheduled, and before anything already queued behind it.

Bun only scanned at the tail of autoTick, after the poll and timer phases, so everything the callback scheduled got to run first. The timer case looked correct only by accident: a setTimeout(…, 0) created inside a timer callback is not due within the same drain, so the tail scan beat it. Once a sibling timer is already due, the same reordering shows up.

Fix

Run the scan at the end of each timer and immediate callback, which is where node's runNextTicks() sits. handleRejectedPromises() now reports whether a handler ran, so the caller can re-drain the microtask queue the handler may have pushed onto, mirroring node's do { drain } while (processPromiseRejections()).

Two placements that look right but aren't:

• EventLoop::exit(), even though that is where the callback's microtasks already drain. exit() is also how native code re-enters JS from inside a JS turn: Interpreter::finish() calls it while ShellPromise.then() is mid-flight, between rej(potentialError) and the super.then() that handles it. Scanning there reports a rejection the caller is about to handle. The enter/exit counter cannot tell the two apart, so only the two batch-drain phases call the new hook.
• The throw site of a callback that threw. exit_maybe_drain_microtasks(false) deliberately skips the drain there, so the scan would see a promise whose .catch() is merely queued and report it. Node marks the failed callback's scope and skips its checkpoint entirely, running it once the immediate batch drains, which is what keeps the timers-immediate-exception fixture's nextTicks deferred past the next immediate. This does the same.

Verification

Differential against node v26, comparing emission order. All of these were wrong before this change:

rejection created in	competing work	before	after / node
setImmediate	setTimeout(…, 0) scheduled by it	later timer, unhandledRejection	unhandledRejection, later timer
setImmediate	sibling setImmediate queued behind it	second immediate, unhandledRejection	unhandledRejection, second immediate
setImmediate	setTimeout(…, 5) already scheduled	5ms timer, unhandledRejection	unhandledRejection, 5ms timer
setTimeout	sibling timer due in the same drain	second timer, unhandledRejection	unhandledRejection, second timer
setImmediate	process.nextTick queued by the handler	nextTick never ran at all	unhandledRejection, nextTick from handler, second immediate
a throwing setImmediate	setTimeout(…, 0) scheduled by it	later timer, unhandledRejection	uncaughtException, unhandledRejection, later timer
a throwing setImmediate ending a batch that started clean	first immediate, later timer, unhandledRejection	first immediate, uncaughtException, unhandledRejection, later timer
a throwing setImmediate that clearImmediates its sibling	later timer, unhandledRejection	uncaughtException, unhandledRejection, later timer

Plus two cases that already matched node on main and are pinned so the checkpoint placement can't drift:

| a throwing setImmediate whose rejection is .catch()ed by a queued microtask | reports nothing (uncaughtException only) |
| a throwing setImmediate with a sibling immediate behind it | uncaughtException, second immediate, unhandledRejection |

All ten are cases in test/js/node/process/process.test.js; the eight in the first table fail on main.

Also re-checked against node and unchanged by this PR: rejections from process.nextTick, queueMicrotask, module top level, fs.readFile callbacks, a setTimeout scheduling another setTimeout, a setTimeout scheduling a setImmediate, a setImmediate scheduling another setImmediate, and nextTick queued from the rejecting callback (still delivered before the rejection).

Known gap, not addressed here

A rejection created in the poll phase (e.g. a socket data handler) is still reported after an already-due timer. Pre-existing; the natural hook is exit(), which is unusable for the reason above.

Suites run against the debug build (no new failures vs. main)

• test/js/node/process/process.test.js
• test/js/node/timers/ (including node's timers-immediate-exception fixture, which pins the deferred-checkpoint behaviour)
• test/js/bun/shell/commands/ (the suite an earlier revision of this PR broke)
• test/js/web/timers/, test/js/node/timers.promises/
• test/js/web/streams/streams.test.js, test/js/bun/http/serve.test.ts
• test/js/bun/test/ (bun's own test-runner suite)
• node's test/parallel/test-promise*-unhandled-*.js, test-promises-unhandled-rejections.js, test-microtask-queue-run-immediate.js, test-timers-immediate.js

The pre-existing failures in those suites (bunshell ls > permission denied directory — the container runs as root, server.requestIP > v6, expect.assertions DOES fail the test, …, …) reproduce identically with src/ reverted to main.

[coderabbitai]

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---

Walkthrough

The PR changes rejected-promise handling to return a boolean across the C++/FFI/Rust boundary, adds post-tick event-loop draining for rejected promises and microtasks, wires that into timer execution paths, and adds process tests covering ordering with timers, immediates, and nextTick.

Changes

Unhandled rejection bool return and post-tick draining

Layer / File(s)	Summary
C++ handleRejectedPromises returns bool src/jsc/bindings/ZigGlobalObject.cpp, src/jsc/bindings/ZigGlobalObject.h	GlobalObject::handleRejectedPromises changes from void to bool, returns false when nothing is queued, tracks whether any handler ran, and handles termination versus reportable exceptions differently while draining.
FFI and Rust binding signature updates src/jsc/bindings/bindings.cpp, src/jsc/bindings/headers.h, src/jsc/JSGlobalObject.rs	The exported JSC__JSGlobalObject__handleRejectedPromises declaration and Rust wrapper now return bool, with the Rust host-call path defaulting to false on failure.
Post-tick microtask draining and timer wiring src/jsc/event_loop.rs, src/runtime/timer/timer_object_internals.rs	A new event-loop helper runs rejected-promise handling after ticks and drains microtasks, and it is called from immediate and timer callback paths.
Process unhandledRejection ordering tests test/js/node/process/process.test.js	Adds subprocess tests that check unhandledRejection output ordering relative to later timers/immediates and process.nextTick work scheduled inside the handler.

Possibly related PRs

• oven-sh/bun#32554: Also changes rejected-promise handling and its draining loop, overlapping with the core handleRejectedPromises update here.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main behavior change to unhandledRejection timing.
Description check	✅ Passed	The description is detailed and includes purpose and verification, even though it does not use the exact template headings.
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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[robobun]

^{Updated 8:05 AM PT - Jul 5th, 2026}

❌ @robobun, your commit 4c2a3b6 has some failures in Build #68550 (All Failures)

---

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[robobun]

@coderabbitai review

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[github-actions]

Found 1 issue this PR may fix:

1. Top level await causes promise rejections inside global.setTimeout to be ignored, allowing execution to continue #22546 - Top-level await causes promise rejections inside setTimeout to be ignored; moving the rejection scan to end-of-tick aligns with Node's processTicksAndRejections and should fix the ordering

If this is helpful, copy the block below into the PR description to auto-close this issue on merge.

Fixes #22546

🤖 Generated with Claude Code

[robobun]

This doesn't fix #22546. That issue is a different bug, and this PR leaves it unchanged.

In that repro the rejection is already reported at the right moment, before the loop's next iteration:

$ bun issue22546.mjs
2
3
error: oops     <- reported promptly, both before and after this PR
oops
4
5
...runs forever

What's broken there is that Bun doesn't abort afterwards, and only when the entry point uses top-level await. Dropping the await makes the same program terminate on the rejection:

	reports the rejection	terminates
await main() (top-level await)	yes	no
main()	yes	yes

Same on both sides of this diff, so the ordering change has no bearing on it. Node aborts in both cases because its default is --unhandled-rejections=throw; Bun's default reporter prints and records a non-zero exit code without stopping the loop, and something about the pending top-level-await module promise is swallowing the termination that otherwise happens. That's worth fixing, but it lives in the reporter/exit path, not in when the scan runs.

[robobun]

Both reviews landed on the exit() placement, which turned out to be wrong for a reason neither of us had: CI went red on every platform with test/js/bun/shell/commands/{exit,false}.test.ts.

EventLoop::exit() is not only the end-of-tick hook. Native code also uses it to re-enter JS from inside a JS turn. Interpreter::finish() does exactly that, and for a synchronous builtin like false the whole thing runs inside ShellPromise.then():

then(onfulfilled, onrejected) {
  this.#run();                                 // interp.run() -> finish() -> rej(potentialError)
  return super.then(onfulfilled, onrejected);  // ...handler attached only here
}

finish()'s loop_.entered() guard drops at depth 0, so the scan ran in the gap and reported a rejection that was about to be handled three statements later. The enter/exit counter can't distinguish "macrotask callback finished" from "native re-entered JS mid-turn", so exit() is off the table. I moved the hook to the two batch-drain phases (run_immediate_task and fire), where nothing sits above us on the JS stack. Shell suite is green again, and the four ordering cases still pass.

Review on the microtask re-drain (event_loop.rs:277) — confirmed and fixed. Worth noting it's worse than the reviewer states: on main the handler's nextTick doesn't just run late, it never runs at all.

node:        unhandledRejection | nextTick from handler | second immediate
bun (main):  second immediate   | unhandledRejection                        <- nextTick dropped
bun (now):   unhandledRejection | nextTick from handler | second immediate

handleRejectedPromises() now returns whether a handler ran, so the new hook can loop report -> drain until quiescent, which is the shape of processTicksAndRejections. That's covered by a new test.

Review on allow_drain_microtask (event_loop.rs:304) — confirmed, deferring. Reproduced exactly as described (uncaughtException, later timer, unhandledRejection; node emits the rejection second), and it is unchanged by this PR. Both suggested fixes mean draining microtasks and scanning on a path that deliberately skips the drain because an exception is live, so it belongs with the uncaught-exception path rather than here. Noted in the PR description along with the poll-phase gap, which has the same root cause as the exit() problem above.

[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@test/js/node/process/process.test.js`:
- Around line 964-1043: The subprocess cases in the current process tests are
independent and should run concurrently instead of sequentially. Update the
`it.each` block and the trailing `it("drains process.nextTick...")` test to use
the concurrent test style recommended by the suite conventions (for example,
`test.concurrent`/`describe.concurrent` or the equivalent in this file), since
each case uses its own `Bun.spawn` and has no shared state.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

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• src/jsc/JSGlobalObject.rs
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• src/jsc/bindings/ZigGlobalObject.h
• src/jsc/bindings/bindings.cpp
• src/jsc/bindings/headers.h
• src/jsc/event_loop.rs
• src/runtime/timer/timer_object_internals.rs
• test/js/node/process/process.test.js

[robobun]

The 🔴 on the throw path was right, and the bug was real. But the suggested fix (drain before the first scan) turns out to break a different node behaviour, so the resolution went a different way. Pushed as 404b0f4.

Confirmed

setImmediate(() => {
  const p = Promise.reject(new Error("x"));
  queueMicrotask(() => p.catch(() => {}));
  throw new Error("boom");
});

node      : uncaught
bun main  : uncaught
bun (old) : uncaught, unhandledRejection, rejectionHandled   <- the regression

Why draining there is wrong

test/js/node/timers/timers-immediate-exception-fixture.js pins the opposite: when an immediate throws, node defers its nextTick queue past the next immediate. Draining at the throw site makes those ticks observe a stale counter and the fixture fails. That's what exit_maybe_drain_microtasks(false) exists to preserve.

So node isn't draining-then-scanning at the throw site. It marks the failed callback's scope and skips the checkpoint entirely, then runs it once the immediate batch drains. Four probes against node v26 pin the contract:

	node
throwing immediate queues a nextTick, sibling immediate behind it	uncaught, second immediate, tick
throwing immediate rejects + schedules a setTimeout(…, 0)	uncaught, unhandledRejection, later timer
throwing immediate rejects, .catch() in a queued microtask	uncaught only
throwing immediate rejects, sibling immediate behind it	uncaught, second immediate, unhandledRejection

Deferred to the end of the batch, not to the throw site (rows 1 and 4) and not to the next loop turn (row 2, where it still beats the timer phase).

What landed

No scan at the throw site; one scan after tick_immediate_tasks' existing post-batch maybe_drain_microtasks(). All four rows above now match node, the fixture passes, and row 2 closes the allow_drain_microtask gap from the earlier review on this PR, which I'd previously deferred. Rows 1, 3 and 4 already matched on main and are now pinned as tests so the placement can't drift.

On the other two comments

Stale PR description (socket row / "five"): already fixed before that review rendered. The description now has six fail-on-main rows plus two regression guards, and calls out the poll-phase gap explicitly.

it.concurrent.each: no change needed. These tests sit inside the describe.concurrent(() => { … }) block at process.test.js:476, and bun's runner resolves ConcurrentMode::Inherit to the parent's setting (bun_test.rs:1694), so plain it() already runs concurrently there. Every sibling test in that block relies on the same inheritance.

[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

src/jsc/event_loop.rs (1)

860-880: 🎯 Functional Correctness | 🔵 Trivial | ⚡ Quick win

Verify batch ordering when a non-last immediate throws.

exception_thrown in the to_run_now loop (line 865) is overwritten on every iteration, so it reflects only the last task's outcome. This PR relies on that value to decide whether handle_rejected_promises_after_tick fires per-task (inside run_immediate_task, when the task didn't throw) or once at the end of the batch (here, when the last task threw).

I traced the "throw-first, succeed-second" case (covered by the new "defers a rejection..." test) and it resolves correctly, because the succeeding second task's own per-task hook ends up scanning the still-global rejection table left by the first (throwing) task. However, the symmetric case — an earlier immediate in the same batch succeeds while a later one throws — isn't exercised by any test here, and depends on this same last-task-wins nuance to route through the post-loop branch correctly.

Since this mechanism is pre-existing (only the new handle_rejected_promises_after_tick call was added to the exception_thrown branch), it's likely fine, but a Node-parity test with the throw at the end of a multi-immediate batch would pin down this coupling for future refactors.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/jsc/event_loop.rs` around lines 860 - 880, Add a regression test around
`run_immediate_task`/`handle_rejected_promises_after_tick` for a multi-immediate
batch where an earlier immediate succeeds and a later one throws, so
`exception_thrown` being overwritten by the last task is covered explicitly.
Verify the batch still routes the rejection handling through the post-loop
`maybe_drain_microtasks` + `handle_rejected_promises_after_tick` path in
`src/jsc/event_loop.rs`, matching Node’s ordering for a throw-at-end case. Keep
the new test symmetric with the existing throw-first/succeed-second coverage to
pin down the last-task-wins behavior.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Outside diff comments:
In `@src/jsc/event_loop.rs`:
- Around line 860-880: Add a regression test around
`run_immediate_task`/`handle_rejected_promises_after_tick` for a multi-immediate
batch where an earlier immediate succeeds and a later one throws, so
`exception_thrown` being overwritten by the last task is covered explicitly.
Verify the batch still routes the rejection handling through the post-loop
`maybe_drain_microtasks` + `handle_rejected_promises_after_tick` path in
`src/jsc/event_loop.rs`, matching Node’s ordering for a throw-at-end case. Keep
the new test symmetric with the existing throw-first/succeed-second coverage to
pin down the last-task-wins behavior.

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[robobun]

Good catch on the exception_thrown coupling, and worth pinning. Added in 1f6e5e3.

You're right that tick_immediate_tasks overwrites exception_thrown each iteration, so the routing between the per-task hook and the post-batch one turns on the last task's outcome. I probed all three combinations against node v26:

batch shape	node	main	this PR
succeed, then throw (ends the batch)	first immediate, uncaughtException, unhandledRejection, later timer	… later timer, unhandledRejection	matches node
throw in the middle, succeed last	first immediate, uncaughtException, third immediate, unhandledRejection	matches	matches
succeed, then throw with a queued .catch()	first immediate, uncaughtException	matches	matches

All three route correctly, for the reason you traced: when the last task didn't throw, its own per-task hook picks up whatever an earlier throwing task left in the rejection table; when it did throw, the post-batch branch does. The first row is the interesting one and it's a fix (main is wrong there).

So rather than add a fourth near-identical block, I folded the throwing-immediate cases into one it.each and added that row. Seven of the nine ordering cases now fail on main; the other two are regression guards for behaviour that already matched.

[robobun]

The exception_thrown finding is real and is fixed in ea7c6e6. Good catch — it defeats the exact ordering one of this PR's own tests asserts.

Confirmed

setImmediate(() => {
  Promise.reject(new Error("x"));
  setTimeout(() => console.log("later timer"), 0);
  clearImmediate(im2);
  throw new Error("boom");
});
var im2 = setImmediate(() => console.log("never"));

node     : uncaughtException, unhandledRejection, later timer
bun main : uncaughtException, later timer, unhandledRejection
bun (before ea7c6e6) : uncaughtException, later timer, unhandledRejection

The cleared sibling takes run_immediate_task's early return, which reports false, and the plain assignment erased the fact that the first task threw. The batch tail then skipped both the drain and the scan.

exception_thrown |= … as suggested, plus a test for the case. It also widens the pre-existing maybe_drain_microtasks() gate from last-task-threw to any-task-threw, which I checked doesn't shift anything: the timers-immediate-exception fixture still passes, and all four throwing-immediate probes plus the 16-case ordering matrix still match node.

On the stale-description comment

Already fixed. Known gap has been singular (poll-phase only) since 404b0f4; the reject-and-throw bullet went away in the same commit that closed it. The description now lists eight fail-on-main rows and two regression guards.

[robobun]

Status: diff is green, one lane is blocked on CI infra

The only red lane on https://buildkite.com/bun/bun/builds/68519 is :darwin: 26 aarch64 - test-bun, and it never ran a test:

Error: buildkite-agent artifact download timed out after 120s for step
'darwin-aarch64-build-bun'. Refusing to continue with a partial download

That is the artifact transfer, not this diff. It is failing identically on every concurrent build in the pipeline, on unrelated branches:

build	branch	failed job
68508	farm/4fa6f111/unhandled-rejection-tick-ordering	:darwin: 26 aarch64 - test-bun
68512	farm/49b9d0bd/memory-usage-heap-invariant	:darwin: 26 aarch64 - test-bun
68513	farm/b70d7484/tls-cipher-config	:darwin: 26 aarch64 - test-bun (×2)
68514	farm/f28e8720/ffi-bytelength-range-error	:darwin: 26 aarch64 - test-bun
68519	this PR	:darwin: 26 aarch64 - test-bun

Everything else that has reported is green, including every shard of the lanes that would catch this change:

• :debian: 13 x64-asan - test-bun — all shards pass
• :windows: 2019 x64 - test-bun, :windows: 2019 x64-baseline - test-bun, :windows: 11 aarch64 - test-bun — all shards pass
• cargo clippy, Format, Lint JavaScript, and every build lane — pass

Those are the same lanes that caught the two real regressions earlier in this PR (the Bun Shell exit-code suite, and the spurious unhandledRejection on a throwing immediate), so they are load-bearing here, not incidental.

Final: I re-rolled once (4c2a3b6f, an empty commit) and build 68550 hit the identical failure on the same shard:

Error: buildkite-agent artifact download timed out after 120s for step
'darwin-aarch64-build-bun'. Refusing to continue with a partial download

That is the only re-roll I'll spend, so no further retriggers from me.

Build 68550 also lost a second darwin shard, to a different network failure, which makes the diagnosis clearer: it isn't one flaky artifact step, the darwin agents can't reach the network.

shard	failure	got as far as
:darwin: 26 aarch64	buildkite-agent artifact download timed out after 120s	no tests
:darwin: 14 x64	fatal: unable to access 'https://github.com/oven-sh/bun.git/': Could not resolve host: github.com	no checkout

282 of 284 lanes passed, and there are no error-level annotations on the build, i.e. not a single test failed anywhere.

The diff itself is green and that is not an inference: the retrigger commit is empty, so build 68519 above ran the byte-identical tree and finished 283 passed / 1 failed, the one failure being this same artifact download, with no error-level annotations anywhere. Every x64-asan and Windows test-bun shard passed, which are the lanes that caught the two real regressions earlier in this PR.

Ready for a maintainer as-is. A re-run once the darwin artifact fetch is healthy should go green without any change to the code.

[robobun]

Cross-reference: #33356 lands in the same area and will conflict with this on merge. Details and a suggested reconciliation are in #33356 (comment).

Short version: that PR fixes a different bug (work a handler defers behind a microtask hop is dropped, rather than emitted in the wrong order), and I verified this branch does not fix its repro, so both are needed. But both add the same report-then-drain loop, plumbed through handleRejectedPromises() -> bool here versus a new has_pending_rejected_promises() extern there, across the same five files. Whichever merges first, the other should rebase and reuse the loop that's already present.