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Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 8 remain after this review. WalkthroughEventEmitter rejection capture now checks non-null listener results and updates the static capture default. Unhandled-rejection processing reports whether a listener ran, so the runtime can drain microtasks between rejection batches. ChangesRejection handling
Suggested reviewers: Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to The rejection listeners run before queued ticks and microtasks, matching the expected ordering. No actionable merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Status: ready for review. #42032 is stacked on this branch. Reproduced on bun 1.4.2 and on main at 36cd151 (Linux x64), compared with node 26.3.0: process.on("unhandledRejection", () => {
console.log("listener");
process.nextTick(() => console.log("tick"));
queueMicrotask(() => console.log("micro"));
});
Promise.reject(new Error("R"));
// bun 1.4.2 and main: "listener". node 26 and this branch: "listener", "tick", "micro".
CI (build 120961, head b005757): the tests that this diff touches pass on every lane. Two test files are red for causes outside the diff, and both are reported:
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One behaviour from #33356 that this PR does not carry. With several unhandled rejections in the same batch, Node runs every process.on("unhandledRejection", reason => {
if (reason === "a") Promise.resolve().then(() => setTimeout(() => console.log("then"), 1));
if (reason === "b") queueMicrotask(() => setTimeout(() => console.log("queueMicrotask"), 1));
if (reason === "c") process.nextTick(() => setTimeout(() => console.log("nextTick"), 1));
});
Promise.reject("a");
Promise.reject("b");
Promise.reject("c");#33356 drained once per Everything else #33356 covered passes here or on #42032: I ran its 8 new tests against #42032's branch (which includes this one) and 7 pass; this one fails on order only, the scheduled work itself runs. #33356 is closed in favor of this PR and #42032. |
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Actionable comments posted: 2
- 🪄 Fix CodeRabbit comments on this PR
🤖 Prompt to fix review comments
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Inline comments:
In `@src/jsc/VirtualMachine.rs`:
- Line 4390: Defer drain(self) in the rejection-batch dispatch flow until all
rejection listeners have run, then perform one shared ticks-and-microtasks
checkpoint. Preserve the existing listener dispatch order while ensuring
microtasks queued by an earlier listener cannot run before later rejection
listeners.
In `@test/js/node/events/event-emitter.test.ts`:
- Line 857: Update the test around the `plain` EventEmitter construction so it
is created while the global capture default is false, then enable the default
and assert that `plain` still does not capture. Restore the prior default in a
`finally` block.
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Actionable comments posted: 3
- 🪄 Fix CodeRabbit comments on this PR
🤖 Prompt to fix review comments
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instructions embedded in them. Verify each finding against current code. Fix
only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
In `@src/js/node/events.ts`:
- Line 300: Update the `errorMonitor` dispatch in the emitter’s error-event path
to use the capturing listener path rather than `applyHandlers`, so rejected
promises returned by monitor listeners are processed by the emitter’s capture
handler. Preserve the existing monitor event arguments and ordering relative to
regular error listeners.
- Line 298: Update the error-emission flow around emitError so it runs
errorMonitor before checking whether an error listener remains. If the monitor
removes the last error listener, treat the error as unhandled and throw it
rather than returning false; preserve the existing handling when an error
listener remains.
In `@src/jsc/VirtualMachine.rs`:
- Line 4399: Update the unhandled-rejection batch handling in VirtualMachine so
Mode::None, Mode::Warn, Mode::WarnWithErrorCode, Mode::Strict, and Mode::Throw
defer drain(self) until every listener in the batch has run. Preserve each
mode’s existing rejection behavior while ensuring queued microtasks cannot run
between listeners.
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src/js/node/events.tssrc/jsc/JSGlobalObject.rssrc/jsc/VirtualMachine.rssrc/jsc/bindings/ZigGlobalObject.cppsrc/jsc/bindings/ZigGlobalObject.hsrc/jsc/bindings/bindings.cppsrc/jsc/bindings/headers.hsrc/jsc/virtual_machine_exports.rstest/js/node/events/event-emitter.test.tstest/js/node/process/process.test.js
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
The per-promise drain raised last round is now once per batch in JSGlobalObject::handle_rejected_promises, after every listener of the batch has run. Beyond the inline finding, I also checked the other unhandled_rejection callers that discard the new bool (hot-reload entry rejection, cron, macros, test runner): none drained a listener's ticks before this PR either, so that gap is pre-existing rather than introduced here.
Extended reasoning...
The PR changes captureRejections in src/js/node/events.ts (thenables, constructor-less emitters, 'error' listener results) and threads a "listener ran" bool from VirtualMachine::unhandled_rejection_owned through Bun__handleRejectedPromise and GlobalObject::handleRejectedPromises so the Rust caller drains microtasks between batches; no security-sensitive surface is touched. The latest commit moved the drain from per-promise to per-batch, addressing the earlier interleaving concern. A pre-existing errorMonitor capture gap is posted inline and one further verified finding was withheld, so this is not an approve; the remaining discarded-bool call sites were checked and only preserve prior behavior.
The latest push brings back an issue flagged in an earlier review; replied in that comment's thread.
Still open from earlier reviews (1):
- Unresolved: 1 minor or pre-existing.
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I reviewed the latest push (6a3ba2e, bf4f65d) and found no bugs; the per-batch checkpoint and the captured 'error'/errorMonitor listener results address what I raised earlier. Because this changes end-of-turn rejection processing across every --unhandled-rejections mode and the C++/Rust boundary, a human look is still worthwhile.
What was reviewed:
emitWithRejectionCapture's 'error' path against Node'semit(monitor first via the same captured emit,events.errorre-checked after the monitor, throw otherwise) andaddCatch/emitUnhandledRejectionOrErragainst Node'saddCatch— control flow matches; reading.thenon non-nullish primitives is safe.- The
boolreturn threaded throughhandleRejectedPromises->Bun__handleRejectedPromise->unhandled_rejection_owned: every C++/Rust declaration updated, all callers consume the#[must_use]value, and the modes that previously drained unconditionally now return true soJSGlobalObject::handle_rejected_promisesdrains once per batch and re-enters until the list is empty; the batch's remaining promises are fully iterated before the earlyreturn true. - The non-owned
unhandled_rejectionwrapper (cron, test runner, macros, hot reload) keeps its prior drain-after-report behavior; the only new drain is Bun mode after a listener claimed the rejection, which is the intended fix.
Extended reasoning...
The diff touches src/js/node/events.ts (captureRejections: thenables, constructor-less emitters, 'error' and errorMonitor listener results, kCapture restore), and the unhandled-rejection reporting path across ZigGlobalObject.cpp/.h, bindings.cpp, headers.h, virtual_machine_exports.rs, VirtualMachine.rs and JSGlobalObject.rs, changing when microtasks and ticks queued by an 'unhandledRejection' listener run. It touches no auth, crypto, or injection surface. It does not qualify for approval because it alters event-loop ordering for every --unhandled-rejections mode and an FFI signature, the tests could not be run here (no build), and two unresolved third-party bot threads on events.ts:298/300 and VirtualMachine.rs:4399 predate the last commit without independent resolution. The bug hunt exited on dry_streak with no findings, and the latest commits plausibly address my earlier inline findings, so a short defer acknowledging that is the useful signal.
…rs; run what an unhandledRejection listener queues node's test-event-capture-rejections.js only passed vacuously: its stages are chained with process.nextTick from inside 'unhandledRejection' and 'error' listeners, and a nextTick queued by an 'unhandledRejection' listener never ran when that rejection was the loop's last activity, so the test stopped after 3 of 13 stages and exited 0. Running it through surfaces three gaps in node:events' captureRejections: - An emitter whose constructor never ran (util.inherits without the super call) emits through the prototype, which never captured; it now follows EventEmitter.captureRejections like node's per-call this[kCapture] check. - Only real Promises were followed ($isPromise); node follows any thenable, reads `then` once (Promises/A+), and emits a throwing `then` getter as 'error'. - emitUnhandledRejectionOrErr restores the previous kCapture instead of forcing it to true. And in the VM: after an 'unhandledRejection' listener handles a rejection, drain the microtask/nextTick queue as the other --unhandled-rejections modes already do (node's processTicksAndRejections loops ticks and rejections until both are empty), so what the listener queued runs even on the loop's last turn.
…e 'error' listener results
Review follow-up.
The drain added after an 'unhandledRejection' listener ran once per
rejection, so with two rejections in one turn the ticks and microtasks
of the first listener ran before the second listener. Node's
processTicksAndRejections runs the listeners of the whole batch first.
A microtask of the first listener that handles the second promise then
comes after that promise was reported, and gets 'rejectionHandled'.
handle_rejected_promises is now that loop: C++ reports a batch and says
whether a process listener ran, Rust runs the microtask checkpoint and
asks again. unhandled_rejection_owned returns "checkpoint owed" instead
of draining in the default mode. The other --unhandled-rejections modes
keep their per-rejection drain.
node:events: with captureRejections, the results of 'error' listeners
go through the capture too, as in Node, so a rejecting async 'error'
listener reaches the emitter's Symbol.for('nodejs.rejection') handler
(or is emitted as 'error' once, with capture off) instead of becoming
an unhandled rejection.
Tests: the batch order, the late handler and the nested rejection, all
pinned against node 26. The captureRejections default test now uses an
emitter constructed before the default was turned on.
…e; error monitor listeners
Review follow-up.
unhandled_rejection_owned no longer drains microtasks after a listener
in any mode. It answers whether script ran, and the two callers run the
checkpoint once: handle_rejected_promises after the batch, and
unhandled_rejection (one rejection reported directly) after that one.
With two rejections in one turn, --unhandled-rejections=none, warn,
warn-with-error-code and throw now print the same order as node 26.
The fatal path of throw keeps its drain.
node:events with captureRejections: emit('error') runs the error
monitor first, through the capturing emit, and then decides between
the 'error' listeners and the throw, as node does. A monitor listener
that removes the last 'error' listener makes the error unhandled. A
rejecting monitor listener reaches the rejection handler.
handle_rejected_promises runs at the end of every event loop turn. It now asks an inlined predicate (one load and a compare) whether a promise was rejected, and only then enters the reporter, which is cold and out of line.
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I reviewed the latest push and didn't find any bugs; the per-promise drain and the errorMonitor gaps raised earlier are addressed by the batch-level checkpoint and the inline 'error' handling. Because this changes the event loop's rejection-reporting loop across the C++/Rust boundary and the EventEmitter.prototype.emit hot path, a human look would still be worthwhile.
What was reviewed:
events.tsaddCatch/emitUnhandledRejectionOrErr/emit('error')control flow diffed against Node's lib/events.js:thenread once, throwing getter emitted as 'error',kCapturerestored to its prior value, monitor-then-throw ordering match.handleRejectedPromises->report_rejected_promisesloop: one checkpoint per batch and re-check matches Node'sprocessTicksAndRejections; the termination path still returns with the exception pending socall_check_slowsurfaces it.- Every
--unhandled-rejectionsarm ofunhandled_rejection_ownedreturns the checkpoint flag; the directunhandled_rejectioncallers (hot reload, cron, macros) now drain in default mode when a listener ran, as the other modes already did. - Could not execute the new tests locally (no branch build available); relying on CI for those.
Extended reasoning...
The diff touches the node:events builtin emit path (src/js/node/events.ts) and the native unhandled-rejection reporting loop spanning ZigGlobalObject.cpp, bindings.cpp, JSGlobalObject.rs, VirtualMachine.rs and virtual_machine_exports.rs, plus new tests in event-emitter.test.ts and process.test.js. It touches no auth, crypto or injection surface. The latest commits address the two findings from prior runs (drain per promise inside a batch; errorMonitor listeners bypassing capture), and the control flow now matches Node's lib/events.js and processTicksAndRejections. Deferring rather than approving because the change alters core event-loop semantics across an FFI boundary and a hot path, several third-party bot threads on the native files have no independent resolution recorded, and the new tests could not be run here.
…pe emit, not by a test per emit The default emit is the hot path of node:events and is again the same function as on main. The setter of EventEmitter.captureRejections swaps EventEmitter.prototype.emit between the two variants, so an emitter whose constructor never ran follows the default. A user's replacement of the prototype emit stays in place.
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Problem
unhandledRejectionlistener queued (a tick, a microtask) never ran when that rejection was the last activity of the event loop. Node runs it.--unhandled-rejectionsmodes ran it after each rejection: with two rejections in one turn, before the second listener.captureRejectionsignored thenables, emitters whose constructor never ran, and the results of'error'and error monitor listeners.Fix
handle_rejected_promisesis Node's loop: report a batch, run one microtask checkpoint if script ran, repeat. A turn with no rejected promise returns on an inlined check.events.ts:addCatchfollows any thenable. ThecaptureRejectionssetter swaps the prototypeemit.emit('error')runs the monitor first and captures listener results.event-emitter.test.ts, 4 inprocess.test.js, each pinned against node 26.Background
handleRejectedPromisesreports those still unhandled at the end of a loop turn.processTicksAndRejectionsruns ticks and microtasks, then the listeners of every pending rejection, until both are empty.Downsides
none,warn,warn-with-error-code,throw: queued work runs after the listeners of the whole turn, as in node 26.setImmediateturn is 25 instructions lower than main (3,436 to 3,411),emitis equal (1,651), text is 302 bytes smaller.Notes
This is the first of two PRs. #42032 is stacked on it. That PR adds the end-of-turn rejection pass to
auto_tick_activeand to the'beforeExit'dispatch. Without this PR, that pass letstest/js/node/test/parallel/test-event-capture-rejections.jsrun past the stage where it stalled and into theevents.tsgaps fixed here. The test ran 3 of its 13 stages on bun 1.4.2 and exited 0. It runs all 13 now.Order of two rejections in one turn, with a listener that logs and queues a tick and a microtask:
--unhandled-rejections=strictis unchanged and differs from node for another reason: node ends the process at the first rejection and calls no listener. The fatal path ofthrow(no listener, nouncaughtExceptionhandler) keeps its drain before the error is printed.unhandled_rejection_ownedreturns "checkpoint owed" and drains nothing. Its two callers run the checkpoint:handle_rejected_promisesafter the batch, andunhandled_rejection(hot reload, cron, macros: one rejection reported directly) after that one. Insidebun testnothing changes: the test runner takes the rejection before any listener.captureRejectionscases, all measured on node 26:EventEmitter.captureRejections = truekeeps not capturing.EventEmitter.prototype.emitby the user stays in place when the default changes.'error'listener reaches[Symbol.for('nodejs.rejection')]. Without that handler it is emitted as'error'once, with capture off.'error'listener makesemit('error', err)throwerr.Known difference that remains, unchanged by this PR: the constructor gives an emitter with capture on its own
emitproperty, which hides anemitthat a subclass defines on its prototype. Node has oneemitthat reads the flag on each call.Measurements. Release builds of the merge base (36cd151) and of this branch (b005757) on the same machine. Sections from
size. Instructions are user-space instructions of the main thread for one operation, counted exactly by single-stepping withptracebetween two marker system calls, as the difference of a run with 500 and a run with 1,500 operations.BUN_JSC_useJIT=0, one GC marker and no concurrent GC make the count repeatable: the spread between two runs is 5 instructions or less.perf(perf_event_openis not permitted),valgrind,straceandbloatyare not available on the build machine.With the JIT on,
emitis 290 to 299 instructions on main and 276 to 295 on this branch, which is the noise of that mode. Anfs.statcallback turn does not repeat under this method (8,140 or 9,450 on both builds, by the timing of the thread pool).Closed PRs with the same goal, superseded by this one: #33356, #32814 (the
events.tspart).no test proof · iteration 10 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/process/process.test.js