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Updated 9:08 AM PT - Oct 21st, 2026
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WalkthroughThe change adds conditional promise waits and promise-aware event-loop ticks. Runtime and test-runner paths use the new wait behavior. Regression tests cover matcher waits involving immediate callbacks, timers, microtasks, and open server handles. ChangesPending Promise Waits
Suggested reviewers: Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly summarizes the main change: promise waits run independently and skip unnecessary polling after settlement. |
| Linked Issues check | |
The changes satisfy the coding requirements in [#45629] and [#45020]. wait_for_promise_while sets aside caller entries, dispatches callbacks as an independent loop, checks promise settlement after e… |
| Out of Scope Changes check | |
The changes remain within the linked promise-wait scope. The shared wait helper updates lint plugins, the REPL, preload and entry-point watcher waits, and related callers that use the same event-loop … |
| Description check | |
The description clearly explains the problem, implementation, scope, tradeoffs, and verification results. It does not use the template headings exactly, but it provides the required information in equ… |
Full details: Linked Issues check
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Explanation
The changes satisfy the coding requirements in [
#45629] and [#45020].wait_for_promise_whilesets aside caller entries, dispatches callbacks as an independent loop, checks promise settlement after each tick, and invokesauto_tick_while_pendingonly while the promise remains pending. The runtime hook reads the promise after immediate callbacks and uses a zero-timeout poll after settlement. The added matcher tests cover.resolves,.rejects, open handles, first-await waits, microtask ordering, and timer progress. The reportedbun_test.test.tstest covers the first-await case.
Full details: Out of Scope Changes check
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Explanation
The changes remain within the linked promise-wait scope. The shared wait helper updates lint plugins, the REPL, preload and entry-point watcher waits, and related callers that use the same event-loop behavior. These changes support the same settlement, interruption, and callback-ordering fix. The supplied whole-PR summary identifies no unrelated product or test changes.
Full details: Description check
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Explanation
The description clearly explains the problem, implementation, scope, tradeoffs, and verification results. It does not use the template headings exactly, but it provides the required information in equivalent sections.
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Beyond the inline findings, I also checked three things and ruled them out: the core::mem::replace of entered_event_loop_count in wait_for_promise_while is restored on both the Settled/GaveUp and Err(Stopped) returns, so a caller's entries are never lost; the --hot reload path in VirtualMachine.rs cannot leave auto_tick_while_pending reading a released promise cell, since the pending check and the tick run under the same Strong; and the split of auto_tick into run_immediates + poll_and_drain_timers preserves the base's ordering and the has_yielded_tasks handoff for the plain auto_tick path.
Extended reasoning...
The change reworks the synchronous promise wait in src/jsc/event_loop.rs, adds a RuntimeHooks slot and auto_tick_while_pending in src/jsc/VirtualMachine.rs and src/runtime/jsc_hooks.rs, and rewires the lint plugin driver's wait in src/runtime/cli/lint_js.rs; it touches no security-sensitive surface. Five confirmed findings are posted inline (the skipped turn tail after an immediate settles the promise, the untouched REPL sibling loop, missing lint coverage, and a wall-clock race in the new test), so a human should look; this note only records what else was examined.
Findings marked 🟡 are optional suggestions and need no follow-up push.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/runtime/cli/repl.rs— REPL users still get the stalled wait this change fixes elsewhere: an awaited expression that an immediate settles parks until unrelated I/O or a timer wakes the loop. The hand-rolled loop inwait_for_promise_or_sigint(src/runtime/cli/repl.rs:1110-1120) keeps callingauto_tick()aftertick(), so once the immediates settle the promise it still polls, and it never sets the caller's entries aside. Fix: route every pending-promise spin through the new turn, herewait_for_promise_while(promise, || !SIGINT_DURING_WAIT.swap(false, AcqRel))mappingGaveUpto the SIGINT return, so the REPL eval wait (repl.rs:1664, repl.rs:1961) gets the same checkpoint order and poll skip as matchers andbun lint. [also at: src/runtime/cli/lint_js.rs:285 - pre-existing: REPL users still get the parked poll this change removes everywhere else, because the REPL's own promise wait was not migrated.; src/jsc/event_loop.rs:1439 - pre-existing: REPL users still get the parked poll this PR removes elsewhere:awaitof a promise an immediate settles hangs in the REPL until unrelated I/O or a timer wakes the loop.]Why this was flagged
A user at the
bun replprompt with an open handle (sayBun.serve({port:0})) evaluatesawait new Promise(r => setImmediate(r)). The eval result is a promise, sowait_for_promise_or_sigintat src/runtime/cli/repl.rs:1107 spins:tick()at repl.rs:1114, thenauto_tick()at repl.rs:1118.auto_tick(src/runtime/jsc_hooks.rs:946) runs the immediates, whose checkpoint resolves the promise, then unconditionally callspoll_and_drain_timers, which parks in the uSockets poll with the open server keeping the loop active and no timer deadline. The prompt does not return until some unrelated I/O or timer wakes the loop. This PR addedauto_tick_while_pending(jsc_hooks.rs:962) andwait_for_promise_while(src/jsc/event_loop.rs:1179) to skip that poll, and converted the matcher waits and thebun lintdriver (src/runtime/cli/lint_js.rs:285), but this sibling loop in the REPL was left onauto_tick(). On the base branch the REPL behaves the same, so this is a gap in the variant matrix of the fix; the PR description names the REPL as a covered caller.Verification: pre-existing (the base already stalls here by the same route). The interactive REPL eval paths (repl.rs:1664, repl.rs:1961) call the hand-rolled
wait_for_promise_or_sigint(src/runtime/cli/repl.rs:1107-1122), which still doesvm.as_mut().event_loop_mut().auto_tick();(repl.rs:1118).auto_tickafter the PR (src/runtime/jsc_hooks.rs:946-951) isrun_immediatesfollowed by an unconditionalpoll_and_drain_timers.
A wait for a promise (expect().resolves, a bundler plugin hook, a macro) ticks the event loop under the caller's frame. It inherited the caller's enter count, so every callback it dispatched was nested and skipped its microtask checkpoint. The reactions ran only at the next tick. An immediate that resolved one hop left the next setImmediate unqueued when the poll deadline was picked, so the poll blocked until something else woke the loop, once per hop, whenever a handle was open. The wait now sets the caller's entries aside while it runs and restores them after. Callbacks it dispatches checkpoint as they do at the top of the loop. The wait also owns its turn. auto_tick runs the immediates, then polls. When an immediate's checkpoint settles the promise, the wait is over, and the poll would park on a deadline nothing in it waits for. The new auto_tick_while_pending hook reads the promise between the two phases and skips the poll once it has settled. auto_tick itself is the same two phases, inlined. The lint driver's hand-rolled wait and the watcher loops of the entry point loaders move onto the same turn.
…ng early The turn of a promise wait skipped the poll, the timers and the rejection report once the immediates had settled the promise. A caller that waits again before it returns to its loop then never ran a timer. The turn now runs in full with a poll that returns at once. The REPL's hand-rolled wait moves onto the shared one. The sentinel timers in the tests widen to 2 s. A test covers a loop of waits that immediates settle beside a pending timer.
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Actionable comments posted: 1
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Inline comments:
Review comments at @test/js/bun/test/expect.test.js:
- Around line 241-243: Remove the arbitrary 500-iteration limit from the loop
awaiting setImmediate in this test, and continue yielding until the done
condition becomes true. Preserve the existing matcher wait and final assertion
behavior.
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📒 Files selected for processing: 5
src/jsc/VirtualMachine.rssrc/runtime/cli/repl.rssrc/runtime/jsc_hooks.rstest/js/bun/test/bun_test.test.tstest/js/bun/test/expect.test.js
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Beyond the inline findings on the two new tests, I checked the reworked settled turn in src/runtime/jsc_hooks.rs: auto_tick_while_pending now routes a settled promise through poll_and_drain_timers(.., no_park = true), so due timers, the after-loop callback and handle_rejected_promises still run on that turn and only the park is skipped. I also checked that wait_for_promise_while restores entered_event_loop_count on the Stopped and GaveUp paths (the restore is unconditional), and that the REPL's SIGINT check moving into keep_waiting keeps the same give-up points as the old loop.
Extended reasoning...
The change reworks EventLoop::wait_for_promise in src/jsc/event_loop.rs into a loop that sets the caller's enter count aside, adds an auto_tick_while_pending hook in src/jsc/VirtualMachine.rs and src/runtime/jsc_hooks.rs that skips the poll park once the immediates settle the promise, and rewires the lint and REPL waits onto it; it touches no security-sensitive surface. The two commits since the prior review address the earlier concern about the settled turn skipping timers and rejection reporting. The remaining inline findings concern timing assumptions in the two new tests, and the core event-loop ordering change is non-trivial enough that a maintainer should still weigh it.
Findings marked 🟡 are optional suggestions and need no follow-up push.
Still open from earlier reviews (2):
- Unresolved: 2 minor or pre-existing.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I also checked that entered_event_loop_count is restored on every exit of wait_for_promise_while (Settled, GaveUp and Err(Stopped) all pass through the single restore after wait_for_pending_promise returns), and that the remaining auto_tick() callers (wait_for_tasks, the test-runner and parallel-runner loops, AnyEventLoop) are not promise waits, so no sibling wait site was missed. The REPL's SIGINT check now runs after the first tick() instead of before it; the result is the same either way since a settled promise ends the wait regardless.
Extended reasoning...
The latest push runs the full turn with a zero-timeout poll on the settled branch, spawns the first-await fixture with the GC timer disabled, and bounds the immediate-settled loop by the clock, which covers the points raised on the earlier versions. The one surviving finding is a pre-existing stall in the mock.module factory wait, not introduced here.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/jsc/VirtualMachine.rs— pre-existing: test files whose asyncmock.modulefactory takes two or moresetImmediatehops still stall about 1 s before tests start when a handle is open. The pre-armedwakeup()at src/jsc/VirtualMachine.rs:5845 only keeps the firstauto_tickfrom parking. The later turns of theJSMock__hasPendingModulePatchesloop in src/runtime/cli/test_command.rs:3001-3004 call plainauto_tick(), whose checkpoint clears the patch and then parks on the next-timer deadline. Fix: make every wait that an immediate's checkpoint can settle poll without parking once it has settled, for this loop too, e.g. a check between the immediates and the poll, orwakeup()before eachauto_tick()of that loop.Why this was flagged
A test file opens a handle at top level (Bun.serve or net.createServer().listen) and calls mock.module("x", async () => { await hop(); await hop(); return {...} }) where hop is a promise settled by setImmediate. load_entry_point_for_test_runner (src/jsc/VirtualMachine.rs:5811-5848) pre-arms the waker at :5845 and runs auto_tick at :5846: hop 1 runs, its checkpoint queues hop 2's immediate, and the poll returns at once. The loop at src/runtime/cli/test_command.rs:3001-3004 sees JSMock__hasPendingModulePatches true and calls auto_tick(): hop 2 runs, its checkpoint resolves the factory and clears hasPendingPatch in the same drain. poll_and_drain_timers (src/runtime/jsc_hooks.rs:1072-1123) then sees no immediates or tasks and parks on get_timeout's deadline with the loop active, until the GC repeating timer (about 1 s) or I/O wakes it. Only after that does the loop re-check the condition and start the tests. The base branch does the same, so this is the shape the PR fixes for wait_for_promise sites left as is at this sibling wait.
Verification: pre-existing: the base has the same stall; the PR fixes this class but leaves this sibling untouched. src/runtime/cli/test_command.rs:3002 calls plain auto_tick() while JSMock__hasPendingModulePatches is true; hop 2's checkpoint clears hasPendingPatch (BunPlugin.cpp:766-770), then poll_and_drain_timers (src/runtime/jsc_hooks.rs:1072-1123) parks on the GC timer deadline, about 1 s.
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Closing: #44833 (merged) settles .resolves and .rejects with promise reactions, so the matchers no longer run the event loop inside the call. The repro of #45629 takes 2 ms per 10 hops on main (3f3a9ff). That is the direction the maintainers chose (#33261). The remaining nested waits (Bun.build and Bun.serve plugin setup) have open PRs that remove the wait (#42680, #42862) instead of improving it. |
Fixes #45629, fixes #45020
Problem
await expect(p).resolvesor.rejects, eachsetImmediatehop waits for the poll when any handle is open: 10 hops take 6 to 7 s, under 1 ms with a plainawait. A wait that is the first await of a file returns about 1 s late.EventLoop::wait_for_promise(src/jsc/event_loop.rs) ticks under the caller's frame and inheritsentered_event_loop_count, so each callback it dispatches is nested andexit_maybe_drain_microtasksskips its checkpoint. The nextsetImmediateis queued only afterauto_tickpicked its poll deadline. When a checkpoint settles the promise,auto_tickstill parks.Fix
Bun.buildplugins, macros, bake, REPL, lint).auto_tick_while_pendingruns the immediates, reads the promise, and once it settled polls with a zero timeout instead of the next-timer deadline. The rest of the turn (timers due, rejections) runs as before.auto_tickis the same two phases, inlined.bun lint's plugin driver, the REPL and the entry point watcher loops use the same wait.test/js/bun/test/expect.test.js(three new tests),bun_test.test.ts(one), the first three fail on main. Also the timers,bun:test, bundler plugin, lint plugin, macro, bake, hot, watch and REPL suites.expect(promise).resolves/.rejectsstalls up to ~1 s persetImmediatehop: the nested event loop blocks in the poll after running immediates #45020 shape A) still parked. This shape reads the promise instead.Background
entered_event_loop_countcountsenter()/exit()pairs around a callback. The outermostexit()runs the microtask checkpoint. A nested pair skips it.auto_tick): one per turn for every top-level-await program. This reads the promise once per turn of a wait.Downsides
awaitincluded: 10000 turns withBun.serveopen, 748 to 816 ms vs 1143 to 1751 ms on main, inside the noise. 0 syscalls added.imm1, imm2, microinsideexpect().resolvessees the change.bloatyand no release build of the base here.Notes
Measurements, debug builds of main (
d0831bc9fe) and this branch, Linux x64,/proc/self/ioandvoluntary_ctxt_switchesfrom/proc/self/status:setImmediatehops insideexpect().resolveswith a listeningnetserver, nested: main 4429 to 5973 ms, 40 to 88 voluntary context switches. Branch: 1.8 to 2.4 ms, 0 context switches. No syscall count changed..rejects: main 5191 ms, 88 context switches. Branch: 6.6 ms, 0.await main()withBun.serve({port: 0})open, 10000 hops: 0write(2)on both, 5 to 8read(2)on both.The reporter's file (
repro.test.jsfrom #45629) on the branch:[child process] plain await 36.2ms | inside expect().resolves 9.6ms | inside expect().rejects 2.0ms,[tcp server] plain 90.8ms | resolves 1.2ms | rejects 1.2ms. The #45020 file: shape Asettle->expect returns 0 to 3ms(main 984 ms), shape Bslowest hop 0 to 4ms(main 896 ms), all 5 rounds of both.Why the first test body of a file is at count 0: the runner calls it through
run_callback_with_result_and_forcefully_drain_microtaskswithout an entry. Every later test body starts from atick()task, which holds count 1, and a body that has awaited anything continues inside a checkpoint, also at count 1.The first push ended the turn right after the immediates once the promise settled. That starved timers and the rejection report for a test body that loops on matcher waits, and moved a due timer behind the next immediate for an entry module whose last top-level
awaitan immediate settles. The turn now runs in full with a zero-timeout poll:timer, immediateas on main and in node, and a loop of 500 immediate-settled waits beside a 10 ms timer ends after about 20.wait_for_promise_whilegives the lint driver theexecution_forbiddenchecks it did not have. A stop there now returns an error instead of spinning.Siblings: #45029 (by the reporter of #45020) makes
auto_ticknot block after a batch of immediates ran. It fixes both shapes of #45020 but keeps the nested checkpoint order and adds a poll per batch to every program. #39268 passes the promise intoauto_tickand runs a checkpoint there. #43356 enters the loop around test callbacks and suspends that entry for matcher waits. ItsRunnerEntry::suspendis not needed with this change. #33261 and #33289 remove the nested wait from matchers. The entry point, macro, plugin and REPL waits stay, and this change is what makes those correct.Suites run with the debug build:
test/js/node/timers/node-timers.test.ts,test/js/bun/test/fake-timers,test-timers.test.ts,bun_test.test.ts,bun-test.test.ts,done-async.test.ts,expect-extend.test.js,test-test.test.ts,test/bundler/bundler_plugin.test.ts,bundler_plugin_chain.test.ts,plugin-error-nested-throw.test.ts,test/cli/lint/js-plugins.test.ts,js-parsers.test.ts,rule-throws.test.ts,robustness.test.ts,test/bundler/transpiler/macro-test.test.ts,test/bake/dev/esm.test.ts,test/cli/hot/hot.test.ts,test/cli/hot/watch.test.ts.[human-review] gate passed · iteration 1 · 7 files touched
fails on main (without fix)
passes on PR (with fix)
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gate history · 1 passed · 0 rejected · iteration 1
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