usockets(epoll/kqueue): nested event-loop tick delivers events the outer tick collected - #40030
usockets(epoll/kqueue): nested event-loop tick delivers events the outer tick collected#40030dylan-conway wants to merge 4 commits into
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…outer tick already collected A handler dispatched from the loop's ready-poll batch can drive the loop again before it returns (bun:test's expect(promise).resolves, Bun.build() waiting for an async plugin setup(), a debugger pause). The nested tick waited into the same loop-global ready_polls array and reset the shared cursor, so when it returned the outer dispatch loop saw the nested batch's count and never dispatched its own remaining entries. Sockets are polled level-triggered and were simply reported again, but a one-shot poll (a child process's stdout pipe) had already been disarmed by the outer wait: its event was lost and the reader hung. - The dispatch cursor is advanced before each entry is dispatched, so [current_ready_poll, num_ready_polls) is always exactly what is left, at any nesting depth. - A tick first dispatches whatever an enclosing tick has left before it waits and reuses the array, and does not park in that case. - kqueue: READ/WRITE kevents for the same poll are folded per entry at dispatch time instead of in a stack array indexed in parallel with ready_polls, which a nested tick would rewrite underneath it. - Bun's FilePoll dispatch receives its event from the loop instead of reading ready_polls[current_ready_poll] itself.
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Updated 8:47 PM PT - Aug 21st, 2026
❌ @dylan-conway, your commit c618111 has 1 failures in
🧪 To try this PR locally: bunx bun-pr 40030That installs a local version of the PR into your bun-40030 --bun |
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Included review availability: 0 reviews are currently available. Your included PR review attempts over the past 7 days set your current allowance at 2 reviews per hour. WalkthroughThe event loop now passes originating ready events directly to callbacks, decodes epoll and kqueue events through shared dispatch logic, preserves unprocessed batches across nested ticks, and adds regression tests for sockets, pipes, pipe servers, and process output. ChangesNested ready-poll dispatch
Suggested reviewers: Merge Risk: 🔵 Low · up to The PR fixes event delivery when handlers re-enter the event loop and includes regression coverage; it is mergeable with owner awareness that the new spawn test may keep a subprocess alive for up to 3000 ms after its assertion completes, adding a bounded test delay. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@packages/bun-usockets/src/eventing/epoll_kqueue.c`:
- Around line 444-448: Update us_loop_run’s us_internal_collect_ready_polls call
to pass KEVENT_FLAG_IMMEDIATE when
us_internal_dispatch_enclosing_ready_polls(loop) forces zero_timeout; preserve
the existing flags for normal timeouts and ensure the epoll build remains
unaffected.
In `@test/js/bun/net/nested-event-loop-fixture.ts`:
- Around line 175-179: Replace the dynamic require calls for path, os, and
node:net in the nested event loop fixture with module-scope imports, then update
their usages to reference those imports while preserving the existing
platform-specific socket name behavior.
In `@test/js/bun/spawn/spawn.test.ts`:
- Around line 836-838: Update the timeout used in the Promise.race around
got[other].promise so it cannot keep the child process alive after success,
either by unrefing the timer or clearing it once the race settles; preserve the
existing 3000 ms timeout and failure handling.
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packages/bun-usockets/src/eventing/epoll_kqueue.cpackages/bun-usockets/src/internal/eventing/epoll_kqueue.hsrc/io/posix_event_loop.rssrc/spawn/process.rssrc/uws_sys/Loop.rstest/js/bun/net/nested-event-loop-fixture.tstest/js/bun/net/socket.test.tstest/js/bun/spawn/spawn.test.ts
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…ef the spawn test's deadline timer, module-scope imports in the fixture
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…xture's module-scope imports, unlink its unix socket path No-Verification-Needed: test-only change
No-Verification-Needed: test-only change
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I reviewed c618111 and didn't find any further issues — the earlier test nits (module-scope imports, Windows skips, unix-socket cleanup) are all addressed. Because this reworks the core epoll/kqueue dispatch loop (cursor pre-increment, kqueue READ/WRITE coalescing moved from a frame-local two-pass to per-entry forward fold, the Bun__internal_dispatch_ready_poll FFI signature) and macOS was compile-checked only, a human look at the C changes is still worthwhile.
What was reviewed:
us_internal_dispatch_ready_polls: cursor is advanced before each dispatch, sous_internal_loop_update_pending_ready_polls(still scanning[current_ready_poll, num_ready_polls)) now correctly covers only not-yet-dispatched entries; the currently-dispatching entry is already read into a local before the increment.- kqueue forward-fold: happens before the handler runs, so a nested tick can't have changed
num_ready_pollsyet; the blanked second entry is skipped by theif (!poll) continueon the next iteration. us_internal_dispatch_enclosing_ready_pollsinus_loop_run_bun_tickruns afterloop_preand beforemi_on_thread_idle_start, so handler allocations don't race the scavenger handoff.- Rust side: the removed
current_ready_event()accessor has no remaining callers; the neweventpointer is copied to the stack before the handler can re-enter and reuseready_polls.
Extended reasoning...
Overview
This PR fixes a nested-event-loop bug in the POSIX usockets dispatch loop: when a handler re-enters us_loop_run_bun_tick (via expect(promise).resolves, Bun.build() with an async plugin, a debugger pause), the inner tick overwrites the shared ready_polls/num_ready_polls/current_ready_poll and the outer dispatch silently drops its remaining entries. For level-triggered sockets the kernel re-reports them; for one-shot polls (child-process pipes) the event is lost forever. The fix: (1) advance current_ready_poll before dispatching each entry so [current_ready_poll, num_ready_polls) is always exactly the undispatched remainder; (2) a nested tick first drains that remainder before collecting its own batch, and forces a zero timeout so it doesn't park; (3) kqueue's READ/WRITE coalescing is moved from a two-pass frame-local coalesced[] array (which the nested-tick fix would have desynchronised) to per-entry forward folding; (4) Bun__internal_dispatch_ready_poll receives the event by pointer instead of reading ready_polls[current_ready_poll] (which is now one past the entry being dispatched). Files: epoll_kqueue.c/.h, posix_event_loop.rs, Loop.rs (removes the now-dead current_ready_event() accessor), process.rs (deletes a comment describing the bug this PR fixes), plus a new fixture and two test wrappers.
Security risks
None identified. This is an internal event-loop scheduling change; no user input parsing, no auth/crypto, no new syscall surface.
Level of scrutiny
High. us_internal_dispatch_ready_polls is the innermost loop of Bun's I/O on Linux/macOS/FreeBSD — every socket, pipe, and process event flows through it. The change is subtle (cursor semantics, kqueue coalescing rework, FFI signature change across the C/Rust boundary) and platform-conditional (#ifdef LIBUS_USE_EPOLL vs kqueue). The PR description states macOS is compile-checked only. A regression here would affect essentially every I/O operation.
Other factors
- All prior review feedback (mine and CodeRabbit's) is addressed as of c618111: module-scope imports, Windows
skipIfgates on both new tests, deadline timer.unref(), and both unix-socketunlink()calls in the fixture. - I traced the interaction between the new pre-increment cursor and
us_internal_loop_update_pending_ready_polls(called fromus_poll_stop/us_poll_change/us_poll_resizeinside handlers): its scan range[current_ready_poll, num_ready_polls)now excludes the entry currently being dispatched, which is correct — that entry was already read into a localpollbefore the increment, and on kqueue its sibling entry was already folded and blanked before the handler runs. - The kqueue forward-fold reads
loop->num_ready_pollsbefore any handler runs, so a nested tick cannot have changed it mid-scan; thebreakafter the first match is justified by the existing "at most 2 kevents per fd" comment. - The
us_internal_dispatch_enclosing_ready_pollscall inus_loop_run_bun_ticksits afterus_internal_loop_preand beforemi_on_thread_idle_start/Bun__JSC_onBeforeWait, so handlers it runs may allocate freely and the subsequentwill_idle_inside_event_loopcomputation correctly sees the forced zero timeout. - Test coverage is good (four fixture cases plus a standalone
Bun.build()repro), but the tests rely onBun.sleepSync(100–200)to force both events into one poll batch — inherently timing-based, though the fixture's own comments justify why no observable signal exists.
Given the criticality of the code path and that macOS/FreeBSD were not exercised locally, this warrants a maintainer's eyes on the C changes rather than automated approval.
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Closing in favor of the other direction: rather than teaching the epoll/kqueue dispatcher to survive a nested tick, make the event loop non-reentrant so the state cannot occur (same conclusion as #32233 / #33261). The diagnosis, the gdb evidence and the two regression tests here stay valid and can be revived as-is if that direction does not pan out. |
2 participants
What does this PR do?
On Linux and macOS, an I/O handler that drives the event loop again before it returns (bun:test's
expect(promise).resolves,Bun.build()waiting for an async pluginsetup(), a debugger pause) could make the loop drop events it had already collected in the same poll batch. The nested tick waited into the same loop-globalready_pollsarray and reset the shared cursor, so when it returned the outer dispatch loop compared against the nested batch's count and never reached its own remaining entries. Sockets are level-triggered and got reported again, which hid it; a one-shot poll such as a child process's stdout pipe had already been disarmed by the outer wait, so its chunk was never delivered and the reader hung.Repro without the test runner: two children answer at once; the first stdout handler calls
Bun.build()with an async pluginsetup()that awaits the other child's output → hangs forever (now a test inspawn.test.ts).Changes:
[current_ready_poll, num_ready_polls)is always exactly what is left, at any nesting depth.ready_polls.Bun__internal_dispatch_ready_pollreceives its event instead of readingready_polls[current_ready_poll].Adds
nested-event-loop-fixture.ts(same file as #40023, pipe case un-gated); whichever lands second rebases trivially.How did you verify your code works?
Linux x64 debug build:
bun bd test test/js/bun/net/nested-event-loop-fixture.ts(4 pass; pipe case fails on 1.4.0),bun bd test test/js/bun/spawn/spawn.test.ts -t "already collected"(fails on 1.4.0), plus socket.test.ts / node-net-server / fetch.stream locally. Confirmed the clobbered cursor in gdb before the change. macOS is compile-checked only; relying on CI there.