child_process: emit 'disconnect' before 'exit' for children with an IPC channel

[robobun]

Superseded by #39479. That PR fixes the same ordering bug where it originates (src/runtime/ipc.rs reports a peer-initiated channel close via a deferred task, so it lands after an exit dispatched in the same poll batch). Fixing it there also corrects Bun.spawn's onExit/onDisconnect order, which this PR does not touch, and it keeps Node's exit-first order in the case where a grandchild still holds the channel open, which a JS-side hold would invert.

Verified on #39479's branch (e741ef7): the two regression tests in this PR pass 3 of 3 with no child_process.ts change, and Bun.spawn goes from onExit, onDisconnect (released binary, 3 of 3) to onDisconnect, onExit (3 of 3). Details in this comment.

Leaving this open as a fallback only. If #39479 lands, the src/ change here should be dropped; the two tests (process.exit() through fork() and through node:cluster) are still valid additional coverage and pass on that branch.

Original description

node:cluster can emit a worker's 'exit' event before 'disconnect'. Node documents (and always produces) the opposite order, and primary-side bookkeeping written against it -- respawn logic, exitedAfterDisconnect checks, draining state -- sees the finalize step before the cleanup step.

It is not cluster-specific: node:cluster just forwards ChildProcess's events, and plain child_process.fork() races the same way.

Repro

// child.js
process.send("bye");
process.exit(3);

// parent.js
const { fork } = require("node:child_process");
const order = [];
const child = fork("./child.js");
child.on("message", () => Bun.sleepSync(100)); // child is already exiting
child.on("disconnect", () => order.push("disconnect"));
child.on("exit", code => order.push("exit:" + code));
child.on("close", () => console.log(order.join(",")));

node:  disconnect,exit:3
bun:   exit:3,disconnect

Without the sleepSync the outcome is racy (roughly 1 run in 3 on an idle machine, more under load). Blocking the loop for a moment after the child's last message is what makes it deterministic: it guarantees the channel EOF and the process exit are observed in the same poll batch.

Cause

The two notifications come from independent sources and take different routes to JS:

• process exit: pidfd/kqueue poll -> onExit -> process.nextTick(...) -> 'exit'
• channel close: socket EOF -> close task -> after-close task -> onDisconnect -> process.nextTick(...) -> 'disconnect'

When both land in the same poll batch, the exit notification reaches the nextTick queue first and wins, because the disconnect one still has two event-loop task hops to go. When the EOF happens to land in an earlier loop iteration, the order comes out right. Node has no such race: it reads the channel's EOF (the kernel closes the child's fds before it signals the parent) and drains the nextTick queue before it processes the exit event.

Fix

ChildProcess holds 'exit' back until the channel has disconnected, which is the order Node produces. Subprocess::on_process_exit always closes the IPC channel, so the disconnect notification is guaranteed to arrive after the process exits -- there is nothing to wait indefinitely for. Children without an IPC channel are unaffected.

Verification

• New tests in test/js/node/cluster.test.ts and test/js/node/child_process/child_process.test.ts assert the order; both print exit:3,disconnect on main and disconnect,exit:3 with the fix.
• test/js/node/test/parallel/test-cluster-*.js (58 files): all pass.
• test/js/node/test/parallel/test-child-process-*.js: no new failures (test-child-process-emfile and test-child-process-windows-hide already fail on main).
• test/js/node/child_process/*, test/js/bun/spawn/spawn.ipc*: pass.

---

no test proof · iteration 6 · Platform-specific test(s) that do not run on this machine. Deferring to CI, which covers all platforms: test/js/node/child_process/child_process.test.ts test/js/node/cluster.test.ts

[coderabbitai]

Walkthrough

Updates ChildProcess IPC lifecycle handling so disconnect occurs before exit. Adds regression tests for forked child processes and cluster workers.

Changes

IPC exit/disconnect ordering

Layer / File(s)	Summary
Defer exit emission until IPC disconnect src/js/node/child_process.ts	Tracks IPC state, defers exit data while the channel remains connected, and emits the pending exit after disconnect.
Regression tests for event ordering test/js/node/child_process/child_process.test.ts, test/js/node/cluster.test.ts	Verifies disconnect precedes exit for forked child processes and cluster workers, including exit status and stderr checks.

Possibly related issues

• IPC message sent right before the child exits is dropped on the waiter-thread exit path (Bun.spawn ipc and child_process.fork) #37849 — Both changes address IPC lifecycle completion before process exit.

Possibly related PRs

• oven-sh/bun#39479 — Directly addresses disconnect-before-exit ordering in child process IPC handling.

Suggested reviewers: jarred-sumner

🚥 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 and concisely states the primary behavior change for children with IPC channels.
Description check	✅ Passed	The description explains the bug, cause, fix, scope, and verification results, despite using different headings from the template.

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[robobun]

^{Updated 8:10 PM PT - Aug 17th, 2026}

✅ @robobun, your commit 8cee2c7b5bd7bf77da6ca06f360ee4aa8c65ce10 passed in Build #100307! 🎉

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[robobun]

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[claude]

No issues found — the fix is small and the native side does guarantee disconnect_ipc fires on exit, but since this reorders lifecycle events in node:child_process it's worth a quick maintainer glance.

Extended reasoning...

Overview

This PR changes src/js/node/child_process.ts to buffer the 'exit' event on ChildProcess until the IPC channel's 'disconnect' has fired, matching Node.js's documented ordering. Three private fields (#hasIpc, #disconnected, #pendingExit) are added; #handleOnExit stashes its arguments when IPC is still connected, and #onDisconnect flushes them via process.nextTick after scheduling the disconnect emit. Two regression tests (one for fork(), one for cluster) reproduce the race deterministically with a Bun.sleepSync in the message handler.

Security risks

None. This is purely event-ordering logic in the Node compat layer; no user input parsing, auth, crypto, or filesystem behavior is touched.

Level of scrutiny

Medium. The diff is ~20 lines and the mechanism is straightforward, but it changes lifecycle event ordering in a core, widely-depended-on Node compat module. I verified the load-bearing invariant — that Subprocess::on_process_exit unconditionally calls disconnect_ipc(true) (src/runtime/api/bun/subprocess.rs:1121) — so a buffered exit cannot be stranded. I also traced the #closesNeeded/#closesGot accounting: #emitExit still contributes one #maybeClose() and #onDisconnect still contributes one, so 'close' fires at the same count as before, just after both events instead of potentially between them.

Other factors

The PR author ran the full Node parallel test suites for test-cluster-* (58 files) and test-child-process-* with no new failures, plus Bun's own IPC tests. The bug-hunting system found nothing. The tests follow harness conventions (tempDir, concurrent pipe drain, combined-object assertions). Given all that, this is close to auto-approvable, but reordering 'exit' relative to 'disconnect' is exactly the kind of change where a maintainer familiar with downstream consumers (timeout handlers, cluster respawn logic, third-party libs) should give it a quick look.

[robobun]

Status: superseded by #39479, converted to draft.

#39479 fixes the same 'exit'-before-'disconnect' race in src/runtime/ipc.rs, where it originates. Verified on its branch (e741ef7):

• the two regression tests from this PR pass 3 of 3 there with no child_process.ts change;
• Bun.spawn with ipc goes from onExit, onDisconnect (released binary, 3 of 3) to onDisconnect, onExit (3 of 3), a path this PR does not cover;
• its CI is green (#100280).

Recommendation: merge #39479. The two tests here (process.exit() through fork() and through node:cluster) are independent of the src/ change and pass on that branch, so they can be carried over if wanted. This PR stays open only as a fallback if #39479 does not land.

Earlier CI history for this branch

Every build of this branch was green on all Linux/Windows/FreeBSD lanes and on every lane that runs the child_process / cluster suites. The recurring reds were macOS agent problems unrelated to the diff: a darwin-aarch64-26 agent timing out in buildkite-agent artifact download before running any test (#68091, #68132), and darwin-14 shards hitting wall-clock limits or agent-environment failures on Bun.serve, bake dev-server, no-orphans, log-test (EACCES on a stale tmpdir) and grpc-js (rustup with no default toolchain) tests (#71235).

[claude]

I didn't find any issues, but this reorders child_process lifecycle events for every IPC-enabled child (deferring exitCode/signalCode assignment and stdio teardown until the native layer delivers onDisconnect), so it's worth a human sanity check on the event-loop timing.

Extended reasoning...

Overview

The PR changes src/js/node/child_process.ts so that when a ChildProcess has an IPC channel, the 'exit' event (and everything #emitExit does: setting exitCode/signalCode, destroying stdin, resuming stdout/stderr, calling #maybeClose) is held back until #onDisconnect has fired. Two new subprocess-based regression tests in test/js/node/child_process/child_process.test.ts and test/js/node/cluster.test.ts assert the disconnect,exit ordering.

Security risks

None. This is purely event-ordering / lifecycle bookkeeping in the Node compat layer; no untrusted input parsing, auth, or crypto is involved.

Level of scrutiny

Medium-high. The diff is small (~25 production lines) and the logic is easy to follow, but it changes the observable ordering of 'disconnect', 'exit', and (indirectly) 'close' for every fork() / cluster worker. Correctness hinges on the native invariant that Subprocess::on_process_exit always calls disconnect_ipc(true) (verified at src/runtime/api/bun/subprocess.rs:1121), otherwise 'exit' would never fire. The #maybeClose accounting is preserved (still exactly one call from the exit path and one from the disconnect path), and the non-IPC path is untouched.

Other factors

• The PR description reports clean runs across all 58 test-cluster-* Node parallel tests and no new test-child-process-* failures, plus the Bun-side IPC tests.
• Side effects that were previously synchronous-ish in onExit (setting .exitCode, destroying stdin) are now deferred by a couple of ticks in the IPC case; I couldn't find any in-tree code that depends on the old timing, but a maintainer familiar with downstream consumers should confirm.
• No prior human review comments; only bot activity on the timeline.

[robobun]

On the one open question from review: whether deferring exitCode/signalCode assignment and stdio teardown by a couple of ticks changes what downstream consumers observe.

It does, and in the direction of Node. Those side effects live in Node's _handle.onexit, which already runs after the channel's 'disconnect' nextTick has drained, so Node never exposes a populated exitCode inside a 'disconnect' handler. Bun used to. Snapshotting the child inside each handler (fork(..., { silent: true }), child sends then process.exit(3)):

inside 'disconnect'	exitCode	signalCode	stdin.destroyed
node v26	null	null	false
bun 1.4.0	3	null	true
this PR	null	null	false

'exit' and 'close' see exitCode: 3 / stdin.destroyed: true in all three. So the deferral removes a divergence rather than adding one.

There is an in-tree consumer that depends on exactly this. internal/cluster/primary.ts's disconnect handler guards removeWorker with worker.isDead(), which reads process.exitCode != null:

             node v26                        bun 1.4.0                      this PR
disconnect   isDead()=false  state=disc.     exit ran first:                isDead()=false  state=disc.
exit         isDead()=true   state=dead      isDead()=true at both          isDead()=true   state=dead

Under the old ordering isDead() is already true when 'disconnect' fires, so removeWorker runs from both handlers instead of one. That double bookkeeping is the "cluster manager occasionally respawns twice" class of bug this PR was filed for.

One divergence this PR does not close, noticed while checking the above: Node sets child.channel = null before emitting 'disconnect' (_disconnect() in lib/internal/child_process.js), Bun leaves it set. internal/cluster/primary.ts carries a worker.process.channel = null line to paper over it, which Node's own primary.js does not have. That is a separate missing state update rather than an ordering problem, so I left it out of this diff. Happy to fold it in here if you'd rather.

[claude]

I didn't find any bugs, but this reorders 'exit'/'disconnect' and defers exitCode/stdio teardown in the IPC path — subtle enough lifecycle semantics that a maintainer familiar with child_process/cluster internals should sign off.

Extended reasoning...

Overview

This PR changes src/js/node/child_process.ts so that when a child has an IPC channel, the 'exit' event is held back until 'disconnect' has fired, matching Node.js's documented ordering. It adds three private fields (#hasIpc, #disconnected, #pendingExit), splits #handleOnExit into a defer check plus #emitExit, and flushes the pending exit from #onDisconnect. Two new subprocess-based regression tests cover fork() and cluster.fork().

I verified the load-bearing invariant the fix relies on: Subprocess::on_process_exit in src/runtime/api/bun/subprocess.rs unconditionally calls self.disconnect_ipc(true) at its tail, so a deferred exit cannot be stranded. The #maybeClose accounting is preserved (still exactly two calls in the IPC-from-node case), and the nextTick ordering in #onDisconnect produces disconnect → exit → close.

Security risks

None. This is event-ordering logic in the Node compat layer; no auth, crypto, untrusted-input parsing, or permission surface is touched.

Level of scrutiny

Medium-high. The diff is small (~21 source lines), but it changes lifecycle event ordering for every IPC-bearing child process and, as the author's own follow-up notes, also shifts when exitCode/signalCode are populated and when stdio is torn down (now after 'disconnect', matching Node). That is the right direction, but it is an observable behavior change that flows into node:cluster and any user code that inspects state inside a 'disconnect' handler. This is not a mechanical/config-style change I'd approve unilaterally.

Other factors

• The bug-hunting pass found nothing; the author ran the full test-cluster-* and test-child-process-* Node parallel suites with no new failures, and CI is green modulo an unrelated macOS artifact-download infra flake.
• No CODEOWNERS entry covers this file.
• The author left an open question in the thread about whether to also fold in the child.channel = null divergence — worth a maintainer's call on scope.

[robobun]

#39479 fixes the same inversion natively (the peer's close is now reported from the read dispatch, ahead of the exit), which also covers Bun.spawn's onDisconnect/onExit and the ECONNRESET and Windows variants. If that lands, the child_process.ts hold-back here is not needed.

[coderabbitai]

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/js/node/child_process.ts`:
- Around line 1565-1570: Update the disconnect handling around `#disconnected` and
`#pendingExit` to defer the emitted-state transition until the queued "disconnect"
callback runs. Track whether the event was emitted with a separate
`#disconnectEmitted` state, set it immediately after emit("disconnect"), and flush
any pending exit from that same callback so "disconnect" is always observed
before "exit".

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[claude]

I reviewed this PR and didn't find any bugs. Because it changes user-observable child_process/cluster lifecycle ordering and the author has flagged #39479 as a native fix that would supersede it, a maintainer should decide whether to land this JS-side hold-back or wait for the native change.

What was reviewed:

• Traced the deferred-exit flow across all onExit/onDisconnect interleavings — #disconnectEmitted flips inside the emitting tick, so a #handleOnExit tick queued first still defers correctly.
• Verified #maybeClose() call-count is preserved (deferred path: flush→#emitExit→maybeClose + the trailing tick = 2, same as before).
• Checked that #hasIpc is only set after a successful spawn, so the synchronous-throw error paths are unaffected.
• The unresolved comment-cop flag on the two-line comment at :1565 looks like an over-trigger; the comment states the invariant CodeRabbit asked for.

Extended reasoning...

Overview

The PR defers ChildProcess's 'exit' emission until after 'disconnect' when an IPC channel exists, matching Node's documented ordering. Three files: src/js/node/child_process.ts (adds #hasIpc, #disconnectEmitted, #pendingExit and splits #handleOnExit/#emitExit), plus regression tests in child_process.test.ts and cluster.test.ts.

Security risks

None. Pure event-ordering change in the Node compat layer; no new inputs, no auth/crypto/fs surface.

Level of scrutiny

Medium-high. This is a Node-compat lifecycle change with downstream consumers (internal/cluster/primary.ts reads exitCode inside its 'disconnect' handler, per the author's own analysis). The #closesNeeded/#closesGot accounting is subtle: the deferred branch returns early from #handleOnExit without calling #maybeClose, and the count is made up by #emitExit inside the pending-flush tick plus the existing third tick in #onDisconnect. I traced every ordering (exit-tick before disconnect-ticks, disconnect-ticks first, disconnect drains fully then exit later) and the total stays at 2, matching pre-PR behavior — but this is exactly the kind of invariant a human should sanity-check.

Other factors

• The author explicitly noted that #39479 fixes the same inversion natively and would make this JS hold-back unnecessary. That is a land-now-vs-wait call a maintainer should make, not a bot.
• One unresolved automated comment remains (comment-cop on the two-line comment at line 1565-1566). It reads as a false positive to me — the comment is two lines and states the ordering invariant CodeRabbit specifically requested — but I'm not the arbiter of that rule.
• My earlier nit about the weak stderr assertion was addressed in 2e8d393; both tests now assert the full {stdout, stderr, exitCode} object.
• CI on the rebased build was green on every lane exercising this code; remaining reds were unrelated macOS flakes.