ipc: run the callbacks of sends still queued when the channel closes

[robobun]

Problem

• subprocess.send(msg, cb) and process.send(msg, cb) never call cb for a message that is still queued when disconnect(), kill(), or the peer closes the IPC channel. util.promisify(subprocess.send) never settles. node v26.3.0 calls each callback with null.
• The cause is SendQueue::run_deferred (src/runtime/ipc.rs:1189). At close it drains the queue with SendHandle::abort_unsent, which closes a pending handle and drops the callbacks.

Fix

• At close, complete each queued entry up to and including the first one that carries a handle. complete calls the callbacks with null on the next tick. Entries behind a handle keep abort_unsent.
• This is node's rule. node calls the callback of every write it submitted, with null even when the close cancels the write (child_process.js#L868-L874). Sends that node parked behind an unacknowledged handle were never submitted, and their callbacks never run (#L818-L853). An existing test pins that second half.
• Verified: six new tests in test/js/node/child_process/child_process_send_cb.test.js and one in child_process_ipc_handle.test.ts. All seven fail on main. Notes lists the other suites I ran.

Background

• SendQueue is the outgoing half of an IPC channel. Its queue holds the messages not fully written. Each entry is a SendHandle: the bytes, an optional handle to pass, and the send() callbacks.
• A message that carries a handle waits in waiting_for_ack until the peer acknowledges it. Nothing behind it is written before that. node does the same with _handleQueue.
• run_deferred runs after the socket closes. It reports the close to the owner, which emits 'disconnect'.

Notes

Repro (three 2 MB messages, then disconnect() or kill() in the same tick):

// bun 773.cjs ; node 773.cjs ; MODE=kill for the kill() variant
const cp = require("child_process");
if (process.argv[2] === "child") { setInterval(() => {}, 1000); setTimeout(() => process.exit(0), 4000); return; }
const MODE = process.env.MODE || "disconnect";
const c = cp.fork(__filename, ["child"], { stdio: ["ignore", "ignore", "inherit", "ipc"] });
const cbs = [];
for (let i = 0; i < 3; i++) c.send({ i, pad: "d".repeat(2_000_000) }, (e) => cbs.push(e ? e.code || e.name : "null"));
if (MODE === "kill") c.kill("SIGKILL"); else c.disconnect();
setTimeout(() => {
  console.log(`mode=${MODE} | send() callbacks invoked: ${cbs.length}/3 [${cbs.join(",")}]`);
  try { c.kill("SIGKILL"); } catch {}
  process.exit(0);
}, 2000);

	disconnect	kill
node v26.3.0	3/3 [null,null,null]	3/3 [null,null,null]
bun 1.4.3-canary (c6b7fcb)	0/3 []	0/3 []
this branch	3/3 [null,null,null]	3/3 [null,null,null]

Why null and not an error. node's write completion is req.oncomplete = () => { ...; callback(null); }. It does not read the status, so a write that the close cancels still reports null. An error here would make await promisify(child.send)(msg) reject on Bun where it resolves on node.

The handle queue. With a large plain message, then a handle, then two plain messages, and the child killed in the same tick, node prints {"before":null,"handle":null,"behind":"never called","behindLarge":"never called"}. The new test in child_process_ipc_handle.test.ts pins that object. The test before it, channel close: written handle callback fires null; unsent queued handle callback never fires, covers a handle that is already written, and still passes.

Order of events. In the child the order is the same as node: 'disconnect', then the callbacks. In the parent the callbacks run before 'disconnect', and node runs them after it. ChildProcess#onDisconnect queues 'disconnect' and #maybeClose as two adjacent ticks from the owner notification, and that notification drains them before run_deferred continues. So the callbacks can go before both ticks or after both. After both means after 'close' whenever 'exit' was already emitted, which is worse: 'close' is the last event. node's exact order needs #onDisconnect to stop queuing #maybeClose next to 'disconnect', and #39479 and #33285 are changing that choreography now. The new tests record the callbacks and 'close' in one list and assert that the callbacks come first. For a killed child node has the same order (callbacks, 'exit', 'close'). After subprocess.disconnect() node never emits 'close' at all.

A separate bug, found in review. disconnect() while a handle send is still queued behind a large message closes on the next tick and drops the handle and everything behind it. node flushes them first. That is about when disconnect() closes, not about which callbacks run at close, and it is tracked and fixed separately.

Not changed. When the owner is finalized with sends still queued (Drop for SendQueue), the callbacks are dropped as before. JavaScript cannot run from a finalizer.

Related. #39479 moves this same block into a new run_after_close method and keeps abort_unsent. The PR that lands second needs a small rebase.

Suites run on the debug build: child_process_send_cb.test.js, child_process_ipc_handle.test.ts, child_process_ipc.test.js, child_process_ipc_large_disconnect.test.js, spawn.ipc.test.ts, spawn.ipc.bun-node.test.ts, spawn.ipc.node-bun.test.ts, cluster.test.ts, and 91 files from test/js/node/test/{parallel,sequential} (test-child-process-* and test-cluster-* that touch send, disconnect, ipc, fork or handles).

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no test proof · iteration 7 · 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_ipc_handle.test.ts

[robobun]

Status

• Reproduced on bun 1.4.3-canary (c6b7fcb) with the script in the PR notes: send() callbacks invoked: 0/3 [] for both disconnect() and kill(). node v26.3.0 prints 3/3 [null,null,null].
• With this branch the same script prints 3/3 [null,null,null] in both modes.
• The seven new tests fail on main and pass on this branch.

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Walkthrough

The IPC send queue now completes callbacks for submitted writes and aborts writes queued behind an unacknowledged handle. Child-process tests cover message and handle callbacks during channel closure and termination.

Changes

IPC send callback completion

Layer / File(s)	Summary
Queue completion handling src/runtime/ipc.rs	SendQueue::run_deferred completes the pending handle and writes before the first unacknowledged handle. It aborts writes queued behind that handle.
Closure and termination regression coverage test/js/node/child_process/child_process_send_cb.test.js, test/js/node/child_process/child_process_ipc_handle.test.ts	Tests cover JSON and advanced serialization, parent or child channel closure, queued socket handles, callback results, and callback ordering before close or exit events.

Suggested reviewers: cirospaciari

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to f9ee3

The IPC callback completion change has no supported remaining merge-blocking risk in the reviewed scope.

🚥 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 summarizes the main change: queued IPC send callbacks now run when the channel closes.
Description check	✅ Passed	The description explains the problem, fix, expected Node.js behavior, test coverage, verification results, and known limitations. It does not use the template headings exactly, but it contains the req…

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Inline comments:
In `@test/js/node/child_process/child_process_ipc_handle.test.ts`:
- Around line 596-597: Update the generated IPC fixtures to use module-scope
imports instead of require calls: create an .mjs parent fixture importing
node:child_process and node:net, and create an .mjs child fixture importing
node:fs. Preserve the existing IPC test behavior while applying this change to
the fixture-generation logic.

In `@test/js/node/child_process/child_process_send_cb.test.js`:
- Around line 70-78: Update the close-based tests in
child_process_send_cb.test.js and the IPC-handle fixture to record callback
invocations and the close event in a shared event array, then assert every
callback event precedes close rather than checking callback state only after
awaiting close. Preserve the existing callback result assertions and use the
child-side exit snapshot as-is.

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[robobun]

^{Updated 10:11 PM PT - Sep 16th, 2026}

✅ @robobun, your commit f9ee3cea09dfb16b09d1b38a861e8dd9ceb9ca8b passed in Build #116857! 🎉

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[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline findings, I also checked that switching queued entries from abort_unsent to complete cannot double-close a passed handle: both paths schedule the same single close_sent_handle tick and each SendHandle is consumed exactly once from the drained queue, so the only behavioral delta is the added callback invocation.

Extended reasoning...

Read SendHandle::complete and SendHandle::abort_unsent in src/runtime/ipc.rs (lines ~790-812): the close_on_complete handling is byte-identical in both, and complete only adds callbacks.call_next_tick. The loop in _onAfterIPCClosed takes ownership of the whole queue via mem::take and moves each item into exactly one of the two methods, so there is no path where a handle is closed twice or a callback fires twice. The remaining concerns (parent-side ordering relative to 'disconnect', and the pre-existing disconnect()-with-queued-handle gap) are already covered by the inline findings.

[claude]

Code review found no issues

No high-confidence issues detected in this change.