Close a MessagePort transferred to a worker that never starts

[robobun]

Problem

• A MessagePort transferred to a worker that never starts stays open. Its peer never emits close, and a message listener on the peer keeps the process alive. It happens when the entry does not resolve, or terminate() stops the thread first. Node closes the port.
• The ports wait in the parent-side proxy. Only a started worker takes WorkerOptions::dataMessagePorts (src/jsc/bindings/webcore/Worker.cpp:302) or drains the inbox m_toWorker. Otherwise both live until a GC collects the Worker.

Fix

• WorkerMessagingProxy::dropUndeliveredWorkerMessages() empties both when the worker reports that it is gone, and after the join when the parent exits first. ~TransferredMessagePort closes each port and notifies its peer.
• No thread races the drop. The worker thread touches neither container after it reports, and the second call runs after the join.
• Verified: test/js/node/worker_threads/worker-transferred-port-close.test.ts (node:test) fails on Bun 1.4.2, and passes with the fix and on Node.js 22 and 24. test/js/web/workers/message-port-pipe.test.ts has two Web Worker tests. Also the MessagePort suites and Node's test-worker-*.
• This PR carries the commits of fix(worker_threads): close ports when worker startup fails #43222 by @steipete, plus the Web Worker and terminate() tests.

Background

• WorkerMessagingProxy is the object that the parent thread and the worker thread share. It lives as long as the Worker object.
• A TransferredMessagePort is a port in transit. If nothing takes it, its destructor closes its side of the pipe and posts close to the peer.
• A node:worker_threads worker gets parentPort and the transferList ports through dataMessagePorts. worker.postMessage() travels over the parentPort pipe.
• A Web Worker has no parentPort. Its postMessage() goes to the proxy inbox m_toWorker.

Notes

Repro. The close listener keeps worker reachable, as a worker pool does:

import { Worker, MessageChannel } from "node:worker_threads";
const { port1, port2 } = new MessageChannel();
const worker = new Worker("/tmp/missing.js", { workerData: { port: port2 }, transferList: [port2] });
worker.on("error", e => console.log("error", e.code));
worker.on("exit", c => console.log("exit", c));
port1.on("message", () => {}); // refs the loop until the port closes
port1.on("close", () => console.log("port1 close, threadId", worker.threadId));

• Node v26.3.0: error MODULE_NOT_FOUND, exit 1, port1 close, threadId -1, then the process exits.
• bun 1.4.3: error MODULE_NOT_FOUND, exit 1, then the process does not exit (still running after 30 s).
• With this PR the output is the same as in Node. In Bun the close on the peer is a posted task, so it always follows the exit event of the worker. Node does not fix that order: the first worker of a process gives exit first, and later workers give the close first (99 to 100 of 100 runs on Node 22, 24 and 26). So the shared test does not assert it.
• If nothing references the Worker, a GC collects it, the proxy dies, and the port closes late. That is why a short script can seem to work on bun 1.4.3.

The same stall on bun 1.4.3, all fixed here:

• worker.postMessage({ port }, [port]) to a node:worker_threads worker whose entry does not resolve. The message waits in the parentPort pipe, and nobody owns the worker side of that pipe.
• The same call on a Web Worker. The message waits in m_toWorker.
• worker.terminate() in the same tick as the constructor, with a port in workerData or in a posted message, for both kinds of worker.

Tests

• worker-transferred-port-close.test.ts imports only Node modules, so the same file runs with node --test and with bun test. Results: Bun 1.4.2 release 0 of 4 (each test times out), this branch 4 of 4, Node.js 22.23.2 and 24.21.0 4 of 4 in 25 of 25 runs each, Node.js 26.3.0 4 of 4.
• It holds the four node:worker_threads cases: the entry does not resolve, and terminate() in the same tick, each with the port in workerData and in a posted message. They were in worker_threads.test.ts (bun:test) in the first version of this PR.
• The two Web Worker tests use bun:test, because Node has no global Worker. They are in test/js/web/workers/message-port-pipe.test.ts, in the existing Worker postMessage inbox block. Both fail on bun 1.4.3 and on a debug build of main, and pass on the debug (ASAN) build of this branch.
• The tests keep the Worker referenced, so they test the fix and not the finalizer. Without that reference, one of them passed on an unfixed debug build after 4.9 s.
• The Web Worker terminate() test uses an entry that never returns (for(;;){}). That worker never reads its inbox, whether terminate() lands before the thread starts or while the entry runs. So the test cannot pass without the fix.
• The node:worker_threads terminate() tests cannot be pinned that way. That thread takes its ports before any user code runs, and a thread that wins the race against terminate() closes them when it exits. So they assert the close only. On an unfixed release build the thread won that race in 0 of 300 runs. The tests where the entry does not resolve are the ones that cannot pass without the fix.

Other checks on the debug (ASAN) build

• The two changed test files together: 19 of 19 pass with the fix (5 of 5 runs). Without the fix the six new tests fail and the other 13 pass.
• worker_threads.test.ts: all pass. worker.test.ts: all pass but terminate() while fs.readFile completions keep arriving, which hits the 5 s test timeout. Its script takes 6.2 s on a debug build with and without this change.
• Pass: message-channel, message-port-closed-leak, message-port-context-destroy-leak, message-port-pipe, worker-postmessage-transfer, worker-transfer-list, worker-transfer-terminate-stress, worker-shutdown-post-leak, worker-async-dispose, worker-terminate-funnels.
• worker-late-completion (2), worker_destruction (3) and worker-terminate-lifetime (1) have failures. The same tests fail the same way on a debug build of main in my environment (5 s timeouts, and one test that needs DNS).
• Node's test/js/node/test/parallel/test-worker-*.js: 109 of 110 pass with bun <file>. test-worker-arraybuffer-zerofill.js needs the bun test runner.
• parentContextWillDestroy: a worker that spawns a child that never starts and then calls process.exit(), 20 runs. A main thread that calls process.exit() while 16 workers have ports in transit, 10 runs. No ASAN report, and the port closes.
• I read the diff for races and lifetime problems and found none. The new code comments are one line each.

Not changed

• m_options.workerDataAndEnvironmentData still lives as long as the proxy. For most data that is memory only.
• A FileHandle in that workerData is different: its fd stays open for the life of the process when the worker never starts. Node closes it. The leak is the same on main, it needs a signal from the proxy to worker_threads.ts, and a separate change handles it.
• m_toParent (worker to parent) still goes with the proxy. It is drained fully before the close event of the worker.

---

[human-review] gate passed · iteration 3 · 4 files touched

fails on main (without fix)

ASAN without fix: 6 FAILED
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/node/worker_threads/worker-transferred-port-close.test.ts test/js/web/workers/message-port-pipe.test.ts
bun test v1.4.3 (b52d51348)

test/js/web/workers/message-port-pipe.test.ts:
(pass) MessagePort pipe > microtasks run between message events (task-source semantics) [33.80ms]
(pass) MessagePort pipe > messages buffered before start() are delivered on start() [14.08ms]
(pass) MessagePort pipe > receiveMessageOnPort pops in FIFO order [134.21ms]
(pass) MessagePort pipe > close() inside onmessage handler still delivers already-queued messages [18.18ms]
(pass) MessagePort pipe > messages queued on a port follow it across transfer [17.73ms]
(pass) MessagePort pipe > same-context re-attach inside handler: inbox follows new wrapper [12.30ms]
(pass) MessagePort pipe > chained transfer delivers through every hop [21.89ms]
(pass) MessagePort pipe > objectTypeCounts drop after close + GC; peer-open pins listening port [1981.68ms]
(pass) MessagePort pipe > port dropped in transit does not pin its peer [860.40ms]
(pass) MessagePort pipe > c
... (truncated)

release without fix: 3 skipped
bun test v1.4.3-canary.1 (8003adbdd)

test/js/web/workers/message-port-pipe.test.ts:
(pass) MessagePort pipe > microtasks run between message events (task-source semantics) [2.51ms]
(pass) MessagePort pipe > messages buffered before start() are delivered on start() [0.23ms]
(pass) MessagePort pipe > receiveMessageOnPort pops in FIFO order [2.36ms]
(pass) MessagePort pipe > close() inside onmessage handler still delivers already-queued messages [0.31ms]
(pass) MessagePort pipe > messages queued on a port follow it across transfer [1.02ms]
(pass) MessagePort pipe > same-context re-attach inside handler: inbox follows new wrapper [0.28ms]
(pass) MessagePort pipe > chained transfer delivers through every hop [0.28ms]
(pass) MessagePort pipe > objectTypeCounts drop after close + GC; peer-open pins listening port [27.45ms]
(pass) MessagePort pipe > port dropped in transit does not pin its peer [16.36ms]
(skip) MessagePort pipe > concurrent MessageChannel creation across workers is race-free
(skip) MessagePort pipe > burst of postMessage across threads delivers every message in order
(skip) Worker postMessage inbox > round-trip burst delivers in order with microtasks betwe
... (truncated)

passes on PR (with fix)

ASAN with fix: all passed
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/js/node/worker_threads/worker-transferred-port-close.test.ts test/js/web/workers/message-port-pipe.test.ts
bun test v1.4.3 (b52d51348)

test/js/web/workers/message-port-pipe.test.ts:
(pass) MessagePort pipe > microtasks run between message events (task-source semantics) [31.54ms]
(pass) MessagePort pipe > messages buffered before start() are delivered on start() [11.77ms]
(pass) MessagePort pipe > receiveMessageOnPort pops in FIFO order [175.41ms]
(pass) MessagePort pipe > close() inside onmessage handler still delivers already-queued messages [26.45ms]
(pass) MessagePort pipe > messages queued on a port follow it across transfer [26.48ms]
(pass) MessagePort pipe > same-context re-attach inside handler: inbox follows new wrapper [18.42ms]
(pass) MessagePort pipe > chained transfer delivers through every hop [24.31ms]
(pass) MessagePort pipe > objectTypeCounts drop after close + GC; peer-open pins listening port [2170.93ms]
(pass) MessagePort pipe > port dropped in transit does not pin its peer [892.48ms]
(pass) MessagePort pipe > c
... (truncated)

release with fix: 3 skipped
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped)
  target       linux-x64-gnu
  build type   Release
  build dir    ./build/release
  revision     96bd38fa5e
  features     lto, baseline

23 deps, 131 codegen, 1176 objects in 926ms

ninja: Entering directory `/workspace/bun/build/release'
[1/144] gen ErrorCode+*.h
[2/144] fetch WebKit (prebuilt)
[WebKit] up to date
[3/144] cc obj/packages/bun-usockets/src/bsd.c.o
[4/144] cc obj/packages/bun-usockets/src/context.c.o
[5/144] cc obj/packages/bun-usockets/src/crypto/openssl.c.o
[6/144] cc obj/packages/bun-usockets/src/eventing/epoll_kqueue.c.o
[7/144] cc obj/packages/bun-usockets/src/eventing/libuv.c.o
[8/144] cc obj/packages/bun-usockets/src/fault_inject.c.o
[9/144] cc obj/packages/bun-usockets/src/loop.c.o
[10/144] cc obj/packages/bun-usockets/src/quic.c.o
[11/144] cc obj/packages/bun-usockets/src/node_quic_shim.c.o
[12/144] cc obj/packages/bun-usockets/src/udp.c.o
[13/144] cc obj/src/jsc/bindings/node/http/llhttp/api.c.o
[14/144] cc obj/src/jsc/bindings/node/http/llhttp/http.c.o
[15/144] cc obj/packages/bun-usockets/src/socket.c.o
[16/144] cc obj/src/jsc/bindings/uv-posix-polyfills
... (truncated)

diff hotspot

src/jsc/bindings/webcore/WorkerMessagingProxy.cpp  | 19 +++++++-
 src/jsc/bindings/webcore/WorkerMessagingProxy.h    |  1 +
 .../worker-transferred-port-close.test.ts          | 50 ++++++++++++++++++++++
 test/js/web/workers/message-port-pipe.test.ts      | 40 ++++++++++++++++-
 4 files changed, 107 insertions(+), 3 deletions(-)

gate history · 3 passed · 1 rejected · iteration 3

evidence per changed file

file                                                      reads  edits  tests
src/jsc/bindings/webcore/WorkerMessagingProxy.cpp             4      2     22
src/jsc/bindings/webcore/WorkerMessagingProxy.h               1      0     22
…de/worker_threads/worker-transferred-port-close.test.ts      1      2     15
test/js/web/workers/message-port-pipe.test.ts                 1      3     11

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---

Walkthrough

The worker messaging proxy releases undelivered messages and transferred ports during worker and parent-context shutdown. Node and web worker tests cover startup failures and termination races.

Changes

Worker message cleanup

Layer / File(s)	Summary
Cleanup helper and shutdown integration src/jsc/bindings/webcore/WorkerMessagingProxy.*	Adds dropUndeliveredWorkerMessages(), releases queued ports and messages, resets drain state, and invokes cleanup during worker and parent-context destruction.
Transferred port lifecycle tests test/js/node/worker_threads/worker-transferred-port-close.test.ts, test/js/web/workers/message-port-pipe.test.ts	Tests port closure after missing entry modules and termination during worker startup or before inbox processing.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to b0ac5

The change releases undelivered ports along the inspected shutdown paths, and the added tests check peer-port closure. No concrete merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: closing MessagePorts transferred to workers that never start.
Description check	✅ Passed	The description explains the problem, implementation, scope, and verification results. It does not use the exact template headings, but it provides the required content in the Problem and Fix sections…
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.

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[alii]

We want a test with node:test so we can check it fails in Bun 1.4.2 and passes with this patch, as well as in Node.js LTS

[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 the lock and thread-affinity concerns in dropUndeliveredWorkerMessages(): MessagePortPipe::close/notifyPeerClosed only take the pipe-side locks and post tasks (no synchronous JS, no re-entry into m_toWorker.lock), and entanglePorts at Worker.cpp:302 moves each RefPtr out of dataMessagePorts, so the parent-side exchange after the worker's final posted task (or after the join) only runs no-op destructors. The WebWorker__create failure path was also checked: the proxy dies with the Worker there, closing the ports via ~WorkerOptions.

Extended reasoning...

Findings-present ruled-out note. The diff adds WorkerMessagingProxy::dropUndeliveredWorkerMessages() and calls it from workerGlobalScopeDestroyedInternal and post-join in parentContextWillDestroy. I read MessagePortPipe::close (worklist-based, locks only m_sides[x].lock, then notifyPeerClosed posts a task) to confirm destroying TransferredMessagePorts on the parent thread cannot deadlock on or re-enter the proxy inbox lock and cannot synchronously run user JS. I confirmed MessagePort::entanglePorts(ScriptExecutionContext&, Vector<TransferredMessagePort>&&) consumes elements via WTF::map over an rvalue, nulling each pipe, so the unlocked std::exchange on m_options.dataMessagePorts is ordered after the worker's writes by the postTaskTo of workerGlobalScopeDestroyed or by WebWorker__join. drainScheduled = false has a consumer in postMessageToWorkerGlobalScope, and drainInbox resets it on an empty queue if a stale drain task lands. The two confirmed inline findings (terminate-before-start test can pass on an unfixed build; pre-existing FileHandle fd leak in undelivered workerData) already signal that a human should look, so this note is informational only.

[robobun]

^{Updated 10:39 PM PT - Sep 18th, 2026}

❌ @alii, your commit 96bd38f has 2 failures in Build #118165 (All Failures):

• test/js/bun/http/serve-body-leak.test.ts - code 1 on 🍎 any aarch64
• test/js/bun/spawn/spawn.test.ts - code 1: stdout reader of an unref'd child and process lifetime > an idle reader stopped at the highwater … on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 43374

That installs a local version of the PR into your bun-43374 executable, so you can run:

bun-43374 --bun

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[alii]

@robobun? #43374 (review)

[robobun]

Sorry, I missed that review. I will add a node:test file for the node:worker_threads cases. I will check that it fails on Bun 1.4.2, passes with this patch, and passes on Node.js 22 and 24.

[robobun]

Done in 242c6ed. test/js/node/worker_threads/worker-transferred-port-close.test.ts uses node:test and imports only Node modules, so the same file runs in both runtimes:

node --test test/js/node/worker_threads/worker-transferred-port-close.test.ts
bun test test/js/node/worker_threads/worker-transferred-port-close.test.ts

Results:

• Bun 1.4.2 (release): 0 of 4 pass. Each test times out, because the port never closes.
• This branch (debug build): 4 of 4 pass.
• Node.js 24.21.0 and 22.23.2: 4 of 4 pass, in 25 of 25 runs each. Node.js 26.3.0 passes too.

The file holds the four node:worker_threads cases, so I removed their bun:test copies from worker_threads.test.ts. The two Web Worker tests use bun:test, because Node has no global Worker. They are in test/js/web/workers/message-port-pipe.test.ts, in the Worker postMessage inbox block (moved there in fc9f99e).

One difference from the first version: the shared test does not assert the order of the worker's exit and the port's close. Node does not fix that order. The first worker of a process gives exit first, and later workers give close first (99 to 100 of 100 runs on Node 22, 24 and 26). Bun with this patch always gives exit first.

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• src/jsc/bindings/webcore/WorkerMessagingProxy.cpp
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• test/js/node/worker_threads/worker-transferred-port-close.test.ts
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[claude]

Code review completed

Nothing new to post: everything this review found is already covered by existing comments on this pull request or didn't merit a separate one.

[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

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[robobun]

CI state for the reviewer. No test of this PR failed on any lane in the last four builds (Linux, ASAN, Windows x64 and aarch64, macOS). Each build is red only on tests that this diff does not touch:

• #118097: test/bake/deinitialization.test.ts on alpine aarch64.
• #118102: test/js/bun/spawn/spawn.test.ts on debian x64-asan. It fails on main too.
• #118108: test/js/bun/http/serve-body-leak.test.ts on macOS aarch64. The other 180 jobs passed.
• #118165 (after the merge of main): the same two again, spawn.test.ts on debian x64-asan and serve-body-leak.test.ts on macOS aarch64. The other 179 jobs passed.

I reported the three tests as breaks on main.

worker_threads.test.ts failed once on the ASAN lane in #118108 and in #118165, and passed on retry both times. The stop-ordering fixture logs in two steps (Atomics.add, then Atomics.store), and terminate() landed between them, which leaves a 0 in the log. #38194 (open since August) reports the same [12, 0] result on main. This PR changes nothing that runs on the worker thread.

I already used my one CI rerun, so I will not push again. The requested node:test file is in, and the diff is ready for review.

[robobun]

I merged main (f4d755a9cf) into this branch to get a current CI run. Buildkite build 118165 ran on 2026-09-19, and the branch was 172 commits behind main. This push changes nothing else. The new run is Buildkite build 122697.

Result of build 122697: 180 jobs passed and one failed. The failed job is debian x64-asan, on test/js/bun/spawn/spawn.test.ts again. That test fails on main too, and this diff does not touch it. No test of this PR failed on any lane.

• The merge has no conflicts. The diff against main is the same as before: 4 files, +107 -3, identical git patch-id. Main changed WorkerMessagingProxy.cpp once since the last merge (Add --disallow-code-generation-from-strings, with a strict level for every way a string becomes code #44171), in WebWorker__create and startWorkerGlobalScope, away from the lines of this PR.
• bun bd test (debug, ASAN) on the merged tree: test/js/node/worker_threads/worker-transferred-port-close.test.ts 4 pass, test/js/web/workers/message-port-pipe.test.ts 15 pass.
• The same two files with bun 1.4.3-canary.1 (367d939d9), which does not have the fix: 0 pass 4 fail, and 10 pass 2 fail 3 skip. Each failure is a 5 s timeout, because the close event of the port does not arrive.
• bun bd test test/js/web/workers/ test/js/node/worker_threads/ on the merged tree: 636 pass, 2 fail. The details are below.

The local failures

• worker-terminate-lifetime.test.ts, "terminate() while dns.lookup() is in flight does not UAF on c-ares channel teardown": a LeakSanitizer report for the node:fs binding box. It also occurs with src/ from main under the debug build (checked at main 4b02e1031d). The suppression in test/leaksan.supp anchors on a frame that is outside the default 30-frame stack in the unoptimized build.
• worker_destruction.test.ts, "bun when Bun.connect is used in a Worker that is terminating": a 5 s timeout in the full run. In three runs of that file alone, 3 to 5 of its 5 tests timed out at 5 s. The host had a load average of about 400 to 500 during those runs. The fixture (worker_thread_check.ts) exits 0 in 6 to 16 s when I run it directly with the merged build, so it is slow here and it does not hang. With src/ from main the file fails in the same way: in 2 interleaved runs each, main gave 2 pass 3 fail and 0 pass 5 fail, and the merged tree gave 0 pass 5 fail twice. The fixture took 8.6 to 17.8 s with main and 12.2 to 14.9 s with the merged tree (4 interleaved runs each, all exit 0, load average 700 to 900). The timeouts do not come from this diff.

CI result: Buildkite build 122697, head b0ac5ed5b9

• All 160 test jobs finished: 159 passed, 1 failed.
• worker-transferred-port-close.test.ts and message-port-pipe.test.ts passed on the first attempt on all 11 lanes, which include debian 13 x64-asan. So did worker_destruction.test.ts, the file that timed out on my machine.
• All 136 worker and MessagePort test files in the run (test/js/web/workers/, test/js/node/worker_threads/, node test-worker* and test-messagechannel*) passed on the first attempt on every lane.
• One test failed: test/js/bun/spawn/spawn.test.ts, "an idle reader stopped at the highwater mark does not keep the process alive (saturating writer)", on debian 13 x64-asan, in all 4 attempts. The child wrote WARNING: ptrace appears to be blocked (is seccomp enabled?). LeakSanitizer may hang. to stderr, and the test expects an empty stderr. This diff does not touch that test.
• That failure is not specific to this branch. Of the last 26 failed builds in the pipeline, 22 have it, on unrelated branches, all created since 2026-10-01 23:14Z. spawn: restart syscalls interrupted by the waiter thread's SIGCHLD handler #43646 (open) describes the cause: a SIGCHLD that interrupts the wait4 of the exit-time LeakSanitizer probe.