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…cess holds (Windows)
A listening socket sent to another process (child.send(msg, server),
cluster SCHED_NONE and TLS workers, listen({ fd })) is open in both
processes, and both wake up for each connection. On Windows, accept()
in the process that does not get the connection waits for the next
one, although the socket is non-blocking. That process's event loop
stops. test-child-process-fork-net-server.js timed out for this reason.
A listener is now marked shared when it is sent over IPC and when it is
made from a descriptor. On Windows, bsd_accept_socket accepts on a
shared listener with AcceptEx. A request that finds no connection is
canceled, and the call returns WSAEWOULDBLOCK. Other listeners, and
POSIX, keep accept().
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Status: the diff is ready for a maintainer. The tests for this change pass on every lane. CI is red on tests that this change does not reach. CI on ee286d1 (build 110122):
How I reproduced it (Windows Server 2019 x64, 16 cores):
With this change, the CI test passed 3000 of 3000 runs in the same loop. |
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Updated 3:47 AM PT - Sep 4th, 2026
⏳ @robobun, your commit ee286d1 is still building in
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info⚙️ Run configurationConfiguration used: Path: .coderabbit.yaml Review profile: ASSERTIVE Plan: Essentials Run ID: 📒 Files selected for processing (2)
Included review availability: Your plan provides up to 10 included reviews per hour; 7 remain after this review. WalkthroughChangesThe listener API now tracks shared descriptors. Windows shared listeners use Shared listener acceptance
Suggested reviewers: Merge Risk: ⚪ Minimal · up to Transferred listeners are marked shared before descriptor duplication, enabling the Windows shared-listener path without identified merge-blocking risk. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Actionable comments posted: 1
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Inline comments:
In `@packages/bun-usockets/src/bsd.c`:
- Around line 839-840: Cache the AcceptEx function pointer in listener-owned
state for shared listeners, so bsd_accept_shared_socket() reuses it instead of
calling WSAIoctl before every AcceptEx invocation. Initialize or refresh the
cache when needed, preserve the existing accept() fallback when pointer lookup
fails, and ensure non-shared listener behavior remains unchanged.
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packages/bun-usockets/src/bsd.cpackages/bun-usockets/src/context.cpackages/bun-usockets/src/internal/internal.hpackages/bun-usockets/src/internal/networking/bsd.hpackages/bun-usockets/src/libusockets.hpackages/bun-usockets/src/loop.csrc/runtime/ipc_host.rssrc/uws_sys/ListenSocket.rstest/js/node/child_process/child_process_ipc_handle.test.ts
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I reviewed this PR and didn't find any bugs. Because it introduces a new Windows-only overlapped-I/O accept path (AcceptEx + CancelIoEx + hEvent low-bit trick + stack-resident OVERLAPPED wait) in the event-loop-critical bsd_accept_socket, a human reviewer familiar with Winsock should still take a look.
What was reviewed:
- Resource handling in
bsd_accept_shared_socket:acceptedandeventare released on every exit path;WSASetLastErroris restored afterclosesocketso the caller sees the real error. - Stack safety of the pending AcceptEx:
WaitForSingleObjectruns afterCancelIoExbefore returning, sooverlapped/addresseson the stack outlive the request. - Flag plumbing:
sharedis initialized to 0 inus_internal_init_listen_socket, set on both the sender (do_send) and adopter (us_socket_group_listen_fd), and threaded through bothbsd_accept_socketcall sites inloop.c; POSIX path is unchanged. - Test: Windows-gated, uses
port: 0, awaits observable events (IPC round-trips) rather than sleeps, and asserts the combined-object beforeexitCode.
Extended reasoning...
Overview
This PR fixes a Windows-specific event-loop hang when a TCP listening socket is shared across processes via IPC handle passing. It adds an unsigned char shared field to us_listen_socket_t, sets it in the two places a listener becomes cross-process (adopted from an fd in us_socket_group_listen_fd, or before being sent over IPC via a new us_listen_socket_set_shared C API called from ipc_host::do_send), threads it as a new parameter through bsd_accept_socket and its two call sites in loop.c, and on Windows routes shared-socket accepts through a new bsd_accept_shared_socket that uses select(0-timeout) → AcceptEx → CancelIoEx → wait, mapping WSA_OPERATION_ABORTED to WSAEWOULDBLOCK. A Windows-only test in child_process_ipc_handle.test.ts proves both processes stay responsive while sharing a listener across 100 sequential connections.
Security risks
None identified. The change is confined to how an already-authorized listening socket is drained on Windows; no new inputs are parsed, no auth or crypto is touched, and the POSIX path is byte-for-byte unchanged. The select() fast-path and SO_UPDATE_ACCEPT_CONTEXT are standard Winsock idioms; the getpeername fallback to AF_UNSPEC on a peer-reset connection matches how the existing macOS addr->len == 0 case is tolerated downstream.
Level of scrutiny
High. This is hand-written Windows overlapped I/O in the accept hot path, with several sharp edges that must all be right at once: the low bit set on hEvent to keep the completion off any IOCP the listening fd may be associated with (via uv_poll), the mandatory wait after CancelIoEx so the kernel's write to the stack-resident OVERLAPPED/address buffer completes before the frame is torn down, and error preservation across closesocket. The code is #ifdef _WIN32-gated and the test is skipIf(!isWindows), so neither is exercised on Linux/macOS CI — the Cross-platform section of the review guidance applies directly. The PR description is unusually thorough (standalone C repro, 20/20 hang → 30/30 pass, CI flake counts before/after), which raises confidence, but the correctness of the AcceptEx dance is the kind of thing a Windows-networking-literate human should sign off on.
Other factors
The struct-field addition is initialized at the single shared init site, so no uninitialized-read risk on existing listen paths. The Rust FFI addition follows the neighboring safe fn pattern in ListenSocket.rs. The test follows harness conventions (tempDir, bunExe/bunEnv, port: 0, await using, combined-object assertion before exit code, no sleeps — progress is driven by IPC acks). No CODEOWNERS entries cover the changed paths. Exit reason was dry_streak, so the hunt ran to completion without findings.
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Problem
test/js/node/test/parallel/test-child-process-fork-net-server.jstimes out on Windows 2019 x64 (builds 109470, 109572, 109981, 110028). In each hang I reproduced, one process is parked inWS2_32!accept<-bsd_accept_socket.net.Serverto the child. Since cluster: port Node's cluster and child_process handle-passing suites (+43 upstream tests; cluster 54 → 85) and implement what they expose — round-robin fd handoff, SCHED_NONE shared handles, UDP clustering, IPC handle passing #31829 both processes hold the listening socket, and both wake up for each connection. The process that does not get the connection callsaccept(), which waits for the next connection although the socket is non-blocking. Its event loop stops.Fix
do_send(src/runtime/ipc_host.rs). Every listener made from a descriptor (us_socket_group_listen_fd) is marked too. On Windows that covers the child here, clusterSCHED_NONEworkers, and TLS cluster workers.bsd_accept_socketaccepts on a shared socket withAcceptEx. A request that finds no connection is canceled, and the call returnsWSAEWOULDBLOCK. Other sockets, and POSIX, keepaccept().child_process_ipc_handle.test.tshangs 20 of 20 runs without the fix and passes 30 of 30 with it. The CI test: 30 timeouts in 3000 runs on the canary, 0 with the fix.test-cluster-shared-leak.js: 12 in 1800, then 0.Background
child.send(msg, server)gives the child a working copy of the server. On Windows the copy comes fromWSADuplicateSocketW.uv_poll, then callsaccept(). That is safe when one process holds the socket.AcceptExis the overlapped accept. libuv accepts with it.Notes
Cause. On Windows,
accept()on a non-blocking listening socket returnsWSAEWOULDBLOCKwhen no connection is pending. When a connection is pending and another process takes it first,accept()waits for the next connection. A standalone C program shows this with two handles of one listening socket, two threads, and 1000 connections:accept()waits on 100 of them, andAcceptExwithCancelIoExon 0 of them. In the hangs, the native stack of the stopped process isNtWaitForSingleObject<-mswsock<-WSAAccept<-accept<-uv_run. The other process is idle inGetQueuedCompletionStatusEx.Culprit. #31829 added handle passing on Windows and this test. The test was flaky since then, and each failure passed on a retry. #41204 stopped the retries for files that
test/flaky-tests.txtdoes not list, so the test is now red.The AcceptEx path. A
select()with a zero timeout runs first, so that no socket is created when nothing is pending. A queued connection is returned by the firstAcceptEx(2000 of 2000 in the C program, for TCP and for AF_UNIX). After the cancel, the function waits on the event, because the request writes to the stack. The low bit ofhEventkeeps the completion off a completion port.Reach.
us_socket_group_listen_fdmarks every adopted descriptor. On Windows this moves clusterSCHED_NONEworkers, TLS cluster workers (a shared-only handle), andlisten({ fd })to theAcceptExpath. Measured on Windows x64 with a release build:test-cluster-shared-leak.js(SCHED_NONE): 12 timeouts in 1800 runs on the canary, 0 in 1800 with the fix.Related PRs. #37815 and #37896 stop cluster workers from sharing a socket on Windows.
child.send(msg, server)andlisten({ fd })must share the socket, as in Node, so this change is in the accept path. The early handle ack that #37815 also fixes is a separate cause. It can still stop aSCHED_NONEworker under load, sotest-cluster-shared-leak.jsstays intest/flaky-tests.txt.Not changed. Two processes that poll one socket cancel each other's exclusive AFD polls in libuv, so both use CPU while idle (about 1 s per 3 s each). That is a separate problem.
The new test. A third process makes the connections, so the two processes that accept are idle when a connection arrives. The test also asserts that each process accepts at least one connection. In 430 runs on Windows, each process accepted between 29 and 71 of the 100.
Test runs. Windows x64 (release and debug) and Linux x64 (debug, ASAN): 139 node tests (
test-child-process-fork*,send*,pass*,ipc*,internal,test-cluster-*,test-net-server*,test-net-listen*),cluster.test.ts,child_process.test.ts,child_process_ipc_handle.test.ts,spawn.ipc*.test.ts,node-net-server.test.ts.no test proof · iteration 2 · 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