wslc: Switch port relay AcceptThread from WaitForMultipleObjects to IO completion ports by benhillis · Pull Request #40042 · microsoft/WSL

benhillis · 2026-03-30T21:19:33Z

Replace the WaitForMultipleObjects-based wait loop in relay::MultiHandleWait with an IO completion port (IOCP), removing the MAXIMUM_WAIT_OBJECTS (64) handle limit for all callers (IORelay, container IO, socket accept, etc.).

Switch the port relay AcceptThread from WaitForMultipleObjects to a direct IOCP, removing the 63-port mapping limit.

relay::MultiHandleWait changes (relay.hpp/cpp)

Add unique_registered_wait RAII type for RegisterWaitForSingleObject handles
Add Register() pure virtual to OverlappedIOHandle, implemented by all subclasses
Handles that support IOCP (pipes, sockets) register directly; handles that don't (console handles, events) use a RegisterWaitForSingleObject bridge
Convert Run() to use GetQueuedCompletionStatus instead of WaitForMultipleObjects
Cancel() posts a key=0 completion to wake Run(); key=0 sets m_cancel on dequeue
Fix member order: m_iocp declared before m_handles for correct destruction

Port relay changes (localhost.cpp)

Associate listen sockets directly with an IOCP via CreateIoCompletionPort
AcceptEx completions go straight to the IOCP — no events or thread pool waits
Stop signal via PostQueuedCompletionStatus key=0
Remove AcceptEvent from PortRelay (no longer needed)
Remove the MAXIMUM_WAIT_OBJECTS port limit check

Copilot

Pull request overview

This PR updates the WSLC localhost port relay to use an I/O completion port (IOCP) for AcceptEx completions instead of WaitForMultipleObjects, removing the 64-handle wait limitation and enabling more than 63 concurrent port mappings.

Changes:

Replace the accept loop with an IOCP-driven AcceptThread and associate each listen socket with a shared completion port.
Remove the relay’s per-port accept event and route both sync/async AcceptEx completions through IOCP.
Update Windows tests to validate mapping/unmapping 100 ports instead of enforcing the prior 63-port limit.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
`src/windows/wslrelay/localhost.cpp`	Switch accept scheduling/completions to IOCP; add stdin `MessageReader` to coordinate exit + message readiness.
`test/windows/WSLCTests.cpp`	Update port-mapping test to validate >63 mappings (now 100).

src/windows/wslrelay/localhost.cpp

test/windows/WSLCTests.cpp

OneBlue

I think this is a good idea, but given that the relay code is not our long term plan, I would recommend making this change at the relay::MultiHandleWait level so it will also solve the 64 handles issue in other places in the code (specifically the container IO thread, which could go over the limit if many Logs() / Exec() calls are made)

If we need to, we can then move the relay to use MultiHandleWait and match the rest of the code

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

src/windows/wslrelay/localhost.cpp

benhillis · 2026-04-01T15:45:54Z

I think this is a good idea, but given that the relay code is not our long term plan, I would recommend making this change at the relay::MultiHandleWait level so it will also solve the 64 handles issue in other places in the code (specifically the container IO thread, which could go over the limit if many Logs() / Exec() calls are made)

If we need to, we can then move the relay to use MultiHandleWait and match the rest of the cod

That's a good call, let me look at that.

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 9 comments.

src/windows/common/relay.cpp

src/windows/wslrelay/localhost.cpp

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

src/windows/wslrelay/localhost.cpp

src/windows/common/relay.cpp

src/windows/wslrelay/localhost.cpp

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

src/windows/common/relay.hpp

src/windows/common/relay.cpp

test/windows/WSLCTests.cpp

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

src/windows/common/relay.cpp

…letion ports Replace the WaitForMultipleObjects-based accept loop in the WSLC port relay with an IO completion port (IOCP). This removes the MAXIMUM_WAIT_OBJECTS (64) handle limit, allowing unlimited port mappings. Changes: - Associate each listen socket with a shared IOCP using the PortRelay pointer as the completion key. - Rewrite AcceptThread to use GetQueuedCompletionStatus instead of WaitForMultipleObjects. On shutdown, cancel all pending I/O and drain completions before returning. - Remove the AcceptEvent from PortRelay (no longer needed with IOCP). - Change ScheduleAccept from bool to void since both sync and async AcceptEx completions are now delivered through the IOCP. - Add a MessageReader that reads stdin on a dedicated thread, allowing the main loop to WaitForMultipleObjects on the exit event and message ready event simultaneously. - Remove the 63-port limit and update tests to validate 100 port mappings.

… AcceptThread - Add InheritHandle() for m_vmTerminatingEvent so the relay child process receives a valid exit event handle via PROC_THREAD_ATTRIBUTE_HANDLE_LIST. - Close parent pipe handles with .reset() instead of .release() so ReadFile in MapRelayPort detects relay process death via pipe EOF. - Move AcceptThread I/O drain into wil::scope_exit so pending accepts are always cancelled even if ScheduleAccept throws during initialization. - Wrap initial ScheduleAccept loop in try/catch to prevent a single port failure from killing the entire accept thread. - Track accept thread liveness with std::atomic<bool> to avoid posting stale IOCP exit signals when the thread has already exited. - Make PortRelay destructor cancel pending I/O instead of __fastfail. - Add <atomic> to precomp.h.

…IO completion ports Replace the WaitForMultipleObjects-based wait loop in relay::MultiHandleWait with an IO completion port (IOCP), removing the MAXIMUM_WAIT_OBJECTS (64) handle limit for all callers. Changes to relay::MultiHandleWait (relay.hpp/cpp): - Add unique_registered_wait RAII type for RegisterWaitForSingleObject handles - Add Register() pure virtual to OverlappedIOHandle, implemented by all subclasses - Handles that support IOCP (pipes, sockets) register directly; handles that don't (console handles, events) use a RegisterWaitForSingleObject bridge - Convert Run() to use GetQueuedCompletionStatus instead of WaitForMultipleObjects - Cancel() posts a key=0 completion to wake Run(); key=0 sets m_cancel on dequeue - Fix member order: m_iocp declared before m_handles for correct destruction Changes to port relay (localhost.cpp): - Replace AcceptThread's WaitForMultipleObjects with a direct IOCP: associate listen sockets via CreateIoCompletionPort, AcceptEx completions go straight to the IOCP, stop signal via PostQueuedCompletionStatus key=0 - Remove the MAXIMUM_WAIT_OBJECTS port limit check - Remove AcceptEvent from PortRelay (no longer needed with direct IOCP)

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

…onModes Two fixes: 1. relay.cpp: Call SetFileCompletionNotificationModes BEFORE CreateIoCompletionPort. If FILE_SKIP_COMPLETION_PORT_ON_SUCCESS is not supported for a handle type (e.g. certain socket types), fall back to event-based mode entirely. Previously the handle was associated with the IOCP without skip-on-success, causing synchronous completions to both be processed inline by Schedule() and queued to the IOCP — a double-processing race that manifests as ERROR_NO_DATA (232) in ContainerLogs/ContainerAttach tests. 2. localhost.cpp: THROW_IF_WIN32_BOOL_FALSE on SetFileCompletionNotificationModes in the port relay AcceptThread. Co-authored-by: Copilot <[email protected]>

ReadHandle, SingleAcceptHandle, and WriteHandle destructors skipped waiting for CancelIoEx completion when RegisteredWithIocp was true. Since OVERLAPPED is a member, this creates a use-after-free race: the kernel may still reference the OVERLAPPED after the destructor frees it. Always wait for the cancellation to complete regardless of IOCP registration. The stale IOCP completion is harmlessly discarded by MultiHandleWait::Run() (handle already removed from m_handles). Co-authored-by: Copilot <[email protected]>

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

src/windows/wslrelay/localhost.cpp

src/windows/common/relay.cpp

…iation SetFileCompletionNotificationModes must be called before CreateIoCompletionPort to prevent a race where a synchronous AcceptEx completion could queue an IOCP packet before the skip mode is enabled, causing double-processing. Co-authored-by: Copilot <[email protected]>

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benhillis commented Mar 30, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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relay::MultiHandleWait changes (relay.hpp/cpp)

Port relay changes (localhost.cpp)

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benhillis commented Apr 1, 2026

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benhillis commented Mar 30, 2026 •

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