Fix UDP socket close hangs on Windows by Guflly · Pull Request #1246 · agronholm/anyio

Guflly · 2026-07-27T07:07:59Z

NOTE Erasing or replacing the contents of this template will result in your pull
request being summarily closed without consideration!

Changes

Fixes #1237.

On Windows Proactor event loops, closing a UDP socket while a datagram send is in flight can leave connection_lost() unscheduled and make aclose() wait indefinitely. Follow the transport close with an event-loop turn and abort(), matching the existing stream cleanup path, so the protocol close event is always delivered.

The regression tests cover connected and unconnected UDP sockets on Windows.

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

You've added tests (in tests/) which would fail without your patch
You've updated the documentation (in docs/), in case of behavior changes or new
features
You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

graingert · 2026-07-27T15:09:31Z

Can you add a test that:

sock = udp.extra(SocketAttribute.raw_socket)
await anyio.aclose_forcefully(udp)
assert sock.fileno() == -1

Guflly · 2026-07-27T21:08:41Z

Added. The test initially left the fd open, so abort now runs in finally and gets a shielded checkpoint. The focused UDP tests pass.

graingert · 2026-07-31T16:09:19Z

Do SocketStreams have the same problem with aclose_forcefully?

for more information, see https://pre-commit.ci

Guflly · 2026-07-31T21:20:40Z

Yes, when a send is in flight. I added the same cleanup pattern and a TCP regression test.

graingert · 2026-08-07T16:39:37Z

        with pytest.raises(ClosedResourceError):
            await stream.send(b"foo")

+    @pytest.mark.parametrize("anyio_backend", asyncio_params)


is there a reason to limit this to asyncio? Surely we should have the same behaviour on asyncio and trio

Suggested change

@pytest.mark.parametrize("anyio_backend", asyncio_params)

graingert · 2026-08-07T16:40:32Z

+    @pytest.mark.skipif(sys.platform != "win32", reason="Windows only")
+    @pytest.mark.parametrize("anyio_backend", asyncio_params)


is there a reason to limit this to asyncio on windows? Surely this should pass on non windows and trio

Suggested change

@pytest.mark.skipif(sys.platform != "win32", reason="Windows only")

@pytest.mark.parametrize("anyio_backend", asyncio_params)

graingert · 2026-08-07T16:41:37Z

            peer.close()

+    @pytest.mark.skipif(sys.platform != "win32", reason="Windows only")
+    @pytest.mark.parametrize("anyio_backend", asyncio_params)


Suggested change

@pytest.mark.parametrize("anyio_backend", asyncio_params)

is there a reason to limit this to asyncio on windows? Surely this should pass on non windows and trio

graingert · 2026-08-07T16:44:28Z

  module name. (The default name for a task spawned with ``TaskGroup.start_soon`` or
  ``TaskGroup.start`` typically includes the module name.)
  (`#1234 <https://github.com/agronholm/anyio/pull/1234>`_; PR by @gschaffner)
+- Fixed UDP socket closing hanging on Windows if a datagram send was still in flight


probably worth something in the news about fixing TCP's aclose_forcefully

graingert · 2026-08-07T17:12:04Z

@@ -1687,6 +1690,11 @@
        self._closed = True
        if not self._transport.is_closing():


I think this is wrong, if aclose_forcefully is called concurrently the second one will return before the socket is closed

graingert · 2026-08-07T17:12:34Z

@@ -1737,6 +1745,11 @@
        self._closed = True
        if not self._transport.is_closing():


I think this is wrong, if aclose_forcefully is called concurrently the second one will return before the socket is closed

graingert · 2026-08-07T17:14:42Z

    async def aclose(self) -> None:
        self._closed = True
        if not self._transport.is_closing():
            try:
                self._transport.write_eof()
            except OSError:
                pass

            self._transport.close()
-            await sleep(0)
-            self._transport.abort()
+            try:
+                await sleep(0)
+            finally:
+                self._transport.abort()
+                await AsyncIOBackend.cancel_shielded_checkpoint()


same here with concurrent aclose_forcefully I think we want this code:

async def aclose(self) -> None: self._closed = True if not self._transport.is_closing(): try: self._transport.write_eof() except OSError: pass self._transport.close() try: await sleep(0) finally: self._transport.abort() await AsyncIOBackend.cancel_shielded_checkpoint()

The previous versions queued sends and then entered a task group, but start_soon() only schedules — the children first run when the host yields at __aexit__, and one event loop iteration is all the proactor needs to reap the overlapped send. close() then took the no-pending-write path and the tests passed on Windows for the wrong reason. anyio puts send()'s checkpoint before transport.sendto(), so the operation is only still in flight if nothing yields afterwards. Drop the pointless 10x send loop (each iteration yielded, so the buffer drained every time) and close directly after a single send, as PR agronholm#1246's own reproducer does. Adds a concurrent variant gated on an Event so the second closer only runs once the first has yielded inside aclose(), which is where its unshielded wait on closed_event is exposed. Neither PR covers that caller. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Fix UDP socket close hangs on Windows

415551d

Fix forced UDP socket closure

57cb4a9

Guflly marked this pull request as ready for review July 27, 2026 21:08

maz and others added 2 commits July 31, 2026 14:20

Fix forced TCP socket closure

51e137e

[pre-commit.ci] auto fixes from pre-commit.com hooks

7928b00

for more information, see https://pre-commit.ci

graingert requested a review from agronholm August 7, 2026 11:12

graingert requested changes Aug 7, 2026

View reviewed changes

graingert mentioned this pull request Aug 12, 2026

concurrent anyio.aclose_forcefully fails to block until socket is closed on asyncio #1273

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2 tasks

graingert-coef mentioned this pull request Aug 13, 2026

CI probe (control): #1246's UDP fix reverted graingert-coef/anyio#2

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agronholm mentioned this pull request Aug 23, 2026

Fixed socket aclose() delayed close, missing checkpoint and Windows hang #1289

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3 tasks

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Fix UDP socket close hangs on Windows - #1246

Fix UDP socket close hangs on Windows#1246
Guflly wants to merge 4 commits into
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Guflly:fix/windows-udp-close

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		@pytest.mark.skipif(sys.platform != "win32", reason="Windows only")
		@pytest.mark.parametrize("anyio_backend", asyncio_params)

		@@ -1687,6 +1690,11 @@
		self._closed = True
		if not self._transport.is_closing():

		@@ -1737,6 +1745,11 @@
		self._closed = True
		if not self._transport.is_closing():

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Guflly commented Jul 27, 2026

Changes

Checklist

Updating the changelog

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