Fixed Condition.wait() not handing over notification when cancelled

Closes #1075.

Author: agronholm

docs/versionhistory.rst

@@ -26,6 +26,8 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 - Fixed cancellation exceptions leaking from a ``CancelScope`` on asyncio when they are
   contained in an exception group alongside non-cancellation exceptions (`#1091
   <https://github.com/agronholm/anyio/issues/1091>`_; PR by @gschaffner)
+- Fixed ``Condition.wait()`` not passing on a notification when the task is cancelled
+  but already received a notification
 
 **4.12.1**
 

src/anyio/_core/_synchronization.py

@@ -335,6 +335,10 @@ async def wait(self) -> None:
         except BaseException:
             if not event.is_set():
                 self._waiters.remove(event)
+            elif self._waiters:
+                # This task was notified by could not act on it, so pass
+                # it on to the next task
+                self._waiters.popleft().set()
 
             raise
         finally:

tests/test_synchronization.py

@@ -2,6 +2,7 @@
 
 import asyncio
 import sys
+from contextlib import AbstractContextManager
 from typing import Any
 
 import pytest
@@ -400,6 +401,24 @@ async def task() -> None:
         assert task_started
         assert not notified
 
+    async def test_notification_handover_on_cancel(self) -> None:
+        condition = Condition()
+
+        async def acquirer(scope: AbstractContextManager[CancelScope]) -> None:
+            with scope:
+                async with condition:
+                    await condition.wait()
+
+        async with create_task_group() as tg:
+            scope1 = CancelScope()
+            scope2 = fail_after(3)
+            tg.start_soon(acquirer, scope1)
+            tg.start_soon(acquirer, scope2)
+            await wait_all_tasks_blocked()
+            async with condition:
+                scope1.cancel()
+                condition.notify(1)
+
     async def test_wait_no_lock(self) -> None:
         condition = Condition()
         with pytest.raises(