Fixed TaskGroup.start() cancellation on asyncio

Fixes #517.

docs/versionhistory.rst

@@ -45,6 +45,7 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 - Fixed ``MemoryObjectReceiveStream.receive()`` causing the receiving task on asyncio to
   remain in a cancelled state if the operation was cancelled after an item was queued to
   be received by the task (but before the task could actually receive the item)
+- Fixed ``TaskGroup.start()`` on asyncio not responding to cancellation from the outside
 - Removed unnecessary extra waiting cycle in ``Event.wait()`` on asyncio in the case
   where the event was not yet set
 

src/anyio/_backends/_asyncio.py

@@ -751,12 +751,17 @@ async def start(
         # point between, the task group is cancelled and this method never proceeds to
         # process the completed future. That's why we have to have a shielded cancel
         # scope here.
-        with CancelScope(shield=True):
-            try:
-                return await future
-            except CancelledError:
-                task.cancel()
-                raise
+        try:
+            return await future
+        except CancelledError:
+            task.cancel()
+            with CancelScope(shield=True):
+                try:
+                    await task
+                except CancelledError:
+                    pass
+
+            raise
 
 
 #

tests/test_taskgroups.py

@@ -259,11 +259,8 @@ def task_fn(*, task_status: TaskStatus = TASK_STATUS_IGNORED) -> None:
             await tg.start(task_fn)  # type: ignore[arg-type]
 
 
-async def test_start_cancel_after_error(anyio_backend_name: str) -> None:
+async def test_start_cancel_after_error() -> None:
     """Regression test for #517."""
-    if anyio_backend_name == "asyncio":
-        pytest.xfail("Known issue with the asyncio backend")
-
     sleep_completed = False
 
     async def sleep_and_raise() -> None: