Fixed task group getting cancelled if start() gets cancelled (#717)

This change fixes the problem by special casing the situation where the Future backing `task_status` was cancelled which only happens when the host task is cancelled.

docs/versionhistory.rst

@@ -10,14 +10,19 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
  portals
 - Added ``__slots__`` to ``AsyncResource`` so that child classes can use ``__slots__``
  (`#733 <https://github.com/agronholm/anyio/pull/733>`_; PR by Justin Su)
+- Fixed two bugs with ``TaskGroup.start()`` on asyncio:
+
+ * Fixed erroneous ``RuntimeError: called 'started' twice on the same task status``
+ when cancelling a task in a TaskGroup created with the ``start()`` method before
+ the first checkpoint is reached after calling ``task_status.started()``
+ (`#706 <https://github.com/agronholm/anyio/issues/706>`_; PR by Dominik Schwabe)
+ * Fixed the entire task group being cancelled if a ``TaskGroup.start()`` call gets
+ cancelled (`#685 <https://github.com/agronholm/anyio/issues/685>`_,
+ `#710 <https://github.com/agronholm/anyio/issues/710>`_)
 - Fixed a race condition that caused crashes when multiple event loops of the same
  backend were running in separate threads and simultaneously attempted to use AnyIO for
  their first time (`#425 <https://github.com/agronholm/anyio/issues/425>`_; PR by David
  Jiricek and Ganden Schaffner)
-- Fixed erroneous ``RuntimeError: called 'started' twice on the same task status``
- when cancelling a task in a TaskGroup created with the ``start()`` method before
- the first checkpoint is reached after calling ``task_status.started()``
- (`#706 <https://github.com/agronholm/anyio/issues/706>`_; PR by Dominik Schwabe)
 - Fixed cancellation delivery on asyncio incrementing the wrong cancel scope's
  cancellation counter when cascading a cancel operation to a child scope, thus failing
  to uncancel the host task (`#716 <https://github.com/agronholm/anyio/issues/716>`_)

src/anyio/_backends/_asyncio.py

@@ -714,6 +714,12 @@ def task_done(_task: asyncio.Task) -> None:
  exc = e
 
  if exc is not None:
+ # The future can only be in the cancelled state if the host task was
+ # cancelled, so return immediately instead of adding one more
+ # CancelledError to the exceptions list
+ if task_status_future is not None and task_status_future.cancelled():
+ return
+
  if task_status_future is None or task_status_future.done():
  if not isinstance(exc, CancelledError):
  self._exceptions.append(exc)

tests/test_taskgroups.py

@@ -22,6 +22,7 @@
  get_current_task,
  move_on_after,
  sleep,
+ sleep_forever,
  wait_all_tasks_blocked,
 )
 from anyio.abc import TaskGroup, TaskStatus
@@ -127,7 +128,6 @@ async def test_no_called_started_twice() -> None:
  async def taskfunc(*, task_status: TaskStatus) -> None:
  task_status.started()
 
- # anyio>4.3.0 should not raise "RuntimeError: called 'started' twice on the same task status"
  async with create_task_group() as tg:
  coro = tg.start(taskfunc)
  tg.cancel_scope.cancel()
@@ -196,9 +196,6 @@ async def taskfunc(*, task_status: TaskStatus) -> None:
  assert not finished
 
 
-@pytest.mark.xfail(
- sys.version_info < (3, 9), reason="Requires a way to detect cancellation source"
-)
 @pytest.mark.parametrize("anyio_backend", ["asyncio"])
 async def test_start_native_host_cancelled() -> None:
  started = finished = False
@@ -1360,6 +1357,24 @@ async def wait_cancel() -> None:
  await cancelled.wait()
 
 
+async def test_start_cancels_parent_scope() -> None:
+ """Regression test for #685 / #710."""
+ started: bool = False
+
+ async def in_task_group(task_status: TaskStatus[None]) -> None:
+ nonlocal started
+ started = True
+ await sleep_forever()
+
+ async with create_task_group() as tg:
+ with CancelScope() as inner_scope:
+ inner_scope.cancel()
+ await tg.start(in_task_group)
+
+ assert started
+ assert not tg.cancel_scope.cancel_called
+
+
 class TestTaskStatusTyping:
  """
  These tests do not do anything at run time, but since the test suite is also checked