Fix task_status future to prevent erroneous RuntimeError (#707)

Fixes #706.

docs/versionhistory.rst

@@ -5,6 +5,10 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
 
 **UNRELEASED**
 
+- 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 erroneous ``TypedAttributeLookupError`` if a typed attribute getter raises
   ``KeyError``
 - Fixed the asyncio backend not respecting the ``PYTHONASYNCIODEBUG`` environment

src/anyio/_backends/_asyncio.py

@@ -620,9 +620,10 @@ def started(self, value: T_contra | None = None) -> None:
         try:
             self._future.set_result(value)
         except asyncio.InvalidStateError:
-            raise RuntimeError(
-                "called 'started' twice on the same task status"
-            ) from None
+            if not self._future.cancelled():
+                raise RuntimeError(
+                    "called 'started' twice on the same task status"
+                ) from None
 
         task = cast(asyncio.Task, current_task())
         _task_states[task].parent_id = self._parent_id

tests/test_taskgroups.py

@@ -123,6 +123,17 @@ async def taskfunc(*, task_status: TaskStatus) -> None:
         assert value is None
 
 
+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()
+        await coro
+
+
 async def test_start_with_value() -> None:
     async def taskfunc(*, task_status: TaskStatus) -> None:
         task_status.started("foo")