Raise a single Cancelled from trio TaskGroups (#593)

docs/versionhistory.rst

@@ -16,6 +16,9 @@ This library adheres to `Semantic Versioning 2.0 <http://semver.org/>`_.
     tasks were spawned and an outer cancellation scope had been cancelled before
   - Ensured that exiting a ``TaskGroup`` always hits a yield point, regardless of
     whether there are running child tasks to be waited on
+  - Task groups on all backends now raise a single cancellation exception when an outer
+    cancel scope is cancelled, and no exceptions other than cancellation exceptions are
+    raised in the group
 - **BACKWARDS INCOMPATIBLE** Changes the pytest plugin to run all tests and fixtures in
   the same task, allowing fixtures to set context variables for tests and other fixtures
 - **BACKWARDS INCOMPATIBLE** Changed ``anyio.Path.relative_to()`` and

src/anyio/_backends/_trio.py

@@ -3,6 +3,7 @@
 import array
 import math
 import socket
+import sys
 import types
 from collections.abc import AsyncIterator, Iterable
 from concurrent.futures import Future
@@ -62,6 +63,9 @@
 from ..abc._eventloop import AsyncBackend
 from ..streams.memory import MemoryObjectSendStream
 
+if sys.version_info < (3, 11):
+    from exceptiongroup import BaseExceptionGroup
+
 T = TypeVar("T")
 T_Retval = TypeVar("T_Retval")
 T_SockAddr = TypeVar("T_SockAddr", str, IPSockAddrType)
@@ -156,6 +160,13 @@ async def __aexit__(
     ) -> bool | None:
         try:
             return await self._nursery_manager.__aexit__(exc_type, exc_val, exc_tb)
+        except BaseExceptionGroup as exc:
+            _, rest = exc.split(trio.Cancelled)
+            if not rest:
+                cancelled_exc = trio.Cancelled._create()  # type: ignore [attr-defined]
+                raise cancelled_exc from exc
+
+            raise
         finally:
             self._active = False
 

tests/test_taskgroups.py

@@ -1129,6 +1129,28 @@ async def main() -> NoReturn:
     assert not caplog.messages
 
 
+async def test_single_cancellation_exc() -> None:
+    """
+    Test that only a single cancellation exception bubbles out of the task group when
+    case it was cancelled via an outer scope and no actual errors were raised.
+
+    """
+    with CancelScope() as outer:
+        try:
+            async with create_task_group() as tg:
+                tg.start_soon(sleep, 5)
+                await wait_all_tasks_blocked()
+                outer.cancel()
+                await sleep(5)
+        except BaseException as exc:
+            if isinstance(exc, get_cancelled_exc_class()):
+                raise
+
+            pytest.fail(f"Raised the wrong type of exception: {exc}")
+        else:
+            pytest.fail("Did not raise a cancellation exception")
+
+
 async def test_start_soon_parent_id() -> None:
     root_task_id = get_current_task().id
     parent_id: int | None = None