Stop raising exception when actor is stopped.

BacgroundService raises BaseExceptionGroup.
We can check if it was cancelled by checking
if all exceptions on BaseGroupException are CancelledError.

Signed-off-by: Elzbieta Kotulska <[email protected]>

Author: ela-kotulska-frequenz

RELEASE_NOTES.md

@@ -17,3 +17,5 @@
 ## Bug Fixes
 
 - Fixed bug with formulas raising exception when stopped.
+
+- Fix a bug that raised `CancelledError` when actor was started with `frequenz.sdk.actor.run` and stopped.

src/frequenz/sdk/actor/_run_utils.py

@@ -38,18 +38,22 @@ async def run(*actors: Actor) -> None:
         done_tasks, pending_tasks = await asyncio.wait(
             pending_tasks, return_when=asyncio.FIRST_COMPLETED
         )
-
         # This should always be only one task, but we handle many for extra safety
         for task in done_tasks:
-            # Cancellation needs to be checked first, otherwise the other methods
-            # could raise a CancelledError
-            if task.cancelled():
+            # BackgroundService returns a BaseExceptionGroup containing multiple
+            # exceptions. The 'await' statement raises these exceptions, and 'except*'
+            # is used to handle them as a group. If the task raises multiple different
+            # exceptions, 'except*' will be invoked multiple times, once for each
+            # exception group.
+            try:
+                await task
+            except* asyncio.CancelledError:
                 _logger.info("Actor %s: Cancelled while running.", task.get_name())
-            elif exception := task.exception():
+            except* BaseException as err:
                 _logger.error(
                     "Actor %s: Raised an exception while running.",
                     task.get_name(),
-                    exc_info=exception,
+                    exc_info=err,
                 )
             else:
                 _logger.info("Actor %s: Finished normally.", task.get_name())

tests/actor/test_actor.py

@@ -390,6 +390,6 @@ async def cancel_actor() -> None:
         (*RUN_INFO, "Actor EchoActor[EchoActor]: Starting..."),
         (*ACTOR_INFO, "Actor EchoActor[EchoActor]: Started."),
         (*ACTOR_INFO, "Actor EchoActor[EchoActor]: Cancelled."),
-        (*RUN_ERROR, "Actor EchoActor[EchoActor]: Raised an exception while running."),
+        (*RUN_INFO, "Actor EchoActor[EchoActor]: Cancelled while running."),
         (*RUN_INFO, "All 1 actor(s) finished."),
     ]