Stop raising exception on BackgroundService cancell

Cancelling BackgroundService should not raise exception.
We cancell it when we stop actor, after calling `actor.wait()`.

Signed-off-by: Elzbieta Kotulska <elzbieta.kotulska@frequenz.com>

Author: ela-kotulska-frequenz

RELEASE_NOTES.md

@@ -9,3 +9,5 @@
 - Fixed a bug that was preventing power proposals to go through if there once existed some proposals with overlapping component IDs, even if the old proposals have expired.
 
 - Fixed a bug that was causing formulas to fallback to CHPs, when the CHP meters didn't have data.  CHPs are not supported in the data sourcing actor and in the client, so we can't fallback to CHPs.
+
+- Fix a bug that caused BackgroundService to raise exception when actor was stopped.

src/frequenz/sdk/actor/_background_service.py

@@ -6,6 +6,7 @@
 import abc
 import asyncio
 import collections.abc
+import logging
 from types import TracebackType
 from typing import Any, Self
 
@@ -251,6 +252,8 @@ async def wait(self) -> None:
                     # This will raise a CancelledError if the task was cancelled or any
                     # other exception if the task raised one.
                     _ = task.result()
+                except asyncio.CancelledError:
+                    logging.debug("Task %s has been cancelled", self.name)
                 except BaseException as error:  # pylint: disable=broad-except
                     exceptions.append(error)
             if exceptions:

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]: Finished normally."),
         (*RUN_INFO, "All 1 actor(s) finished."),
     ]