Cancel pending tasks in __del__ methods only if possible (#85)

This helps eliminate a lot of warnings when running tests, where
`Select` and `Merge` objects get destroyed after the event loop has
closed.

Author: sahas-subramanian-frequenz

src/frequenz/channels/util/_merge.py

@@ -46,7 +46,8 @@ def __init__(self, *args: Receiver[T]) -> None:
     def __del__(self) -> None:
         """Cleanup any pending tasks."""
         for task in self._pending:
-            task.cancel()
+            if not task.done() and task.get_loop().is_running():
+                task.cancel()
 
     async def stop(self) -> None:
         """Stop the `Merge` instance and cleanup any pending tasks."""

src/frequenz/channels/util/_merge_named.py

@@ -34,7 +34,8 @@ def __init__(self, **kwargs: Receiver[T]) -> None:
     def __del__(self) -> None:
         """Cleanup any pending tasks."""
         for task in self._pending:
-            task.cancel()
+            if not task.done() and task.get_loop().is_running():
+                task.cancel()
 
     async def stop(self) -> None:
         """Stop the `MergeNamed` instance and cleanup any pending tasks."""

src/frequenz/channels/util/_select.py

@@ -108,7 +108,8 @@ def __init__(self, **kwargs: Receiver[Any]) -> None:
     def __del__(self) -> None:
         """Cleanup any pending tasks."""
         for task in self._pending:
-            task.cancel()
+            if not task.done() and task.get_loop().is_running():
+                task.cancel()
 
     async def stop(self) -> None:
         """Stop the `Select` instance and cleanup any pending tasks."""