Rework task exceptions loop. (#755)

The original reason to do this was because mypy was
having a difficult time ensuring that the final raised
exception was valid.  This fixes that problem.

But this also should be much faster.  Before we were iterating
over the list 3 times; once to the collect the exceptions,
once to filter out None, and once to them print them out.
Instead, this just iterates once, dispatching everything
properly.

Signed-off-by: Chris Lalancette <[email protected]>

launch/launch/launch_service.py

@@ -327,14 +327,20 @@ def _on_exception(loop, context):
                         return_when=asyncio.FIRST_COMPLETED
                     )
                     # Propagate exception from completed tasks
-                    completed_tasks_exceptions = [task.exception() for task in completed_tasks]
-                    completed_tasks_exceptions = list(filter(None, completed_tasks_exceptions))
-                    if completed_tasks_exceptions:
-                        self.__logger.debug('An exception was raised in an async action/event')
-                        # in case there is more than one completed_task, log other exceptions
-                        for completed_tasks_exception in completed_tasks_exceptions[1:]:
-                            self.__logger.error(completed_tasks_exception)
-                        raise completed_tasks_exceptions[0]
+                    exception_to_raise = None
+                    for task in completed_tasks:
+                        exc = task.exception()
+                        if exc is None:
+                            continue
+
+                        if exception_to_raise is None:
+                            self.__logger.debug('An exception was raised in an async action/event')
+                            exception_to_raise = exc
+                        else:
+                            self.__logger.error(exc)
+
+                    if exception_to_raise is not None:
+                        raise exception_to_raise
 
                 except KeyboardInterrupt:
                     continue

launch/test/launch/frontend/test_substitutions.py

@@ -21,6 +21,7 @@
 from launch import SomeSubstitutionsType
 from launch import Substitution
 from launch.actions import ExecuteProcess
+from launch.frontend import Parser
 from launch.frontend.expose import expose_substitution
 from launch.frontend.parse_substitution import parse_if_substitutions
 from launch.frontend.parse_substitution import parse_substitution
@@ -317,13 +318,13 @@ def test_parse_if_substitutions():
         parse_if_substitutions(['$(test asd)', 1, 1.0])
 
 
-class MockParser:
+class MockParser(Parser):
 
-    def parse_substitution(self, value: Text) -> SomeSubstitutionsType:
+    def parse_substitution(self, value: Text) -> List[Substitution]:
         return parse_substitution(value)
 
 
-def test_execute_process_parse_cmd_line():
+def test_execute_process_parse_cmd_line() -> None:
     """Test ExecuteProcess._parse_cmd_line."""
     parser = MockParser()