Errors in callbacks when running async is not calling _on_error()

[sveinse]

If a Notifier is created using an asyncio loop, any errors that occurs in the receive callbacks will not call the Listener._on_error() callback. It does when not running asyncio.

Looking at Notifier._rx_thread(): When a loop is present it will call Notifier._on_message_received() using the self._loop.call_soon_threadsafe() call. However the except Exception as exc: in line 127 will never be reached, because any errors in the call_soon_threadsafe() callback does not return the exceptions for the function it calls. This in turn implies that when asyncio is enabled, the Listener._on_error() callback won't be called either which is unexpected.

https://github.com/hardbyte/python-can/blob/654a02ae24bfc50bf1bb1fad7aab4aa88763d302/can/notifier.py#L111-L137

The fix would be to either encapsulate Notifier._on_message_received() in an except block and call the _on_error() call back from it. Another solution is to make it call an extra handler when running async:

    def _rx_thread(self, bus: BusABC) -> None:
        # determine message handling callable early, not inside while loop
        if self._loop:
            def rx_handler(msg: Message) -> None:
                try:
                    self._on_message_received(msg)
                except Exception as exc:
                    if not self._on_error(exc):
                        raise
                    else:
                        # It was handled, so only log it
                        logger.debug("suppressed exception: %s", exc)
            handle_message: Callable[[Message], Any] = functools.partial(
                self._loop.call_soon_threadsafe,
                rx_handler,  # type: ignore[arg-type]
            )
        else:
            handle_message = self._on_message_received
        ...

Probably related to #1865

[sveinse]

In addition, exception that occur in async callbacks, are currently not handled due to the loop.create_task() method. If combined with #1938, the process of running an async function in Notifier._on_message_received() could look like this:

def _task_done(task: asyncio.Task) -> None:
    self._tasks.discard(task)
    if (exc := task.exception) is not None:
        self.exception = exc
        if self._loop is not None:
            self._loop.call_soon_threadsafe(self._on_error, exc)

task = self._loop.create_task(res)
self._tasks.add(task)
task.add_done_callback(_task_done)