Async notifier does not handle errors

notifier.py の Notifier._on_message_available と AsyncBufferedReader から始めます。bus.recv() のエラーが _on_error() に届くまでの流れと、待機中の coroutine がメッセージキューをどのように消費するかを追跡します。非同期の受信エラーが listener に届き、待機中の coroutine がブロックされたままにならずに処理でき、かつメッセージの順序が維持されれば完了です。

Greetings! I'm using python-can to talk to my inverter's battery.

Describe the bug

In class Notifier:

    def _on_message_available(self, bus: BusABC) -> None:
        if msg := bus.recv(0):
            self._on_message_received(msg)

When an exception occurs, for example the CAN interface going down or being unplugged, bus.recv() raises but the exception is not handled. The threaded version does call _on_error(), so I guess the async version should too. I fixed it by inheriting the class, but these 3 lines of code would be better in notifier.py.

class Notifier_e (can.Notifier):
    def _on_message_available(self, bus):
        try:
            super()._on_message_available(bus)
        except Exception as exc:
            if not self._on_error(exc):
                # If it was not handled, raise the exception here
                raise

Then _on_error() passes the exception to the listeners, for example AsyncBufferedReader, but it doesn't handle exceptions either: instead the coroutine waiting for messages simply stays stuck forever. So I extended it in the same way:

class AsyncBufferedReader_e( can.AsyncBufferedReader ):
    def on_error( self, exc ):
        self.buffer.put_nowait( exc )

    async def __anext__(self):
        m = await self.buffer.get()
        if isinstance( m, BaseException ):
            raise m
        return m

It puts the exception into the queue with messages, so when the coroutine waiting on these messages gets to the exception, it is raised, and the coroutine can process it. This preserves order between messages and exceptions, so messages received and pushed into the queue before the exception occurs will be processed.

I use it to cleanly disconnect the CAN interface, then connect again when it is plugged back into the USB port.

I'm not sure about Exception vs BaseException. I guess it would be useful to pass KeyboardInterrupt to the waiting coroutine, so it would be terminated in the same way as a Ctrl-C occurring during a blocking recv().

Have a nice day!