Websocket `_send_heartbeat` raises AttributeError

[pjknkda]

Long story short

Websocket heartbeat sender task raises AttributeError when connecting with slow clients.

class WebSocketResponse(StreamResponse):
    ...

    def _reset_heartbeat(self) -> None:
        self._cancel_heartbeat()

        if self._heartbeat is not None:
            self._heartbeat_cb = call_later(
                self._send_heartbeat, self._heartbeat, self._loop)

    def _send_heartbeat(self) -> None:
        if self._heartbeat is not None and not self._closed:
            # fire-and-forget a task is not perfect but maybe ok for
            # sending ping. Otherwise we need a long-living heartbeat
            # task in the class.
            self._loop.create_task(self._writer.ping())  # type: ignore
            ...

    async def prepare(self, request: BaseRequest) -> AbstractStreamWriter:
        ...
        protocol, writer = self._pre_start(request)
        payload_writer = await super().prepare(request)
        assert payload_writer is not None
        self._post_start(request, protocol, writer)
        ...

    def _pre_start(self, request: BaseRequest) -> Tuple[str, WebSocketWriter]:
        self._loop = request._loop

        headers, protocol, compress, notakeover = self._handshake(
            request)

        self._reset_heartbeat()
        ...

    def _post_start(self, request: BaseRequest,
                    protocol: str, writer: WebSocketWriter) -> None:
        self._ws_protocol = protocol
        self._writer = writer
        ...

I guess that the exception is caused by the race condition in the following steps

1. In WebSocketResponse._pre_start, WebSocketResponse._reset_heartbeat is called and WebSocketResponse._send_heartbeat is scheduled to be executed.
2. Because await super().prepare can take a long time, even super().prepare task is not finished, WebSocketResponse._send_heartbeat can be executed.
3. However, self._writer which is used by WebSocketResponse._send_heartbeat is not initialized yet because self._post_start is not called yet.

Expected behaviour

The exception should not be raised.

Actual behaviour

Exception in callback <bound method WebSocketResponse._send_heartbeat of <WebSocketResponse Switching Protocols ...>>
handle: <TimerHandle WebSocketResponse._send_heartbeat>
Traceback (most recent call last):
  File "uvloop/cbhandles.pyx", line 265, in uvloop.loop.TimerHandle._run
  File "/usr/local/lib/python3.7/site-packages/aiohttp/web_ws.py", line 101, in _send_heartbeat
    self._loop.create_task(self._writer.ping())  # type: ignore
AttributeError: 'NoneType' object has no attribute 'ping'

Steps to reproduce

1. Set a very short websocket heartbeat interval.
2. Insert a random async delay (which is larger than the heartbeat interval) into StreamResponse.prepare to simulate slow client.
3. Connect a websocket client to the server.

Your environment

• aiohttp 3.6.0
• uvloop 0.13.0
• python 3.7.1 on linux

[asvetlov]

Interesting, thanks for the report.

[erik-megarad]

I'm also seeing this. The issue is that self._reset_heartbeat() is being called during _pre_start, which is before the writer is set. This error will occur if the writer isn't set before the first heartbeat is sent.

[asvetlov]

Fixed by #4323