ERROR:asyncio:Task exception was never retrieved

[hainm]

hi,
I am trying voila (latest release from conda-forge). It's pretty nice.
The error is not serious but I still see a series of warning/error in terminal:

[Voila] Kernel shutdown: 82a56940-98ac-4280-9376-41f2d80fbe27
ERROR:asyncio:Task exception was never retrieved
future: <Task finished coro=<WebSocketProtocol13.write_message.<locals>.wrapper() done, defined at /scr2/nguyen/miniconda3/lib/python3.7/site-packages/tornado/websocket.py:1102> exception=WebSocketClosedError()>
Traceback (most recent call last):
  File "/scr2/nguyen/miniconda3/lib/python3.7/site-packages/tornado/websocket.py", line 1104, in wrapper
    await fut
tornado.iostream.StreamClosedError: Stream is closed

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/scr2/nguyen/miniconda3/lib/python3.7/site-packages/tornado/websocket.py", line 1106, in wrapper
    raise WebSocketClosedError()
tornado.websocket.WebSocketClosedError
WARNING:tornado.general:Got events for closed stream None

cheers

[maartenbreddels]

Yeah, thansk for opening this issue, I've almost become blind to that warning. My guess is this is an issue upstream in jupyter_server. In any case, we keep this open until fixes.

[jtpio]

This seems to be triggered by:

voila/share/jupyter/voila/templates/default/static/main.js

Line 44 in 919d57d

kernel.shutdown()

The shutdown call doesn't make it to the server all the time (especially with Firefox), which leads to other issues such as memory consumption (as already mentioned in #173, #209, #479).

But when it does, a request to shut down the kernel is sent over Websocket, but the browser tab is closed immediately after, closing the WS connection abruptly.

[maartenbreddels]

So the websocket is closed from the browser side before the shutdown is send, maybe the shutdown call is async?

[jtpio]

Yes it looks like it is:

https://github.com/jupyterlab/jupyterlab/blob/f5e5b0e50873042c101e913c1403aab5c8b9447c/packages/services/src/kernel/default.ts#L456-L465

[jtpio]

We might actually want to dispose the kernel with kernel.dispose() in the beforeunload handler, which should clear the socket properly.

[jtpio]

Opened #525.