Errors after user logs out

Hi @yuvipanda, @manics et al:

First of all, this is a great extension! I'm looking to use it for summer schools and a few classes that require running legacy X11 apps.

One UX problem I stumbled upon -- if a user logs out (by hitting 'Applications|Log out' in the XFCE GUI), that shuts down vncserver. When that happens, here's what I see on the console:

Killing Xvnc process ID 44
127.0.0.1 - - [28/Jun/2020 08:12:27] 127.0.0.1:34195: Target closed connection
127.0.0.1 - - [28/Jun/2020 08:12:27] 127.0.0.1:34195: Closed target
[E 08:12:27.250 NotebookApp] Uncaught exception GET /desktop/websockify (172.17.0.1)
    HTTPServerRequest(protocol='http', host='127.0.0.1:8888', method='GET', uri='/desktop/websockify', version='HTTP/1.1', remote_ip='172.17.0.1')
    Traceback (most recent call last):
      File "/opt/conda/lib/python3.7/site-packages/tornado/websocket.py", line 1098, in write_message
        fut = self._write_frame(True, opcode, message, flags=flags)
      File "/opt/conda/lib/python3.7/site-packages/tornado/websocket.py", line 1075, in _write_frame
        return self.stream.write(frame)
      File "/opt/conda/lib/python3.7/site-packages/tornado/iostream.py", line 546, in write
        self._check_closed()
      File "/opt/conda/lib/python3.7/site-packages/tornado/iostream.py", line 1035, in _check_closed
        raise StreamClosedError(real_error=self.error)
    tornado.iostream.StreamClosedError: Stream is closed

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "/opt/conda/lib/python3.7/site-packages/tornado/websocket.py", line 649, in _run_callback
        result = callback(*args, **kwargs)
      File "/opt/conda/lib/python3.7/site-packages/jupyter_server_proxy/handlers.py", line 86, in on_message
        self.ws.write_message(message, binary=isinstance(message, bytes))
      File "/opt/conda/lib/python3.7/site-packages/tornado/websocket.py", line 1506, in write_message
        return self.protocol.write_message(message, binary=binary)
      File "/opt/conda/lib/python3.7/site-packages/tornado/websocket.py", line 1100, in write_message
        raise WebSocketClosedError()
    tornado.websocket.WebSocketClosedError
Wrapped command exited (or daemon). Returned 0
In exit
Closing socket listening at :5901

More problematically, the next time the user tries to start the desktop (by clicking 'New|desktop' in Jupyter), the notebook server seems unaware that vncserver has exited:

[I 08:12:57.125 NotebookApp] Client sent subprotocols: ['binary']
[I 08:12:57.127 NotebookApp] Trying to establish websocket connection to ws://localhost:5901/websockify
[E 08:12:57.134 NotebookApp] Exception in callback functools.partial(<bound method IOLoop._discard_future_result of <tornado.platform.asyncio.AsyncIOMainLoop object at 0x7fad5f24ec90>>, <Task finished coro=<ProxyHandler.proxy_open.<locals>.start_websocket_connection() done, defined at /opt/conda/lib/python3.7/site-packages/jupyter_server_proxy/handlers.py:307> exception=OSError(99, 'Cannot assign requested address')>)
    Traceback (most recent call last):
      File "/opt/conda/lib/python3.7/site-packages/tornado/tcpclient.py", line 143, in on_connect_done
        stream = future.result()
    tornado.iostream.StreamClosedError: Stream is closed

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "/opt/conda/lib/python3.7/site-packages/tornado/ioloop.py", line 743, in _run_callback
        ret = callback()
      File "/opt/conda/lib/python3.7/site-packages/tornado/ioloop.py", line 767, in _discard_future_result
        future.result()
      File "/opt/conda/lib/python3.7/site-packages/jupyter_server_proxy/handlers.py", line 313, in start_websocket_connection
        subprotocols=self.subprotocols)
      File "/opt/conda/lib/python3.7/site-packages/tornado/iostream.py", line 1226, in connect
        self.socket.connect(address)
    OSError: [Errno 99] Cannot assign requested address

(and vnclite just hangs waiting to connect).

Can you tell what's going on here? I have zero experience with coding tornado apps and Jupyter extensions (a lot with scientific python, though), but I'm willing to try to develop a fix it if you can point me in the right direction.

yuvipanda / jupyter-desktop-server

Errors after user logs out #16