[FIXED] Tornado: Task exception was never retrieved

Issue

I'm getting an Exception in a Tornado WebSocket server but It gives no information in the trace to know which line of code or which step in my program it is originating from. I would like to find out so that I try-catch the origin of the exception.

Error Trace: (No mention of any part of my files)

[E 200527 21:07:19 base_events:1608] Task exception was never retrieved
    future: <Task finished coro=<WebSocketProtocol13.write_message.<locals>.wrapper() done, defined at /usr/local/lib/python3.7/site-packages/tornado/websocket.py:1102> exception=WebSocketClosedError()>
    Traceback (most recent call last):
    File "/usr/local/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 "/usr/local/lib/python3.7/site-packages/tornado/websocket.py", line 1106, in wrapper
        raise WebSocketClosedError()
    tornado.websocket.WebSocketClosedError

[E 200527 21:07:19 base_events:1608] Task exception was never retrieved

That Same Group of Traceback repeats over 16 times.

Here is my code:

import tornado.iostream
import tornado.web
import tornado.gen
import tornado.websocket
import asyncio

class SocketHandler(tornado.websocket.WebSocketHandler):
    waiters = set()
    def initialize(self):
        self.client_name = "newly_connected"

    def get_compression_options(self):
        # Non-None enables compression with default options.
        return {}

    def open(self):
        print('connection opened')
        # SocketHandler.waiters.add(self)

    def on_close(self):
        print("CLOSED!", self.client_name)
        try:
            SocketHandler.waiters.remove(self)
        except KeyError:
            print('tried removing new client')

    def check_origin(self, origin):
        # Override the origin check if needed
        return True

    @classmethod
    async def send_updates(cls, message):
        if len(cls.waiters) < 2:
            while True:
                chat = {}
                # Prevent RuntimeError: Set changed size during iteration
                waiters_copy = cls.waiters.copy()
                for waiter in waiters_copy:
                    try:
                        await waiter.write_message(chat)
                    except tornado.websocket.WebSocketClosedError:
                        pass
                    except tornado.iostream.StreamClosedError:
                        pass
                    except Exception as e:
                        print('Exception e:', waiter.client_name)
                        pass

                # sleep a bit
                await asyncio.sleep(0.05)
        else:
            print('broadcast loop already running')

    async def on_message(self, message):
        print("RECEIVED :", message)
        self.client_name = message
        self.first_serve_cache_on_connnect()
        SocketHandler.waiters.add(self)
        await SocketHandler.send_updates(message)

    def first_serve_cache_on_connnect(self):
        print('serving cache on connect')
        temp_calc_results = self.namespace.results
        try:
            self.write_message(temp_calc_results)
        except Exception as e:
            pass

I have tried catching the exceptions that may cause any error while sending messages to the websocket clients but this error still happens when clients connect to the server.

Solution

The message "task exception was never retrieved" is not about a missing try/except block, it's about a missing await. In first_serve_cache_on_connect, you call write_message without await, so first_serve_cache_on_connect is no longer running by the time the exception is raised and it has nowhere to go but the logs.

This is basically harmless, but if you want to clean up your logs, you need to make first_serve_cache_on_connect an async def coroutine, call it with await in on_message, and call write_message with await.

Answered By - Ben Darnell