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Handle websocket disconnect in event loop

See original GitHub issue

It seems that the starlette.websockets.WebSocketDisconnect exception is not raised when a client disconnects.

This can cause orphaned async tasks as multiple clients connect/disconnect. As someone first referenced on StackOverflow, it can also result in Uvicorn’s shutdown/reload to block.

Example

#!/usr/bin/env python

import asyncio

from starlette.applications import Starlette
from starlette.responses import HTMLResponse

app = Starlette()


@app.route('/')
async def index(req):
    """Return basic websocket connection template at base route"""
    return HTMLResponse('''
        <!DOCTYPE html>
        <html>
            <head>
                <title>WebSocket Example</title>
            </head>
            <body>
                Intentionally empty. Messages echoed to js console.
                <script>
                ws = new WebSocket("ws://localhost:8000/ws");
                ws.onmessage = e => {
                    console.log(e);
                };
                </script>
            </body>
        </html>
        ''')


@app.websocket_route('/ws')
async def ws(websocket):
    await websocket.accept()

    try:
        while True:
            # Expected behavior: print "Looping..." until client
            # disconnects, then close websocket
            print(f'Looping. Client state: {websocket.client_state}')
            # await websocket.send_text('Hello world!')
            await asyncio.sleep(1)
    finally:
        await websocket.close()


if __name__ == '__main__':
    import uvicorn
    uvicorn.run(app, log_level='debug')

After accessing the base route to create a websocket connection, closing the window/connection does not raise starlette.websockets.WebSocketDisconnect, leaves the loop running indefinitely, and does not modify websocket.client_state. Additional connections start additional loops.

If you add the line to send the Hello world! message over the websocket after the client has disconnected, websockets.exceptions.ConnectionClosedOK will be raised and can be handled. However, this likely won’t be a workaround for all use cases.

Issue Analytics

State:
Created 4 years ago
Reactions:6
Comments:11 (6 by maintainers)

Top GitHub Comments

8reactions

euri10commented, Dec 18, 2019

you can subclass WebSocketEndpoint @benfasoli , here’s an example of a dummy counter

@app.websocket_route("/counter")
class WebSocketTicks(WebSocketEndpoint):
    async def on_connect(self, websocket: WebSocket) -> None:
        await websocket.accept()
        self.ticker_task = asyncio.create_task(self.tick(websocket))
        logger.debug("connected")

    async def on_disconnect(self, websocket: WebSocket, close_code: int) -> None:
        self.ticker_task.cancel()
        logger.debug("disconnected")

    async def on_receive(self, websocket: WebSocket, data: typing.Any) -> None:
        await websocket.send_json({"Message: ": data})

    async def tick(self, websocket: WebSocket) -> None:
        counter = 0
        while True:
            logger.debug(counter)
            await websocket.send_json({"counter": counter})
            counter += 1
            await asyncio.sleep(1)

1reaction

euri10commented, Feb 14, 2020

understood , the 40 to 50s you see is exactly what is described here: https://github.com/aaugustin/websockets/blob/3bab7fd155636c73b79b258de752b36687bba347/src/websockets/protocol.py#L803-L810

close_timeout default to 10s so maybe setting it to a lower value might help

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