"Invalid frame header" error in browser

[ekw]

First off, the example echo server example works for me. But when I try to modify it for my needs, I get an "Invalid frame header" error in browser at the end of the connection. Instead of a forever loop as in the echo example, I want to make a connection, send data, receive data and end the connection. I can send and receive data fine, but always get an "Invalid frame header" error in the browser at the end of the connection. My websocket endpoint is this:

@sock.route('/ws')
def ws(sock):
    data = sock.receive()  
    app.logger.info(data)   # "HI" from client received fine
    sock.send(1)  
    sock.send(2)
    sock.send(3)
    sock.close(message="BYE") # Error occurs in browser with or without this close()

On the client side:

const socket = new WebSocket('wss://myhost/ws');
socket.addEventListener('message', ev => { console.log(ev); })
socket.onopen = () => socket.send("HI");

I see the "HI" on the server, and I see all the integer data on the client. But the browser gets an error after the integer data is sent. It doesn't matter how many sends I do from the server, all the data is received by the client fine. It's only at the end that I always get an error on the browser.

On Chrome, the error is "Invalid frame header". On Firefox, the error is "The connection to wss://host/ws was interrupted while the page was loading."

I am using:
Python 3.8.10
gunicorn 20.1.0
gevent 21.12.0
nginx 1.18.0

gunicorn is being proxied by nginx. (As I'm detailing these versions, I now wonder if it's gunicorn/nginx not closing the connection properly.)

Does anyone know how to prevent the error from occurring?

edit: I took nginx and wss out of the equation and the error still occurs.

[miguelgrinberg]

I'm not sure if this can be fixed. The issue happens because Flask, which does not recognize WebSocket requests, tries to issue a standard HTTP response when the connection ends.

[ekw]

OK, thanks for looking into it. Do the reason and message parameters in socket close function do anything at the moment (accessible by client)?

[miguelgrinberg]

I've made some improvements. Could you please test the main branch of this repo and report back?

Here is a summary:

• If you use the eventlet or gunicorn servers, closing the connection works cleanly.
• If you use the gevent server, closing the connection works cleanly from the browser's side, but the server shows a stack trace. I will need to spend more time investigating if that can be addressed.
• If you use the werkzeug/Flask server, closing the connection remains broken. This requires fixes for Flask and/or Werkzeug, which do not know how to end a WebSocket request properly.

[ekw]

It works, thank you! I am using gunicorn proxied by nginx.