raise ConnectionRefusedError() actually raises exception, allows connection

[ryanwc]

When I do as suggested in the docs and raise ConnectionRefusedError('some message') to refuse an unauthenticated connection, all that happens is the Flask logger prints the exception and the connection proceeds as normal. I have no special error handlers installed on the Flask app.

Here is my code:

server:

from flask_socketio import ConnectionRefusedError

from main import app, socketio


@socketio.on("connect")
def connect():
    app.logger.info("got con")
    raise ConnectionRefusedError('unauthorized')

client (javascript):

console.log('creating new socket');
const socket = io('http://localhost:5000');
socket.on("connect_error", (err) => {
  console.log("connect_error");
});
socket.on("connect", () => {
  console.log("connect succeeded");
});

Expected server log output:

[2021-11-13 16:24:21,533] INFO in collaborate: got con

Actual server log output:

[2021-11-13 16:24:21,533] INFO in collaborate: got con
[2021-11-13 16:24:21,533] ERROR in collaborate: socketio exception:
Traceback (most recent call last):
  File "/.../venv/lib/python3.9/site-packages/flask_socketio/__init__.py", line 762, in _handle_event
    ret = handler(auth)
  File "/the_server_file.py", line 14, in connect
    raise ConnectionRefusedError('unauthorized')
socketio.exceptions.ConnectionRefusedError: unauthorized

Expected client log output:

creating new socket
connect_error

Actual client log output:

creating new socket
connect succeeded

And then the client can use the connection.

The setup seems incredibly simple, yet it's not working as expected. Is this a bug? If not, what am I doing wrong?

[ryanwc]

By the way, return False does the expected thing (i.e. client's connect_error handler fires). But ideally I want to use the exception to return a custom message.

[ryanwc]

Aha! I figured it out.

Above, I actually provided minimal code I thought was important. But, I started stepping through the source of flask_socketio with pdb, and noticed self.exception_handlers, into which exception handlers are installed via a decorator in the app's code.

Then I remembered... I had done this:

@socketio.on_error()
def error_handler(err: Exception):
    """Handle any socketio error."""
    app.logger.error(f"socketio exception:\n{traceback.format_exc()}")

I commented that out, and raise ConnectionRefusedError() worked as expected. So that solves my mystery.