threading implementation with `process_request` raise ConnectionClosedError

[Flowrey]

I'm using:

• Python: 3.12
• websockets: 12.0

When using the threading implementation like in this code:

import http
from websockets.sync.server import serve, ServerConnection, Request, Response
from websockets.http11 import datastructures

def health_check(websocket: ServerConnection, request: Request):
    if request.path == "/healthz":
        return Response(http.HTTPStatus.OK, "OK", datastructures.Headers([]), b"OK")
    else:
        return None


def echo(websocket):
    for message in websocket:
        websocket.send(message)

def main():
    with serve(echo, "localhost", 8765, process_request=health_check) as server:
        server.serve_forever()

main()

Any call to the /healthz endpoint works but raise a ConnectionCloseError:

connection handler failed
Traceback (most recent call last):
  File "/home/flowrey/Code/poc/venv/lib64/python3.12/site-packages/websockets/sync/server.py", line 499, in conn_handler
    handler(connection)
  File "/home/flowrey/Code/poc/server.py", line 13, in echo
    for message in websocket:
  File "/home/flowrey/Code/poc/venv/lib64/python3.12/site-packages/websockets/sync/connection.py", line 162, in __iter__
    yield self.recv()
          ^^^^^^^^^^^
  File "/home/flowrey/Code/poc/venv/lib64/python3.12/site-packages/websockets/sync/connection.py", line 201, in recv
    raise self.protocol.close_exc from self.recv_events_exc
websockets.exceptions.ConnectionClosedError: no close frame received or sent

[Mauro Antonino]

I noticed that it is receiving empty bytes as a data response, and this causes the socket to close.

websockets/src/websockets/sync/connection.py

Lines 549 to 550 in e217458

 if data == b"": 
     break 

websockets/src/websockets/sync/connection.py

Lines 614 to 616 in e217458

 finally: 
     # This isn't expected to raise an exception. 
     self.close_socket() 

Another thing is that when the status code is not 101, the state does not end up being OPEN, which then closes the connection as well.

websockets/src/websockets/server.py

Lines 541 to 547 in e217458

 if response.status_code == 101: 
     assert self.state is CONNECTING 
     self.state = OPEN 
 else: 
     self.send_eof() 
     self.parser = self.discard() 
     next(self.parser)  # start coroutine 

websockets/src/websockets/sync/server.py

Lines 143 to 146 in e217458

 if self.protocol.state is not OPEN: 
     self.recv_events_thread.join(self.close_timeout) 
     self.close_socket() 
     self.recv_events_thread.join() 

I have made this PR that solved this error for me, but I don't know if it broke tests or other functinalities
#1440

[Aymeric Augustin]

The change causes tests to hang -- because you removed the code paths that manages the end of a connection.

The issue looks legit but that's not the right fix.

[Mauro Antonino]

I think I figured it out now. The code is handling all connections as WebSocket connections. When it is an HTTP connection and a request is made, it is supposed to close the connection, which differs from a WebSocket, where it is expected to keep the connection alive. I created a checker to verify if it is a WebSocket before handling it.

I have created this new PR with this change:
#1443