`python -m websockets` blocks until open_timeout when connection fails

[aaugustin]

$ python -m websockets wss://.../
^C
Traceback (most recent call last):
  File "/Users/myk/dev/websockets/src/websockets/sync/client.py", line 378, in connect
    connection.handshake(
    ~~~~~~~~~~~~~~~~~~~~^
        additional_headers,
        ^^^^^^^^^^^^^^^^^^^
        user_agent_header,
        ^^^^^^^^^^^^^^^^^^
        deadline.timeout(),
        ^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/myk/dev/websockets/src/websockets/sync/client.py", line 101, in handshake
    raise self.protocol.handshake_exc
  File "/Users/myk/dev/websockets/src/websockets/client.py", line 325, in parse
    self.process_response(response)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/Users/myk/dev/websockets/src/websockets/client.py", line 142, in process_response
    raise InvalidStatus(response)
websockets.exceptions.InvalidStatus: server rejected WebSocket connection: HTTP 302

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/myk/dev/websockets/src/websockets/__main__.py", line 159, in <module>
    main()
    ~~~~^^
  File "/Users/myk/dev/websockets/src/websockets/__main__.py", line 131, in main
    websocket = connect(args.uri)
  File "/Users/myk/dev/websockets/src/websockets/sync/client.py", line 385, in connect
    connection.recv_events_thread.join()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/Users/myk/.pyenv/versions/3.13.1/lib/python3.13/threading.py", line 1092, in join
    self._handle.join(timeout)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^
KeyboardInterrupt

Connecting to the same server directly with websockets.sync.client.connect doesn't block.

[aaugustin]

Actually this is a race condition.

When this line:

websockets/src/websockets/sync/connection.py

Line 814 in 8c9f6fc

data = self.socket.recv(self.recv_bufsize)

runs before this line:

websockets/src/websockets/sync/client.py

Line 384 in 8c9f6fc

connection.close_socket()

closing the socket doesn't interrupt recv() because recv() isn't running in the main thread — and interruption is based on signals which are only received by the main thread.

This is an unexpected side effect of migrating python -m websockets from the asyncio to the sync implementation.

[aaugustin]

The main constraint of python -m websockets is that both reading from stdin and from the WebSocket connection must be interruptible. Since input() and the sync implementation (in its current version) are only interruptible by signals, both would need to run in the main thread, which would require running two processes.

The most elegant option could be to go back to an asyncio implementation and do non-blocking read of sys.stdin. This blog post suggests that it's doable — solution at the bottom.

[aaugustin]

Outside of python -m websockets, the problem could still happen for a any connection running in a non-main thread, including all server connections. This requires investigation.