Handle rejected handshakes cleanly in the threading server

[apoorvdarshan]

Summary

In the :mod:threading server, when process_request or process_response rejects the opening handshake by returning an HTTP response, handshake() returns normally and handshake_exc stays None (it's only set for failures, not deliberate rejections). conn_handler then reached:

assert connection.protocol.state is OPEN

After a rejection the state is CONNECTING or CLOSED, never OPEN, so the assertion failed. The resulting AssertionError was swallowed by the surrounding except Exception: block, which closed the raw socket without going through close_socket() or joining recv_events_thread. Nothing was logged, so the failure was silent.

With python -O, the assertion was stripped instead, so the connection handler ran on the already-closed connection: start_keepalive() and handler(connection) executed against a CLOSED connection and every rejection logged "connection handler failed".

This is the exact case the asyncio server already handles explicitly (asyncio/server.py), so the two implementations had diverged.

Fix

Replace the assertion with an explicit branch that takes the rejection/close path cleanly, mirroring the asyncio server and using the threading server's own cleanup idiom:

if connection.protocol.state is not OPEN:
    # process_request or process_response rejected the handshake.
    # The connection is already closing or closed.
    connection.close_socket()
    connection.recv_events_thread.join()
    return

This also makes the # pragma: no cover on the outer except Exception: accurate again: that branch no longer runs on every rejection.

Tests / Verification

• Guarded tests/sync/test_server.py::test_process_request_returns_response with self.assertNoLogs("websockets", logging.ERROR), matching its asyncio counterpart. On the unpatched server this fails under python -O (the handler runs on the rejected connection and logs an error); with the fix it passes under both python and python -O.
• python -m unittest tests.sync.test_server passes (50 tests) under both python and python -O.
• tests.sync.test_client, tests.sync.test_router, and tests.asyncio.test_server remain green.
• Coverage of src/websockets/sync/server.py stays at 100% (the new branch is exercised by the rejection and success paths).
• ruff format --check, ruff check, and mypy --strict are clean.
• Added a changelog entry under 16.2.

Disclosure: prepared with AI assistance; reviewed and verified locally.