Handle rejected handshakes cleanly in the threading server

docs/project/changelog.rst

@@ -33,6 +33,13 @@ notice.
 
 *In development*
 
+Bug fixes
+.........
+
+* Handled ``process_request`` or ``process_response`` rejecting the opening
+  handshake cleanly in the :mod:`threading` implementation, instead of raising
+  a silenced :exc:`AssertionError`.
+
 .. _16.1:
 
 16.1

src/websockets/sync/server.py

@@ -605,7 +605,13 @@ def protocol_select_subprotocol(
                 connection.recv_events_thread.join()
                 return
 
-            assert connection.protocol.state is OPEN
+            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
+
             try:
                 connection.start_keepalive()
                 handler(connection)

tests/sync/test_server.py

@@ -151,14 +151,15 @@ def process_request(ws, request):
         def handler(ws):
             self.fail("handler must not run")
 
-        with run_server(handler, process_request=process_request) as server:
-            with self.assertRaises(InvalidStatus) as raised:
-                with connect(get_uri(server)):
-                    self.fail("did not raise")
-            self.assertEqual(
-                str(raised.exception),
-                "server rejected WebSocket connection: HTTP 403",
-            )
+        with self.assertNoLogs("websockets", logging.ERROR):
+            with run_server(handler, process_request=process_request) as server:
+                with self.assertRaises(InvalidStatus) as raised:
+                    with connect(get_uri(server)):
+                        self.fail("did not raise")
+                self.assertEqual(
+                    str(raised.exception),
+                    "server rejected WebSocket connection: HTTP 403",
+                )
 
     def test_process_request_raises_exception(self):
         """Server returns an error if process_request raises an exception."""