Force legacy ws subprotocol when using gateway (#1311)

Co-authored-by: Emmanuel Pignot <[email protected]>
Co-authored-by: Zachary Sailer <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Zachary Sailer <[email protected]>

jupyter_server/gateway/connections.py

@@ -13,7 +13,7 @@
 from tornado.escape import json_decode, url_escape, utf8
 from tornado.httpclient import HTTPRequest
 from tornado.ioloop import IOLoop
-from traitlets import Bool, Instance, Int
+from traitlets import Bool, Instance, Int, Unicode
 
 from ..services.kernels.connection.base import BaseKernelWebsocketConnection
 from ..utils import url_path_join
@@ -31,6 +31,11 @@ class GatewayWebSocketConnection(BaseKernelWebsocketConnection):
 
     retry = Int(0)
 
+    # When opening ws connection to gateway, server already negotiated subprotocol with notebook client.
+    # Same protocol must be used for client and gateway, so legacy ws subprotocol for client is enforced here.
+
+    kernel_ws_protocol = Unicode("", allow_none=True, config=True)
+
     async def connect(self):
         """Connect to the socket."""
         # websocket is initialized before connection