enable a way to run a task when an io_loop is created

jupyter_server/pytest_plugin.py

@@ -336,7 +336,7 @@ def client_fetch(*parts, headers={}, params={}, **kwargs):
 
 
 @pytest.fixture
-def jp_ws_fetch(jp_serverapp, jp_auth_header, jp_http_port, jp_base_url):
+def jp_ws_fetch(jp_serverapp, http_server_client, jp_auth_header, jp_http_port, jp_base_url):
     """Sends a websocket request to a test server.
 
     The fixture is a factory; it can be called like

jupyter_server/serverapp.py

@@ -2052,6 +2052,7 @@ def initialize(self, argv=None, find_extensions=True, new_httpserver=True, start
         self.init_webapp()
         self.init_terminals()
         self.init_signal()
+        self.init_ioloop()
         self.load_server_extensions()
         self.init_mime_overrides()
         self.init_shutdown_no_activity()
@@ -2331,7 +2332,6 @@ def _cleanup(self):
 
     def start_ioloop(self):
         """Start the IO Loop."""
-        self.io_loop = ioloop.IOLoop.current()
         if sys.platform.startswith('win'):
             # add no-op to wake every 5s
             # to handle signals that may be ignored by the inner loop
@@ -2344,6 +2344,10 @@ def start_ioloop(self):
         finally:
             self._cleanup()
 
+    def init_ioloop(self):
+        """init self.io_loop so that an extension can use it by io_loop.call_later() to create background tasks"""
+        self.io_loop = ioloop.IOLoop.current()
+
     def start(self):
         """ Start the Jupyter server app, after initialization