tornado 5 fixes

- ._running private attribute is removed. We don't need it anymore,
  since we were only using it while the application was run in a background thread.
- call blocking cleanup in a thread because asyncio doesn't allow multiple loops in one thread.

jupyterhub/app.py

@@ -1625,9 +1625,6 @@ class JupyterHub(Application):
         if not self.io_loop:
             return
         if self.http_server:
-            if self.io_loop._running:
-                self.io_loop.add_callback(self.http_server.stop)
-            else:
             self.http_server.stop()
         self.io_loop.add_callback(self.io_loop.stop)
 

jupyterhub/tests/mocking.py

@@ -1,5 +1,6 @@
 """mock utilities for testing"""
 
+from concurrent.futures import ThreadPoolExecutor
 import os
 import sys
 from tempfile import NamedTemporaryFile
@@ -229,7 +230,21 @@ class MockHub(JupyterHub):
 
     def stop(self):
         super().stop()
-        IOLoop().run_sync(self.cleanup)
+
+        # run cleanup in a background thread
+        # to avoid multiple eventloops in the same thread errors from asyncio
+
+        def cleanup():
+            loop = IOLoop.current()
+            loop.run_sync(self.cleanup)
+            loop.close()
+
+        pool = ThreadPoolExecutor(1)
+        f = pool.submit(cleanup)
+        # wait for cleanup to finish
+        f.result()
+        pool.shutdown()
+
         # ignore the call that will fire in atexit
         self.cleanup = lambda : None
         self.db_file.close()