Fix asyncio error when opening notebooks

This is a fix for jupyter#6164

`nest_asyncio` must be applied before any async tasks have been created
otherwise there could be tasks queued that are unpatched, and thus do
not respect nested loops. An example of an unpatched task can be seen in
the original issue:

```
<Task pending coro=<HTTP1ServerConnection._server_request_loop() running at /apps/python3/lib/python3.7/site-packages/tornado/http1connection.py:823>
```
which originates from Tornado.

A similar issue was reported in `nest-asyncio`: erdewit/nest_asyncio#22
where the solution is to call `apply` on import so that unpatched tasks
do not get created.

notebook/notebookapp.py

@@ -2092,7 +2092,34 @@ def _init_asyncio_patch(self):
 
         FIXME: if/when tornado supports the defaults in asyncio,
                remove and bump tornado requirement for py38
+
+        With the introduction of the async kernel, the existing sync kernel
+        requires the use of nested loops in order to run code synchronously.
+        This is done in `jupyter_client` using the helper util `run_sync`:
+
+        ref: https://github.com/jupyter/jupyter_client/blob/f453b51eeeff9e905c583b7da3905c0e35cfbdf0/jupyter_client/utils.py#L11
+
+        which creates a new event loop and relies on `nest_asyncio` patching
+        to allow nested loops. This requires that *all* potential tasks are
+        patched before executing. When only some tasks are patched it leads to
+        the following issue:
+
+        ref: https://github.com/jupyter/notebook/issues/6164
+
+        So we must call `nest_asyncio.apply()` method as early as possible. It
+        is preferable to do this in the consuming application rather than the
+        `jupyter_client` as it is a global patch and would impact all consumers
+        rather than just the ones that rely on synchronous kernel behavior.
         """
+        import nest_asyncio
+
+        try:
+            nest_asyncio.apply()
+        except RuntimeError:
+            # nest_asyncio requires a running loop in order to patch.
+            # In tests the loop may not have been created yet.
+            pass
+
         if sys.platform.startswith("win") and sys.version_info >= (3, 8):
             import asyncio
             try:

notebook/tests/launchnotebook.py

@@ -167,10 +167,16 @@ def start_thread():
                     token=cls.token,
                     **bind_args
                 )
-                if 'asyncio' in sys.modules:
-                          app._init_asyncio_patch()
-                          import asyncio
-                          asyncio.set_event_loop(asyncio.new_event_loop())
+                if "asyncio" in sys.modules:
+                    app._init_asyncio_patch()
+                    import asyncio
+
+                    asyncio.set_event_loop(asyncio.new_event_loop())
+                    # Patch the current loop in order to match production
+                    # behavior
+                    import nest_asyncio
+
+                    nest_asyncio.apply()
                 # don't register signal handler during tests
                 app.init_signal = lambda : None
                 # clear log handlers and propagate to root for nose to capture it

setup.py

@@ -121,6 +121,7 @@
         'jupyter_client>=5.3.4',
         'nbformat',
         'nbconvert',
+        'nest-asyncio>=1.5',
         'ipykernel', # bless IPython kernel for now
         'Send2Trash>=1.8.0',
         'terminado>=0.8.3',