Merge pull request #5047 from minrk/py38-tornado

workaround tornado py38 compatibility issue

notebook/notebookapp.py

@@ -1707,8 +1707,42 @@ class NotebookApp(JupyterApp):
             pc = ioloop.PeriodicCallback(self.shutdown_no_activity, 60000)
             pc.start()
 
+    def _init_asyncio_patch(self):
+        """set default asyncio policy to be compatible with tornado
+
+        Tornado 6 (at least) is not compatible with the default
+        asyncio implementation on Windows
+
+        Pick the older SelectorEventLoopPolicy on Windows
+        if the known-incompatible default policy is in use.
+
+        do this as early as possible to make it a low priority and overrideable
+
+        ref: https://github.com/tornadoweb/tornado/issues/2608
+
+        FIXME: if/when tornado supports the defaults in asyncio,
+               remove and bump tornado requirement for py38
+        """
+        if sys.platform.startswith("win") and sys.version_info >= (3, 8):
+            import asyncio
+            try:
+                from asyncio import (
+                    WindowsProactorEventLoopPolicy,
+                    WindowsSelectorEventLoopPolicy,
+                )
+            except ImportError:
+                pass
+                # not affected
+            else:
+                if type(asyncio.get_event_loop_policy()) is WindowsProactorEventLoopPolicy:
+                    # WindowsProactorEventLoopPolicy is not compatible with tornado 6
+                    # fallback to the pre-3.8 default of Selector
+                    asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy())
+
     @catch_config_error
     def initialize(self, argv=None):
+        self._init_asyncio_patch()
+
         super(NotebookApp, self).initialize(argv)
         self.init_logging()
         if self._dispatching:

notebook/tests/launchnotebook.py

@@ -140,9 +140,6 @@ class NotebookTestBase(TestCase):
 
         started = Event()
         def start_thread():
-            if 'asyncio' in sys.modules:
-                import asyncio
-                asyncio.set_event_loop(asyncio.new_event_loop())
             app = cls.notebook = NotebookApp(
                 port=cls.port,
                 port_retries=0,
@@ -156,6 +153,10 @@ class NotebookTestBase(TestCase):
                 allow_root=True,
                 token=cls.token,
             )
+            if 'asyncio' in sys.modules:
+                app._init_asyncio_patch()
+                import asyncio
+                asyncio.set_event_loop(asyncio.new_event_loop())
             # don't register signal handler during tests
             app.init_signal = lambda : None
             # clear log handlers and propagate to root for nose to capture it