Revert "Enable ProactorEventLoop on windows for ipykernel"

ipykernel/kernelapp.py

@@ -624,6 +624,43 @@ def configure_tornado_logger(self):
         handler.setFormatter(formatter)
         logger.addHandler(handler)
 
+    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.
+
+        Support for Proactor via a background thread is available in tornado 6.1,
+        but it is still preferable to run the Selector in the main thread
+        instead of the background.
+
+        do this as early as possible to make it a low priority and overridable
+
+        ref: https://github.com/tornadoweb/tornado/issues/2608
+
+        FIXME: if/when tornado supports the defaults in asyncio without threads,
+               remove and bump tornado requirement for py38.
+               Most likely, this will mean a new Python version
+               where asyncio.ProactorEventLoop supports add_reader and friends.
+
+        """
+        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())
+
     def init_pdb(self):
         """Replace pdb with IPython's version that is interruptible.
 
@@ -643,6 +680,7 @@ def init_pdb(self):
     @catch_config_error
     def initialize(self, argv=None):
         """Initialize the application."""
+        self._init_asyncio_patch()
         super().initialize(argv)
         if self.subapp is not None:
             return