Catch concurrent.futures.CancelledError in websocket code. (#12150)

* Catch concurrent.futures.CancelledError in websocket code.

* Added a comment about the use of futures.CancelledError

homeassistant/components/websocket_api.py

@@ -5,6 +5,7 @@ For more details about this component, please refer to the documentation at
 https://home-assistant.io/developers/websocket_api/
 """
 import asyncio
+from concurrent import futures
 from contextlib import suppress
 from functools import partial
 import json
@@ -120,6 +121,11 @@ BASE_COMMAND_MESSAGE_SCHEMA = vol.Schema({
                                   TYPE_PING)
 }, extra=vol.ALLOW_EXTRA)
 
+# Define the possible errors that occur when connections are cancelled.
+# Originally, this was just asyncio.CancelledError, but issue #9546 showed
+# that futures.CancelledErrors can also occur in some situations.
+CANCELLATION_ERRORS = (asyncio.CancelledError, futures.CancelledError)
+
 
 def auth_ok_message():
     """Return an auth_ok message."""
@@ -231,7 +237,7 @@ class ActiveConnection:
     def _writer(self):
         """Write outgoing messages."""
         # Exceptions if Socket disconnected or cancelled by connection handler
-        with suppress(RuntimeError, asyncio.CancelledError):
+        with suppress(RuntimeError, *CANCELLATION_ERRORS):
             while not self.wsock.closed:
                 message = yield from self.to_write.get()
                 if message is None:
@@ -363,7 +369,7 @@ class ActiveConnection:
             self.log_error(msg)
             self._writer_task.cancel()
 
-        except asyncio.CancelledError:
+        except CANCELLATION_ERRORS:
             self.debug("Connection cancelled by server")
 
         except asyncio.QueueFull: