Add a test for the graceful CancelledError handling. The user app should _never_ see a CancelledError bubble up, nor should they be able to catch it, because the response is already sent at that point.

tests/test_response_timeout.py

@@ -7,6 +7,7 @@ from sanic.config import Config
 Config.RESPONSE_TIMEOUT = 1
 response_timeout_app = Sanic('test_response_timeout')
 response_timeout_default_app = Sanic('test_response_timeout_default')
+response_handler_cancelled_app = Sanic('test_response_handler_cancelled')
 
 
 @response_timeout_app.route('/1')
@@ -36,3 +37,29 @@ def test_default_server_error_response_timeout():
     request, response = response_timeout_default_app.test_client.get('/1')
     assert response.status == 503
     assert response.text == 'Error: Response Timeout'
+
+
+response_handler_cancelled_app.flag = False
+
+
+@response_handler_cancelled_app.exception(asyncio.CancelledError)
+def handler_cancelled(request, exception):
+    # If we get a CancelledError, it means sanic has already sent a response,
+    # we should not ever have to handle a CancelledError.
+    response_handler_cancelled_app.flag = True
+    return text("App received CancelledError!", 500)
+    # The client will never receive this response, because the socket
+    # is already closed when we get a CancelledError.
+
+
+@response_handler_cancelled_app.route('/1')
+async def handler_3(request):
+    await asyncio.sleep(2)
+    return text('OK')
+
+
+def test_response_handler_cancelled():
+    request, response = response_handler_cancelled_app.test_client.get('/1')
+    assert response.status == 503
+    assert response.text == 'Error: Response Timeout'
+    assert response_handler_cancelled_app.flag is False