Modifications the handle_request function to detect and gracefully handle the case that the request_handler Task is canceled by the sanic server while it is handling the request. One common occurrence of this is when the server issues a ResponseTimeout error, it also cancels the response_handler Task.

The Canceled exception handler purposely sets `response` to `None` to drop references to the handler coroutine, in an attempt to preemptively release resources.
This commit also fixes a possible reference-before-assignment of the `response` variable in the `handle_request` function.
Finally, another byproduct of this change is that ResponseMiddleware will no longer run if the `response` is `None`.

sanic/app.py

@@ -571,6 +571,10 @@ class Sanic:
 
         :return: Nothing
         """
+        # Define `response` var here to remove warnings about
+        # allocation before assignment below.
+        response = None
+        cancelled = False
         try:
             # -------------------------------------------- #
             # Request Middleware
@@ -597,6 +601,13 @@ class Sanic:
                 response = handler(request, *args, **kwargs)
                 if isawaitable(response):
                     response = await response
+        except CancelledError:
+            # If response handler times out, the server handles the error
+            # and cancels the handle_request job.
+            # In this case, the transport is already closed and we cannot
+            # issue a response.
+            response = None
+            cancelled = True
         except Exception as e:
             # -------------------------------------------- #
             # Response Generation Failed
@@ -622,13 +633,22 @@ class Sanic:
             # -------------------------------------------- #
             # Response Middleware
             # -------------------------------------------- #
+            # Don't run response middleware if response is None
+            if response is not None:
                 try:
                     response = await self._run_response_middleware(request,
                                                                    response)
+                except CancelledError:
+                    # Response middleware can timeout too, as above.
+                    response = None
+                    cancelled = True
                 except BaseException:
                     error_logger.exception(
-                    'Exception occurred in one of response middleware handlers'
+                        'Exception occurred in one of response '
+                        'middleware handlers'
                     )
+            if cancelled:
+                raise CancelledError()
 
         # pass the response to the correct callback
         if isinstance(response, StreamingHTTPResponse):