Capture the Deferred for request cancellation in _AsyncResource

changelog.d/12694.misc

@@ -0,0 +1 @@
+Capture the `Deferred` for request cancellation in `_AsyncResource`.

synapse/http/server.py

@@ -344,7 +344,9 @@ def __init__(self, extract_context: bool = False):
 
     def render(self, request: SynapseRequest) -> int:
         """This gets called by twisted every time someone sends us a request."""
-        defer.ensureDeferred(self._async_render_wrapper(request))
+        request.render_deferred = defer.ensureDeferred(
+            self._async_render_wrapper(request)
+        )
         return NOT_DONE_YET
 
     @wrap_async_request_handler