Capture the `Deferred` for request cancellation in `_AsyncResource` (#12694)

All async request processing goes through `_AsyncResource`, so this is
the only place where a `Deferred` needs to be captured for cancellation.

Unfortunately, the same isn't true for determining whether a request
can be cancelled. Each of `RestServlet`, `BaseFederationServlet`,
`DirectServe{Html,Json}Resource` and `ReplicationEndpoint` have
different wrappers around the method doing the request handling and they
all need to be handled separately.

Signed-off-by: Sean Quah <seanq@element.io>

changelog.d/12694.misc

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

synapse/http/server.py

@@ -344,7 +344,9 @@ class _AsyncResource(resource.Resource, metaclass=abc.ABCMeta):
 
     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

synapse/http/site.py

@@ -92,11 +92,12 @@ class SynapseRequest(Request):
         # we can't yet create the logcontext, as we don't know the method.
         self.logcontext: Optional[LoggingContext] = None
 
-        # The `Deferred` to cancel if the client disconnects early. Expected to be set
-        # by `Resource.render`.
+        # The `Deferred` to cancel if the client disconnects early and
+        # `is_render_cancellable` is set. Expected to be set by `Resource.render`.
         self.render_deferred: Optional["Deferred[None]"] = None
-        # A boolean indicating whether `_render_deferred` should be cancelled if the
-        # client disconnects early. Expected to be set during `Resource.render`.
+        # A boolean indicating whether `render_deferred` should be cancelled if the
+        # client disconnects early. Expected to be set by the coroutine started by
+        # `Resource.render`, if rendering is asynchronous.
         self.is_render_cancellable = False
 
         global _next_request_seq