Use the filtered version of an event when responding to /context requests for that event

changelog.d/6553.bugfix

@@ -0,0 +1 @@
+Fix a bug causing responses to the `/context` client endpoint to not use the pruned version of the event the request is for.

synapse/handlers/room.py

@@ -907,7 +907,10 @@ def filter_evts(events):
 
         results["events_before"] = yield filter_evts(results["events_before"])
         results["events_after"] = yield filter_evts(results["events_after"])
-        results["event"] = event
+        # filter_evts can return a pruned event in case the user is allowed to see that
+        # there's something there but not see the content, so use the event that's in
+        # `filtered` rather than the event we retrieved from the datastore.
+        results["event"] = filtered[0]
 
         if results["events_after"]:
             last_event_id = results["events_after"][-1].event_id

synapse/visibility.py

@@ -52,7 +52,8 @@ def filter_events_for_client(
     apply_retention_policies=True,
 ):
     """
-    Check which events a user is allowed to see
+    Check which events a user is allowed to see. If the user can see the event but its
+    sender asked for their data to be erased, prune the content of the event.
 
     Args:
         storage