fix MSC3912-relation-based-redaction for room versions > 10

changelog.d/19782.misc

@@ -0,0 +1 @@
+Fix [MSC3912](https://github.com/matrix-org/matrix-spec-proposals/pull/3912) (relation based redaction) for room versions greater than 10. Contributed by @famedly.

synapse/handlers/relations.py

@@ -38,6 +38,7 @@
 from synapse.logging.opentracing import trace
 from synapse.storage.databases.main.relations import ThreadsNextBatch, _RelatedEvent
 from synapse.streams.config import PaginationConfig
+from synapse.synapse_rust.room_versions import RoomVersion
 from synapse.types import JsonDict, Requester, UserID
 from synapse.util.async_helpers import gather_results
 from synapse.visibility import filter_and_transform_events_for_client
@@ -211,6 +212,7 @@ async def redact_events_related_to(
         event_id: str,
         initial_redaction_event: EventBase,
         relation_types: list[str],
+        room_version: RoomVersion,
     ) -> None:
         """Redacts all events related to the given event ID with one of the given
         relation types.
@@ -228,6 +230,8 @@ async def redact_events_related_to(
                 event_id.
             relation_types: The types of relations to look for. If "*" is in the list,
                 all related events will be redacted regardless of the type.
+            room_version: The RoomVersion of the room, for deciding where the 'redacts'
+                key should go in the event dict
 
         Raises:
             ShadowBanError if the requester is shadow-banned
@@ -244,16 +248,24 @@ async def redact_events_related_to(
             )
 
         for related_event_id in related_event_ids:
+            # Depending on the room version involved, the "redacts" key can go in one of
+            # two places. If we only use what was provided in the initial event, it will
+            # only target an event that was already redacted and nothing will happen.
+            new_redaction_content = dict(initial_redaction_event.content)
+            event_dict: JsonDict = {
+                "type": EventTypes.Redaction,
+                "content": new_redaction_content,
+                "room_id": initial_redaction_event.room_id,
+                "sender": requester.user.to_string(),
+            }
+            if room_version.updated_redaction_rules:
+                event_dict["content"].update({"redacts": related_event_id})
+            else:
+                event_dict["redacts"] = related_event_id
             try:
                 await self._event_creation_handler.create_and_send_nonmember_event(
                     requester,
-                    {
-                        "type": EventTypes.Redaction,
-                        "content": initial_redaction_event.content,
-                        "room_id": initial_redaction_event.room_id,
-                        "sender": requester.user.to_string(),
-                        "redacts": related_event_id,
-                    },
+                    event_dict,
                     ratelimit=False,
                 )
             except SynapseError as e:

synapse/rest/client/room.py

@@ -1438,6 +1438,7 @@ async def _do(
                         event_id=event_id,
                         initial_redaction_event=event,
                         relation_types=with_relations,
+                        room_version=room_version,
                     )
 
             event_id = event.event_id