Allow room creator to send MSC2716 related events in existing room versions

changelog.d/10566.feature

@@ -0,0 +1 @@
+Allow room creators to send historical events specified by [MSC2716](https://github.com/matrix-org/matrix-doc/pull/2716) in existing room versions.

synapse/handlers/federation_event.py

@@ -1023,9 +1023,15 @@ async def _handle_marker_event(self, origin: str, marker_event: EventBase):
             return
 
         # Skip processing a marker event if the room version doesn't
-        # support it.
+        # support it or the event is not from the room creator.
         room_version = await self.store.get_room_version(marker_event.room_id)
-        if not room_version.msc2716_historical:
+        create_event = await self.store.get_create_event_for_room(marker_event.room_id)
+        room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
+        if (
+            not room_version.msc2716_historical
+            or not self.hs.config.experimental.msc2716_enabled
+            or marker_event.sender != room_creator
+        ):
             return
 
         logger.debug("_handle_marker_event: received %s", marker_event)

synapse/handlers/message.py

@@ -1393,6 +1393,9 @@ async def persist_and_notify_client_event(
                 allow_none=True,
             )
 
+            room_version = await self.store.get_room_version_id(event.room_id)
+            room_version_obj = KNOWN_ROOM_VERSIONS[room_version]
+
             # we can make some additional checks now if we have the original event.
             if original_event:
                 if original_event.type == EventTypes.Create:
@@ -1404,16 +1407,35 @@ async def persist_and_notify_client_event(
                 if original_event.type == EventTypes.ServerACL:
                     raise AuthError(403, "Redacting server ACL events is not permitted")
 
+                # Add a little safety stop-gap to prevent people from trying to
+                # redact MSC2716 related events when they're in a room version
+                # which does not support it yet. We allow people to use MSC2716
+                # events in existing room versions but only from the room
+                # creator since it does not require any changes to the auth
+                # rules and in effect, the redaction algorithm . In the
+                # supported room version, we add the `historical` power level to
+                # auth the MSC2716 related events and adjust the redaction
+                # algorthim to keep the `historical` field around (redacting an
+                # event should only strip fields which don't affect the
+                # structural protocol level).
+                is_msc2716_event = (
+                    original_event.type == EventTypes.MSC2716_INSERTION
+                    or original_event.type == EventTypes.MSC2716_CHUNK
+                    or original_event.type == EventTypes.MSC2716_MARKER
+                )
+                if not room_version_obj.msc2716_historical and is_msc2716_event:
+                    raise AuthError(
+                        403,
+                        "Redacting MSC2716 events is not supported in this room version",
+                    )
+
             prev_state_ids = await context.get_prev_state_ids()
             auth_events_ids = self._event_auth_handler.compute_auth_events(
                 event, prev_state_ids, for_verification=True
             )
             auth_events_map = await self.store.get_events(auth_events_ids)
             auth_events = {(e.type, e.state_key): e for e in auth_events_map.values()}
 
-            room_version = await self.store.get_room_version_id(event.room_id)
-            room_version_obj = KNOWN_ROOM_VERSIONS[room_version]
-
             if event_auth.check_redaction(
                 room_version_obj, event, auth_events=auth_events
             ):

synapse/storage/databases/main/events.py

@@ -1770,10 +1770,21 @@ def _handle_insertion_event(self, txn: LoggingTransaction, event: EventBase):
             # Not a insertion event
             return
 
-        # Skip processing a insertion event if the room version doesn't
-        # support it.
+        # Skip processing an insertion event if the room version doesn't
+        # support it or the event is not from the room creator.
         room_version = self.store.get_room_version_txn(txn, event.room_id)
-        if not room_version.msc2716_historical:
+        room_creator = self.db_pool.simple_select_one_onecol_txn(
+            txn,
+            table="rooms",
+            keyvalues={"room_id": event.room_id},
+            retcol="creator",
+            allow_none=True,
+        )
+        if (
+            not room_version.msc2716_historical
+            or not self.hs.config.experimental.msc2716_enabled
+            or event.sender != room_creator
+        ):
             return
 
         next_chunk_id = event.content.get(EventContentFields.MSC2716_NEXT_CHUNK_ID)
@@ -1822,9 +1833,20 @@ def _handle_chunk_event(self, txn: LoggingTransaction, event: EventBase):
             return
 
         # Skip processing a chunk event if the room version doesn't
-        # support it.
+        # support it or the event is not from the room creator.
         room_version = self.store.get_room_version_txn(txn, event.room_id)
-        if not room_version.msc2716_historical:
+        room_creator = self.db_pool.simple_select_one_onecol_txn(
+            txn,
+            table="rooms",
+            keyvalues={"room_id": event.room_id},
+            retcol="creator",
+            allow_none=True,
+        )
+        if (
+            not room_version.msc2716_historical
+            or not self.hs.config.experimental.msc2716_enabled
+            or event.sender != room_creator
+        ):
             return
 
         chunk_id = event.content.get(EventContentFields.MSC2716_CHUNK_ID)