Fix missing sync events during historical batch imports (#12319)

Discovered after much in-depth investigation in #12281.

Closes: #12281
Closes: #3305

Signed off by: Nick Mills-Barrett nick@beeper.com

synapse/storage/databases/main/stream.py

@@ -758,6 +758,32 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             "get_room_event_before_stream_ordering", _f
         )
 
+    async def get_last_event_in_room_before_stream_ordering(
+        self,
+        room_id: str,
+        end_token: RoomStreamToken,
+    ) -> Optional[EventBase]:
+        """Returns the last event in a room at or before a stream ordering
+
+        Args:
+            room_id
+            end_token: The token used to stream from
+
+        Returns:
+            The most recent event.
+        """
+
+        last_row = await self.get_room_event_before_stream_ordering(
+            room_id=room_id,
+            stream_ordering=end_token.stream,
+        )
+        if last_row:
+            _, _, event_id = last_row
+            event = await self.get_event(event_id, get_prev_content=True)
+            return event
+
+        return None
+
     async def get_current_room_stream_token_for_room_id(
         self, room_id: Optional[str] = None
     ) -> RoomStreamToken: