Fix 404 on /sync when the last event is a redaction of an unknown/purged event (#12905)

Currently, we try to pull the event corresponding to a sync token from the database. However, when
we fetch redaction events, we check the target of that redaction (because we aren't allowed to send
redactions to clients without validating them). So, if the sync token points to a redaction of an event
that we don't have, we have a problem.

It turns out we don't really need that event, and can just work with its ID and metadata, which
sidesteps the whole problem.

synapse/storage/databases/main/state.py

@@ -54,6 +54,7 @@ class EventMetadata:
     room_id: str
     event_type: str
     state_key: Optional[str]
+    rejection_reason: Optional[str]
 
 
 def _retrieve_and_check_room_version(room_id: str, room_version_id: str) -> RoomVersion:
@@ -167,17 +168,22 @@ class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
             )
 
             sql = f"""
-                SELECT e.event_id, e.room_id, e.type, se.state_key FROM events AS e
+                SELECT e.event_id, e.room_id, e.type, se.state_key, r.reason
+                FROM events AS e
                 LEFT JOIN state_events se USING (event_id)
+                LEFT JOIN rejections r USING (event_id)
                 WHERE {clause}
             """
 
             txn.execute(sql, args)
             return {
                 event_id: EventMetadata(
-                    room_id=room_id, event_type=event_type, state_key=state_key
+                    room_id=room_id,
+                    event_type=event_type,
+                    state_key=state_key,
+                    rejection_reason=rejection_reason,
                 )
-                for event_id, room_id, event_type, state_key in txn
+                for event_id, room_id, event_type, state_key, rejection_reason in txn
             }
 
         result_map: Dict[str, EventMetadata] = {}