Upsert redactions in case they already exists

changelog.d/10343.bugfix

@@ -0,0 +1 @@
+Fix errors during backfill caused by previously purged redaction events. Contributed by Andreas Rammhold (@andir).

synapse/storage/databases/main/events.py

@@ -1580,11 +1580,11 @@ def _store_redaction(self, txn, event):
         # invalidate the cache for the redacted event
         txn.call_after(self.store._invalidate_get_event_cache, event.redacts)
 
-        self.db_pool.simple_insert_txn(
+        self.db_pool.simple_upsert_txn(
             txn,
             table="redactions",
+            keyvalues={"event_id": event.event_id},
             values={
-                "event_id": event.event_id,
                 "redacts": event.redacts,
                 "received_ts": self._clock.time_msec(),
             },

synapse/storage/databases/main/purge_events.py

@@ -215,6 +215,7 @@ def _purge_history_txn(
             "event_relations",
             "event_search",
             "rejections",
+            "redactions",
         ):
             logger.info("[purge] removing events from %s", table)