Upsert redactions in case they already exists

Occasionally, in combination with retention, redactions aren't deleted
from the database whenever they are due for deletion. The server will
eventually try to backfill the deleted events and trip over the already
existing redaction events.

Switching to an UPSERT for those events allows us to recover from there
situations. The retention code still needs fixing but that is outside of
my current comfort zone on this code base.

This is related to #8707 where the error was discussed already.

Signed-off-by: Andreas Rammhold <andreas@rammhold.de>

changelog.d/10343.bugfix

@@ -0,0 +1 @@
+Fix duplicate key error during backfilling of redaction events that were not deleted by the history purge job. 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(),
             },