Add a shortcut exit when no events to purge

Author: reivilibre

synapse/storage/databases/main/purge_events.py

@@ -239,6 +239,15 @@ def _purge_history_txn(
 
         txn.execute("SELECT event_id, should_delete FROM events_to_purge")
         event_rows = txn.fetchall()
+
+        if len(event_rows) == 0:
+            logger.info("[purge] no events found to purge")
+
+            # For the sake of cleanliness: drop the temp table.
+            # This will commit the txn in sqlite, so make sure to keep this actually last.
+            txn.execute("DROP TABLE events_to_purge")
+            return
+
         logger.info(
             "[purge] found %i events before cutoff, of which %i can be deleted",
             len(event_rows),