Fix message duplication if something goes wrong after persisting the event (#8476)

Should fix #3365.

1 files changed, 6 insertions, 3 deletions

diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 5ac2fc5656..455acd7669 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -2966,17 +2966,20 @@ class FederationHandler(BaseHandler):
             return result["max_stream_id"]
         else:
             assert self.storage.persistence
-            max_stream_token = await self.storage.persistence.persist_events(
+
+            # Note that this returns the events that were persisted, which may not be
+            # the same as were passed in if some were deduplicated due to transaction IDs.
+            events, max_stream_token = await self.storage.persistence.persist_events(
                 event_and_contexts, backfilled=backfilled
             )
 
             if self._ephemeral_messages_enabled:
-                for (event, context) in event_and_contexts:
+                for event in events:
                     # If there's an expiry timestamp on the event, schedule its expiry.
                     self._message_handler.maybe_schedule_expiry(event)
 
             if not backfilled:  # Never notify for backfilled events
-                for event, _ in event_and_contexts:
+                for event in events:
                     await self._notify_persisted_event(event, max_stream_token)
 
             return max_stream_token.stream