Limit the number of events sent over replication when persisting events.

changelog.d/10082.bugfix

@@ -0,0 +1 @@
+Fixed a bug causing replication requests to fail when receiving a lot of events via federation.

synapse/handlers/federation.py

@@ -91,6 +91,7 @@
     get_domain_from_id,
 )
 from synapse.util.async_helpers import Linearizer, concurrently_execute
+from synapse.util.iterutils import batch_iter
 from synapse.util.retryutils import NotRetryingDestination
 from synapse.util.stringutils import shortstr
 from synapse.visibility import filter_events_for_server
@@ -3053,13 +3054,15 @@ async def persist_events_and_notify(
         """
         instance = self.config.worker.events_shard_config.get_instance(room_id)
         if instance != self._instance_name:
-            result = await self._send_events(
-                instance_name=instance,
-                store=self.store,
-                room_id=room_id,
-                event_and_contexts=event_and_contexts,
-                backfilled=backfilled,
-            )
+            # Limit the number of events sent over federation.
+            for batch in batch_iter(event_and_contexts, 50):
+                result = await self._send_events(
+                    instance_name=instance,
+                    store=self.store,
+                    room_id=room_id,
+                    event_and_contexts=batch,
+                    backfilled=backfilled,
+                )
             return result["max_stream_id"]
         else:
             assert self.storage.persistence