Fix a bug where the joined hosts for a given event were not being properly cached (#14125)

changelog.d/14125.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in v1.69.0rc1 where the joined hosts for a given event were not being properly cached.

synapse/handlers/federation_event.py

@@ -414,7 +414,9 @@ class FederationEventHandler:
 
         # First, precalculate the joined hosts so that the federation sender doesn't
         # need to.
-        await self._event_creation_handler.cache_joined_hosts_for_event(event, context)
+        await self._event_creation_handler.cache_joined_hosts_for_events(
+            [(event, context)]
+        )
 
         await self._check_for_soft_fail(event, context=context, origin=origin)
         await self._run_push_actions_and_persist_event(event, context)

synapse/handlers/message.py

@@ -1390,7 +1390,7 @@ class EventCreationHandler:
                             extra_users=extra_users,
                         ),
                         run_in_background(
-                            self.cache_joined_hosts_for_event, event, context
+                            self.cache_joined_hosts_for_events, events_and_context
                         ).addErrback(
                             log_failure, "cache_joined_hosts_for_event failed"
                         ),
@@ -1491,13 +1491,14 @@ class EventCreationHandler:
                 await self.store.remove_push_actions_from_staging(event.event_id)
             raise
 
-    async def cache_joined_hosts_for_event(
-        self, event: EventBase, context: EventContext
+    async def cache_joined_hosts_for_events(
+        self, events_and_context: List[Tuple[EventBase, EventContext]]
     ) -> None:
-        """Precalculate the joined hosts at the event, when using Redis, so that
+        """Precalculate the joined hosts at each of the given events, when using Redis, so that
         external federation senders don't have to recalculate it themselves.
         """
 
+        for event, _ in events_and_context:
             if not self._external_cache.is_enabled():
                 return
 
@@ -1546,7 +1547,9 @@ class EventCreationHandler:
                     expiry_ms=60 * 60 * 1000,
                 )
 
-            self._external_cache_joined_hosts_updates[state_entry.state_group] = None
+                self._external_cache_joined_hosts_updates[
+                    state_entry.state_group
+                ] = None
 
     async def _validate_canonical_alias(
         self,