Faster joins: Fix spurious errors on incremental sync

changelog.d/15232.bugfix

@@ -0,0 +1 @@
+Faster joins: Fix a bug introduced in Synapse 1.66 where spurious "Failed to find memberships ..." errors would be logged.

synapse/handlers/sync.py

@@ -1226,16 +1226,21 @@ async def _find_missing_partial_state_memberships(
                 continue
 
             event_with_membership_auth = events_with_membership_auth[member]
+            is_create = (
+                event_with_membership_auth.is_state()
+                and event_with_membership_auth.type == EventTypes.Create
+            )
             is_join = (
                 event_with_membership_auth.is_state()
                 and event_with_membership_auth.type == EventTypes.Member
                 and event_with_membership_auth.state_key == member
                 and event_with_membership_auth.content.get("membership")
                 == Membership.JOIN
             )
-            if not is_join:
+            if not is_create and not is_join:
                 # The event must include the desired membership as an auth event, unless
-                # it's the first join event for a given user.
+                # it's the `m.room.create` event for a room or the first join event for
+                # a given user.
                 missing_members.add(member)
             auth_event_ids.update(event_with_membership_auth.auth_event_ids())