1 files changed, 10 insertions, 0 deletions
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 4b3f1ea059..e1df9b3106 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -98,6 +98,9 @@ class TimelineBatch:
     prev_batch: StreamToken
     events: List[EventBase]
     limited: bool
+    # A mapping of event ID to the bundled aggregations for the above events.
+    # This is only calculated if limited is true.
+    bundled_aggregations: Optional[Dict[str, Dict[str, Any]]] = None
 
     def __bool__(self) -> bool:
         """Make the result appear empty if there are no updates. This is used
@@ -630,10 +633,17 @@ class SyncHandler:
 
             prev_batch_token = now_token.copy_and_replace("room_key", room_key)
 
+        # Don't bother to bundle aggregations if the timeline is unlimited,
+        # as clients will have all the necessary information.
+        bundled_aggregations = None
+        if limited or newly_joined_room:
+            bundled_aggregations = await self.store.get_bundled_aggregations(recents)
+
         return TimelineBatch(
             events=recents,
             prev_batch=prev_batch_token,
             limited=limited or newly_joined_room,
+            bundled_aggregations=bundled_aggregations,
         )
 
     async def get_state_after_event(