Omit bump stamp when it is unchanged

synapse/handlers/sliding_sync/__init__.py

@@ -1048,22 +1048,34 @@ async def get_room_sync_data(
                     )
                 )
 
-        # Figure out the last bump event in the room
-        #
-        # By default, just choose the membership event position for any non-join membership
-        bump_stamp = room_membership_for_user_at_to_token.event_pos.stream
+        # Figure out the last bump event in the room. If the bump stamp hasn't
+        # changed we omit it from the response.
+        bump_stamp = None
+
+        always_return_bump_stamp = (
+            room_membership_for_user_at_to_token.membership != Membership.JOIN
+            or limited is not False
+            or initial
+        )
+
         # If we're joined to the room, we need to find the last bump event before the
         # `to_token`
         if room_membership_for_user_at_to_token.membership == Membership.JOIN:
-            # Try and get a bump stamp, if not we just fall back to the
-            # membership token.
+            # Try and get a bump stamp
             new_bump_stamp = await self._get_bump_stamp(
-                room_id, to_token, timeline_events
+                room_id,
+                to_token,
+                timeline_events,
+                check_non_timeline=always_return_bump_stamp,
             )
             if new_bump_stamp is not None:
                 bump_stamp = new_bump_stamp
 
-        if bump_stamp < 0:
+        if bump_stamp is None and always_return_bump_stamp:
+            # By default, just choose the membership event position for any non-join membership
+            bump_stamp = room_membership_for_user_at_to_token.event_pos.stream
+
+        if bump_stamp is not None and bump_stamp < 0:
             # We never want to send down negative stream orderings, as you can't
             # sensibly compare positive and negative stream orderings (they have
             # different meanings).
@@ -1156,14 +1168,22 @@ async def get_room_sync_data(
 
     @trace
     async def _get_bump_stamp(
-        self, room_id: str, to_token: StreamToken, timeline: List[EventBase]
+        self,
+        room_id: str,
+        to_token: StreamToken,
+        timeline: List[EventBase],
+        check_non_timeline: bool,
     ) -> Optional[int]:
-        """Get a bump stamp for the room, if we have a bump event
+        """Get a bump stamp for the room, if we have a bump event and it has
+        changed.
 
         Args:
-            room_id
-            to_token: The upper bound of token to return
-            timeline: The list of events we have fetched.
+            room_id to_token: The upper bound of token to return timeline: The
+                list of events we have fetched. limited: If the timeline was
+                limited.
+            check_non_timeline: Whether we need to check for bump stamp for
+                events before the timeline if we didn't find a bump stamp in
+                the timeline events.
         """
 
         # First check the timeline events we're returning to see if one of
@@ -1183,6 +1203,11 @@ async def _get_bump_stamp(
                 if new_bump_stamp > 0:
                     return new_bump_stamp
 
+        if not check_non_timeline:
+            # If we are not a limited sync, then we know the bump stamp can't
+            # have changed.
+            return None
+
         # We can quickly query for the latest bump event in the room using the
         # sliding sync tables.
         latest_room_bump_stamp = await self.store.get_latest_bump_stamp_for_room(

synapse/rest/client/sync.py

@@ -1010,11 +1010,13 @@ async def encode_rooms(
         serialized_rooms: Dict[str, JsonDict] = {}
         for room_id, room_result in rooms.items():
             serialized_rooms[room_id] = {
-                "bump_stamp": room_result.bump_stamp,
                 "notification_count": room_result.notification_count,
                 "highlight_count": room_result.highlight_count,
             }
 
+            if room_result.bump_stamp is not None:
+                serialized_rooms[room_id]["bump_stamp"] = room_result.bump_stamp
+
             if room_result.joined_count is not None:
                 serialized_rooms[room_id]["joined_count"] = room_result.joined_count
 

synapse/types/handlers/sliding_sync.py

@@ -158,6 +158,7 @@ class RoomResult:
                 name changes to mark the room as unread and bump it to the top. For
                 encrypted rooms, we just have to consider any activity as a bump because we
                 can't see the content and the client has to figure it out for themselves.
+                This is only included if there has been a change.
             joined_count: The number of users with membership of join, including the client's
                 own user ID. (same as sync `v2 m.joined_member_count`)
             invited_count: The number of users with membership of invite. (same as sync v2
@@ -193,7 +194,7 @@ class StrippedHero:
         limited: Optional[bool]
         # Only optional because it won't be included for invite/knock rooms with `stripped_state`
         num_live: Optional[int]
-        bump_stamp: int
+        bump_stamp: Optional[int]
         joined_count: Optional[int]
         invited_count: Optional[int]
         notification_count: int