Order heroes by stream_ordering (as spec'ed)

changelog.d/17435.bugfix

@@ -0,0 +1 @@
+Order `heroes` by `stream_ordering` as the Matrix specification states (applies to `/sync`).

synapse/storage/databases/main/roommember.py

@@ -279,8 +279,19 @@ def _get_users_in_room_with_profiles(
 
     @cached(max_entries=100000)  # type: ignore[synapse-@cached-mutable]
     async def get_room_summary(self, room_id: str) -> Mapping[str, MemberSummary]:
-        """Get the details of a room roughly suitable for use by the room
+        """
+        Get the details of a room roughly suitable for use by the room
         summary extension to /sync. Useful when lazy loading room members.
+
+        Returns the total count of members in the room by membership type, and a
+        truncated list of members (the heroes). This will be the first 6 members of the
+        room:
+        - We want 5 heroes plus 1, in case one of them is the
+        calling user.
+        - They are ordered by `stream_ordering`, which are joined or
+        invited. When no joined or invited members are available, this also includes
+        banned and left users.
+
         Args:
             room_id: The room ID to query
         Returns:
@@ -308,23 +319,36 @@ def _get_room_summary_txn(
             for count, membership in txn:
                 res.setdefault(membership, MemberSummary([], count))
 
-            # we order by membership and then fairly arbitrarily by event_id so
-            # heroes are consistent
-            # Note, rejected events will have a null membership field, so
-            # we we manually filter them out.
+            # Order by membership (joins -> invites -> leave (former insiders) ->
+            # everything else (outsiders like bans/knocks), then by `stream_ordering` so
+            # the first members in the room show up first and to make the sort stable
+            # (consistent heroes).
+            #
+            # Note: rejected events will have a null membership field, so we we manually
+            # filter them out.
             sql = """
                 SELECT state_key, membership, event_id
                 FROM current_state_events
                 WHERE type = 'm.room.member' AND room_id = ?
                     AND membership IS NOT NULL
                 ORDER BY
-                    CASE membership WHEN ? THEN 1 WHEN ? THEN 2 ELSE 3 END ASC,
-                    event_id ASC
+                    CASE membership WHEN ? THEN 1 WHEN ? THEN 2 WHEN ? THEN 3 ELSE 4 END ASC,
+                    event_stream_ordering ASC
                 LIMIT ?
             """
 
-            # 6 is 5 (number of heroes) plus 1, in case one of them is the calling user.
-            txn.execute(sql, (room_id, Membership.JOIN, Membership.INVITE, 6))
+            txn.execute(
+                sql,
+                (
+                    room_id,
+                    # Sort order
+                    Membership.JOIN,
+                    Membership.INVITE,
+                    Membership.LEAVE,
+                    # 6 is 5 (number of heroes) plus 1, in case one of them is the calling user.
+                    6,
+                ),
+            )
             for user_id, membership, event_id in txn:
                 summary = res[membership]
                 # we will always have a summary for this membership type at this
@@ -1509,10 +1533,19 @@ def extract_heroes_from_room_summary(
 ) -> List[str]:
     """Determine the users that represent a room, from the perspective of the `me` user.
 
+    This function expects `MemberSummary.members` to already be sorted by
+    `stream_ordering` like the results from `get_room_summary(...)`.
+
     The rules which say which users we select are specified in the "Room Summary"
     section of
     https://spec.matrix.org/v1.4/client-server-api/#get_matrixclientv3sync
 
+
+    Args:
+        details: Mapping from membership type to member summary. We expect
+            `MemberSummary.members` to already be sorted by `stream_ordering`.
+        me: The user for whom we are determining the heroes for.
+
     Returns a list (possibly empty) of heroes' mxids.
     """
     empty_ms = MemberSummary([], 0)
@@ -1527,11 +1560,11 @@ def extract_heroes_from_room_summary(
         r[0] for r in details.get(Membership.LEAVE, empty_ms).members if r[0] != me
     ] + [r[0] for r in details.get(Membership.BAN, empty_ms).members if r[0] != me]
 
-    # FIXME: order by stream ordering rather than as returned by SQL
+    # We expect `MemberSummary.members` to already be sorted by `stream_ordering`
     if joined_user_ids or invited_user_ids:
-        return sorted(joined_user_ids + invited_user_ids)[0:5]
+        return (joined_user_ids + invited_user_ids)[0:5]
     else:
-        return sorted(gone_user_ids)[0:5]
+        return gone_user_ids[0:5]
 
 
 @attr.s(slots=True, auto_attribs=True)

tests/rest/client/test_sync.py

@@ -2160,18 +2160,15 @@ def test_rooms_meta_heroes_max(self) -> None:
 
         # Room2 doesn't have a name so we should see `heroes` populated
         self.assertIsNone(channel.json_body["rooms"][room_id1].get("name"))
-        # FIXME: Remove this basic assertion and uncomment the better assertion below
-        # after https://github.com/element-hq/synapse/pull/17435 merges
-        self.assertEqual(len(channel.json_body["rooms"][room_id1].get("heroes", [])), 5)
-        # self.assertCountEqual(
-        #     [
-        #         hero["user_id"]
-        #         for hero in channel.json_body["rooms"][room_id1].get("heroes", [])
-        #     ],
-        #     # Heroes should be the first 5 users in the room (excluding the user
-        #     # themselves, we shouldn't see `user1`)
-        #     [user2_id, user3_id, user4_id, user5_id, user6_id],
-        # )
+        self.assertCountEqual(
+            [
+                hero["user_id"]
+                for hero in channel.json_body["rooms"][room_id1].get("heroes", [])
+            ],
+            # Heroes should be the first 5 users in the room (excluding the user
+            # themselves, we shouldn't see `user1`)
+            [user2_id, user3_id, user4_id, user5_id, user6_id],
+        )
         self.assertEqual(
             channel.json_body["rooms"][room_id1]["joined_count"],
             7,