Tie-break child ordering on the child room id

_child_events_comparison_key's final tie-break used the room the
m.space.child event lives in, which is the same for every child of a
space and so never broke a tie (leaving ties in arbitrary DB order). Use
the child room id (the event's state key), per the spec, matching the
cached path.

synapse/handlers/room_summary.py

@@ -1351,7 +1351,7 @@ def _child_events_comparison_key(
 
     1. The 'order' key, if it is valid.
     2. The 'origin_server_ts' of the 'm.space.child' event.
-    3. The 'room_id'.
+    3. The child room ID (the event's state key).
 
     Args:
         child: The event for generating a comparison key.
@@ -1361,7 +1361,7 @@ def _child_events_comparison_key(
             False if the ordering is valid.
             The 'order' field or None if it is not given or invalid.
             The 'origin_server_ts' field.
-            The room ID.
+            The child room ID.
     """
     order = child.content.get("order")
     # If order is not a string or doesn't meet the requirements, ignore it.
@@ -1371,4 +1371,8 @@ def _child_events_comparison_key(
         order = None
 
     # Items without an order come last.
-    return order is None, order, child.origin_server_ts, child.room_id
+    #
+    # NB the final tie-break is the CHILD room id (the state key), not the room
+    # the event lives in — that is the same for every child of a space, so it
+    # never actually broke a tie.
+    return order is None, order, child.origin_server_ts, child.state_key

tests/handlers/test_room_summary.py

@@ -51,8 +51,14 @@ from tests.unittest import override_config
 def _create_event(
     room_id: str, order: Any | None = None, origin_server_ts: int = 0
 ) -> mock.Mock:
+    """Build a fake m.space.child event pointing at `room_id`.
+
+    The child room id is the event's state key (the event itself lives in the
+    parent space).
+    """
     result = mock.Mock(name=room_id)
-    result.room_id = room_id
+    result.state_key = room_id
+    result.room_id = "!parent:test"
     result.content = {}
     result.origin_server_ts = origin_server_ts
     if order is not None: