Fix bump stamp for non-joined rooms (#17674)

We should only look for bump stamps in joined rooms, otherwise we should
just use the membership stream ordering.

changelog.d/17674.bugfix

@@ -0,0 +1 @@
+Fix bug where we returned the wrong `bump_stamp` for invites in sliding sync response, causing incorrect ordering of invites in the room list.

synapse/handlers/sliding_sync/__init__.py

@@ -975,7 +975,11 @@ class SlidingSyncHandler:
                     )
                 )
 
-        # Figure out the last bump event in the room
+        # By default, just choose the membership event position
+        bump_stamp = room_membership_for_user_at_to_token.event_pos.stream
+
+        # Figure out the last bump event in the room if we're in the room.
+        if room_membership_for_user_at_to_token.membership == Membership.JOIN:
             last_bump_event_result = (
                 await self.store.get_last_event_pos_in_room_before_stream_ordering(
                     room_id,
@@ -984,8 +988,6 @@ class SlidingSyncHandler:
                 )
             )
 
-        # By default, just choose the membership event position
-        bump_stamp = room_membership_for_user_at_to_token.event_pos.stream
             # But if we found a bump event, use that instead
             if last_bump_event_result is not None:
                 _, new_bump_event_pos = last_bump_event_result

tests/rest/client/sliding_sync/test_rooms_meta.py

@@ -755,3 +755,48 @@ class SlidingSyncRoomsMetaTestCase(SlidingSyncBase):
         response_body, _ = self.do_sync(sync_body, tok=user1_tok)
 
         self.assertGreater(response_body["rooms"][room_id]["bump_stamp"], 0)
+
+    def test_rooms_bump_stamp_invites(self) -> None:
+        """
+        Test that `bump_stamp` is present and points to the membership event,
+        and not later events, for non-joined rooms
+        """
+
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        room_id = self.helper.create_room_as(
+            user2_id,
+            tok=user2_tok,
+        )
+
+        # Invite user1 to the room
+        invite_response = self.helper.invite(room_id, user2_id, user1_id, tok=user2_tok)
+
+        # More messages happen after the invite
+        self.helper.send(room_id, "message in room1", tok=user2_tok)
+
+        # We expect the bump_stamp to match the invite.
+        invite_pos = self.get_success(
+            self.store.get_position_for_event(invite_response["event_id"])
+        )
+
+        # Doing an SS request should return a `bump_stamp` of the invite event,
+        # rather than the message that was sent after.
+        sync_body = {
+            "lists": {
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [],
+                    "timeline_limit": 5,
+                }
+            }
+        }
+        response_body, _ = self.do_sync(sync_body, tok=user1_tok)
+
+        self.assertEqual(
+            response_body["rooms"][room_id]["bump_stamp"], invite_pos.stream
+        )