Sharded DDL running under versions 4.4 and earlier can possibly fail after having written some config.chunks entries, but before it has written the config.collection entry to join them. Such chunks would become unreachable and are effectively garbage.

Since they are unreachable for the upgrade process as well, it means we cannot assign UUID to them and because of this they would result in a duplicate key error on the uuid, min index. Because of this, they should be deleted after we have patched up all the chunks reachable from config.collections entries.