private boolean canUpgradeOsOf(NodeType type) {
        return nodeRepository().nodes()
                               .list(Node.State.ready, Node.State.active)
                               .nodeType(type)
                               .changingVersion()
                               .isEmpty();
    }
Returns whether to allow OS upgrade of nodes of given type
node-repository/src/main/java/com/yahoo/vespa/hosted/provision/maintenance/OsUpgradeActivator.java