public ClusterStateBundle getClusterStateBundle() {
        synchronized(timer) {
            // In a two-phase state activation scenario, bundles are added to `clusterStateBundles` _before_
            // the version has been activated. Since we want this method to only return _activated_ bundles
            // we filter out versions that are not yet activated. In a non two-phase scenario the activated
            // version is implicitly the same as the most recently received bundle, so the filter is a no-op.
            return clusterStateBundles.stream()
                    .filter(b -> b.getVersion() <= activatedClusterStateVersion)
                    .findFirst() // Most recent cluster state bundle first in list
                    .orElse(null);
        }
    }
Returns the latest system state version received, or empty if none are received yet.
clustercontroller-core/src/test/java/com/yahoo/vespa/clustercontroller/core/DummyVdsNode.java