public Status running() {
            if (state != State.READY && state != State.FAILED)
                throw new IllegalStateException("Current state must be READY or FAILED when changing to RUNNING");
            return new Status(startedAt, null, progress, State.RUNNING, null);
        }
Returns a copy of this, in state RUNNING.
clustercontroller-reindexer/src/main/java/ai/vespa/reindexing/Reindexing.java