public Status successful(Instant now) {
            if (state != State.RUNNING)
                throw new IllegalStateException("Current state must be RUNNING when changing to SUCCESSFUL");
            return new Status(startedAt, requireNonNull(now), null, State.SUCCESSFUL, null);
        }
Returns a copy of this with the given end instant, in state SUCCESSFUL.
clustercontroller-reindexer/src/main/java/ai/vespa/reindexing/Reindexing.java