public Lock lockReindexing(String cluster) throws ReindexingLockException {
        try {
            return curator.lock(lockPath(cluster), lockTimeout);
        }
        catch (UncheckedTimeoutException e) {
            throw new ReindexingLockException(e);
        }
    }
This lock must be held to manipulate reindexing state, or by whoever has a running visitor.
clustercontroller-reindexer/src/main/java/ai/vespa/reindexing/ReindexingCurator.java