void handleDeadWorkers(Set<ServerName> serverNames) {
    synchronized (deadWorkersLock) {
      if (deadWorkers == null) {
        deadWorkers = new HashSet<>(100);
      }
      deadWorkers.addAll(serverNames);
    }
    LOG.info("dead splitlog workers " + serverNames);
  }
Returns null on success, existing task on error
hbase-server/src/main/java/org/apache/hadoop/hbase/master/SplitLogManager.java