code: void reportTaskFinished(TaskAttemptID taskid, boolean commitPending) {
      TaskInProgress tip;
      synchronized (this) {
        tip = tasks.get(taskid);
      }
      if (tip != null) {
        tip.reportTaskFinished(commitPending);
      } else {
        LOG.warn("Unknown child task finished: "+taskid+". Ignored.");
      }
    }
docs: The task is no longer running.  It may not have completed successfully