/** How many {@link MergeThread}s have kicked off (this is use to name them). */
  protected int mergeThreadCount;

  /**
   * Returns the number of merge threads that are alive, ignoring the calling thread if it is a
   * merge thread. Note that this number is ≤ {@link #mergeThreads} size.
   *
   * @lucene.internal
   */
  public synchronized int mergeThreadCount() {
    Thread currentThread = Thread.currentThread();
    int count = 0;
    for (MergeThread mergeThread : mergeThreads) {
      if (currentThread != mergeThread
          && mergeThread.isAlive()
          && mergeThread.merge.isAborted() == false) {
        count++;
      }
    }
    return count;
  }