foo.java

public abstract class MergeScheduler {
void merge(IndexWriter writer, MergeTrigger trigger, boolean newMergesFound){
  if (acquirePermit()) {
    try {
      doMerg(writer, trigger, newMergesFound);
    } finally {
      releasePermit();
    }
  }
}

private boolean acquirePermit() {
  if (stall) {
    maybeStall();
    return true;
  }
  return semaphore.tryAcquire();
}

proteced void maybeStall() {
  semaphore.acquire();
}

protected void releasePermit(){
  semaphore.release();
}

}