@Override
    public void postFlush(ObserverContext<? extends RegionCoprocessorEnvironment> c, Store store,
      StoreFile resultFile, FlushLifeCycleTracker tracker) throws IOException {
      if (TRACKER != null) {
        assertSame(tracker, TRACKER);
      }
      // inject here so we can make a flush request to fail because of we already have a flush
      // ongoing.
      CountDownLatch arrive = ARRIVE;
      if (arrive != null) {
        arrive.countDown();
        try {
          BLOCK.await();
        } catch (InterruptedException e) {
          throw new InterruptedIOException();
        }
      }
    }
Confirm that the function of FlushLifeCycleTracker is OK as we do not use it in our own code.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestFlushLifeCycleTracker.java