@Override
    protected void doRegionCompactionPrep() throws IOException {
      compactionsWaiting.countDown();
      try {
        compactionsBlocked.await();
      } catch (InterruptedException ex) {
        throw new IOException();
      }
      super.doRegionCompactionPrep();
    }
An override of HRegion that allows us park compactions in a holding pattern and then when
appropriate for the test, allow them proceed again.
hbase-server/src/test/java/org/apache/hadoop/hbase/TestIOFencing.java