@Override
    public InternalScanner preFlush(ObserverContext<? extends RegionCoprocessorEnvironment> c,
      Store store, InternalScanner scanner, FlushLifeCycleTracker tracker) throws IOException {
      return null;
    }
Coprocessor that returns null when preCompact or preFlush is called.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestRegionObserverPreFlushAndPreCompact.java