public ScanInfo preFlushScannerOpen(HStore store, FlushLifeCycleTracker tracker)
    throws IOException {
    if (coprocEnvironments.isEmpty()) {
      return store.getScanInfo();
    }
    CustomizedScanInfoBuilder builder = new CustomizedScanInfoBuilder(store.getScanInfo());
    execOperation(new RegionObserverOperationWithoutResult() {
      @Override
      public void call(RegionObserver observer) throws IOException {
        observer.preFlushScannerOpen(this, store, builder, tracker);
      }
    });
    return builder.build();
  }
Invoked before create StoreScanner for flush.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.java