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