public ScanInfo preCompactScannerOpen(HStore store, ScanType scanType,
    CompactionLifeCycleTracker tracker, CompactionRequest request, User user) throws IOException {
    if (coprocEnvironments.isEmpty()) {
      return store.getScanInfo();
    }
    CustomizedScanInfoBuilder builder = new CustomizedScanInfoBuilder(store.getScanInfo());
    execOperation(new RegionObserverOperationWithoutResult(user) {
      @Override
      public void call(RegionObserver observer) throws IOException {
        observer.preCompactScannerOpen(this, store, scanType, builder, tracker, request);
      }
    });
    return builder.build();
  }
Called prior to opening store scanner for compaction.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.java