private InternalScanner postCompactScannerOpen(CompactionRequestImpl request, ScanType scanType,
    InternalScanner scanner, User user) throws IOException {
    if (store.getCoprocessorHost() == null) {
      return scanner;
    }
    return store.getCoprocessorHost().preCompact(store, scanner, scanType, request.getTracker(),
      request, user);
  }
Calls coprocessor, if any, to create scanners - after normal scanner creation.
@param request  Compaction request.
@param scanType Scan type.
@param scanner  The default scanner created for compaction.
@return Scanner scanner to use (usually the default); null if compaction should not proceed.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/compactions/Compactor.java