Review

class IndexFileDeleter {
  void decRef(String fileName) throws IOException {
    RefCount rc = getRefCount(fileName);
    if (infoStream != null && VERBOSE_REF_COUNTS) {
      message("  DecRef \"" + fileName + "\": pre-decr count is " + rc.count);
    }
    if (0 == rc.DecRef()) {
      // This file is no longer referenced by any past
      // commit points nor by the in-memory SegmentInfos:
      deleteFile(fileName);
      refCounts.remove(fileName);
    }
  }

}