code: private static String convertMetatadataFileName(String oldFileName) {
      Matcher matcher = PRE_GENSTAMP_META_FILE_PATTERN.matcher(oldFileName);
      if (matcher.matches()) {
        //return the current metadata file name
        return BlockWithChecksumFileWriter.getMetaFileName(matcher.group(1),
                                         Block.GRANDFATHER_GENERATION_STAMP);
      }
      return oldFileName;
    }
docs: This is invoked on target file names when upgrading from pre generation
  stamp version (version -13) to correct the metatadata file name.
  @param oldFileName
  @return the new metadata file name with the default generation stamp.