code: private String getOldCodeId(FileStatus srcStat ) throws IOException {
      if (codec.id.equals("xor") || codec.id.equals("rs")) {
        return codec.id;
      } else {
        // Search for xor/rs parity files
        if (ParityFilePair.getParityFile(
          Codec.getCodec("xor"), srcStat, this.conf) != null)
          return "xor";
        if (ParityFilePair.getParityFile(
          Codec.getCodec("rs"), srcStat, this.conf) != null)
          return "rs";
      }
      return null;
    }
docs: Return the old code id to construct a old decoder