code: private static boolean checkAlreadyCompactedBasedOnCompletionFile(FileSystem fs, Dataset dataset) {
      Path filePath = new Path(dataset.outputPath(), MRCompactor.COMPACTION_COMPLETE_FILE_NAME);
      try {
        return fs.exists(filePath);
      } catch (IOException e) {
        LOG.error("Failed to verify the existence of file " + filePath, e);
        return false;
      }
    }
docs: When completion file strategy is used, a compaction completion means there is a file named
  {@link MRCompactor#COMPACTION_COMPLETE_FILE_NAME} in its {@link Dataset#outputPath()}.