code: func (w *DataFileWriter) Close() error {
  	err := w.Flush() // flush anything remaining
  	if err == nil {
  		// Do an empty flush to signal end of data file format
  		err = w.actuallyFlush()
  
  		if err == nil {
  			// Clean up references.
  			w.output, w.outputEnc, w.datumWriter = nil, nil, nil
  			w.blockBuf, w.blockEnc = nil, nil
  		}
  	}
  	return err
  }
docs: // Close this DataFileWriter.
  // This is required to finish out the data file format.
  // After Close() is called, this DataFileWriter cannot be used anymore.