code: func NewFileWriter() *FileLogWriter {
  	w := &FileLogWriter{
  		Filename: "",
  		Format:   log15.LogfmtFormat(),
  		Maxlines: 1000000,
  		Maxsize:  1 << 28, //256 MB
  		Daily:    true,
  		Maxdays:  7,
  		Rotate:   true,
  	}
  	// use MuxWriter instead direct use os.File for lock write when rotate
  	w.mw = new(MuxWriter)
  	return w
  }
docs: // create a FileLogWriter returning as LoggerInterface.