code: func (l *Logger) mill() {
  	l.startMill.Do(func() {
  		l.millCh = make(chan bool, 1)
  		go l.millRun()
  	})
  	select {
  	case l.millCh <- true:
  	default:
  	}
  }
docs: // mill performs post-rotation compression and removal of stale log files,
  // starting the mill goroutine if necessary.