code: func (h *execWorker) start() {
  	ticker := h.clock.Tick(h.period)
  	h.readyCh <- struct{}{} // For testing.
  
  	for {
  		select {
  		// If the command takes > period, the command runs continuously.
  		case <-ticker:
  			logf("Worker running %v to serve %v", h.probeCmd, h.probePath)
  			output, err := h.exec.Command("sh", "-c", h.probeCmd).CombinedOutput()
  			ts := h.clock.Now()
  			func() {
  				h.mutex.Lock()
  				defer h.mutex.Unlock()
  				h.result = execResult{output, err, ts}
  			}()
  		case <-h.stopCh:
  			return
  		}
  	}
  }
docs: // start attempts to run the probeCmd every `period` seconds.
  // Meant to be called as a goroutine.