code: func (sc *StubController) called() {
  	pc, _, _, ok := runtime.Caller(1)
  	if !ok {
  		panic("Failed to find caller of function")
  	}
  	// longName looks like 'github.com/docker/swarmkit/agent/exec.(*StubController).Prepare:1'
  	longName := runtime.FuncForPC(pc).Name()
  	parts := strings.Split(longName, ".")
  	tail := strings.Split(parts[len(parts)-1], ":")
  	sc.calls[tail[0]]++
  }
docs: // If function A calls updateCountsForSelf,
  // The callCount[A] value will be incremented