使用有缓存的 chan，但 sync.WaitGroup 在 wait()时死住了

func fetch(url string, channel chan string, wg sync.WaitGroup) {
	defer wg.Done()
	resp, err := http.Get(url)
	if (err != nil) || (resp.StatusCode != http.StatusOK) {
		temp := fmt.Sprintf("Cannot fetch %s", url)
		channel <- temp
		fmt.Println("put one")
	}
}

func preFetchAd(ad *Ad) error {
	var wg sync.WaitGroup
	count := 0
	count = count + len(ad.Urls)
	for _, urls := range ad.Urls2 {
		count = count + len(urls)
	}
	c := make(chan string, count)
	//fmt.Println(count)
	errs := []string{}
	for _, url := range ad.Urls {
		wg.Add(1)
		go fetch(url.L, c, wg)
	}
	for _, urls := range ad.Urls2 {
		for _, url := range urls {
			wg.Add(1)
			go fetch(url.L, c, wg)
		}
	}

	wg.Wait()
	close(c)

	for err := range c {
		errs = append(errs, err)
	}

	if len(errs) == 0 {
		return nil
	}
	e := strings.Join(errs, "\n")
	return errors.New(e)
}

我先预算了个数，创建了足够缓存的 chan，但似乎依旧阻塞了？