问递归的Goroutines，告诉Go停止从频道阅读的最简洁的方法是什么？

EN

// Safe HashSet Version
type SafeHashSet struct {
    sync.Mutex
    urls map[string]bool //Primarily we wanted use this as an hashset, so the value of map is not significant to us
}

var (
    urlSet     SafeHashSet
    urlCounter int64
)

// Adds an URL to the Set, returns true if new url was added (if not present already)
func (m *SafeHashSet) add(newUrl string) bool {
    m.Lock()
    defer m.Unlock()
    _, ok := m.urls[newUrl]
    if !ok {
        m.urls[newUrl] = true
        return true
    }
    return false
}


// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {

    // Decrement the atomic url counter, when this crawl function exits
    defer atomic.AddInt64(&urlCounter, -1)

    if depth <= 0 {
        return
    }

    // Don't Process a url if it is already processed
    isNewUrl := urlSet.add(url)

    if !isNewUrl {
        fmt.Printf("skip: \t%s\n", url)
        return
    }


    body, urls, err := fetcher.Fetch(url)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Printf("found: \t%s %q\n", url, body)

    for _, u := range urls {
        atomic.AddInt64(&urlCounter, 1)
        // Crawl parallely
        go Crawl(u, depth-1, fetcher)
    }
    return
}

func main() {
    urlSet = SafeHashSet{urls: make(map[string]bool)}

    atomic.AddInt64(&urlCounter, 1)
    go Crawl("https://golang.org/", 4, fetcher)

    for atomic.LoadInt64(&urlCounter) > 0 {
        time.Sleep(100 * time.Microsecond)
    }
    fmt.Println("Exiting")
}