让sync.WaitGroup支持并发数量限制

// WaitGroup 实现一个简单的goroutine池
type WaitGroup struct {
 size      int
 pool      chan byte
 waitCount int64
 waitGroup sync.WaitGroup // 底层还是利用sync.WaitGroup去做并发控制
}
// NewWaitGroup 创建一个带有size的并发池 当size为<=0时，直接走sync.WaitGroup逻辑
func NewWaitGroup(size int) *WaitGroup {
 wg := &WaitGroup{
  size: size,
 }
 if size > 0 {
  wg.pool = make(chan byte, size)
 }
 return wg
}
// BlockAdd 将“1”推入并发池中，如果池已满，则阻塞，这样就打到了限制并发的目的
func (wg *WaitGroup) BlockAdd() {
 if wg.size > 0 {
  wg.pool <- 1
 }
 wg.waitGroup.Add(1)
}
// Done 代表一个并发结束
func (wg *WaitGroup) Done() {
 if wg.size > 0 {
  <-wg.pool
 }
 wg.waitGroup.Done()
}
// Wait 等待所有并发goroutine结束
func (wg *WaitGroup) Wait() {
 wg.waitGroup.Wait()
}
// PendingCount 返回所有pending状态的goroutine数量
func (wg *WaitGroup) PendingCount() int64 {
 return int64(len(wg.pool))
}
func main() {
 urls := []string{
  "https://www.a.com/",
  "https://www.b.com",
  "https://www.c.com",
  "https://www.d.com/",
  "https://www.e.com",
  "https://www.f.com",
 }
 wg := NewWaitGroup(2)
 for _, url := range urls {
  wg.BlockAdd()
  go func(url string) {
   defer wg.Done()
   res, err := http.Get(url)
   if err != nil {
    fmt.Printf("%s: result: %v\n", url, err)
    return
   }
   defer res.Body.Close()
  }(url)
 }
 wg.Wait()
 fmt.Println("Finished")
}