控制协程(goroutine)的并发数量

// main_chan.go
func main() {
    var wg sync.WaitGroup
    ch := make(chan struct{}, 3)
    for i := 0; i < 10; i++ {
        ch <- struct{}{}
        wg.Add(1)
        go func(i int) {
            defer wg.Done()
            log.Println(i)
            time.Sleep(time.Second)
            <-ch
        }(i)
    }
    wg.Wait()
}