test.go

package main

import (
	"fmt"
	"net/http"
	"sync"
)

type LimitedRoundTripper struct {
	inflight chan interface{}
	rt       http.RoundTripper
}

func NewLimitedRoundTripper(n int, rt http.RoundTripper) *LimitedRoundTripper {
	return &LimitedRoundTripper{
		inflight: make(chan interface{}, n),
		rt:       rt,
	}
}

func (l *LimitedRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
	defer func() {
		<-l.inflight
	}()
	fmt.Println("waiting")
	l.inflight <- nil
	fmt.Println("requesting")
	r, err := l.rt.RoundTrip(req)
	fmt.Println("done")
	return r, err
}

func main() {
	c := http.Client{Transport: NewLimitedRoundTripper(3, http.DefaultTransport)}
	wg := sync.WaitGroup{}
	for i := 0; i < 10; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			r, _ := c.Get("http://localistico.com")
			r.Body.Close()
		}()
	}
	wg.Wait()
}