reproduction for Go issue #16808

16808.go

package main

import (
	"log"
	"net"
	"net/http"
	"os"
	"os/exec"
	"strconv"
	"time"
)

func access(url string) {
	transport := &http.Transport{
		Proxy: http.ProxyFromEnvironment,
		DialContext: (&net.Dialer{
			Timeout:   time.Second * 15, //DNS + Connect
			KeepAlive: time.Second * 30,
		}).DialContext,
		MaxIdleConns:          100,
		IdleConnTimeout:       5 * time.Second,
		ExpectContinueTimeout: 1 * time.Second,
	}
	client := http.Client{
		Transport: transport,
		CheckRedirect: func(req *http.Request, via []*http.Request) error {
			return http.ErrUseLastResponse
		}, //Since we now use high-level client we must stop redirects.
	}

	req, err := http.NewRequest("GET", url, nil)
	if err != nil {
		log.Fatal(err)
	}
	resp, err := client.Do(req)
	if err != nil {
		log.Fatal(err)
	}
	log.Println(resp.Proto)
	resp.Body.Close()
}

func lsof() {
	pid := os.Getpid()
	cmd := exec.Command("lsof", "-p", strconv.Itoa(pid))
	out, err := cmd.Output()
	if err != nil {
		log.Fatal(err)
	}
	log.Println(string(out))
}

func main() {
	access(os.Args[1])
	lsof()
	time.Sleep(6 * time.Second)
	lsof()
}