golang 1.7 http client trace example

most of code copy from official go/src/net/http/transport_test.go

package main

import (
	"bytes"
	"context"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"net/http/httptrace"
	"sync"
	"time"
)

func main() {
	req, err := http.NewRequest("HEAD", "https://www.google.com", nil)
	if err != nil {
		log.Println(err.Error())
		return
	}

	var t1 time.Time

	var mu sync.Mutex
	var buf bytes.Buffer
	logf := func(format string, args ...interface{}) {
		mu.Lock()
		defer mu.Unlock()
		fmt.Fprintf(&buf, time.Since(t1).String()+" "+format, args...)
		buf.WriteByte('
')
	}

	ctx := context.Background()
	trace := &httptrace.ClientTrace{
		GetConn:              func(hostPort string) { t1 = time.Now(); logf("Getting conn for %v ...", hostPort) },
		GotConn:              func(ci httptrace.GotConnInfo) { logf("got conn: %+v", ci) },
		GotFirstResponseByte: func() { logf("first response byte") },
		PutIdleConn:          func(err error) { logf("PutIdleConn = %v", err) },
		DNSStart:             func(e httptrace.DNSStartInfo) { logf("DNS start: %+v", e) },
		DNSDone:              func(e httptrace.DNSDoneInfo) { logf("DNS done: %+v", e) },
		ConnectStart:         func(network, addr string) { logf("ConnectStart: Connecting to %s %s ...", network, addr) },
		ConnectDone: func(network, addr string, err error) {
			if err != nil {
				logf("ConnectDone: %v", err)
			}
			logf("ConnectDone: connected to %s %s = %v", network, addr, err)
		},
		Wait100Continue: func() { logf("Wait100Continue") },
		Got100Continue:  func() { logf("Got100Continue") },
		WroteRequest: func(e httptrace.WroteRequestInfo) {
			logf("WroteRequest: %+v", e)
		},
	}
	req = req.WithContext(httptrace.WithClientTrace(ctx, trace))

	client := new(http.Client)
	t1 = time.Now()
	resp, err := client.Do(req)
	if err != nil {
		log.Println(err.Error())
		return
	}
	defer resp.Body.Close()
	ioutil.ReadAll(resp.Body)
	fmt.Println(buf.String())
	fmt.Println(time.Since(t1), resp.Status)
}

955ns Getting conn for www.google.com:443 ...
181.143µs DNS start: {Host:www.google.com}
23.113771ms DNS done: {Addrs:[{IP:216.58.194.164 Zone:} {IP:2607:f8b0:4005:804::2004 Zone:}] Err:<nil> Coalesced:false}
23.172604ms ConnectStart: Connecting to tcp 216.58.194.164:443 ...
25.058901ms ConnectDone: connected to tcp 216.58.194.164:443 = <nil>
108.660729ms got conn: {Conn:0xc420067500 Reused:false WasIdle:false IdleTime:0s}
108.809026ms WroteRequest: {Err:<nil>}
153.106683ms first response byte

154.284484ms 200 OK