a.go

package main

import (
	"context"
	"net/http"
	"time"
)

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	req, err := http.NewRequestWithContext(ctx, "GET", "http://qq.com", nil)
	check(err)
	timer := time.AfterFunc(time.Second*20, func() {
		cancel()
	})
	timer.Reset(time.Second * 30) // 重设超时
	cancel()                      // 手工取消
	resp, err := http.DefaultClient.Do(req)
	check(err)
	defer resp.Body.Close()
}

func check(err error) {
	if err != nil {
		panic(err)
	}
}