a.go

package main

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

func main() {
	req, err := http.NewRequest("GET", "http://qq.com", nil)
	check(err)
	ch := make(chan struct{})
	req.Cancel = ch
	var closeOnce sync.Once
	cancel := func() {
		closeOnce.Do(func() {
			close(ch)
		})
	}
	timer := time.AfterFunc(time.Second*20, func() {
		cancel()
	})
	timer.Reset(time.Second * 30) // 重设超时
	cancel()                      // 手工取消
}

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