ioutil。ReadAll导致goroutine泄漏

为什么在终止之前我有不止一个goroutine,即使我关闭了 resp.body ,而我只使用阻塞呼叫?如果我不消费 resp.Body 它只以一个goroutine结束。

package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
    "runtime"
    "time"
)

func fetch() {
    client := http.Client{Timeout: time.Second * 10}
    url := "http://example.com"
    req, err := http.NewRequest("POST", url, nil)
    resp, err := client.Do(req)
    if err != nil {
        panic(err)
    }
    defer resp.Body.Close()
    _, err = ioutil.ReadAll(resp.Body)
    if err != nil {
        panic(err)
    }
}

func main() {
    fmt.Println("#Goroutines:", runtime.NumGoroutine())
    fetch()
    // runtime.GC()
    time.Sleep(time.Second * 5)
    fmt.Println("#Goroutines:", runtime.NumGoroutine())
}

输出:

#Goroutines: 1
#Goroutines: 3