为什么`nbytes，err：= io.Copy（ioutil.Discard，resp.Body）`总是返回0？

I've written a simple program that fetch a list of url to store them inside some files. In this example google and gmail. I always run same command in different software version. Program is stored inside goFetchAll: this is the name of compiled version of the algorithm.

• go build
• ./goFetchAll http://www.google.com http://www.gmail.com

0.23s        0  http://www.google.com
1.15s        0  http://www.gmail.com

The second number should be the number of bytes of content. But it is alway 0.

package main

import (
    "fmt"
    "io"
    "io/ioutil"
    "net/http"
    "os"
    "strings"
    "time"
)

func main() {
    start := time.Now()

    ch := make(chan string)

    for _, url := range os.Args[1:] {
        go fetch(url, ch)
    }

    for range os.Args[1:] {
        fmt.Println(<-ch)
    }

    secs := time.Since(start).Seconds()
    fmt.Sprintf("%.2fs elapsed
", secs)
}

func fetch(url string, ch chan<- string) {
    start := time.Now()
    resp, err := http.Get(url)
    if err != nil {
        ch <- fmt.Sprint(err)
        return
    }

    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        ch <- fmt.Sprintf("Cant catch content")
        return
    }

    nbytes, err := io.Copy(ioutil.Discard, resp.Body)
    defer resp.Body.Close()
    if err != nil {
        ch <- fmt.Sprintf("while reading %s: %v", url, err)
        return
    }

    secs := time.Since(start).Seconds()
    ch <- fmt.Sprintf("%.2fs  %7d  %s", secs, nbytes, url)

    // store on file
    filename := string(url)
    filename = strings.Replace(filename, ":", "", -1)
    filename = strings.Replace(filename, "//", "-", -1)
    filename = strings.Replace(filename, "/", "", -1)
    filename = strings.Replace(filename, ".", "-", -1)
    filename = "downloads/" + filename + ".html"

    f, err := os.Create(filename)
    f.Write(body)
    defer f.Close()
    if err != nil {
        ch <- fmt.Sprintf("while writing %s: %v", url, err)
        return
    }
}

I've also an older version of same script that actually works:

0.25s    10363  http://www.google.com
0.89s    66576  http://www.gmail.com

package main

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

func main() {
    start := time.Now()

    ch := make(chan string)

    for _, url := range os.Args[1:] {
        go fetch(url, ch)
    }

    for range os.Args[1:] {
        fmt.Println(<-ch)
    }

    fmt.Println("%.2fs elapsed
", time.Since(start).Seconds())
}

func fetch(url string, ch chan<- string) {
    start := time.Now()
    resp, err := http.Get(url)
    if err != nil {
        ch <- fmt.Sprint(err)
        return
    }

    nbytes, err := io.Copy(ioutil.Discard, resp.Body)
    resp.Body.Close()
    if err != nil {
        ch <- fmt.Sprintf("whioe reading %s: %v", url, err)
        return
    }

    secs := time.Since(start).Seconds()
    ch <- fmt.Sprintf("%.2fs  %7d  %s", secs, nbytes, url)
}

Can someone explain why the newest version always count 0 seconds?

My partial solution is the following. I've just request again http.Get(url)

resp, err := http.Get(url)
nbytes, err := io.Copy(ioutil.Discard, resp.Body)
defer resp.Body.Close() // dont leak resources
if err != nil {
    ch <- fmt.Sprintf("while reading %s: %v", url, err)
    return
}

resp, err = http.Get(url)
body, err := ioutil.ReadAll(resp.Body)
if err != nil {
    ch <- fmt.Sprintf("Cant catch content")
    return
}