Golang abort HTTP request after header

main.go

package main

import (
	"crypto/tls"
	"fmt"
	"net/http"
	"time"
)

// Make a GET request for a large (24 MB) file.  We only care about a single header field ("ETag")
// and want to abort the download as soon as the header is available.  This may be necessary
// for servers that don't support "If-None-Match".
//
// In my testing, I can fetch the header in about 250 ms and close the body immediately (<1 ms)
// thereafter.

func main() {
	transport := &http.Transport{
		TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
	}
	httpclient := &http.Client{Transport: transport}

	req, err := http.NewRequest("GET", "https://resources.lendingclub.com/SecondaryMarketAllNotes.csv", nil)

	// how long does it take to fetch the header?
	start := time.Now()
	resp, err := httpclient.Do(req)
	if err != nil {
		panic(err)
	}
	fmt.Println(resp.Header.Get("etag"))
	fmt.Printf("Header took: %s\n", time.Now().Sub(start))

	// how long to close (discard) the body
	start = time.Now()
	resp.Body.Close()
	fmt.Printf("Close took: %s\n", time.Now().Sub(start))
}