gistfile1.go

package main

import (
  "flag"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
)

var url string = "http://repo.bukkit.org/index.html"

func main() {
	flag.StringVar(&url, "url", url, "URL to test against")
	flag.Parse()

	resp, err := http.Get(url)
	//defer resp.Body.Close()

	if err != nil {
		log.Fatal(err)
	}

	fmt.Printf("%#v\n", resp)
	if resp.Request != nil {
		fmt.Printf("%#v\n", resp.Request)
	}

	_, err = ioutil.ReadAll(resp.Body)
	if err != nil {
		log.Fatal(err)
	}
}

Fails with "use of closed network connection" on:

• http://217.19.223.2:20001
• http://localhost:8080/_ah/admin/datastore (my local appengine development server)
• http://repo.bukkit.org/index.html

and succeeds without error on all other domains I've tested, notably:

• http://www.example.com
• http://a.tiles.mapbox.com/v3/marker/pin-l-bicycle+94eeff.png

In all the "fail" URLs above, curl -v will report

* additional stuff not fine transfer.c:1037: 0 0
* HTTP 1.0, assume close after body

In the two notable "succeed" URLs above, curl -v will, in addition to the previous message, report

* HTTP/1.0 connection set to keep alive!
< Connection: Keep-Alive

The failure case, then, seems to be HTTP 1.0 servers that do not report Connection: Keep-Alive.