http - multiple response.WriteHeader calls in really simple example?

Question

I have the most basic net/http program that I'm using to learn the namespace in Go:

package main

import (
    "fmt"
    "log"
    "net/http"
)

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        fmt.Println(r.URL)
        go HandleIndex(w, r)
    })

    fmt.Println("Starting Server...")
    log.Fatal(http.ListenAndServe(":5678", nil))
}

func HandleIndex(w http.ResponseWriter, r *http.Request) {
    w.WriteHeader(200)
    w.Write([]byte("Hello, World!"))
}

When I run the program and connect to localhost:5678 in Chrome, I get this in the console:

Starting Server...
/
2015/01/15 13:41:29 http: multiple response.WriteHeader calls
/favicon.ico
2015/01/15 13:41:29 http: multiple response.WriteHeader calls

But I don't see how that's possible. I print the URL, start up a new goroutine, write the header once, and give it a static body of Hello, World! It seems like one of two things is happening. Either something behind the scenes is writing another header or somehow HandleIndex is called twice for the same request. What can I do to stop writing multiple headers?

EDIT: It seems to have something to do with the go HandleIndex(w, r) line because if I remove go and just make it a function call instead of a goroutine, I don't get any issues and the browser gets it's data. With it being a goroutine, I get the multiple WriteHeader error and the browser doesn't show "Hello World." Why is making this a goroutine breaking it?

Answer 1

Waitting for answers