multiple response.WriteHeader calls in really simple example?

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?

Take a look at the anonymous function you register as the handler of incoming requests:

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

It prints the URL (to the standard output) then calls HandleIndex() in a new goroutine and continues execution.

If you have a handler function where you do not set the response status before the first call to Write, Go will automatically set the response status to 200 (HTTP OK). If the handler function does not write anything to the response (and does not set the response status and completes normally), that is also treated as a successful handling of the request and the response status 200 will be sent back. Your anonymous function does not set it, it does not even write anything to the response. So Go will do just that: set the response status to 200 HTTP OK.

Note that handling each request runs in its own goroutine.

So if you call HandleIndex in a new goroutine, your original anonymous function will continue: it will end and so the response header will be set - meanwhile (concurrently) your started new goroutine will also set the response header - hence the "multiple response.WriteHeader calls" error.

If you remove the "go", your HandleIndex function will set the response header in the same goroutine before your handler function returns, and the "net/http" will know about this and will not try to set the response header again, so the error you experienced will not happen.

You already received a correct answer which addresses your problem, I will give some information about the general case (such error appears often).

From the documentation, you see that WriteHeader sends an http status code and you can't send more than 1 status code. If you Write anything this is equivalent to sending 200 status code and then writing things.

So the message that you see appears if you either user w.WriteHeader more than once explicitly or uses w.Write before w.WriteHeader.

From the documentation:

// WriteHeader sends an HTTP response header with status code. 
// If WriteHeader is not called explicitly, the first call to Write  
// will trigger an implicit WriteHeader(http.StatusOK).

What is happening in your case is that you are launching go HandleIndex from the handler.
The first handler finishes. The standard WriteHeader writes to the ResponseWriter. Then the go routine HandleIndex is launched and it also tries to write a header and write.

Just remove the go from HandleIndex and it will work.

the root cause is that you called WriteHeader more than once. from the source codes

func (w *response) WriteHeader(code int) {
	if w.conn.hijacked() {
		w.conn.server.logf("http: response.WriteHeader on hijacked connection")
		return
	}
	if w.wroteHeader {
		w.conn.server.logf("http: multiple response.WriteHeader calls")
		return
	}
	w.wroteHeader = true
	w.status = code

	if w.calledHeader && w.cw.header == nil {
		w.cw.header = w.handlerHeader.clone()
	}

	if cl := w.handlerHeader.get("Content-Length"); cl != "" {
		v, err := strconv.ParseInt(cl, 10, 64)
		if err == nil && v >= 0 {
			w.contentLength = v
		} else {
			w.conn.server.logf("http: invalid Content-Length of %q", cl)
			w.handlerHeader.Del("Content-Length")
		}
	}
}

so when you wrote once, the variable wroteHeader would be true, then you wrote header again, it wouldn't be effective and gave a warning "http: multiple respnse.WriteHeader calls".
actually the function Write also calls WriteHeader, so putting the function WriteHeader after the function Write also causes that error, and the later WriteHeader doesn't work.

from your case, go handleindex runs in another thread and the original already returns, if you do nothing, it will call WriteHeader to set 200. when running handleindex, it calls another WriteHeader, at that time wroteHeader is true, then the message "http: multiple response.WriteHeader calls" is output.

Yes, use HandleIndex(w, r) instead of go HandleIndex(w, r) will fix your issue, I think you have already figured that out.

The reason is simple, when handling multiple requests at the same time, the http server will start multiple goroutines, and your handler function will be called separately in each of the goroutines without blocking others.
You don't need to start your own goroutine in the handler, unless you practically need it, but that will be another topic.

Because modern browsers send an extra request for /favicon.ico which is also handled in your / request handler.

If you ping your server with curl for example, you'll see only one request being sent:

 curl localhost:5678

To be sure you can add an EndPoint in your http.HandleFunc

http.HandleFunc("/Home", func(w http.ResponseWriter, r *http.Request)