Implementing GitHub Badges in Golang

I have asked this question before and didn't receive satisfactory answer, so this time I'd try to be more specific.

I would like to implement a server in golang which outputs dynamic status updates in the form of svg. (Think "Build Passing/Failing" GitHub Badges.) The purpose is that one should be able to embed a link to the server's address in GitHub Readme and the Readme should update automatically depending on the server state.

Here's the golang code that I came up with but it doesn't seem to work with GitHub aggressive caching. Do I need to add more Cache-Control headers? Do I need to add ETag?

I'm using the following to embed the image in GitHub Readme.

[![Mine](http://58dcd0b5.ngrok.com/view)]()

Ideally, I would like to see the GitHub Readme change the image every time I load it -- flipping between the two images "correct"/"wrong". (This is just a proof of concept.)

package main

import (
	"log"
	"net/http"
	_ "time"
)
var mymap map[string][]byte

var state bool = false


func viewHandler(w http.ResponseWriter, r *http.Request) {
	log.Printf("State %v", state)
	state = !state
	w.Header().Set("Content-Type", "image/svg+xml")
	w.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate")
	if state {
		w.Write(mymap["correct"])
	} else {
		w.Write(mymap["wrong"])
	}
}

func main() {
	mymap = make(map[string][]byte)
	mymap["correct"] = []byte(`<svg xmlns="http://www.w3.org/2000/svg" width="104" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><mask id="a"><rect width="104" height="20" rx="3" fill="#fff"/></mask><g mask="url(#a)"><path fill="#555" d="M0 0h54v20H0z"/><path fill="#4c1" d="M54 0h50v20H54z"/><path fill="url(#b)" d="M0 0h104v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="11"><text x="28" y="15" fill="#010101" fill-opacity=".3">solution</text><text x="28" y="14">solution</text><text x="78" y="15" fill="#010101" fill-opacity=".3">correct</text><text x="78" y="14">correct</text></g></svg>`)
	mymap["wrong"] = []byte(`<svg xmlns="http://www.w3.org/2000/svg" width="99" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><mask id="a"><rect width="99" height="20" rx="3" fill="#fff"/></mask><g mask="url(#a)"><path fill="#555" d="M0 0h54v20H0z"/><path fill="#e05d44" d="M54 0h45v20H54z"/><path fill="url(#b)" d="M0 0h99v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="11"><text x="28" y="15" fill="#010101" fill-opacity=".3">solution</text><text x="28" y="14">solution</text><text x="75.5" y="15" fill="#010101" fill-opacity=".3">wrong</text><text x="75.5" y="14">wrong</text></g></svg>`)

	mux := http.NewServeMux()
	mux.HandleFunc("/view", viewHandler)
	http.ListenAndServe(":8085", mux)
}

Here's what travis are serving for their images:

Age:0
Cache-Control:no-cache
Content-Length:0
Date:Mon, 30 Mar 2015 07:49:10 GMT
ETag:"88e168c2d5cdb30ee9af739765e78e4d"
Expires:Mon, 30 Mar 2015 07:49:10 GMT
Keep-Alive:timeout=10, max=48
Last-Modified:Wed, 07 Jan 2015 11:26:53 GMT
Timing-Allow-Origin:https://github.com
X-Timer:S1427701750.146025,VS0,VE156

It might be a good start to try these and see what works.

Here is a Github issue about it : https://github.com/github/markup/issues/224

> Assets must include Cache-Control: no-cache and ETag headers. If a
> badge is not updating, then it means they are not properly setting
> these headers.

and

> The assets may also need to include either an ETag or Expires header.
> Fastly's docs on Cache-Control say that no-cache means "re-validate
> before serving this content", but it doesn't specify what it does to
> "re-validate". I'm guessing it is re-validating, but there's no
> indication that the asset has changed, so it continues to serves the
> cached asset.
>
> An ETag would be the biggest win, since it would guarantee that the
> cache gets refreshed when it changes, but still saves bandwidth.

Following this commit to shields.io server, I made the following changes to the above code and it works now.

w.Header().Set("Date", date)
w.Header().Set("Expires", date)

For completeness (and in case someone wants to try it out), here's the complete code. (Also on GitHub.)

package main

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

var mymap map[string][]byte

var state bool = false

func viewHandler(w http.ResponseWriter, r *http.Request) {
	date := time.Now().Format(http.TimeFormat)
	log.Printf("%v", date)
	log.Printf("State %v", state)
	state = !state
	w.Header().Set("Content-Type", "image/svg+xml")
	w.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate")
	w.Header().Set("Date", date)
	w.Header().Set("Expires", date)
	if state {
		w.Write(mymap["correct"])
	} else {
		w.Write(mymap["wrong"])
	}
}

func main() {
	mymap = make(map[string][]byte)
	mymap["correct"] = []byte(`<svg xmlns="http://www.w3.org/2000/svg" width="104" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><mask id="a"><rect width="104" height="20" rx="3" fill="#fff"/></mask><g mask="url(#a)"><path fill="#555" d="M0 0h54v20H0z"/><path fill="#4c1" d="M54 0h50v20H54z"/><path fill="url(#b)" d="M0 0h104v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="11"><text x="28" y="15" fill="#010101" fill-opacity=".3">solution</text><text x="28" y="14">solution</text><text x="78" y="15" fill="#010101" fill-opacity=".3">correct</text><text x="78" y="14">correct</text></g></svg>`)
	mymap["wrong"] = []byte(`<svg xmlns="http://www.w3.org/2000/svg" width="99" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><mask id="a"><rect width="99" height="20" rx="3" fill="#fff"/></mask><g mask="url(#a)"><path fill="#555" d="M0 0h54v20H0z"/><path fill="#e05d44" d="M54 0h45v20H54z"/><path fill="url(#b)" d="M0 0h99v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="11"><text x="28" y="15" fill="#010101" fill-opacity=".3">solution</text><text x="28" y="14">solution</text><text x="75.5" y="15" fill="#010101" fill-opacity=".3">wrong</text><text x="75.5" y="14">wrong</text></g></svg>`)

	mux := http.NewServeMux()
	mux.HandleFunc("/view", viewHandler)
	log.Println("Server started. Listening on 8085...")
	http.ListenAndServe(":8085", mux)
}