HTTP Basic Auth example in Go (based on http://stackoverflow.com/a/21937924/556573 + bespoke middleware implementation)

authserver.go

package main

import (
	"encoding/base64"
	"github.com/gorilla/mux"
	"net/http"
	"strings"
)

func main() {

	r := mux.NewRouter()
	r.HandleFunc("/form", use(myHandler, basicAuth))
	http.Handle("/", r)

	http.ListenAndServe(":9900", nil)
}

// use provides a cleaner interface for chaining middleware for single routes.
// Middleware functions are simple HTTP handlers (w http.ResponseWriter, r *http.Request)
//
//  r.HandleFunc("/login", use(loginHandler, rateLimit, csrf))
//  r.HandleFunc("/form", use(formHandler, csrf))
//  r.HandleFunc("/about", aboutHandler)
// 
// See https://gist.github.com/elithrar/7600878#comment-955958 for how to extend it to suit simple http.Handler's
func use(h http.HandlerFunc, middleware ...func(http.HandlerFunc) http.HandlerFunc) http.HandlerFunc {
	for _, m := range middleware {
		h = m(h)
	}

	return h
}

func myHandler(w http.ResponseWriter, r *http.Request) {

	w.Write([]byte("Authenticated!"))
	return
}

// Leverages nemo's answer in http://stackoverflow.com/a/21937924/556573
func basicAuth(h http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {

		w.Header().Set("WWW-Authenticate", `Basic realm="Restricted"`)

		s := strings.SplitN(r.Header.Get("Authorization"), " ", 2)
		if len(s) != 2 {
			http.Error(w, "Not authorized", 401)
			return
		}

		b, err := base64.StdEncoding.DecodeString(s[1])
		if err != nil {
			http.Error(w, err.Error(), 401)
			return
		}

		pair := strings.SplitN(string(b), ":", 2)
		if len(pair) != 2 {
			http.Error(w, "Not authorized", 401)
			return
		}

		if pair[0] != "username" || pair[1] != "password" {
			http.Error(w, "Not authorized", 401)
			return
		}

		h.ServeHTTP(w, r)
	}
}

Greate post, Just have a concern,
w.Header().Set("WWW-Authenticate", Basic realm="Restricted")
this line, should it be unset when the user is authenticated , just before the
h.ServeHTTP(w, r)
https://developer.mozilla.org/en-US/docs/Web/HTTP/Authentication
Seems there is no need to set the WWW-Authenticate after user has passed the authentication.

Any idea how I can use your middleware with static fileserver? See here: https://stackoverflow.com/questions/50863974/gorilla-mux-middleware-for-static-fileserver

There is a BasicAuth() method in the http.Request struct too which does the splitting and decoding stuff.
https://golang.org/pkg/net/http/#Request.BasicAuth

Usage:

func basicAuth(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        user, pass, _ := r.BasicAuth()
        
        if *flagUser != user || *flagPass != pass {
            w.Header().Set("WWW-Authenticate", `Basic realm="Restricted"`)
            http.Error(w, "Unauthorized.", http.StatusUnauthorized)
            return
        }

        h.ServeHTTP(w, r)
    })
}