Experiment to get average time of SRI hash calculation from visiting browsers

sri-profile.go

package main

import (
	"crypto/sha512"
	"encoding/base64"
	"fmt"
	"log"
	"math/rand"
	"net/http"
	"strings"
	"sync"
	"time"
)

// Number of tests to average
const nTests = 10

// Amount of random junk at end of CSS file
const cssPadding = 100 * 1024

var wg sync.WaitGroup
var resource []byte
var t0, t1 time.Time

func main() {
	http.HandleFunc("/", handler)
	log.Println("Serving on :8080")
	http.ListenAndServe(":8080", nil)
}

func computeHash(b []byte) string {
	h := sha512.Sum512(b)
	return "SHA512-" + base64.StdEncoding.EncodeToString(h[:])
}

func handler(w http.ResponseWriter, r *http.Request) {
	if r.Method != "GET" {
		return
	}
	if strings.HasSuffix(r.URL.Path, ".css") {
		css(w, r)
		return
	}
	if strings.HasSuffix(r.URL.Path, ".png") {
		img(w, r)
		return
	}
	if r.URL.Path != "/" {
		return
	}
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	flusher, ok := w.(http.Flusher)
	if !ok {
		return
	}
	for i := 0; i < nTests; i++ {
		w.Write([]byte(fmt.Sprintf("<div id=\"e%d\"></div>\n", i)))
	}
	flusher.Flush()
	total := time.Duration(0)
	for i := 0; i < nTests; i++ {
		wg.Add(1)
		s := "<link rel=\"stylesheet\" href=\"/c%d.css\" integrity=\"%s\">\n"
		hash, data := createResource(i)
		resource = data
		w.Write([]byte(fmt.Sprintf(s, i, hash)))
		flusher.Flush()
		wg.Wait()
		total += t1.Sub(t0)
	}
	log.Println(total / time.Duration(nTests))
}

func css(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "text/css")
	w.Header().Set("Cache-Control", "no-cache, no-store, must-revalidate")
	w.Header().Set("Pragma", "no-cache")
	w.Header().Set("Expires", "0")
	w.Write(resource)
	t0 = time.Now()
}

func img(w http.ResponseWriter, r *http.Request) {
	t1 = time.Now()
	wg.Done()
}

func createResource(i int) (string, []byte) {
	data := []byte(fmt.Sprintf("#e%d{background:url(\"i%d.png\")}\n", i, i))
	garbage := make([]byte, cssPadding)
	rand.Read(garbage)
	data = append(data, garbage...)
	hash := computeHash(data)
	return hash, data
}