Fix: Do not cache index, cache SRI hashes

Signed-off-by: Knut Ahlers <knut@ahlers.me>

main.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"bytes"
 	"embed"
 	"fmt"
 	"mime"
@@ -27,8 +26,6 @@ var (
 		VersionAndExit bool   `flag:"version" default:"false" description:"Print version information and exit"`
 	}
 
-	cachedIndex []byte
-
 	product = "ots"
 	version = "dev"
 )
@@ -93,11 +90,6 @@ func assetDelivery(w http.ResponseWriter, r *http.Request) {
 }
 
 func handleIndex(w http.ResponseWriter, r *http.Request) {
-	if cachedIndex != nil {
-		w.Write(cachedIndex)
-		return
-	}
-
 	indexTpl, err := assets.ReadFile("frontend/index.html")
 	if err != nil {
 		http.Error(w, "404 not found", http.StatusNotFound)
@@ -110,14 +102,10 @@ func handleIndex(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	buf := new(bytes.Buffer)
-	if err = tpl.Execute(buf, struct{ Vars map[string]string }{Vars: getJSVars(r)}); err != nil {
+	if err = tpl.Execute(w, struct{ Vars map[string]string }{Vars: getJSVars(r)}); err != nil {
 		http.Error(w, errors.Wrap(err, "parsing template").Error(), http.StatusInternalServerError)
 		return
 	}
-
-	cachedIndex = buf.Bytes()
-	w.Write(buf.Bytes())
 }
 
 func getJSVars(r *http.Request) map[string]string {

tplFuncs.go

@@ -4,14 +4,22 @@ import (
 	"crypto/sha512"
 	"encoding/base64"
 	"path"
+	"sync"
 	"text/template"
 )
 
-var tplFuncs = template.FuncMap{
-	"SRIHash": assetSRIHash,
-}
+var (
+	sriCacheStore = newSRICache()
+	tplFuncs      = template.FuncMap{
+		"SRIHash": assetSRIHash,
+	}
+)
 
 func assetSRIHash(assetName string) string {
+	if sri, ok := sriCacheStore.Get(assetName); ok {
+		return sri
+	}
+
 	data, err := assets.ReadFile(path.Join("frontend", assetName))
 	if err != nil {
 		panic(err)
@@ -21,5 +29,29 @@ func assetSRIHash(assetName string) string {
 	h.Write(data)
 	sum := h.Sum(nil)
 
-	return "sha384-" + base64.StdEncoding.EncodeToString(sum)
+	sri := "sha384-" + base64.StdEncoding.EncodeToString(sum)
+	sriCacheStore.Set(assetName, sri)
+	return sri
+}
+
+type sriCache struct {
+	c map[string]string
+	l sync.RWMutex
+}
+
+func newSRICache() *sriCache { return &sriCache{c: map[string]string{}} }
+
+func (s *sriCache) Get(assetName string) (string, bool) {
+	s.l.RLock()
+	defer s.l.RUnlock()
+
+	h, ok := s.c[assetName]
+	return h, ok
+}
+
+func (s *sriCache) Set(assetName, hash string) {
+	s.l.Lock()
+	defer s.l.Unlock()
+
+	s.c[assetName] = hash
 }