Static asset caching

backend/routes/routes.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 	"shelves/backend/httpx"
 	"strings"
+	"time"
 
 	"shelves"
 )
@@ -28,9 +29,28 @@ func redirectSetup(next http.Handler) http.Handler {
 	})
 }
 
+func ServeStatic() http.Handler {
+	inner := http.StripPrefix("/static/", http.FileServerFS(shelves.Frontend))
+	startup := time.Now().UTC()
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ims := r.Header.Get("If-Modified-Since")
+		imsTime, _ := time.Parse(http.TimeFormat, ims)
+		imsTime = imsTime.Add(time.Second * 10)
+
+		if imsTime.Before(startup) {
+			w.Header().Add("Last-Modified", startup.Format(http.TimeFormat))
+			w.Header().Add("Cache-Control", "public, max-age=0, stale-while-revalidate=9999999")
+			inner.ServeHTTP(w, r)
+		} else {
+			w.WriteHeader(http.StatusNotModified)
+		}
+	})
+}
+
 func Routes() http.Handler {
 	mux := http.NewServeMux()
-	mux.Handle("GET /static/", http.StripPrefix("/static/", http.FileServerFS(shelves.Frontend)))
+	mux.Handle("GET /static/", ServeStatic())
 	mux.HandleFunc("GET /{$}", HomeGet)
 	mux.HandleFunc("GET /setup", SetupGet)
 	mux.HandleFunc("POST /setup", SetupPost)