Improve caching

static.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"html/template"
+	"io/fs"
+	"net/http"
+	"strings"
+)
+
+// assetVersions maps each embedded asset (path relative to assets/, e.g.
+// "page/toc.js") to a short hash of its content. Templates reference assets
+// through assetURL, which appends the hash as ?v=…, so a versioned URL names
+// one exact file content and can be cached forever; a rebuilt asset gets a
+// new URL.
+var assetVersions = hashAssets()
+
+func hashAssets() map[string]string {
+	versions := map[string]string{}
+	_ = fs.WalkDir(assets, "assets", func(p string, d fs.DirEntry, err error) error {
+		if err != nil || d.IsDir() {
+			return err
+		}
+		b, err := assets.ReadFile(p)
+		if err != nil {
+			return err
+		}
+		sum := sha256.Sum256(b)
+		versions[strings.TrimPrefix(p, "assets/")] = hex.EncodeToString(sum[:])[:12]
+		return nil
+	})
+	return versions
+}
+
+// assetURL is the "asset" template func: {{asset "style.css"}} renders
+// /_/style.css?v=<hash>. An unknown name fails the template so a typo or a
+// removed file surfaces as a render error instead of a silent 404.
+func assetURL(name string) (string, error) {
+	v, ok := assetVersions[name]
+	if !ok {
+		return "", fmt.Errorf("unknown asset %q", name)
+	}
+	return "/_/" + name + "?v=" + v, nil
+}
+
+// assetFuncs is for the standalone fragment templates that do not take the
+// full tmplFuncs.
+var assetFuncs = template.FuncMap{"asset": assetURL}
+
+// serveStatic serves the embedded assets under /_/. Embedded files carry no
+// modtime, so http.FileServer alone sends no validators and browsers refetch
+// every asset on every page. Instead:
+//   - a URL carrying the current ?v= hash is immutable;
+//   - fonts are immutable too (referenced unversioned from style.css, never
+//     change);
+//   - anything else (unversioned or stale ?v=, e.g. icons from CSS) gets the
+//     content hash as ETag and revalidates, answered with 304 when unchanged.
+func serveStatic() http.Handler {
+	staticFS, _ := fs.Sub(assets, "assets")
+	static := http.StripPrefix("/_/", http.FileServer(http.FS(staticFS)))
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		name := strings.TrimPrefix(r.URL.Path, "/_/")
+		v, ok := assetVersions[name]
+		switch {
+		case ok && r.URL.Query().Get("v") == v, strings.HasPrefix(name, "fonts/"):
+			w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
+		case ok:
+			w.Header().Set("Cache-Control", "no-cache")
+			w.Header().Set("ETag", `"`+v+`"`)
+		}
+		static.ServeHTTP(w, r)
+	})
+}