datascape/static.go

package main

import (
	"bytes"
	"compress/gzip"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"html/template"
	"io/fs"
	"net/http"
	"strings"
	"time"
)

// 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
}

// assetGzip holds a gzip copy of every embedded asset that compresses well,
// keyed like assetVersions. Built once at startup so serving compressed costs
// no CPU per request; already-compressed files (fonts) are left out.
var assetGzip = gzipAssets()

func gzipAssets() map[string][]byte {
	out := map[string][]byte{}
	_ = 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
		}
		var buf bytes.Buffer
		zw, _ := gzip.NewWriterLevel(&buf, gzip.BestCompression)
		zw.Write(b)
		zw.Close()
		// Not worth a Content-Encoding round trip unless it saves ≥10%.
		if buf.Len() < len(b)*9/10 {
			out[strings.TrimPrefix(p, "assets/")] = buf.Bytes()
		}
		return nil
	})
	return out
}

// acceptsGzip reports whether the client accepts a gzip response body.
func acceptsGzip(r *http.Request) bool {
	for _, enc := range strings.Split(r.Header.Get("Accept-Encoding"), ",") {
		enc, q, _ := strings.Cut(strings.TrimSpace(enc), ";")
		if strings.TrimSpace(enc) == "gzip" && strings.ReplaceAll(q, " ", "") != "q=0" {
			return true
		}
	}
	return false
}

// 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.
//
// Assets with a precompressed copy (see assetGzip) are sent gzipped to
// clients that accept it; the gzip variant gets its own ETag.
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]
		gz, hasGz := assetGzip[name]
		useGz := hasGz && acceptsGzip(r)
		if hasGz {
			w.Header().Set("Vary", "Accept-Encoding")
		}
		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")
			if useGz {
				w.Header().Set("ETag", `"`+v+`-gz"`)
			} else {
				w.Header().Set("ETag", `"`+v+`"`)
			}
		}
		if useGz {
			w.Header().Set("Content-Encoding", "gzip")
			// ServeContent derives Content-Type from the name's extension.
			http.ServeContent(w, r, name, time.Time{}, bytes.NewReader(gz))
			return
		}
		static.ServeHTTP(w, r)
	})
}