Add thumbnailing for photo grids

thumb.go

@@ -0,0 +1,224 @@
+package main
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+	"path"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+)
+
+// Thumbnailer produces a thumbnail of a source file. Implementations register
+// themselves in init() by appending to thumbnailers. The first registered
+// handler whose CanHandle returns true is used.
+type Thumbnailer interface {
+	CanHandle(ext string) bool
+	Generate(src io.Reader, dst io.Writer, width int) error
+}
+
+var thumbnailers []Thumbnailer
+
+// thumbCacheDir is set from the -cache flag at startup.
+var thumbCacheDir string
+
+const thumbURLPrefix = "/_thumb"
+
+// Cache is content-addressed: the cache path is derived from the SHA-256 of
+// the source file. Renames and moves reuse the same cache entry; overwriting
+// a file with new content produces a new digest and regenerates.
+var (
+	thumbLocks   = map[string]*sync.Mutex{}
+	thumbLocksMu sync.Mutex
+
+	digestCache  = map[string]digestEntry{}
+	digestCacheMu sync.Mutex
+)
+
+// digestEntry remembers the digest of a source file so repeated requests do
+// not re-hash the whole file. The (mtime, size) pair invalidates the cache
+// when the file is overwritten in place.
+type digestEntry struct {
+	mtime time.Time
+	size  int64
+	hex   string
+}
+
+func findThumbnailer(name string) Thumbnailer {
+	ext := strings.ToLower(filepath.Ext(name))
+	if ext == "" {
+		return nil
+	}
+	for _, t := range thumbnailers {
+		if t.CanHandle(ext) {
+			return t
+		}
+	}
+	return nil
+}
+
+// hasThumbnail reports whether a file name has a registered thumbnailer.
+func hasThumbnail(name string) bool {
+	return findThumbnailer(name) != nil
+}
+
+// thumbURL builds a thumbnail URL for a wiki file. filePath must be URL-style
+// (slash-separated, leading slash), as already used on page links.
+func thumbURL(filePath string, width int) string {
+	return thumbURLPrefix + filePath + "?w=" + strconv.Itoa(width)
+}
+
+func (h *handler) handleThumb(w http.ResponseWriter, r *http.Request) {
+	raw := strings.TrimPrefix(r.URL.Path, thumbURLPrefix)
+	if raw == "" || raw == "/" {
+		http.NotFound(w, r)
+		return
+	}
+	decoded, err := url.PathUnescape(raw)
+	if err != nil {
+		http.Error(w, "bad path", http.StatusBadRequest)
+		return
+	}
+	cleanPath := path.Clean(decoded)
+
+	srcFS := filepath.Join(h.root, filepath.FromSlash(cleanPath))
+	rel, err := filepath.Rel(h.root, srcFS)
+	if err != nil || strings.HasPrefix(rel, "..") {
+		http.Error(w, "Forbidden", http.StatusForbidden)
+		return
+	}
+
+	srcInfo, err := os.Stat(srcFS)
+	if err != nil || srcInfo.IsDir() {
+		http.NotFound(w, r)
+		return
+	}
+
+	t := findThumbnailer(srcFS)
+	if t == nil {
+		http.NotFound(w, r)
+		return
+	}
+
+	width := 300
+	if s := r.URL.Query().Get("w"); s != "" {
+		if n, err := strconv.Atoi(s); err == nil && n > 0 && n <= 2000 {
+			width = n
+		}
+	}
+
+	digest, data, err := sourceDigest(srcFS, srcInfo)
+	if err != nil {
+		log.Printf("thumb digest %s: %v", rel, err)
+		http.Error(w, "thumbnail failed", http.StatusInternalServerError)
+		return
+	}
+
+	cacheFS := filepath.Join(thumbCacheDir, digest[:2], fmt.Sprintf("%s.%d.jpg", digest, width))
+	if _, err := os.Stat(cacheFS); err == nil {
+		serveThumb(w, r, cacheFS)
+		return
+	}
+
+	lock := thumbLock(cacheFS)
+	lock.Lock()
+	defer lock.Unlock()
+
+	if _, err := os.Stat(cacheFS); err == nil {
+		serveThumb(w, r, cacheFS)
+		return
+	}
+
+	var src io.Reader
+	if data != nil {
+		src = bytes.NewReader(data)
+	} else {
+		f, err := os.Open(srcFS)
+		if err != nil {
+			log.Printf("thumb open %s: %v", rel, err)
+			http.Error(w, "thumbnail failed", http.StatusInternalServerError)
+			return
+		}
+		defer f.Close()
+		src = f
+	}
+
+	if err := generateThumb(t, src, cacheFS, width); err != nil {
+		log.Printf("thumb %s: %v", rel, err)
+		http.Error(w, "thumbnail failed", http.StatusInternalServerError)
+		return
+	}
+	serveThumb(w, r, cacheFS)
+}
+
+// sourceDigest returns the SHA-256 hex digest of a source file's content.
+// On a cache hit (path + mtime + size unchanged) the returned data is nil,
+// so the caller knows to open the file itself. On a miss the file is read
+// once and the contents are returned for the caller to reuse.
+func sourceDigest(srcFS string, info os.FileInfo) (string, []byte, error) {
+	digestCacheMu.Lock()
+	d, ok := digestCache[srcFS]
+	digestCacheMu.Unlock()
+	if ok && d.mtime.Equal(info.ModTime()) && d.size == info.Size() {
+		return d.hex, nil, nil
+	}
+
+	data, err := os.ReadFile(srcFS)
+	if err != nil {
+		return "", nil, err
+	}
+	sum := sha256.Sum256(data)
+	h := hex.EncodeToString(sum[:])
+
+	digestCacheMu.Lock()
+	digestCache[srcFS] = digestEntry{mtime: info.ModTime(), size: info.Size(), hex: h}
+	digestCacheMu.Unlock()
+
+	return h, data, nil
+}
+
+func serveThumb(w http.ResponseWriter, r *http.Request, cacheFS string) {
+	w.Header().Set("Cache-Control", "public, max-age=86400")
+	http.ServeFile(w, r, cacheFS)
+}
+
+func generateThumb(t Thumbnailer, src io.Reader, cacheFS string, width int) error {
+	if err := os.MkdirAll(filepath.Dir(cacheFS), 0755); err != nil {
+		return err
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(cacheFS), ".thumb-*")
+	if err != nil {
+		return err
+	}
+	tmpName := tmp.Name()
+	if err := t.Generate(src, tmp, width); err != nil {
+		tmp.Close()
+		os.Remove(tmpName)
+		return err
+	}
+	if err := tmp.Close(); err != nil {
+		os.Remove(tmpName)
+		return err
+	}
+	return os.Rename(tmpName, cacheFS)
+}
+
+func thumbLock(key string) *sync.Mutex {
+	thumbLocksMu.Lock()
+	defer thumbLocksMu.Unlock()
+	m, ok := thumbLocks[key]
+	if !ok {
+		m = &sync.Mutex{}
+		thumbLocks[key] = m
+	}
+	return m
+}