Video thumbnail generation

thumb.go

@@ -26,6 +26,14 @@ type Thumbnailer interface {
 	Generate(src io.Reader, dst io.Writer, width int) error
 }
 
+// PathThumbnailer is an optional capability for thumbnailers whose source must
+// be a seekable file on disk rather than a stream (e.g. ffmpeg for video). When
+// a Thumbnailer also implements this, handleThumb passes the file path directly
+// and skips reading the file into memory and content-hashing it.
+type PathThumbnailer interface {
+	GenerateFromPath(srcPath string, dst io.Writer, width int) error
+}
+
 var thumbnailers []Thumbnailer
 
 // thumbCacheDir is set from the -cache flag at startup.
@@ -116,11 +124,39 @@ func (h *handler) handleThumb(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	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
+	// Path thumbnailers (video/ffmpeg) take the source path directly and key
+	// the cache on path+mtime+size, so the file is never read into memory.
+	// Content thumbnailers (images) stay content-addressed so renames reuse
+	// the cache entry. Either way, generation runs through the closure below.
+	var (
+		digest string
+		write  func(dst io.Writer) error
+	)
+	if pt, ok := t.(PathThumbnailer); ok {
+		digest = pathDigest(srcFS, srcInfo)
+		write = func(dst io.Writer) error { return pt.GenerateFromPath(srcFS, dst, width) }
+	} else {
+		d, data, err := sourceDigest(srcFS, srcInfo)
+		if err != nil {
+			log.Printf("thumb digest %s: %v", rel, err)
+			http.Error(w, "thumbnail failed", http.StatusInternalServerError)
+			return
+		}
+		digest = d
+		write = func(dst io.Writer) error {
+			var src io.Reader
+			if data != nil {
+				src = bytes.NewReader(data)
+			} else {
+				f, err := os.Open(srcFS)
+				if err != nil {
+					return err
+				}
+				defer f.Close()
+				src = f
+			}
+			return t.Generate(src, dst, width)
+		}
 	}
 
 	cacheFS := filepath.Join(thumbCacheDir, digest[:2], fmt.Sprintf("%s.%d.jpg", digest, width))
@@ -138,21 +174,7 @@ func (h *handler) handleThumb(w http.ResponseWriter, r *http.Request) {
 		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 {
+	if err := generateThumb(cacheFS, write); err != nil {
 		log.Printf("thumb %s: %v", rel, err)
 		http.Error(w, "thumbnail failed", http.StatusInternalServerError)
 		return
@@ -160,6 +182,14 @@ func (h *handler) handleThumb(w http.ResponseWriter, r *http.Request) {
 	serveThumb(w, r, cacheFS)
 }
 
+// pathDigest keys the cache for path-based thumbnailers on the source path plus
+// its mtime and size, so the (potentially large) file is never read into memory
+// just to hash it. Overwriting the file changes mtime/size and busts the entry.
+func pathDigest(srcFS string, info os.FileInfo) string {
+	sum := sha256.Sum256(fmt.Appendf(nil, "%s\x00%d\x00%d", srcFS, info.ModTime().UnixNano(), info.Size()))
+	return hex.EncodeToString(sum[:])
+}
+
 // 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
@@ -191,7 +221,10 @@ func serveThumb(w http.ResponseWriter, r *http.Request, cacheFS string) {
 	http.ServeFile(w, r, cacheFS)
 }
 
-func generateThumb(t Thumbnailer, src io.Reader, cacheFS string, width int) error {
+// generateThumb writes a thumbnail to cacheFS atomically: write builds the
+// image into a temp file in the same dir, which is renamed into place only on
+// success so readers never see a partial file.
+func generateThumb(cacheFS string, write func(dst io.Writer) error) error {
 	if err := os.MkdirAll(filepath.Dir(cacheFS), 0755); err != nil {
 		return err
 	}
@@ -200,7 +233,7 @@ func generateThumb(t Thumbnailer, src io.Reader, cacheFS string, width int) erro
 		return err
 	}
 	tmpName := tmp.Name()
-	if err := t.Generate(src, tmp, width); err != nil {
+	if err := write(tmp); err != nil {
 		tmp.Close()
 		os.Remove(tmpName)
 		return err