perf: cache generated image thumbnails

.gitignore

@@ -0,0 +1,5 @@
+# Local build artifacts
+bin/
+
+# Local/debug runtime configuration
+chfs.ini

internal/api/server.go

@@ -56,6 +56,8 @@ type Server struct {
 	Log                *opslog.Logger
 	UserStore          *auth.UserStore
 	UserHome           string
+	ThumbnailCache     *thumbnailCache
+	thumbnailCacheOnce sync.Once
 }
 type fileInfo struct {
 	Name      string `json:"name"`
@@ -1176,20 +1178,36 @@ func (s *Server) thumbnail(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	defer f.Close()
+	fi, err := f.Stat()
+	if err != nil {
+		http.NotFound(w, r)
+		return
+	}
+	key := thumbnailCacheKey{Path: file, Size: fi.Size(), ModTime: fi.ModTime().UnixNano()}
+	s.thumbnailCacheOnce.Do(func() {
+		if s.ThumbnailCache == nil {
+			s.ThumbnailCache = newThumbnailCache()
+		}
+	})
+	data, err := s.ThumbnailCache.Do(key, func() ([]byte, error) {
 		src, _, err := image.Decode(f)
 		if err != nil {
-		http.Error(w, "thumbnail unavailable", http.StatusUnsupportedMediaType)
-		return
+			return nil, err
 		}
 		thumb := transform.Resize(src, 320, 240, transform.Linear)
 		var buf bytes.Buffer
 		if err := png.Encode(&buf, thumb); err != nil {
-		http.Error(w, "thumbnail unavailable", http.StatusInternalServerError)
+			return nil, err
+		}
+		return append([]byte(nil), buf.Bytes()...), nil
+	})
+	if err != nil {
+		http.Error(w, "thumbnail unavailable", http.StatusUnsupportedMediaType)
 		return
 	}
 	w.Header().Set("Content-Type", "image/png")
 	w.Header().Set("Cache-Control", "private, max-age=300")
-	_, _ = w.Write(buf.Bytes())
+	_, _ = w.Write(data)
 }
 
 func (s *Server) fileHandler(w http.ResponseWriter, r *http.Request) {

internal/api/thumbnail_cache.go

@@ -0,0 +1,103 @@
+package api
+
+import (
+	"container/list"
+	"sync"
+)
+
+type thumbnailCacheKey struct {
+	Path    string
+	Size    int64
+	ModTime int64
+}
+
+type thumbnailCacheEntry struct {
+	key  thumbnailCacheKey
+	data []byte
+}
+
+type thumbnailFlight struct {
+	done chan struct{}
+	data []byte
+	err  error
+}
+
+type thumbnailCache struct {
+	mu       sync.Mutex
+	items    map[thumbnailCacheKey]*list.Element
+	order    *list.List
+	flights  map[thumbnailCacheKey]*thumbnailFlight
+	bytes    int64
+	maxBytes int64
+	maxItems int
+}
+
+func newThumbnailCache() *thumbnailCache {
+	return &thumbnailCache{
+		items:    make(map[thumbnailCacheKey]*list.Element),
+		order:    list.New(),
+		flights:  make(map[thumbnailCacheKey]*thumbnailFlight),
+		maxBytes: 32 << 20,
+		maxItems: 64,
+	}
+}
+
+func (c *thumbnailCache) Do(key thumbnailCacheKey, fn func() ([]byte, error)) ([]byte, error) {
+	c.mu.Lock()
+	if e, ok := c.items[key]; ok {
+		c.order.MoveToFront(e)
+		data := append([]byte(nil), e.Value.(thumbnailCacheEntry).data...)
+		c.mu.Unlock()
+		return data, nil
+	}
+	if f, ok := c.flights[key]; ok {
+		c.mu.Unlock()
+		<-f.done
+		return append([]byte(nil), f.data...), f.err
+	}
+	f := &thumbnailFlight{done: make(chan struct{})}
+	c.flights[key] = f
+	c.mu.Unlock()
+
+	data, err := fn()
+	if err == nil && len(data) > 0 && int64(len(data)) <= c.maxBytes {
+		c.mu.Lock()
+		c.putLocked(key, data)
+		f.data = append([]byte(nil), data...)
+		f.err = err
+		delete(c.flights, key)
+		close(f.done)
+		c.mu.Unlock()
+		return data, nil
+	}
+	c.mu.Lock()
+	f.err = err
+	delete(c.flights, key)
+	close(f.done)
+	c.mu.Unlock()
+	return data, err
+}
+
+func (c *thumbnailCache) putLocked(key thumbnailCacheKey, data []byte) {
+	if e, ok := c.items[key]; ok {
+		old := e.Value.(thumbnailCacheEntry)
+		c.bytes -= int64(len(old.data))
+		e.Value = thumbnailCacheEntry{key: key, data: append([]byte(nil), data...)}
+		c.bytes += int64(len(data))
+		c.order.MoveToFront(e)
+	} else {
+		e := c.order.PushFront(thumbnailCacheEntry{key: key, data: append([]byte(nil), data...)})
+		c.items[key] = e
+		c.bytes += int64(len(data))
+	}
+	for c.bytes > c.maxBytes || c.order.Len() > c.maxItems {
+		e := c.order.Back()
+		if e == nil {
+			break
+		}
+		entry := e.Value.(thumbnailCacheEntry)
+		delete(c.items, entry.key)
+		c.bytes -= int64(len(entry.data))
+		c.order.Remove(e)
+	}
+}

internal/api/thumbnail_cache_test.go

@@ -0,0 +1,72 @@
+package api
+
+import (
+	"errors"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+func TestThumbnailCacheDeduplicatesConcurrentBuilds(t *testing.T) {
+	c := newThumbnailCache()
+	key := thumbnailCacheKey{Path: "photo.jpg", Size: 10, ModTime: 20}
+	var builds atomic.Int32
+	const workers = 24
+	var wg sync.WaitGroup
+	results := make([][]byte, workers)
+	errs := make([]error, workers)
+	wg.Add(workers)
+	for i := 0; i < workers; i++ {
+		go func(i int) {
+			defer wg.Done()
+			results[i], errs[i] = c.Do(key, func() ([]byte, error) {
+				builds.Add(1)
+				return []byte("thumbnail"), nil
+			})
+		}(i)
+	}
+	wg.Wait()
+	if builds.Load() != 1 {
+		t.Fatalf("thumbnail builder ran %d times, want 1", builds.Load())
+	}
+	for i := range results {
+		if errs[i] != nil || string(results[i]) != "thumbnail" {
+			t.Fatalf("worker %d got result %q, err=%v", i, results[i], errs[i])
+		}
+	}
+}
+
+func TestThumbnailCacheDoesNotCacheErrors(t *testing.T) {
+	c := newThumbnailCache()
+	key := thumbnailCacheKey{Path: "broken.jpg"}
+	wantErr := errors.New("decode failed")
+	var builds atomic.Int32
+	for i := 0; i < 2; i++ {
+		_, err := c.Do(key, func() ([]byte, error) {
+			builds.Add(1)
+			return nil, wantErr
+		})
+		if !errors.Is(err, wantErr) {
+			t.Fatalf("got err %v, want %v", err, wantErr)
+		}
+	}
+	if builds.Load() != 2 {
+		t.Fatalf("error result was cached; builder ran %d times", builds.Load())
+	}
+}
+
+func TestThumbnailCacheEvictsByBound(t *testing.T) {
+	c := newThumbnailCache()
+	c.maxItems = 2
+	for i := 0; i < 3; i++ {
+		key := thumbnailCacheKey{Path: string(rune('a' + i))}
+		if _, err := c.Do(key, func() ([]byte, error) { return []byte("x"), nil }); err != nil {
+			t.Fatal(err)
+		}
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if len(c.items) != 2 || c.order.Len() != 2 {
+		t.Fatalf("cache size=%d/list=%d, want 2/2", len(c.items), c.order.Len())
+	}
+}