perf: cache generated image thumbnails

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)
+	}
+}