add album art caching

backend/imagemanager.go

@@ -1,36 +1,75 @@
 package backend
 
 import (
+	"fmt"
 	"image"
+	"image/jpeg"
+	"log"
+	"os"
+	"path/filepath"
 
+	"github.com/20after4/configdir"
 	"github.com/bluele/gcache"
 	subsonic "github.com/dweymouth/go-subsonic"
 )
 
 type ImageManager struct {
 	s              *subsonic.Client
+	cacheDir       string
 	thumbnailCache gcache.Cache
 }
 
-func NewImageManager(s *subsonic.Client) *ImageManager {
+func NewImageManager(s *subsonic.Client, cacheDir string) *ImageManager {
 	cache := gcache.New(100).LRU().Build()
+	if err := configdir.MakePath(cacheDir); err != nil {
+		log.Println("failed to create album cover cache dir")
+		cacheDir = ""
+	}
 	return &ImageManager{
 		s:              s,
+		cacheDir:       cacheDir,
 		thumbnailCache: cache,
 	}
 }
 
 func (i *ImageManager) GetAlbumThumbnail(albumID string) (image.Image, error) {
+	// in-memory cache
 	if i.thumbnailCache.Has(albumID) {
 		if img, err := i.thumbnailCache.Get(albumID); err == nil {
 			return img.(image.Image), nil
 		}
 	}
-	// TODO: on disc cache
-	img, err := i.s.GetCoverArt(albumID, map[string]string{"size": "250"})
+
+	// on disc cache
+	path := filepath.Join(i.cacheDir, fmt.Sprintf("%s.jpg", albumID))
+	if i.cacheDir != "" {
+		if _, err := os.Stat(path); err == nil {
+			// serve image from on-disc cache
+			// TODO: image may have changed on server.
+			//    first, return cached image, then fetch fresh img from server in background
+			if f, err := os.Open(path); err == nil {
+				defer f.Close()
+				if img, _, err := image.Decode(f); err == nil {
+					i.thumbnailCache.Set(albumID, img)
+					return img, nil
+				}
+			}
+		}
+	}
+
+	// fetch from server
+	img, err := i.s.GetCoverArt(albumID, map[string]string{"size": "300"})
 	if err != nil {
 		return nil, err
 	}
+	if i.cacheDir != "" {
+		if f, err := os.Create(path); err == nil {
+			defer f.Close()
+			if err := jpeg.Encode(f, img, nil /*options*/); err != nil {
+				log.Printf("failed to cache image: %s", err.Error())
+			}
+		}
+	}
 	i.thumbnailCache.Set(albumID, img)
 	return img, nil
 }