fix(artwork): return correct timestamp when disc or album coverart changes (#5378)

* fix(artwork): return imagesUpdatedAt in LastUpdated when cover art changes

When cover art (cover.jpg) is updated in an album folder, the HTTP
Last-Modified header was incorrectly returning album.UpdatedAt (which
only tracks media file changes) instead of imagesUpdatedAt (which
tracks cover art changes).

This caused browsers to use their cached cover art because the
Last-Modified header didn't change, even though the actual cover art
image data was new (due to cache key changing based on imagesUpdatedAt).

The fix ensures LastUpdated() returns a.lastUpdate (which is the max of
album.UpdatedAt and imagesUpdatedAt) instead of always returning
album.UpdatedAt.

Fixes navidrome/navidrome#5377

* refactor tests

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(artwork): return imagesUpdatedAt in disc LastUpdated

The discArtworkReader had the same bug as albumArtworkReader (fixed in
9a741859f): LastUpdated() returned album.UpdatedAt while Key() used the
max of album.UpdatedAt and ImagesUpdatedAt. This mismatch caused browsers
to keep stale disc cover art in cache when only the image file changed.

Also strengthen the album LastUpdated tests and add matching tests for
the disc reader. The tests use DescribeTable and were verified to fail
when the fix is reverted.

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Deluan <deluan@navidrome.org>

core/artwork/artwork_internal_test.go

@@ -7,12 +7,11 @@ import (
 	"image/jpeg"
 	"image/png"
 	"io"
-
 	"os"
 	"path/filepath"
+	"time"
 
 	_ "github.com/gen2brain/webp"
-
 	"github.com/navidrome/navidrome/conf"
 	"github.com/navidrome/navidrome/conf/configtest"
 	"github.com/navidrome/navidrome/log"
@@ -146,6 +145,51 @@ var _ = Describe("Artwork", func() {
 				Entry(nil, " embedded , front.* , cover.*,folder.*", "tests/fixtures/artist/an-album/test.mp3"),
 			)
 		})
+		Context("LastUpdated", func() {
+			// Regression test for #5377: LastUpdated feeds the HTTP Last-Modified header.
+			// It must return max(album.UpdatedAt, ImagesUpdatedAt) so browsers revalidate
+			// cached cover art when only the image file changes.
+			now := time.Now().Truncate(time.Second)
+			DescribeTable("returns the max of album.UpdatedAt and ImagesUpdatedAt",
+				func(albumUpdatedAt, imagesUpdatedAt, expected time.Time) {
+					album := model.Album{ID: "al1", UpdatedAt: albumUpdatedAt}
+					folderRepo.result = []model.Folder{{ImagesUpdatedAt: imagesUpdatedAt}}
+					ds.Album(ctx).(*tests.MockAlbumRepo).SetData(model.Albums{album})
+
+					ar, err := newAlbumArtworkReader(ctx, aw, album.CoverArtID(), nil)
+					Expect(err).ToNot(HaveOccurred())
+					Expect(ar.LastUpdated()).To(Equal(expected))
+				},
+				Entry("album newer than images", now, now.Add(-1*time.Hour), now),
+				Entry("images newer than album", now.Add(-24*time.Hour), now.Add(-1*time.Hour), now.Add(-1*time.Hour)),
+				Entry("equal timestamps", now, now, now),
+			)
+		})
+	})
+	Describe("discArtworkReader", func() {
+		Context("LastUpdated", func() {
+			// Regression test for #5377: same bug as albumArtworkReader — disc covers
+			// must also revalidate when the image file changes, not only when media files do.
+			now := time.Now().Truncate(time.Second)
+			DescribeTable("returns the max of album.UpdatedAt and ImagesUpdatedAt",
+				func(albumUpdatedAt, imagesUpdatedAt, expected time.Time) {
+					album := model.Album{ID: "al1", UpdatedAt: albumUpdatedAt}
+					folderRepo.result = []model.Folder{{ImagesUpdatedAt: imagesUpdatedAt}}
+					ds.Album(ctx).(*tests.MockAlbumRepo).SetData(model.Albums{album})
+					ds.MediaFile(ctx).(*tests.MockMediaFileRepo).SetData(model.MediaFiles{
+						{ID: "mf1", AlbumID: "al1", DiscNumber: 1, Path: "tests/fixtures/test.mp3"},
+					})
+
+					artID := model.NewArtworkID(model.KindDiscArtwork, model.DiscArtworkID("al1", 1), nil)
+					dr, err := newDiscArtworkReader(ctx, aw, artID)
+					Expect(err).ToNot(HaveOccurred())
+					Expect(dr.LastUpdated()).To(Equal(expected))
+				},
+				Entry("album newer than images", now, now.Add(-1*time.Hour), now),
+				Entry("images newer than album", now.Add(-24*time.Hour), now.Add(-1*time.Hour), now.Add(-1*time.Hour)),
+				Entry("equal timestamps", now, now, now),
+			)
+		})
 	})
 	Describe("artistArtworkReader", func() {
 		Context("Multiple covers", func() {

core/artwork/reader_album.go

@@ -72,7 +72,7 @@ func (a *albumArtworkReader) Key() string {
 	)
 }
 func (a *albumArtworkReader) LastUpdated() time.Time {
-	return a.album.UpdatedAt
+	return a.lastUpdate
 }
 
 func (a *albumArtworkReader) Reader(ctx context.Context) (io.ReadCloser, string, error) {

core/artwork/reader_disc.go

@@ -116,7 +116,7 @@ func (d *discArtworkReader) Key() string {
 }
 
 func (d *discArtworkReader) LastUpdated() time.Time {
-	return d.album.UpdatedAt
+	return d.lastUpdate
 }
 
 func (d *discArtworkReader) Reader(ctx context.Context) (io.ReadCloser, string, error) {