feat(metadata): add download attribution to comments

go_backend/download_comment_test.go

@@ -0,0 +1,34 @@
+package gobackend
+
+import "testing"
+
+func TestBuildDownloadedFileCommentKeepsSourceLinkAndAddsCredit(t *testing.T) {
+	const source = "https://music.apple.com/us/album/example/123"
+	got := buildDownloadedFileComment(source, "https://music.amazon.com/albums/example")
+	want := source + "\n" + downloadCommentCredit
+	if got != want {
+		t.Fatalf("comment = %q, want %q", got, want)
+	}
+}
+
+func TestBuildDownloadedFileCommentUsesProviderCommentWhenSourceIsEmpty(t *testing.T) {
+	const provider = "https://music.amazon.com/albums/example"
+	got := buildDownloadedFileComment("", provider)
+	want := provider + "\n" + downloadCommentCredit
+	if got != want {
+		t.Fatalf("comment = %q, want %q", got, want)
+	}
+}
+
+func TestBuildDownloadedFileCommentAddsCreditWithoutLink(t *testing.T) {
+	if got := buildDownloadedFileComment("", ""); got != downloadCommentCredit {
+		t.Fatalf("comment = %q, want credit only", got)
+	}
+}
+
+func TestBuildDownloadedFileCommentDoesNotDuplicateCredit(t *testing.T) {
+	original := "https://example.test/album/1\n" + downloadCommentCredit
+	if got := buildDownloadedFileComment(original, ""); got != original {
+		t.Fatalf("credit was duplicated: %q", got)
+	}
+}

go_backend/exports_download.go

@@ -134,6 +134,24 @@ type DownloadResult struct {
 	RequiresContainerConversion bool
 }
 
+const downloadCommentCredit = "Downloaded with SpotiFLAC Mobile. Enjoy your music!"
+
+func buildDownloadedFileComment(sourceComment, providerComment string) string {
+	comment := strings.TrimSpace(sourceComment)
+	if comment == "" {
+		comment = strings.TrimSpace(providerComment)
+	}
+	for _, line := range strings.Split(strings.ReplaceAll(comment, "\r\n", "\n"), "\n") {
+		if strings.EqualFold(strings.TrimSpace(line), downloadCommentCredit) {
+			return comment
+		}
+	}
+	if comment == "" {
+		return downloadCommentCredit
+	}
+	return comment + "\n" + downloadCommentCredit
+}
+
 func buildDownloadSuccessResponse(
 	req DownloadRequest,
 	result DownloadResult,
@@ -187,10 +205,7 @@ func buildDownloadSuccessResponse(
 	if composer == "" {
 		composer = req.Composer
 	}
-	comment := result.Comment
-	if comment == "" {
-		comment = req.Comment
-	}
+	comment := buildDownloadedFileComment(req.Comment, result.Comment)
 
 	albumType := result.AlbumType
 	if albumType == "" {

go_backend/extension_fallback_metadata_test.go

@@ -34,8 +34,9 @@ func TestOverlayExtensionReleaseMetadataFillsMissingRequestFields(t *testing.T)
 		"song.m4a",
 		false,
 	)
+	expectedComment := track.Comment + "\n" + downloadCommentCredit
 	if response.UPC != track.UPC || response.AlbumType != "single" ||
-		!response.Explicit || response.Comment != track.Comment {
+		!response.Explicit || response.Comment != expectedComment {
 		t.Fatalf("enriched metadata was lost in download response: %#v", response)
 	}
 }

lib/providers/download_queue_provider_embedding.dart

@@ -528,6 +528,7 @@ extension _DownloadQueueEmbedding on DownloadQueueNotifier {
     final backendComment = normalizeOptionalString(
       backendResult['comment']?.toString(),
     );
+    final resolvedComment = backendComment ?? sourceComment;
     final backendAlbumType = normalizeOptionalString(
       backendResult['album_type']?.toString(),
     );
@@ -572,7 +573,7 @@ extension _DownloadQueueEmbedding on DownloadQueueNotifier {
         (sourceGenre == null && backendGenre != null) ||
         (sourceLabel == null && backendLabel != null) ||
         (sourceCopyright == null && backendCopyright != null) ||
-        (sourceComment == null && backendComment != null) ||
+        resolvedComment != sourceComment ||
         (baseTrack.explicit != true && backendExplicit) ||
         (sourceUpc == null && backendUpc != null);
 
@@ -604,7 +605,7 @@ extension _DownloadQueueEmbedding on DownloadQueueNotifier {
       genre: sourceGenre ?? backendGenre,
       label: sourceLabel ?? backendLabel,
       copyright: sourceCopyright ?? backendCopyright,
-      comment: sourceComment ?? backendComment,
+      comment: resolvedComment,
       source: baseTrack.source,
       itemType: baseTrack.itemType,
       audioQuality: baseTrack.audioQuality,