Add discTitles to OpenSubsonic responses

server/subsonic/responses/.snapshots/Responses AlbumWithSongsID3 with data should match .JSON

@@ -21,6 +21,16 @@
     "musicBrainzId": "1234",
     "isCompilation": true,
     "sortName": "sorted album",
+    "discTitles": [
+      {
+        "disc": 1,
+        "title": "disc 1"
+      },
+      {
+        "disc": 2,
+        "title": "disc 2"
+      }
+    ],
     "song": [
       {
         "id": "1",

server/subsonic/responses/.snapshots/Responses AlbumWithSongsID3 with data should match .XML

@@ -2,6 +2,8 @@
   <album id="1" name="album" artist="artist" genre="rock" userRating="0" musicBrainzId="1234" isCompilation="true" sortName="sorted album">
     <genres name="rock"></genres>
     <genres name="progressive"></genres>
+    <discTitles disc="1" title="disc 1"></discTitles>
+    <discTitles disc="2" title="disc 2"></discTitles>
     <song id="1" isDir="true" title="title" album="album" artist="artist" track="1" year="1985" genre="Rock" coverArt="1" size="8421341" contentType="audio/flac" suffix="flac" starred="2016-03-02T20:30:00Z" transcodedContentType="audio/mpeg" transcodedSuffix="mp3" duration="146" bitRate="320" isVideo="false" bpm="127" comment="a comment" sortName="sorted song" mediaType="song" musicBrainzId="4321">
       <genres name="rock"></genres>
       <genres name="progressive"></genres>

server/subsonic/responses/.snapshots/Responses AlbumWithSongsID3 without data should match .JSON

@@ -11,6 +11,7 @@
     "genres": [],
     "musicBrainzId": "",
     "isCompilation": false,
-    "sortName": ""
+    "sortName": "",
+    "discTitles": []
   }
 }

server/subsonic/responses/responses.go

@@ -224,6 +224,7 @@ type AlbumID3 struct {
 	MusicBrainzId string     `xml:"musicBrainzId,attr"    json:"musicBrainzId"`
 	IsCompilation bool       `xml:"isCompilation,attr"    json:"isCompilation"`
 	SortName      string     `xml:"sortName,attr"         json:"sortName"`
+	DiscTitles    DiscTitles `xml:"discTitles"            json:"discTitles"`
 }
 
 type ArtistWithAlbumsID3 struct {
@@ -459,12 +460,7 @@ type ItemGenre struct {
 type ItemGenres []ItemGenre
 
 func (i ItemGenres) MarshalJSON() ([]byte, error) {
-	if len(i) == 0 {
-		return json.Marshal([]ItemGenre{})
-	}
-	type Alias []ItemGenre
-	a := (Alias)(i)
-	return json.Marshal(a)
+	return marshalJSONArray(i)
 }
 
 type ReplayGain struct {
@@ -475,3 +471,24 @@ type ReplayGain struct {
 	BaseGain     float64 `xml:"baseGain,omitempty,attr"     json:"baseGain,omitempty"`
 	FallbackGain float64 `xml:"fallbackGain,omitempty,attr" json:"fallbackGain,omitempty"`
 }
+
+type DiscTitle struct {
+	Disc  int    `xml:"disc,attr,omitempty" json:"disc,omitempty"`
+	Title string `xml:"title,attr,omitempty" json:"title,omitempty"`
+}
+
+type DiscTitles []DiscTitle
+
+func (d DiscTitles) MarshalJSON() ([]byte, error) {
+	return marshalJSONArray(d)
+}
+
+// marshalJSONArray marshals a slice of any type to JSON. If the slice is empty, it is marshalled as an
+// empty array instead of null.
+func marshalJSONArray[T any](v []T) ([]byte, error) {
+	if len(v) == 0 {
+		return json.Marshal([]T{})
+	}
+	a := v
+	return json.Marshal(a)
+}

server/subsonic/responses/responses_test.go

@@ -176,6 +176,7 @@ var _ = Describe("Responses", func() {
 					Id: "1", Name: "album", Artist: "artist", Genre: "rock",
 					Genres:        []ItemGenre{{Name: "rock"}, {Name: "progressive"}},
 					MusicBrainzId: "1234", IsCompilation: true, SortName: "sorted album",
+					DiscTitles: DiscTitles{{Disc: 1, Title: "disc 1"}, {Disc: 2, Title: "disc 2"}},
 				}
 				t := time.Date(2016, 03, 2, 20, 30, 0, 0, time.UTC)
 				songs := []Child{{