fix(jellyfin): honor repeated Fields query params (#5811)

The Fields param was read with StringOr, which keeps only one value, so a
request sending it as repeated params (Fields=Genres&Fields=MediaSources) — as
Finamp and Feishin do — lost all but the first. Field-gated data like
MediaSources was then omitted for Audio items even though the client asked for
it; the comma-separated form happened to work because ParseFields splits on
commas. Real Jellyfin accepts both forms.

ParseFields is now variadic and a parseFields helper reads every repeated value
via req.Values.Strings, applied to the item list, single-item, and playlist
endpoints.

server/jellyfin/dto/fields.go

@@ -7,12 +7,15 @@ import "strings"
 // omits those unless the client asks for them.
 type Fields map[string]struct{}
 
-// ParseFields splits the comma-separated Fields param into a lowercased set.
-func ParseFields(csv string) Fields {
+// ParseFields builds a lowercased set from the Fields param. It accepts each value comma-separated
+// (Fields=a,b) and across repeated params (Fields=a&Fields=b), both of which real Jellyfin honors.
+func ParseFields(values ...string) Fields {
 	f := Fields{}
-	for name := range strings.SplitSeq(csv, ",") {
-		if name = strings.TrimSpace(strings.ToLower(name)); name != "" {
-			f[name] = struct{}{}
+	for _, csv := range values {
+		for name := range strings.SplitSeq(csv, ",") {
+			if name = strings.TrimSpace(strings.ToLower(name)); name != "" {
+				f[name] = struct{}{}
+			}
 		}
 	}
 	return f

server/jellyfin/dto/fields_test.go

@@ -0,0 +1,26 @@
+package dto
+
+import (
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("ParseFields", func() {
+	It("parses a single comma-separated value", func() {
+		f := ParseFields("Genres,MediaSources")
+		Expect(f.Has("Genres")).To(BeTrue())
+		Expect(f.Has("MediaSources")).To(BeTrue())
+	})
+
+	It("parses fields spread across repeated params", func() {
+		f := ParseFields("Genres", "MediaSources", "SortName")
+		Expect(f.Has("Genres")).To(BeTrue())
+		Expect(f.Has("MediaSources")).To(BeTrue())
+		Expect(f.Has("SortName")).To(BeTrue())
+	})
+
+	It("returns an empty set for no values", func() {
+		Expect(ParseFields()).To(BeEmpty())
+		Expect(ParseFields("")).To(BeEmpty())
+	})
+})

server/jellyfin/e2e/browsing_test.go

@@ -62,6 +62,16 @@ var _ = Describe("Browsing", func() {
 			}
 		})
 
+		// Clients (Finamp, Feishin) send Fields as repeated params rather than one comma-separated
+		// value; real Jellyfin accepts both, so a later Fields=MediaSources must still take effect.
+		It("honors MediaSources when Fields is sent as repeated params", func() {
+			q := queryResult(get("/Items?IncludeItemTypes=Audio&Recursive=true&Fields=Genres&Fields=MediaSources"))
+			Expect(q.Items).ToNot(BeEmpty())
+			for _, it := range q.Items {
+				Expect(it.MediaSources).To(HaveLen(1))
+			}
+		})
+
 		It("lists all album artists", func() {
 			q := queryResult(get("/Items?IncludeItemTypes=MusicArtist&Recursive=true"))
 			Expect(q.TotalRecordCount).To(Equal(4))

server/jellyfin/items.go

@@ -224,13 +224,20 @@ type itemsQuery struct {
 	albumIds         []string
 }
 
+// parseFields reads the Fields param, accepting both repeated params (Fields=a&Fields=b) and a
+// comma-separated value (Fields=a,b), matching real Jellyfin. StringOr would keep only one value.
+func parseFields(p *req.Values) dto.Fields {
+	values, _ := p.Strings("fields")
+	return dto.ParseFields(values...)
+}
+
 // parseItemsQuery also resolves the entity types (inferring them from the parent when
 // IncludeItemTypes is absent) and the library scope. Query keys are read lowercase because
 // normalizeQueryKeys folded them (Jellyfin binds case-insensitively).
 func (api *Router) parseItemsQuery(ctx context.Context, r *http.Request) itemsQuery {
 	p := req.Params(r)
 	q := itemsQuery{
-		fields:    dto.ParseFields(p.StringOr("fields", "")),
+		fields:    parseFields(p),
 		ids:       decodedQueryIDs(r, "ids"),
 		rawTypes:  p.StringOr("includeitemtypes", ""),
 		search:    searchTerm(p),
@@ -730,7 +737,7 @@ func (api *Router) itemsByIDs(ctx context.Context, ids []string, fields dto.Fiel
 
 func (api *Router) getItem(w http.ResponseWriter, r *http.Request) {
 	id := api.resolveItemID(r.Context(), dto.DecodeID(chi.URLParam(r, "itemId")))
-	fields := dto.ParseFields(req.Params(r).StringOr("fields", ""))
+	fields := parseFields(req.Params(r))
 	if item, ok := api.resolveItemByID(r.Context(), id, fields); ok {
 		api.ok(w, r, item)
 		return

server/jellyfin/playlists.go

@@ -191,7 +191,7 @@ func (api *Router) getPlaylistItems(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	p := req.Params(r)
-	fields := dto.ParseFields(p.StringOr("fields", ""))
+	fields := parseFields(p)
 	res, err := api.playlistTrackPage(repo, fields, p.IntOr("startindex", 0), p.IntOr("limit", 0))
 	if err != nil {
 		api.internalError(w, r, err)