fix(server): return 404 instead of 500 for non-existent playlists

The native API endpoints GET /playlist/{id}/tracks and
GET /playlist/{id}/tracks/{id} were panicking with a nil pointer
dereference (resulting in a 500) when the playlist did not exist.
This happened because Tracks() returns nil for missing playlists,
and the nil repository was passed directly to the rest handler.
Extracted a shared playlistTracksHandler that checks for nil and
returns 404 early. Added tests covering both the error and happy paths.

server/nativeapi/playlists.go

@@ -19,47 +19,33 @@ import (
 
 type restHandler = func(rest.RepositoryConstructor, ...rest.Logger) http.HandlerFunc
 
-func getPlaylist(ds model.DataStore) http.HandlerFunc {
-	// Add a middleware to capture the playlistId
-	wrapper := func(handler restHandler) http.HandlerFunc {
-		return func(w http.ResponseWriter, r *http.Request) {
-			constructor := func(ctx context.Context) rest.Repository {
-				plsRepo := ds.Playlist(ctx)
-				plsId := chi.URLParam(r, "playlistId")
-				p := req.Params(r)
-				start := p.Int64Or("_start", 0)
-				return plsRepo.Tracks(plsId, start == 0)
-			}
-
-			handler(constructor).ServeHTTP(w, r)
-		}
-	}
-
+func playlistTracksHandler(ds model.DataStore, handler restHandler, refreshSmartPlaylist func(*http.Request) bool) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		accept := r.Header.Get("accept")
-		if strings.ToLower(accept) == "audio/x-mpegurl" {
+		plsId := chi.URLParam(r, "playlistId")
+		tracks := ds.Playlist(r.Context()).Tracks(plsId, refreshSmartPlaylist(r))
+		if tracks == nil {
+			http.Error(w, "not found", http.StatusNotFound)
+			return
+		}
+		handler(func(ctx context.Context) rest.Repository { return tracks }).ServeHTTP(w, r)
+	}
+}
+
+func getPlaylist(ds model.DataStore) http.HandlerFunc {
+	handler := playlistTracksHandler(ds, rest.GetAll, func(r *http.Request) bool {
+		return req.Params(r).Int64Or("_start", 0) == 0
+	})
+	return func(w http.ResponseWriter, r *http.Request) {
+		if strings.ToLower(r.Header.Get("accept")) == "audio/x-mpegurl" {
 			handleExportPlaylist(ds)(w, r)
 			return
 		}
-		wrapper(rest.GetAll)(w, r)
+		handler(w, r)
 	}
 }
 
 func getPlaylistTrack(ds model.DataStore) http.HandlerFunc {
-	// Add a middleware to capture the playlistId
-	wrapper := func(handler restHandler) http.HandlerFunc {
-		return func(w http.ResponseWriter, r *http.Request) {
-			constructor := func(ctx context.Context) rest.Repository {
-				plsRepo := ds.Playlist(ctx)
-				plsId := chi.URLParam(r, "playlistId")
-				return plsRepo.Tracks(plsId, true)
-			}
-
-			handler(constructor).ServeHTTP(w, r)
-		}
-	}
-
-	return wrapper(rest.Get)
+	return playlistTracksHandler(ds, rest.Get, func(*http.Request) bool { return true })
 }
 
 func createPlaylistFromM3U(playlists core.Playlists) http.HandlerFunc {

server/nativeapi/playlists_test.go

@@ -0,0 +1,167 @@
+package nativeapi
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"time"
+
+	"github.com/deluan/rest"
+	"github.com/navidrome/navidrome/conf"
+	"github.com/navidrome/navidrome/conf/configtest"
+	"github.com/navidrome/navidrome/consts"
+	"github.com/navidrome/navidrome/core/auth"
+	"github.com/navidrome/navidrome/model"
+	"github.com/navidrome/navidrome/server"
+	"github.com/navidrome/navidrome/tests"
+	. "github.com/onsi/ginkgo/v2"
+	. "github.com/onsi/gomega"
+)
+
+type mockPlaylistTrackRepo struct {
+	model.PlaylistTrackRepository
+	tracks model.PlaylistTracks
+}
+
+func (m *mockPlaylistTrackRepo) Count(...rest.QueryOptions) (int64, error) {
+	return int64(len(m.tracks)), nil
+}
+
+func (m *mockPlaylistTrackRepo) ReadAll(...rest.QueryOptions) (any, error) {
+	return m.tracks, nil
+}
+
+func (m *mockPlaylistTrackRepo) EntityName() string {
+	return "playlist_track"
+}
+
+func (m *mockPlaylistTrackRepo) NewInstance() any {
+	return &model.PlaylistTrack{}
+}
+
+func (m *mockPlaylistTrackRepo) Read(id string) (any, error) {
+	for _, t := range m.tracks {
+		if t.ID == id {
+			return &t, nil
+		}
+	}
+	return nil, rest.ErrNotFound
+}
+
+var _ = Describe("Playlist Tracks Endpoint", func() {
+	var (
+		router   http.Handler
+		ds       *tests.MockDataStore
+		plsRepo  *tests.MockPlaylistRepo
+		userRepo *tests.MockedUserRepo
+		w        *httptest.ResponseRecorder
+	)
+
+	BeforeEach(func() {
+		DeferCleanup(configtest.SetupConfig())
+		conf.Server.SessionTimeout = time.Minute
+
+		plsRepo = &tests.MockPlaylistRepo{}
+		userRepo = tests.CreateMockUserRepo()
+
+		ds = &tests.MockDataStore{
+			MockedPlaylist: plsRepo,
+			MockedUser:     userRepo,
+			MockedProperty: &tests.MockedPropertyRepo{},
+		}
+
+		auth.Init(ds)
+
+		testUser := model.User{
+			ID:          "user-1",
+			UserName:    "testuser",
+			Name:        "Test User",
+			IsAdmin:     false,
+			NewPassword: "testpass",
+		}
+		err := userRepo.Put(&testUser)
+		Expect(err).ToNot(HaveOccurred())
+
+		nativeRouter := New(ds, nil, nil, nil, tests.NewMockLibraryService(), tests.NewMockUserService(), nil, nil)
+		router = server.JWTVerifier(nativeRouter)
+		w = httptest.NewRecorder()
+	})
+
+	createAuthenticatedRequest := func(method, path string) *http.Request {
+		req := httptest.NewRequest(method, path, nil)
+		testUser := model.User{ID: "user-1", UserName: "testuser"}
+		token, err := auth.CreateToken(&testUser)
+		Expect(err).ToNot(HaveOccurred())
+		req.Header.Set(consts.UIAuthorizationHeader, "Bearer "+token)
+		return req
+	}
+
+	Describe("GET /playlist/{playlistId}/tracks", func() {
+		It("returns 404 when playlist does not exist", func() {
+			req := createAuthenticatedRequest("GET", "/playlist/non-existent/tracks")
+			router.ServeHTTP(w, req)
+
+			Expect(w.Code).To(Equal(http.StatusNotFound))
+		})
+
+		It("returns tracks when playlist exists", func() {
+			plsRepo.TracksReturn = &mockPlaylistTrackRepo{
+				tracks: model.PlaylistTracks{
+					{ID: "1", MediaFileID: "mf-1", PlaylistID: "pls-1"},
+					{ID: "2", MediaFileID: "mf-2", PlaylistID: "pls-1"},
+				},
+			}
+
+			req := createAuthenticatedRequest("GET", "/playlist/pls-1/tracks")
+			router.ServeHTTP(w, req)
+
+			Expect(w.Code).To(Equal(http.StatusOK))
+
+			var response []model.PlaylistTrack
+			err := json.Unmarshal(w.Body.Bytes(), &response)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(response).To(HaveLen(2))
+			Expect(response[0].ID).To(Equal("1"))
+			Expect(response[1].ID).To(Equal("2"))
+		})
+	})
+
+	Describe("GET /playlist/{playlistId}/tracks/{id}", func() {
+		It("returns 404 when playlist does not exist", func() {
+			req := createAuthenticatedRequest("GET", "/playlist/non-existent/tracks/1")
+			router.ServeHTTP(w, req)
+
+			Expect(w.Code).To(Equal(http.StatusNotFound))
+		})
+
+		It("returns the track when playlist exists", func() {
+			plsRepo.TracksReturn = &mockPlaylistTrackRepo{
+				tracks: model.PlaylistTracks{
+					{ID: "1", MediaFileID: "mf-1", PlaylistID: "pls-1"},
+				},
+			}
+
+			req := createAuthenticatedRequest("GET", "/playlist/pls-1/tracks/1")
+			router.ServeHTTP(w, req)
+
+			Expect(w.Code).To(Equal(http.StatusOK))
+
+			var response model.PlaylistTrack
+			err := json.Unmarshal(w.Body.Bytes(), &response)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(response.ID).To(Equal("1"))
+			Expect(response.MediaFileID).To(Equal("mf-1"))
+		})
+
+		It("returns 404 when track does not exist in playlist", func() {
+			plsRepo.TracksReturn = &mockPlaylistTrackRepo{
+				tracks: model.PlaylistTracks{},
+			}
+
+			req := createAuthenticatedRequest("GET", "/playlist/pls-1/tracks/999")
+			router.ServeHTTP(w, req)
+
+			Expect(w.Code).To(Equal(http.StatusNotFound))
+		})
+	})
+})