playback: fix crash (#5240)

when requesting a recording with the mp4 format, if there are two
tracks and the second track has no sample, the server crashed. This
fixes the issue.

internal/playback/muxer_mp4.go

@@ -94,12 +94,15 @@ func (w *muxerMP4) flush() error {
 		return recordstore.ErrNoSegmentsFound
 	}
 
-	h := pmp4.Presentation{
-		Tracks: make([]*pmp4.Track, len(w.tracks)),
+	var tracks []*pmp4.Track
+	for _, track := range w.tracks {
+		if len(track.Samples) != 0 {
+			tracks = append(tracks, &track.Track)
+		}
 	}
 
-	for i, track := range w.tracks {
-		h.Tracks[i] = &track.Track
+	h := pmp4.Presentation{
+		Tracks: tracks,
 	}
 
 	return h.Marshal(w.w)

internal/playback/muxer_mp4_test.go

@@ -0,0 +1,59 @@
+package playback
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/bluenviron/mediacommon/v2/pkg/formats/fmp4"
+	"github.com/bluenviron/mediacommon/v2/pkg/formats/mp4"
+	"github.com/bluenviron/mediamtx/internal/test"
+	"github.com/stretchr/testify/require"
+)
+
+// Test that tracks with no samples are not included in the output
+func TestMuxerMP4EmptyTracks(t *testing.T) {
+	var buf bytes.Buffer
+
+	mux := &muxerMP4{
+		w: &buf,
+	}
+
+	init := &fmp4.Init{
+		Tracks: []*fmp4.InitTrack{
+			{
+				ID:        1,
+				TimeScale: 90000,
+				Codec: &mp4.CodecH264{
+					SPS: test.FormatH264.SPS,
+					PPS: test.FormatH264.PPS,
+				},
+			},
+			{
+				ID:        2,
+				TimeScale: 48000,
+				Codec:     &mp4.CodecOpus{},
+			},
+		},
+	}
+
+	mux.writeInit(init)
+
+	// Only write samples to the first track
+	mux.setTrack(1)
+	err := mux.writeSample(0, 0, false, 5, func() ([]byte, error) {
+		return []byte{0x01, 0x02, 0x03, 0x04, 0x05}, nil
+	})
+	require.NoError(t, err)
+
+	err = mux.writeSample(90000, 0, true, 5, func() ([]byte, error) {
+		return []byte{0x06, 0x07, 0x08, 0x09, 0x0a}, nil
+	})
+	require.NoError(t, err)
+
+	mux.writeFinalDTS(180000)
+
+	err = mux.flush()
+	require.NoError(t, err)
+
+	require.Greater(t, buf.Len(), 0)
+}