Merge pull request #198 from gopxl/clear-mixer-during-callback

Fix panic when clearing Mixer during mixing

mixer.go

@@ -45,12 +45,12 @@ func (m *Mixer) Stream(samples [][2]float64) (n int, ok bool) {
 	for len(samples) > 0 {
 		toStream := min(len(tmp), len(samples))
 
-		// clear the samples
+		// Clear the samples
 		clear(samples[:toStream])
 
 		snMax := 0
 		for si := 0; si < len(m.streamers); si++ {
-			// mix the stream
+			// Mix the stream
 			sn, sok := m.streamers[si].Stream(tmp[:toStream])
 			for i := range tmp[:sn] {
 				samples[i][0] += tmp[i][0]
@@ -61,12 +61,16 @@ func (m *Mixer) Stream(samples [][2]float64) (n int, ok bool) {
 			}
 
 			if sn < toStream || !sok {
-				// remove drained streamer
-				last := len(m.streamers) - 1
-				m.streamers[si] = m.streamers[last]
-				m.streamers[last] = nil
-				m.streamers = m.streamers[:last]
-				si--
+				// Remove drained streamer.
+				// Check the length of m.streamers again in case the call to Stream()
+				// had a callback which clears the Mixer.
+				if len(m.streamers) > 0 {
+					last := len(m.streamers) - 1
+					m.streamers[si] = m.streamers[last]
+					m.streamers[last] = nil
+					m.streamers = m.streamers[:last]
+					si--
+				}
 
 				if m.stopWhenEmpty && len(m.streamers) == 0 {
 					return n + snMax, true

mixer_test.go

@@ -96,6 +96,17 @@ func TestMixer_PlaysSilenceWhenNoStreamersProduceSamples(t *testing.T) {
 	assert.Equal(t, make([][2]float64, 10), samples)
 }
 
+// Bug: https://github.com/gopxl/beep/issues/197#issuecomment-2468951277
+func TestMixer_CanClearDuringCallback(t *testing.T) {
+	m := beep.Mixer{}
+
+	m.Add(beep.Callback(func() {
+		m.Clear()
+	}))
+
+	testtools.CollectNum(10, &m)
+}
+
 func BenchmarkMixer_MultipleStreams(b *testing.B) {
 	s1, _ := testtools.RandomDataStreamer(b.N)
 	s2, _ := testtools.RandomDataStreamer(b.N)