internal/mux: preserve buffered audio on an unsupported seek

internal/mux/mux.go

@@ -361,6 +361,12 @@ func (p *Player) Seek(offset int64, whence int) (int64, error) {
 }
 
 func (p *playerImpl) Seek(offset int64, whence int) (int64, error) {
+	// Check if the source implements io.Seeker.
+	s, ok := p.src.(io.Seeker)
+	if !ok {
+		return 0, errors.New("mux: the source must implement io.Seeker")
+	}
+
 	defer p.mux.signal()
 
 	p.m.Lock()
@@ -387,11 +393,6 @@ func (p *playerImpl) Seek(offset int64, whence int) (int64, error) {
 		p.reportSourceErrorIfDrainedImpl()
 	}
 
-	// Check if the source implements io.Seeker.
-	s, ok := p.src.(io.Seeker)
-	if !ok {
-		return 0, errors.New("mux: the source must implement io.Seeker")
-	}
 	return s.Seek(offset, whence)
 }
 

internal/mux/mux_test.go

@@ -1003,3 +1003,35 @@ func TestPartialSampleAtEOFStopsPlayer(t *testing.T) {
 		t.Errorf("IsRegistered after EOF: got %v; want false", got)
 	}
 }
+
+func TestSeekNonSeekerPreservesBuffer(t *testing.T) {
+	for _, playing := range []bool{false, true} {
+		t.Run(fmt.Sprintf("playing=%t", playing), func(t *testing.T) {
+			src := []byte{0, 64, 128, 192, 255}
+			m := mux.New(48000, 1, mux.FormatUnsignedInt8)
+			p := newPlayer(t, m, bytes.NewBuffer(src))
+			p.Play()
+			waitForBufferedSize(t, p, len(src))
+			if !playing {
+				p.PauseAndStopReading()
+			}
+			if _, err := p.Seek(0, io.SeekStart); err == nil {
+				t.Fatal("Seek on a non-seekable source succeeded")
+			}
+			if got := p.BufferedSize(); got != len(src) {
+				t.Fatalf("BufferedSize after failed Seek: got %d; want %d", got, len(src))
+			}
+			if got := p.IsPlaying(); got != playing {
+				t.Fatalf("IsPlaying: got %t; want %t", got, playing)
+			}
+			p.Play()
+			buf := make([]float32, len(src))
+			m.ReadFloat32s(buf)
+			for i, got := range buf {
+				if want := float32(src[i])/128 - 1; got != want {
+					t.Errorf("sample %d: got %v; want %v", i, got, want)
+				}
+			}
+		})
+	}
+}