Implement a workaround to signal Opus support (mumble) (#1764)

* Mumble: Implement a workaround to signal Opus support without pulling in the CGO gopus dependency.

* mumble: lowercase error messages

* mumble: Add link to #1750 in bridge/mumble/codec.go

bridge/mumble/codec.go

@@ -0,0 +1,70 @@
+package bmumble
+
+import (
+	"fmt"
+
+	"layeh.com/gumble/gumble"
+)
+
+// This is a dummy implementation of a Gumble audio codec which claims
+// to implement Opus, but does not actually do anything.  This serves
+// as a workaround until https://github.com/layeh/gumble/pull/61 is
+// merged.
+// See https://github.com/42wim/matterbridge/issues/1750 for details.
+
+const (
+	audioCodecIDOpus = 4
+)
+
+func registerNullCodecAsOpus() {
+	codec := &NullCodec{
+		encoder: &NullAudioEncoder{},
+		decoder: &NullAudioDecoder{},
+	}
+	gumble.RegisterAudioCodec(audioCodecIDOpus, codec)
+}
+
+type NullCodec struct {
+	encoder *NullAudioEncoder
+	decoder *NullAudioDecoder
+}
+
+func (c *NullCodec) ID() int {
+	return audioCodecIDOpus
+}
+
+func (c *NullCodec) NewEncoder() gumble.AudioEncoder {
+	e := &NullAudioEncoder{}
+	return e
+}
+
+func (c *NullCodec) NewDecoder() gumble.AudioDecoder {
+	d := &NullAudioDecoder{}
+	return d
+}
+
+type NullAudioEncoder struct{}
+
+func (e *NullAudioEncoder) ID() int {
+	return audioCodecIDOpus
+}
+
+func (e *NullAudioEncoder) Encode(pcm []int16, mframeSize, maxDataBytes int) ([]byte, error) {
+	return nil, fmt.Errorf("not implemented")
+}
+
+func (e *NullAudioEncoder) Reset() {
+}
+
+type NullAudioDecoder struct{}
+
+func (d *NullAudioDecoder) ID() int {
+	return audioCodecIDOpus
+}
+
+func (d *NullAudioDecoder) Decode(data []byte, frameSize int) ([]int16, error) {
+	return nil, fmt.Errorf("not implemented")
+}
+
+func (d *NullAudioDecoder) Reset() {
+}

bridge/mumble/mumble.go

@@ -185,6 +185,7 @@ func (b *Bmumble) doConnect() error {
 		gumbleConfig.Password = password
 	}
 
+	registerNullCodecAsOpus()
 	client, err := gumble.DialWithDialer(new(net.Dialer), b.GetString("Server"), gumbleConfig, &b.tlsConfig)
 	if err != nil {
 		return err