Fix backchannel reconnection issue

cmd/rtsp/rtsp.go

@@ -84,10 +84,10 @@ func rtspHandler(url string) (streamer.Producer, error) {
 		}
 
 		// second try without backchannel, we need to reconnect
+		conn.Backchannel = false
 		if err = conn.Dial(); err != nil {
 			return nil, err
 		}
-		conn.Backchannel = false
 		if err = conn.Describe(); err != nil {
 			return nil, err
 		}

cmd/streams/producer.go

@@ -64,6 +64,9 @@ func (p *Producer) GetTrack(media *streamer.Media, codec *streamer.Codec) *strea
 	}
 
 	track := p.element.GetTrack(media, codec)
+	if track == nil {
+		return nil
+	}
 
 	for _, t := range p.tracks {
 		if track == t {

pkg/rtsp/conn.go

@@ -362,21 +362,25 @@ func (c *Conn) SetupMedia(
 	var res *tcp.Response
 	res, err = c.Do(req)
 	if err != nil {
-		// Dahua VTO2111D fail on this step because of backchannel
+		// some Dahua/Amcrest cameras fail here because two simultaneous
+		// backchannel connections
 		if c.Backchannel {
-			if err = c.Dial(); err != nil {
-				return nil, err
-			}
 			c.Backchannel = false
-			if err = c.Describe(); err != nil {
+			if err := c.Dial(); err != nil {
 				return nil, err
 			}
-			res, err = c.Do(req)
+			if err := c.Describe(); err != nil {
+				return nil, err
+			}
+
+			for _, newMedia := range c.Medias {
+				if newMedia.Control == media.Control {
+					return c.SetupMedia(newMedia, newMedia.Codecs[0])
+				}
+			}
 		}
 
-		if err != nil {
-			return nil, err
-		}
+		return nil, err
 	}
 
 	if c.Session == "" {