Fix transport-wide CC feedback when simulcast SSRCs are missing (#2908)

src/ice.c

@@ -3979,6 +3979,7 @@ static gboolean janus_ice_outgoing_transport_wide_cc_feedback(gpointer user_data
 	janus_ice_handle *handle = (janus_ice_handle *)user_data;
 	janus_ice_peerconnection *pc = handle->pc;
 
+	guint32 ssrc_peer = 0;
 	janus_ice_peerconnection_medium *medium = NULL;
 	if(pc) {
 		/* Find inbound video medium */
@@ -3989,14 +3990,31 @@ static gboolean janus_ice_outgoing_transport_wide_cc_feedback(gpointer user_data
 		while (g_hash_table_iter_next(&iter, NULL, &value)) {
 			janus_ice_peerconnection_medium *m = value;
 			if(m && m->type == JANUS_MEDIA_VIDEO && m->recv) {
-				medium = m;
-				break;
+				/* If a medium (or simulcast layer, if applicable) has not received data, its SSRC may be unknown. */
+				/* Pick the first valid SSRC we find across all considered mediums */
+				int i = 0;
+				for(i = 0; i < 3; i++) {
+					if(m->ssrc_peer[i] != 0) {
+						ssrc_peer = m->ssrc_peer[i];
+						medium = m;
+						break;
+					}
+				}
+
+				/* Stop if we found a valid SSRC/medium to use */
+				if(medium && ssrc_peer)
+					break;
 			}
 		}
 		janus_mutex_unlock(&handle->mutex);
 	}
 
-	if(pc && pc->do_transport_wide_cc && medium) {
+	if(medium == NULL) {
+		JANUS_LOG(LOG_HUGE, "No medium with a valid peer SSRC found for transport-wide CC feedback\n");
+		return G_SOURCE_CONTINUE;
+	}
+
+	if(pc && pc->do_transport_wide_cc) {
 		/* Create a transport wide feedback message */
 		size_t size = 1300;
 		char rtcpbuf[1300];
@@ -4065,7 +4083,7 @@ static gboolean janus_ice_outgoing_transport_wide_cc_feedback(gpointer user_data
 			guint8 feedback_packet_count = pc->transport_wide_cc_feedback_count++;
 			/* Create RTCP packet */
 			int len = janus_rtcp_transport_wide_cc_feedback(rtcpbuf, size,
-				medium->ssrc, medium->ssrc_peer[0], feedback_packet_count, packets_to_process);
+				medium->ssrc, ssrc_peer, feedback_packet_count, packets_to_process);
 			/* Enqueue it, we'll send it later */
 			if(len > 0) {
 				janus_plugin_rtcp rtcp = { .mindex = medium->mindex, .video = TRUE, .buffer = rtcpbuf, .length = len };