res_pjsip_session: Reinvite using active stream topology if none requested.

When a connected line update is sent to an endpoint we do not request
a specific stream topology to be used. Previously this resulted in the
configured stream topology being used which may actually differ from the
currently negotiated topology. PJSIP is helpful in this regard in that
it will fill in any missing streams with removed ones. This results in
our own state not matching the SDP, though, and we do not apply the
negotiated SDP.

This change tweaks the code to use the actively negotiated stream
topology if it is present with a fallback to the configured one. This
results in the SDP and the state having matching information and the
world is happy.

ASTERISK*27397

Change-Id: I7a57117f0183479e6884b7bf3a53bb8c7464f604

res/res_pjsip_session.c

@@ -3872,10 +3872,15 @@ static struct pjmedia_sdp_session *create_local_sdp(pjsip_inv_session *inv, stru
 
 	if (!session->pending_media_state->topology || !ast_stream_topology_get_count(session->pending_media_state->topology)) {
 		/* We've encountered a situation where we have been told to create a local SDP but noone has given us any indication
-		 * of what kind of stream topology they would like. As a fallback we use the topology from the configured endpoint.
+		 * of what kind of stream topology they would like. We try to not alter the current state of the SDP negotiation
+		 * by using what is currently negotiated. If this is unavailable we fall back to what is configured on the endpoint.
 		 */
 		ast_stream_topology_free(session->pending_media_state->topology);
-		session->pending_media_state->topology = ast_stream_topology_clone(session->endpoint->media.topology);
+		if (session->active_media_state->topology) {
+			session->pending_media_state->topology = ast_stream_topology_clone(session->active_media_state->topology);
+		} else {
+			session->pending_media_state->topology = ast_stream_topology_clone(session->endpoint->media.topology);
+		}
 		if (!session->pending_media_state->topology) {
 			return NULL;
 		}