res_pjsip_session: Preserve label on incoming re-INVITE.

When a re-INVITE is received we create a new set of
streams that are then swapped in as the active streams.
We did not preserve the SDP label from the previous
streams, resulting in the label getting lost.

This change ensures that if an SDP label is present
on the previous stream then it is set on the new stream.

ASTERISK-28953

Change-Id: I9dd63b88b562fe96ce5c791a3dae5bcaca258445

res/res_pjsip_session.c

@@ -762,6 +762,14 @@ static int handle_incoming_sdp(struct ast_sip_session *session, const pjmedia_sd
 				ast_stream_free(stream);
 				return -1;
 			}
+			if (existing_stream) {
+				const char *stream_label = ast_stream_get_metadata(existing_stream, "SDP:LABEL");
+
+				if (!ast_strlen_zero(stream_label)) {
+					ast_stream_set_metadata(stream, "SDP:LABEL", stream_label);
+				}
+			}
+
 			/* For backwards compatibility with the core the default audio stream is always sendrecv */
 			if (!ast_sip_session_is_pending_stream_default(session, stream) || strcmp(media, "audio")) {
 				if (pjmedia_sdp_media_find_attr2(remote_stream, "sendonly", NULL)) {
@@ -1968,6 +1976,14 @@ static int sdp_requires_deferral(struct ast_sip_session *session, const pjmedia_
 			return -1;
 		}
 
+		if (existing_stream) {
+			const char *stream_label = ast_stream_get_metadata(existing_stream, "SDP:LABEL");
+
+			if (!ast_strlen_zero(stream_label)) {
+				ast_stream_set_metadata(stream, "SDP:LABEL", stream_label);
+			}
+		}
+
 		session_media = ast_sip_session_media_state_add(session, session->pending_media_state, ast_media_type_from_str(media), i);
 		if (!session_media) {
 			return -1;