mgcp: Make sure endpoint is free on CRCX

The MSC is aware of the assigned endpoints at all times, so it
will not assign an occupied endpoint to someone else. However,
if it has just restarted, there maybe lingering open endpoints.

This patch introduces a fairly simple soultion. Before a new
endpoint is seized (CRCX), a DLCX is send in advance. If the
endpoint was still occupied with a dead connection, it will
be freed. If it was free anyway, the DLCX will just have no
effect.

Change-Id: I7990e9e7c4c8101b8772ab5505a7cc11f8f7cd23

src/libmsc/msc_ifaces.c

@@ -193,6 +193,7 @@ static int conn_iu_rab_act_cs(struct gsm_trans *trans)
 	struct gsm_subscriber_connection *conn = trans->conn;
 	struct mgcpgw_client *mgcp = conn->network->mgcpgw.client;
 	struct msgb *msg;
+	struct msgb *msg_dlcx;
 
 	/* HACK. where to scope the RAB Id? At the conn / subscriber /
 	 * ue_conn_ctx? */
@@ -205,6 +206,16 @@ static int conn_iu_rab_act_cs(struct gsm_trans *trans)
 	 * and config. */
 	conn->iu.mgcp_rtp_port_ue = 4000 + 2 * conn->iu.mgcp_rtp_endpoint;
 
+	/* Since we know now the endpoint number, we enforce a DLCX on tha
+	 * endpoint in order to ensure that this endpoint is not occupied
+	 * with some old connection that was not properly cleared during
+	 * some crash or restart event */
+	msg_dlcx = mgcp_msg_dlcx(mgcp, conn->iu.mgcp_rtp_endpoint);
+	if (mgcpgw_client_tx(mgcp, msg_dlcx, NULL, NULL))
+		LOGP(DMGCP, LOGL_ERROR,
+		     "Failed to send DLCX message for %s\n",
+		     vlr_subscr_name(trans->vsub));
+
 	/* Establish the RTP stream first as looping back to the originator.
 	 * The MDCX will patch through to the counterpart. TODO: play a ring
 	 * tone instead. */