mid_registrar_save(): Fix shm memleak with no t_relay()

Since mid_registrar_save() is stateful by definition, it needs the SIP
transaction to properly function, so always forcing its creation if the
REGISTER is about to be forwarded is not that significant of a change,
if any at all.

Thus, this patch fixes a SHM memleak on the following type of logic:

    mid_registrar_save();

    # script terminates without having created the transaction,
    # so mid_registrar's "prepped" data does not get freed on T deletion
    exit;

modules/mid_registrar/save.c

@@ -1861,6 +1861,28 @@ static int prepare_forward(struct sip_msg *msg, udomain_t *d,
 {
 	struct mid_reg_info *mri;
 	struct to_body *to, *from;
+	int rc;
+
+	rc = tmb.t_newtran(msg);
+	switch (rc) {
+	case 1:
+		break;
+
+	case E_SCRIPT:
+		LM_DBG("%.*s transaction already exists, continuing...\n",
+		       msg->REQ_METHOD_S.len, msg->REQ_METHOD_S.s);
+		break;
+
+	case 0:
+		LM_INFO("absorbing %.*s retransmission, use t_check_trans() "
+		        "earlier\n", msg->REQ_METHOD_S.len, msg->REQ_METHOD_S.s);
+		return 0;
+
+	default:
+		LM_ERR("internal error %d while creating %.*s transaction\n",
+		       rc, msg->REQ_METHOD_S.len, msg->REQ_METHOD_S.s);
+		return -1;
+	}
 
 	LM_DBG("from: '%.*s'\n", msg->from->body.len, msg->from->body.s);
 	LM_DBG("Call-ID: '%.*s'\n", msg->callid->body.len, msg->callid->body.s);
@@ -2471,11 +2493,6 @@ int mid_reg_save(struct sip_msg *msg, udomain_t *d, str *flags_str,
 		return -1;
 	}
 
-	if (((int (*)(struct sip_msg *))tmb.t_check_trans)(msg) == 0) {
-		LM_INFO("absorbing retransmission, use t_check_trans() earlier!\n");
-		return 0;
-	}
-
 	rerrno = R_FINE;
 	memset(&sctx, 0, sizeof sctx);
 

modules/tm/t_hooks.h

@@ -153,6 +153,9 @@ struct cell;
  *  to any other piece of code except the transaction cleanup routine. All
  *  registered callbacks of this type will be invoked by the cleanup routine,
  *  after which the transaction object will be freed.
+ *    Note: if you schedule data for deletion using this callback, then you may
+ *          have to ensure a transaction always exists beforehand, using
+ *          tmb.t_newtran()
  *
  * TMCB_MSG_MATCHED_IN -- triggered whenever there is an
  *  incoming SIP message matching the transaction. It may be

modules/tm/tm_load.h

@@ -54,7 +54,7 @@ struct tm_binds {
 	 *		1 (success)
 	 *		0 (retransmission)
 	 *	  < 0 (error)
-	 *			* E_SCRIPT (T already exists)
+	 *			* E_SCRIPT (the current transaction (@T) is already populated)
 	 *			* (others)
 	 */
 	tnewtran_f       t_newtran;