Proper exit if bsc_nat_fsm fails to start

Instead of segfaulting later on, properly exit if the bsc_nat_fsm does
not start. One reason for having it fail could be a missing mgw pool
config block.

Related: SYS#5560
Change-Id: Ia8bbe6ae908d5c3ce49f71b43c950497aeebb6d6

include/osmocom/bsc_nat/bsc_nat_fsm.h

@@ -20,5 +20,5 @@
 #pragma once
 
 void bsc_nat_fsm_alloc(struct bsc_nat *bsc_nat);
-void bsc_nat_fsm_start(struct bsc_nat *bsc_nat);
+int bsc_nat_fsm_start(struct bsc_nat *bsc_nat);
 void bsc_nat_fsm_stop(struct bsc_nat *bsc_nat);

src/osmo-bsc-nat/bsc_nat_fsm.c

@@ -474,9 +474,18 @@ void bsc_nat_fsm_alloc(struct bsc_nat *bsc_nat)
 	OSMO_ASSERT(bsc_nat->fi);
 }
 
-void bsc_nat_fsm_start(struct bsc_nat *bsc_nat)
+int bsc_nat_fsm_start(struct bsc_nat *bsc_nat)
 {
-	osmo_fsm_inst_dispatch(bsc_nat->fi, BSC_NAT_FSM_EV_START, NULL);
+	int rc = osmo_fsm_inst_dispatch(bsc_nat->fi, BSC_NAT_FSM_EV_START, NULL);
+	if (rc)
+		return rc;
+
+	/* st_starting_on_enter() doesn't change to STARTED if e.g. vty config
+	 * is incomplete */
+	if (bsc_nat->fi->state != BSC_NAT_FSM_ST_STARTED)
+		return -1;
+
+	return 0;
 }
 
 void bsc_nat_fsm_stop(struct bsc_nat *bsc_nat)

src/osmo-bsc-nat/main.c

@@ -198,7 +198,8 @@ int main(int argc, char **argv)
 	main_vty_init(argc, argv);
 	signal_handler_init();
 
-	bsc_nat_fsm_start(g_bsc_nat);
+	if (bsc_nat_fsm_start(g_bsc_nat) < 0)
+		exit(1);
 
 	if (msc_alloc_from_addr_book() < 0)
 		exit(1);