northd: Fix Crash when deleting ports with BFD session.

When deleting ports with BFD sessions without removing the BFD
northbound object northd will crash after recompute because the
southbound BFD object does get removed when all ports are deleted.

Fix by checking for the southbound objects existence for stale BFD
entries before attempting to delete them.

Reported-at: https://redhat.atlassian.net/browse/FDP-4218
Fixes: 15c9c9f ("northd: Add bfd, static_routes, route_policies and bfd_sync nodes to I-P engine.")
Signed-off-by: Jacob Tanenbaum <jtanenba@redhat.com>
Signed-off-by: Ales Musil <amusil@redhat.com>
(cherry picked from commit 0f75487)

Author: JacobTanenbaum

northd/northd.c

@@ -10757,7 +10757,7 @@ bfd_table_sync(struct ovsdb_idl_txn *ovnsb_txn,
     }
 
     HMAP_FOR_EACH_POP (bfd_e, hmap_node, &sync_bfd_connections) {
-        if (bfd_e->stale) {
+        if (bfd_e->stale && bfd_e->sb_bt) {
             sbrec_bfd_delete(bfd_e->sb_bt);
         }
         bfd_erase_entry(bfd_e);

tests/ovn-northd.at

@@ -4167,6 +4167,11 @@ check as northd ovn-appctl -t ovn-northd inc-engine/clear-stats
 bfd_route_policy_uuid=$(fetch_column nb:bfd _uuid logical_port=r0-sw9)
 AT_CHECK([ovn-nbctl list logical_router_policy | sed s/,//g | grep -q "$bfd_route_policy_uuid"])
 
+# Check that the logical router port can be deleted and the bfd
+# recompute does not crash northd.
+check ovn-nbctl --wait=sb lrp-del r0-sw2
+check ovn-nbctl --wait=sb sync
+
 OVN_CLEANUP_NORTHD
 AT_CLEANUP
 ])