northd: Fix action parsing in build_lb_vip_actions().

Fix the following error reported in build_lb_vip_actions() if the CMS
configure a load balancer with no backends:

2025-01-21T18:18:26.668Z|04152|lflow|WARN|error parsing actions "flags.force_snat_for_lb = 1; drop;": Syntax error at `drop' expecting action.

The issue can be triggered with the following reproducer in ovn-sandbox:

$ovn-nbctl create load_balancer name=lb-empty vips:\"172.168.10.30\"=\"\" protocol=tcp options:skip_snat=true
$ovn-nbctl lr-lb-add lr0 lb-empty
$grep Syntax ./sandbox/ovn-controller.log 2025-01-22T17:13:49.709Z|00047|lflow|WARN|error parsing actions
"flags.skip_snat_for_lb = 1; drop;": Syntax error at `drop' expecting action.

Reported-at: https://issues.redhat.com/browse/FDP-1095
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: Ales Musil <amusil@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
(cherry picked from commit c0af418)

northd/northd.c

@@ -3802,10 +3802,14 @@ build_lb_vip_actions(const struct ovn_northd_lb *lb,
     const char *enclose = is_lb_action ? ");" : "";
 
     if (!ls_dp) {
-        ds_put_format(skip_snat_action, "flags.skip_snat_for_lb = 1; %s%s",
+        if (!drop) {
+            ds_put_cstr(skip_snat_action, "flags.skip_snat_for_lb = 1; ");
+            ds_put_cstr(force_snat_action, "flags.force_snat_for_lb = 1; ");
+        }
+        ds_put_format(skip_snat_action, "%s%s",
                       ds_cstr(action),
                       is_lb_action ? "; skip_snat);" : enclose);
-        ds_put_format(force_snat_action, "flags.force_snat_for_lb = 1; %s%s",
+        ds_put_format(force_snat_action, "%s%s",
                       ds_cstr(action),
                       is_lb_action ? "; force_snat);" : enclose);
     }

tests/ovn-northd.at

@@ -6566,7 +6566,7 @@ check ovn-nbctl --wait=sb set load_balancer lb6 options:skip_snat=true
 
 AT_CHECK([ovn-sbctl dump-flows lr0 | grep "lr_in_dnat" | ovn_strip_lflows], [0], [dnl
   table=??(lr_in_dnat         ), priority=0    , match=(1), action=(next;)
-  table=??(lr_in_dnat         ), priority=110  , match=(ct.new && !ct.rel && ip4 && ip4.dst == 172.168.10.30), action=(flags.skip_snat_for_lb = 1; drop;)
+  table=??(lr_in_dnat         ), priority=110  , match=(ct.new && !ct.rel && ip4 && ip4.dst == 172.168.10.30), action=(drop;)
   table=??(lr_in_dnat         ), priority=50   , match=(ct.est && !ct.rel && !ct.new && !ct.rpl && ct_mark.natted), action=(next;)
   table=??(lr_in_dnat         ), priority=50   , match=(ct.rel && !ct.est && !ct.new && !ct.rpl), action=(ct_commit_nat;)
   table=??(lr_in_dnat         ), priority=70   , match=(ct.est && !ct.rel && !ct.new && !ct.rpl && ct_mark.natted && ct_mark.force_snat == 1), action=(flags.force_snat_for_lb = 1; next;)
@@ -6582,7 +6582,7 @@ check ovn-nbctl --wait=sb set logical_router lr0 options:lb_force_snat_ip="route
 
 AT_CHECK([ovn-sbctl dump-flows lr0 | grep "lr_in_dnat" | ovn_strip_lflows], [0], [dnl
   table=??(lr_in_dnat         ), priority=0    , match=(1), action=(next;)
-  table=??(lr_in_dnat         ), priority=110  , match=(ct.new && !ct.rel && ip4 && ip4.dst == 172.168.10.30), action=(flags.force_snat_for_lb = 1; drop;)
+  table=??(lr_in_dnat         ), priority=110  , match=(ct.new && !ct.rel && ip4 && ip4.dst == 172.168.10.30), action=(drop;)
   table=??(lr_in_dnat         ), priority=50   , match=(ct.est && !ct.rel && !ct.new && !ct.rpl && ct_mark.natted), action=(next;)
   table=??(lr_in_dnat         ), priority=50   , match=(ct.rel && !ct.est && !ct.new && !ct.rpl), action=(ct_commit_nat;)
   table=??(lr_in_dnat         ), priority=70   , match=(ct.est && !ct.rel && !ct.new && !ct.rpl && ct_mark.natted && ct_mark.force_snat == 1), action=(flags.force_snat_for_lb = 1; next;)