[OVN] Changing public network MTU does not update router port gateway_mtu option

[Roman Safronov]

High level description:
If a neutron router is connected to a provider network 'A' and private geneve networks, a change of the provider network MTU does not always update router port gateway_mtu option even if provider network has MTU smaller than the private network.

Reproduction scenario:

openstack router create r1
openstack router set --external-gateway public r1
openstack network create net1
openstack subnet create --subnet-range 192.168.100.0/24 --network net1 subnet1
openstack router add subnet r1 subnet1

stack@devstack:~/devstack$ crudini --get /etc/neutron/plugins/ml2/ml2_conf.ini ovn ovn_emit_need_to_frag
True

stack@devstack:~/devstack$ openstack router show r1 -c id -f value
610f7fcb-40c2-4221-b535-8f5548de6fa5
stack@devstack:~/devstack$ openstack port list --device-id=610f7fcb-40c2-4221-b535-8f5548de6fa5 --device-owner=network:router_gateway -c ID -f value
c765a42c-9736-45b8-a2c9-c362bc221119

stack@devstack:~/devstack$ openstack network show net1 -c mtu -f value
1372
stack@devstack:~/devstack$ openstack network show public -c mtu -f value
1330
stack@devstack:~/devstack$ sudo ovn-nbctl --db=unix:/usr/local/var/run/ovn/ovnnb_db.sock get logical_router_port lrp-c765a42c-9736-45b8-a2c9-c362bc221119 options:gateway_mtu
"1330"

i.e. Private network has bigger mtu than the public one so the case described in https://bugs.launchpad.net/neutron/+bug/1951559 is not relevant and gateway_mtu option is expected to be updated after public mtu change

stack@devstack:~/devstack$ openstack network set --mtu 1300 public
stack@devstack:~/devstack$ openstack network show public -c mtu -f value
1300
stack@devstack:~/devstack$ sudo ovn-nbctl --db=unix:/usr/local/var/run/ovn/ovnnb_db.sock get logical_router_port lrp-c765a42c-9736-45b8-a2c9-c362bc221119 options:gateway_mtu
"1330"
stack@devstack:~/devstack$ date
Wed Apr 10 14:46:32 UTC 2024
stack@devstack:~/devstack$ sudo ovn-nbctl --db=unix:/usr/local/var/run/ovn/ovnnb_db.sock get logical_router_port lrp-c765a42c-9736-45b8-a2c9-c362bc221119 options:gateway_mtu
"1330"
Wed Apr 10 14:50:04 UTC 2024
stack@devstack:~/devstack$ sudo ovn-nbctl --db=unix:/usr/local/var/run/ovn/ovnnb_db.sock get logical_router_port lrp-c765a42c-9736-45b8-a2c9-c362bc221119 options:gateway_mtu
"1330"
stack@devstack:~/devstack$ openstack network show public -c mtu -f value
1300
Wed Apr 10 14:54:25 UTC 2024
stack@devstack:~/devstack$ openstack network show public -c mtu -f value
1300
stack@devstack:~/devstack$ sudo ovn-nbctl --db=unix:/usr/local/var/run/ovn/ovnnb_db.sock get logical_router_port lrp-c765a42c-9736-45b8-a2c9-c362bc221119 options:gateway_mtu
"1330"
stack@devstack:~/devstack$ date
Wed Apr 10 14:57:30 UTC 2024
stack@devstack:~/devstack$ sudo ovn-nbctl --db=unix:/usr/local/var/run/ovn/ovnnb_db.sock get logical_router_port lrp-c765a42c-9736-45b8-a2c9-c362bc221119 options:gateway_mtu
"1330"
stack@devstack:~/devstack$ date
Wed Apr 10 15:13:36 UTC 2024
stack@devstack:~/devstack$ sudo ovn-nbctl --db=unix:/usr/local/var/run/ovn/ovnnb_db.sock get logical_router_port lrp-c765a42c-9736-45b8-a2c9-c362bc221119 options:gateway_mtu
"1330"

i.e. Router port gateway_mtu option remains unchanged for a long time after value in neutron db has been changed.

Found on master devstack deployed on ubuntu-jammy.