The haproxy service gets stuck in defunct state (Zombie) after an instance is migrated or deleted from compute node

Issue

• The haproxy service gets stuck in defunct state (Zombie) after an instance is migrated or deleted from a given compute node such as:

root       82445  0.0  0.0 339200  1880 ?        Sl   13:26   0:00      \_ /usr/bin/docker-containerd-shim-current f8bd73468edc5331b94dcfcb67a14803fff815e56dd761147be08376206e143e /var/run/docker/libcontainerd/f
42435      82460  0.2  2.6 282076 76408 ?        Ss   13:26   0:10      |   \_ /usr/bin/python2 /usr/bin/networking-ovn-metadata-agent --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/n
42435      82685  0.0  2.3 278352 69868 ?        S    13:26   0:00      |       \_ /usr/bin/python2 /usr/bin/networking-ovn-metadata-agent --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugi
42435      82686  0.0  2.3 278352 69868 ?        S    13:26   0:00      |       \_ /usr/bin/python2 /usr/bin/networking-ovn-metadata-agent --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugi
root       82703  0.0  1.1 185684 32452 ?        S    13:26   0:00      |       \_ /usr/bin/python2 /bin/privsep-helper --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/networking-ovn/n
42435      99990  0.0  0.0      0     0 ?        Zs   14:27   0:00      |       \_ [haproxy] <defunct>

• This issue happens when the last VM is deleted or migrated away from that compute node and the following lots can be seen in /var/log/containers/neutron/ovn-metadata-agent.log:

2018-09-25 14:19:39.746 51 DEBUG oslo.privsep.daemon [-] privsep: reply[140578945681520]: (4, ['ovnmeta-9a7dba41-d243-4bc4-83aa-06bed0521408']) loop /usr/lib/python2.7/site-packages/oslo_privsep/daemon.py:456
2018-09-25 14:19:39.747 1 INFO networking_ovn.agent.metadata.agent [-] Cleaning up ovnmeta-9a7dba41-d243-4bc4-83aa-06bed0521408 namespace which is not needed anymore
2018-09-25 14:19:39.747 1 DEBUG neutron.agent.linux.utils [-] Running command: ['sudo', 'neutron-rootwrap', '/etc/neutron/rootwrap.conf', 'kill', '-9', '125'] create_process /usr/lib/python2.7/site-packages/neut
ron/agent/linux/utils.py:92