Prevent neutron-l3-agent killing keepalived on restart

Systemd processes use a default KillMode of 'control-group'
which causes all other processes spawned during execution to be
killed on service stop. Neutron expects the keepalived processes
it starts to remain running in order to prevent data-plane
interruptions for HA routers.

This change switches the systemd KillMode to process in order to
prevent this issue. In doing so we also have to clean up
non-keepalived processes started by neutron so that upon restart
everything is running from the latest virtualenv which may have
changed during an upgrade.

Change-Id: I958fda17e6207553466d8a7512e35c30b122c22c
Closes-Bug: #1846198
Depends-On: https://review.opendev.org/771770
(cherry picked from commit a640ae8cc9)

handlers/main.yml

@@ -49,6 +49,27 @@
     - "Restart neutron services"
     - "venv changed"
 
+# NOTE
+# When restarting neutron-l3-agent, a non-default systemd KillMode of 'process' is used
+# to prevent Keepalived from exiting and causing a data-plane outage. As a result of this
+# some neutron processes remain running. In the case of an upgrade, these remaining
+# processes will be running code from the previous version. This step ensures these
+# orphaned processes are cleaned up correctly.
+- name: Run neutron-l3-agent process cleanup
+  shell: |
+    for ns_pid in $(cat /sys/fs/cgroup/pids/neutron.slice/neutron-l3-agent.service/cgroup.procs); do
+      echo $(readlink -f "/proc/$ns_pid/exe") | grep -qv "keepalived"
+      if [ $? -eq 0 ]; then
+        if kill -9 "$ns_pid"; then
+          logger -s "old neutron-l3-agent pid found and has been cleaned up on: \"$ns_pid\""
+        fi
+      fi
+    done
+  when: "'neutron-l3-agent' in (filtered_neutron_services | map(attribute='service_key') | list)"
+  listen:
+    - "Restart neutron services"
+    - "venv changed"
+
 - name: Perform a DB contract
   command: "{{ neutron_bin }}/neutron-db-manage upgrade --contract"
   become: yes

vars/main.yml

@@ -415,7 +415,7 @@ neutron_services:
     execstarts: "{{ neutron_bin }}/neutron-l3-agent --config-file {{ neutron_conf_dir }}/neutron.conf --config-file {{ neutron_conf_dir }}/l3_agent.ini"
     config_overrides: "{{ neutron_l3_agent_ini_overrides }}"
     config_type: "ini"
-    init_config_overrides: "{{ neutron_l3_agent_init_overrides }}"
+    init_config_overrides: "{{ _neutron_l3_agent_init_defaults | combine(neutron_l3_agent_init_overrides) }}"
     start_order: 3
   neutron-bgp-dragent:
     group: neutron_bgp_dragent
@@ -508,6 +508,15 @@ neutron_services:
     init_config_overrides: "{{ neutron_ovn_metadata_agent_init_overrides }}"
     start_order: 3
 
+###
+### Overrides for services where required
+###
+
+# Ensure that keepalived processes are not stopped on exit
+_neutron_l3_agent_init_defaults:
+  Service:
+    KillMode: process
+
 ###
 ### Internals: Drivers mappings
 ###