Configure nofile OS limit for memcached server role

This fix adds a new override `memcached_connections` which is
automatically calculated from the number of memcached
connection limit plus additional 1k to configure the OS nofile limit.
Without proper nofile limit configuration, memcached will crash
in order to support higher parallel connection TCP/Memcache counts.

Related-To: https://review.openstack.org/#/c/317030/

Change-Id: Ib9a3a5fdf567e6024819ebcfd574a5d1d15e5e23
Closes-Bug: #1582288

playbooks/roles/memcached_server/defaults/main.yml

@@ -28,6 +28,7 @@ memcached_listen: "127.0.0.1"
 memcached_log: /var/log/memcached.log
 memcached_connections: 1024
 memcached_threads: 4
+memcached_file_limits: "{{ memcached_connections | int + 1024 }}"
 
 memcached_apt_packages:
   - memcached

playbooks/roles/memcached_server/tasks/memcached_config.yml

@@ -36,3 +36,19 @@
   notify: Restart memcached
   tags:
     - memcached-config
+
+- name: Configure soft file limits
+  lineinfile:
+    dest: "/etc/security/limits.conf"
+    regexp: "^memcache.*soft.*nofile.*"
+    state: "present"
+    line: "memcache\tsoft\tnofile\t\t{{ memcached_file_limits }}"
+    insertbefore: "^# End of file"
+
+- name: Configure hard file limits
+  lineinfile:
+    dest: "/etc/security/limits.conf"
+    regexp: "^memcache.*hard.*nofile.*"
+    state: "present"
+    line: "memcache\thard\tnofile\t\t{{ memcached_file_limits }}"
+    insertbefore: "^# End of file"

releasenotes/notes/memcached_server-add-nofile-setting-504e0c50e10a4ea6.yaml

@@ -0,0 +1,9 @@
+---
+features:
+  - The openstack-ansible-memcached_server role includes
+    a new override,`memcached_connections` which is
+    automatically calculated from the number of memcached
+    connection limit plus additional 1k to configure
+    the OS nofile limit. Without proper nofile limit
+    configuration, memcached will crash in order to support
+    higher parallel connection TCP/Memcache counts.