Fix memcache caching w/ multiple cache servers

We found a bug recently where either oslo.cache or python-memcached
aren't using the `backend_argument` properly with more than one
memcached server defined. Until we get the memcached client libraries
figured out, `memcache_servers` works just the same for a single
memcached instance and it works defined with a ring of memcached
instances.

The current variable used for the directive memcache_servers was
pointing to localhost servers, that were historically used for
UUID token cache. Only the ``keystone_cache_backend_argument``
has the right list of servers, but the variable's content is
already formatted to match the cache_backend_argument directive,
and therefore needs editing to be used in ``memcache_servers``.

This is far too fragile, and simplification was needed. This
patch moves to a new variable (with a graceful deprecation
cycle), ``keystone_cache_servers``, a simple list containing the
servers.

The variable ``keystone_memcached_max_compare_and_set_retry``
wasn't used and was therefore removed too.

Related-Bug: 1743036
Closes-Bug: 1681695
Co-Authored-By: Lance Bragstad <lbragstad@gmail.com>

Change-Id: I85ebce8b41dd440e1866a08aa1329b3df798c04f

defaults/main.yml

@@ -88,11 +88,6 @@ keystone_resource_driver: sql
 
 keystone_bind_address: 0.0.0.0
 
-## Memcached servers used within keystone.
-# String or Comma separated list of servers.
-keystone_memcached_servers: 127.0.0.1
-keystone_memcached_max_compare_and_set_retry: 16
-
 ## Database info
 keystone_database_connection_string: >-
   mysql+pymysql://{{ keystone_galera_user }}:{{ keystone_container_mysql_password }}@{{ keystone_galera_address }}/{{ keystone_galera_database }}?charset=utf8{% if keystone_galera_use_ssl | bool %}&ssl_ca={{ keystone_galera_ssl_ca_cert }}{% endif %}
@@ -253,8 +248,12 @@ keystone_external_ssl: false
 keystone_secure_proxy_ssl_header: HTTP_X_FORWARDED_PROTO
 
 ## Caching
-# If set this will enable dog pile cache for keystone.
-# keystone_cache_backend_argument: url:127.0.0.1:11211
+# This is a list of strings, each string contains a cache server's
+# information (IP:port for example)
+# The cache_servers default backend is memcached, so this variable
+# should point to a list of memcached servers.
+# If empty, caching is disabled.
+keystone_cache_servers: []
 
 ## LDAP Section
 # Define Keystone LDAP domain configuration here.

releasenotes/notes/deprecated-memcached-and-backend-caching-vars-88c48117b232b37e.yaml

@@ -0,0 +1,6 @@
+---
+deprecations:
+  - |
+    The variables ``keystone_memcached_servers`` and
+    ``keystone_cache_backend_argument`` have been deprecated in favor of
+    ``keystone_cache_servers``, a list of servers for caching purposes.

tasks/main.yml

@@ -21,6 +21,20 @@
   tags:
     - always
 
+- name: Handle deprecation of keystone_memcached_servers
+  set_fact:
+    keystone_cache_servers: "{{ keystone_cache_servers + (_ | deprecated(keystone_memcached_servers, 'keystone_memcached_servers', 'keystone_cache_servers', 'queens')).split(',') }}"
+  when: keystone_memcached_servers is defined
+  tags:
+    - always
+
+- name: Handle deprecation of keystone_cache_backend_argument
+  set_fact:
+    keystone_cache_servers: "{{ keystone_cache_servers + (_ | deprecated(keystone_cache_backend_argument.replace('url:',''), 'keystone_cache_backend_argument', 'keystone_cache_servers', 'queens')).split(',') }}"
+  when: keystone_cache_backend_argument is defined
+  tags:
+    - always
+
 - name: Gather variables for each operating system
   include_vars: "{{ item }}"
   with_first_found:

templates/keystone.conf.j2

@@ -32,10 +32,15 @@ driver = messagingv2
 transport_url = rabbit://{% for host in keystone_rabbitmq_telemetry_servers.split(',') %}{{ keystone_rabbitmq_telemetry_userid }}:{{ keystone_rabbitmq_telemetry_password }}@{{ host }}:{{ keystone_rabbitmq_telemetry_port }}{% if not loop.last %},{% else %}/{{ keystone_rabbitmq_telemetry_vhost }}{% endif %}{% endfor %}
 {% endif %}
 
-{% if keystone_cache_backend_argument is defined %}
+{% if keystone_cache_servers | length > 0 %}
 [cache]
 backend = dogpile.cache.memcached
-backend_argument = {{ keystone_cache_backend_argument }}
+# FIXME(lbragstad): Some strange behaviors have been reported when using
+# multiple memcached instances with backend_argument. This has been documented
+# in https://bugs.launchpad.net/oslo.cache/+bug/1743036
+# For the time being, memcache_servers works with a single memcached instance
+# and multiple instances.
+memcache_servers = {{ keystone_cache_servers | join(',') }}
 config_prefix = cache.keystone
 distributed_lock = True
 enabled = true