Add Memcache relation adapter and registration

Add relation adapter for memcache based coordination backends
as used by gnocchi and designate.

Register adapter for any 'coordinator_memcached' named relations.

Change-Id: Id6fd584de4fd524b86dfa71db95f868408830ac9
Related-Bug: 1729236

charms_openstack/adapters.py

@@ -145,6 +145,22 @@ class OpenStackRelationAdapter(object):
                             self.relation, name)()))(meth_name))
 
 
+class MemcacheRelationAdapter(OpenStackRelationAdapter):
+
+    """
+    Adapter for the MemcacheRequires relation interface.
+    """
+
+    interface_type = 'memcache'
+
+    @property
+    def url(self):
+        hosts = sorted(self.relation.memcache_hosts())
+        if hosts:
+            return "memcached://{}:11211?timeout=5".format(hosts[0])
+        return None
+
+
 class RabbitMQRelationAdapter(OpenStackRelationAdapter):
     """
     Adapter for the RabbitMQRequires relation interface.
@@ -1096,6 +1112,7 @@ class OpenStackAPIRelationAdapters(OpenStackRelationAdapters):
         'amqp': RabbitMQRelationAdapter,
         'shared_db': DatabaseRelationAdapter,
         'cluster': PeerHARelationAdapter,
+        'coordinator_memcached': MemcacheRelationAdapter,
     }
 
     def __init__(self, relations, options=None, options_instance=None,

unit_tests/test_charms_openstack_adapters.py

@@ -102,6 +102,26 @@ class TestOpenStackRelationAdapter(unittest.TestCase):
         self.assertEqual(i.b, 4)
 
 
+class FakeMemcacheRelation():
+
+    auto_accessors = ['private_address']
+    relation_name = 'memcache'
+
+    def private_address(self):
+        return 'private-address'
+
+    def memcache_hosts(self):
+        return ['host1', 'host2']
+
+
+class TestMemcacheRelationAdapter(unittest.TestCase):
+
+    def test_class(self):
+        fake = FakeMemcacheRelation()
+        memcache = adapters.MemcacheRelationAdapter(fake)
+        self.assertEqual(memcache.url, 'memcached://host1:11211?timeout=5')
+
+
 class FakeRabbitMQRelation():
 
     auto_accessors = ['vip', 'private_address']