clustering: guard on hostname presentation

Deployment environments may have random features with regards to
forward/reverse hostname lookups; the charm does endeavour to
write reliable hostname/ip maps into /etc/hosts however not all
functions that assess the ready-ness of the cluster to form
look at whether this has been presented.

Update the is_sufficient_peers check to validate with this key
and use during changes on the cluster relation to avoid cluster
host discovery for unresolvable hostnames as a side effect of
peers within the cluster discovering each other outside of the
charm code.

Change-Id: I27d6356dba4340388211c6dfed383417c35542e0
Closes-Bug: 1812568

hooks/rabbit_utils.py

@@ -974,7 +974,7 @@ def is_sufficient_peers():
     """Sufficient number of expected peers to build a complete cluster
 
     If min-cluster-size has been provided, check that we have sufficient
-    number of peers as expected for a complete cluster.
+    number of peers who have presented a hostname for a complete cluster.
 
     If not defined assume a single unit.
 
@@ -988,7 +988,10 @@ def is_sufficient_peers():
         # Include this unit
         units = 1
         for rid in relation_ids('cluster'):
-            units += len(related_units(rid))
+            for unit in related_units(rid):
+                if relation_get(attribute='hostname',
+                                rid=rid, unit=unit):
+                    units += 1
 
         if units < min_size:
             log("Insufficient number of peer units to form cluster "

hooks/rabbitmq_server_relations.py

@@ -435,8 +435,9 @@ def cluster_changed(relation_id=None, remote_unit=None):
             'rabbitmq cluster config.', level=INFO)
         return
 
-    # NOTE(freyes): all the nodes need to marked as 'clustered' (LP: #1691510)
-    rabbit.cluster_with()
+    if rabbit.is_sufficient_peers():
+        # NOTE(freyes): all the nodes need to marked as 'clustered' (LP: #1691510)
+        rabbit.cluster_with()
 
     if not is_leader() and is_relation_made('nrpe-external-master'):
         update_nrpe_checks()

unit_tests/test_rabbit_utils.py

@@ -471,6 +471,7 @@ class UtilsTests(CharmTestCase):
     @mock.patch.object(rabbit_utils, 'config')
     def test_is_sufficient_peers(self, mock_config, mock_relation_ids,
                                  mock_related_units, mock_is_leader):
+        self.relation_get.return_value = None
         # With leadership Election
         mock_is_leader.return_value = False
         _config = {'min-cluster-size': None}
@@ -484,6 +485,7 @@ class UtilsTests(CharmTestCase):
         mock_config.side_effect = lambda key: _config.get(key)
         self.assertFalse(rabbit_utils.is_sufficient_peers())
 
+        self.relation_get.side_effect = ['testhost0', 'testhost1']
         mock_is_leader.return_value = False
         mock_related_units.return_value = ['test/0', 'test/1']
         _config = {'min-cluster-size': 3}