Fix broken v4 caching due to leader-get asymmetry

leader-get decodes using json, but leader-set just sets the keys. This
wasn't taken into consideration when fetching all the keys to filter for
cached keys when a relation is leaving.  This is resolved in this patch.

func-test-pr: openstack-charmers/zaza-openstack-tests#1153

Change-Id: I2d44ec0c43c1ecffd9ac77a1162ead4e4a01aabe
(cherry picked from commit d925ac7)
(cherry picked from commit 0a18ac2)

src/lib/charm/vault_pki.py

@@ -565,7 +565,14 @@ def _fetch(key):
         """
         value = hookenv.leader_get(key)
         if value:
-            return json.loads(value)
+            if key is not None:
+                # load the value that was json serialised in _store()
+                return json.loads(value)
+            else:
+                # due to a weird asymetry been leader_set and leader_get,
+                # leader_get() already deserialises as json so if no key was
+                # specified, it's already been deserialised.
+                return value
         return ""
 
     @staticmethod

unit_tests/test_lib_charm_vault_pki.py

@@ -856,6 +856,21 @@ def test_certcache__fetch(self, mock_leader_get):
         self.assertEqual(vault_pki.CertCache(request)._fetch("mine"),
                          'the-value')
 
+    @patch.object(vault_pki.hookenv, 'leader_get')
+    def test_certcache__fetch_none(self, mock_leader_get):
+        request = self._default_request()
+        # due to weird asymetry between leader_get and leader_set, if no
+        # attribute is passed to leader_get() then the result is the
+        # deserialised set of key, values as a dictionary.
+        leader_get = {
+            'a': 'an-a',
+            'b': 'an-b',
+        }
+
+        mock_leader_get.return_value = leader_get
+        self.assertEqual(vault_pki.CertCache(request)._fetch(None), leader_get)
+        mock_leader_get.assert_called_once_with(None)
+
     @patch.object(vault_pki.hookenv, 'leader_set')
     def test_certcache__store(self, mock_leader_set):
         request = self._default_request()