Retrieve root password from leaderdb if needed

Within update_root_password if there is no root password in the
config() object then cfg['root-password'] will return None.
set_mysql_root_password is then called with None which triggers a
TypeError. The desired root password is in the leaderdb so fallback
to retrieving it from there if it is not in config().

unit_tests/test_utils.py was also updated to fix the behaviour of
config.changed to mimic that of the real method in charm helpers.

Change-Id: I8d22a66f335c0c8e893cda699a55056476e6d9d5

hooks/percona_utils.py

@@ -841,7 +841,7 @@ def update_root_password():
     m_helper = get_db_helper()
 
     # password that needs to be set
-    new_root_passwd = cfg['root-password']
+    new_root_passwd = cfg['root-password'] or root_password()
     m_helper.set_mysql_root_password(new_root_passwd)
 
     # check the password was changed

unit_tests/test_percona_utils.py

@@ -640,3 +640,17 @@ class TestUpdateBootstrapUUID(CharmTestCase):
 
         mock_leader_set.assert_called_with(
             settings={'mysql.passwd': new_password})
+
+    @mock.patch.object(percona_utils, 'leader_get')
+    @mock.patch.object(percona_utils, 'get_db_helper')
+    def test_update_root_password_None(self, mock_get_db_helper,
+                                       mock_leader_get):
+        # Test fix for 1744961
+        my_mock = mock.Mock()
+        mock_get_db_helper.return_value = my_mock
+        self.config.side_effect = self.test_config.get
+        leader_config = {'root-password': 'leaderpass'}
+        mock_leader_get.side_effect = lambda k: leader_config[k]
+
+        percona_utils.update_root_password()
+        my_mock.set_mysql_root_password.assert_called_once_with('leaderpass')

unit_tests/test_utils.py

@@ -83,6 +83,8 @@ class TestConfig(object):
             self.config_prev.pop(k)
 
     def changed(self, k):
+        if not self.config_prev:
+            return True
         return self.get(k) != self.previous(k)
 
     def get(self, attr=None):