Ensure update config function can handle new headers

The linked bug occurred due to a new header for the config file being in
the upgraded charm, but not in the previous version of the charm.  As
the config file is read, updated, and then written, new headings in the
INI file would crash the charm code with a KeyError.

This patch just sets the header and parameter if it is missing.

Closes-Bug: #1927981
Change-Id: I18a6a4143ee0a1144eade5caa50611b802cba28a

src/lib/charm/openstack/mysql_router.py

@@ -635,7 +635,12 @@ class MySQLRouterCharm(charms_openstack.charm.OpenStackCharm):
         config.read(self.mysqlrouter_conf)
         for heading in parameters.keys():
             for param in parameters[heading].keys():
-                config[heading][param] = parameters[heading][param]
+                # BUG LP#1927981 - heading may not exist during a charm upgrade
+                # Handle missing heading via direct assignment in except.
+                try:
+                    config[heading][param] = parameters[heading][param]
+                except KeyError:
+                    config[heading] = {param: parameters[heading][param]}
         ch_core.hookenv.log("Writing {}".format(
             self.mysqlrouter_conf))
         with open(self.mysqlrouter_conf, 'w') as configfile:

unit_tests/test_lib_charm_openstack_mysql_router.py

@@ -661,6 +661,35 @@ class TestMySQLRouterCharm(test_utils.PatchHelper):
         _mock_config_parser.write.assert_called_once_with(
             self.mock_open()().__enter__())
 
+    def test_update_config_parameters_missing_heading(self):
+        # test fix for Bug LP#1927981
+        class FakeConfigParser(dict):
+            def read(*args, **kwargs):
+                pass
+
+            def write(*args, **kwargs):
+                pass
+
+        current_config = {"DEFAULT": {"client_ssl_mode": "NONE"}}
+        fake_config = FakeConfigParser(current_config)
+
+        self.patch_object(mysql_router.configparser, "ConfigParser",
+                          return_value=fake_config)
+
+        # metadata_cache:jujuCluster didn't exist in the previous config so the
+        # header needs to be created (c.f. BUG LP#1927981)
+        _params = {
+            "DEFAULT": {"client_ssl_mode": "PREFERRED"},
+            "metadata_cache:jujuCluster": {"thing": "a-thing"},
+        }
+
+        mrc = mysql_router.MySQLRouterCharm()
+        # should not throw a key error.
+        mrc.update_config_parameters(_params)
+        self.assertIn('metadata_cache:jujuCluster', fake_config)
+        self.assertEqual(fake_config['metadata_cache:jujuCluster'],
+                         {"thing": "a-thing"})
+
     def test_config_changed(self):
         _config_data = {
             "ttl": '5',