[fix] Update related config when template default_values are changed

The config related to a template were not being flagged for update
when default_values were changed.
Moreover, during debugging, I found out that in some cases
the action was being performed even if there were no real changes,
the only difference being the new value was an instance of dict
while the value loaded from the model is an instance of OrderedDict.

This change fixes both issues.

openwisp_controller/config/base/template.py

@@ -1,3 +1,4 @@
+import json
 from collections import OrderedDict
 from copy import copy
 
@@ -112,8 +113,10 @@ def save(self, *args, **kwargs):
         update_related_config_status = False
         if not self._state.adding:
             current = self.__class__.objects.get(pk=self.pk)
-            for attr in ['backend', 'config']:
-                if getattr(self, attr) != getattr(current, attr):
+            for attr in ['backend', 'config', 'default_values']:
+                new_value = _get_value_for_comparison(getattr(self, attr))
+                current_value = _get_value_for_comparison(getattr(current, attr))
+                if new_value != current_value:
                     update_related_config_status = True
                     break
         # save current changes
@@ -209,3 +212,13 @@ def __get_clone_name(self):
 # It's allowed to be blank because VPN client templates can be
 # automatically generated via the netjsonconfig library if left empty.
 AbstractTemplate._meta.get_field('config').blank = True
+
+
+def _get_value_for_comparison(value):
+    """
+    if value is a nested OrderedDict, convert it to dict
+    so two simple dicts can be compared
+    """
+    if not isinstance(value, OrderedDict):
+        return value
+    return json.loads(json.dumps(value))

openwisp_controller/config/tests/test_template.py

@@ -657,12 +657,28 @@ def test_task_called(self, mocked_task):
             conf.set_status_applied()
             mocked_task.assert_not_called()
 
-        with self.subTest('task is called when template is assigned to conf'):
+        with self.subTest('task is called when template conf is changed'):
             template.config['interfaces'][0]['name'] = 'eth1'
             template.full_clean()
             template.save()
             mocked_task.assert_called_with(template.pk)
 
+        mocked_task.reset_mock()
+
+        with self.subTest('task is called when template default_values are changed'):
+            template.refresh_from_db()
+            template.default_values = {'a': 'a'}
+            template.full_clean()
+            template.save()
+            mocked_task.assert_called_with(template.pk)
+
+        mocked_task.reset_mock()
+
+        with self.subTest('task is not called when there are no changes'):
+            template.full_clean()
+            template.save()
+            mocked_task.assert_not_called()
+
     @mock.patch.object(task_logger, 'warning')
     def test_task_failure(self, mocked_warning):
         update_template_related_config_status.delay(uuid.uuid4())