[fix] Allow updating templates with invalid configurations

Previously, fixing an invalid template configuration via the UI was
blocked due to the cache invalidation mechanism. This mechanism
attempted to evaluate the existing configuration, triggering a
ValidationError and preventing updates.

(cherry picked from commit bd3d11a)

openwisp_controller/config/base/template.py

@@ -6,6 +6,7 @@
 from django.db import models, transaction
 from django.utils.translation import gettext_lazy as _
 from jsonfield import JSONField
+from netjsonconfig.exceptions import ValidationError as NetjsonconfigValidationError
 from swapper import get_model_name
 from taggit.managers import TaggableManager
 
@@ -115,7 +116,14 @@ def save(self, *args, **kwargs):
             if hasattr(self, 'backend_instance'):
                 del self.backend_instance
             current = self.__class__.objects.get(pk=self.pk)
-            update_related_config_status = self.checksum != current.checksum
+            try:
+                current_checksum = current.checksum
+            except NetjsonconfigValidationError:
+                # If the Netjsonconfig library upgrade changes the schema,
+                # the old configuration may become invalid, raising an exception.
+                # Setting the checksum to None forces related configurations to update.
+                current_checksum = None
+            update_related_config_status = self.checksum != current_checksum
         # save current changes
         super().save(*args, **kwargs)
         # update relations

openwisp_controller/config/tests/test_template.py

@@ -7,6 +7,7 @@
 from django.db import transaction
 from django.test import TestCase, TransactionTestCase
 from netjsonconfig import OpenWrt
+from netjsonconfig.exceptions import ValidationError as NetjsonconfigValidationError
 from swapper import load_model
 
 from openwisp_utils.tests import catch_signal
@@ -733,3 +734,18 @@ def test_task_timeout(self, mocked_update_related_config_status):
             template.save()
             mocked_error.assert_called_once()
         mocked_update_related_config_status.assert_called_once()
+
+    def test_fixing_wrong_configuration(self):
+        template = self._create_template()
+        # create a wrong configuration
+        Template.objects.update(config={'interfaces': [{'name': 'eth0', 'type': ''}]})
+        # Ensure the configuration raises ValidationError
+        with self.assertRaises(NetjsonconfigValidationError):
+            template.refresh_from_db()
+            del template.backend_instance
+            template.checksum
+
+        del template.backend_instance
+        template.config = {'interfaces': [{'name': 'eth0', 'type': 'ethernet'}]}
+        template.full_clean()
+        template.save()