[API] Handle AppRegistryNotReady (#12571)

Explicit handling for AppRegistryNotReady error

src/backend/InvenTree/InvenTree/exceptions.py

@@ -7,6 +7,7 @@ import traceback
 from typing import Optional
 
 from django.conf import settings
+from django.core.exceptions import AppRegistryNotReady
 from django.core.exceptions import ValidationError as DjangoValidationError
 from django.utils.translation import gettext_lazy as _
 
@@ -121,6 +122,24 @@ def exception_handler(exc, context):
         # If sentry.io fails, we don't want to crash the server!
         pass
 
+    # The Django app registry can be transiently un-ready while the plugin
+    # registry is reloading apps (see plugin.registry.PluginsRegistry._reload_apps).
+    # Any request handled by another thread/worker during that window can trip
+    # this - it is not a real server error, so ask the client to retry shortly
+    # rather than surfacing a 500.
+    if isinstance(exc, AppRegistryNotReady):
+        response = Response(
+            {
+                'error': 'AppRegistryNotReady',
+                'detail': _(
+                    'Server is temporarily reloading, please retry the request'
+                ),
+            },
+            status=503,
+        )
+        response['Retry-After'] = '1'
+        return response
+
     # Catch any django validation error, and re-throw a DRF validation error
     if isinstance(exc, DjangoValidationError):
         exc = DRFValidationError(detail=serializers.as_serializer_error(exc))

src/backend/InvenTree/InvenTree/test_api.py

@@ -4,12 +4,15 @@ from base64 import b64encode
 from pathlib import Path
 from tempfile import TemporaryDirectory
 
+from django.core.exceptions import AppRegistryNotReady
+from django.test import TestCase
 from django.urls import reverse
 
 from rest_framework import status
 
 from InvenTree.api import read_license_file
 from InvenTree.api_version import INVENTREE_API_VERSION
+from InvenTree.exceptions import exception_handler
 from InvenTree.unit_test import InvenTreeAPITestCase, InvenTreeTestCase
 from InvenTree.version import inventreeApiText, parse_version_text
 from users.ruleset import RULESET_NAMES
@@ -66,6 +69,25 @@ class HTMLAPITests(InvenTreeTestCase):
             self.assertEqual(response.status_code, 404)
 
 
+class ExceptionHandlerTests(TestCase):
+    """Tests for the custom DRF exception handler."""
+
+    def test_app_registry_not_ready(self):
+        """AppRegistryNotReady should be reported as a transient 503, not a 500.
+
+        Regression test: this can be raised on a request served by one thread while
+        the plugin registry is mid-reload on another (see
+        plugin.registry.PluginsRegistry._reload_apps, which briefly clears Django's
+        app registry) - it is not a genuine server error, so the client should be
+        told to retry rather than seeing a hard failure.
+        """
+        response = exception_handler(AppRegistryNotReady(), {})
+
+        self.assertEqual(response.status_code, 503)
+        self.assertEqual(response.data['error'], 'AppRegistryNotReady')
+        self.assertEqual(response['Retry-After'], '1')
+
+
 class ApiAccessTests(InvenTreeAPITestCase):
     """Tests for various access scenarios with the InvenTree API."""
 

src/backend/InvenTree/plugin/base/ui/tests.py

@@ -1,5 +1,8 @@
 """Unit tests for base mixins for plugins."""
 
+from unittest import mock
+
+from django.core.exceptions import AppRegistryNotReady
 from django.urls import reverse
 
 from common.models import InvenTreeSetting
@@ -86,6 +89,24 @@ class UserInterfaceMixinTests(InvenTreeAPITestCase):
         response = self.get(url)
         self.assertEqual(len(response.data), 3)
 
+    def test_ui_feature_list_app_registry_not_ready(self):
+        """A mid-reload AppRegistryNotReady should surface as a 503, not a 500.
+
+        Regression test: the plugin registry can force-reload Django's app registry
+        (e.g. when ENABLE_PLUGINS_INTERFACE is toggled - see
+        common.setting.system.reload_plugin_registry) while other requests are still
+        being served, so this endpoint's own settings lookup can race that reload.
+        """
+        url = reverse('api-plugin-ui-feature-list', kwargs={'feature': 'dashboard'})
+
+        with mock.patch(
+            'plugin.base.ui.api.get_global_setting', side_effect=AppRegistryNotReady()
+        ):
+            response = self.get(url, expected_code=503)
+
+        self.assertEqual(response.data['error'], 'AppRegistryNotReady')
+        self.assertEqual(response['Retry-After'], '1')
+
     def test_ui_panels(self):
         """Test that the sample UI plugin provides custom panels."""
         from part.models import Part