Fixed #17667 -- Prevented app loading from skipping nonexistent apps after the first try

Thanks ea2100@ for the report and akaariai for the patch.

django/db/models/loading.py

@@ -105,9 +105,9 @@ class AppCache(object):
         Loads the app with the provided fully qualified name, and returns the
         model module.
         """
+        app_module = import_module(app_name)
         self.handled.add(app_name)
         self.nesting_level += 1
-        app_module = import_module(app_name)
         try:
             models = import_module('%s.%s' % (app_name, MODELS_MODULE_NAME))
         except ImportError:

tests/app_loading/tests.py

@@ -7,7 +7,8 @@ import time
 from unittest import TestCase
 
 from django.conf import Settings
-from django.db.models.loading import cache, load_app, get_model, get_models
+from django.db.models.loading import cache, load_app, get_model, get_models, AppCache
+from django.test.utils import override_settings
 from django.utils._os import upath
 
 
@@ -61,12 +62,33 @@ class EggLoadingTest(TestCase):
         egg_name = '%s/brokenapp.egg' % self.egg_dir
         sys.path.append(egg_name)
         self.assertRaises(ImportError, load_app, 'broken_app')
+        raised = None
         try:
             load_app('broken_app')
         except ImportError as e:
+            raised = e
+
         # Make sure the message is indicating the actual
         # problem in the broken app.
-            self.assertTrue("modelz" in e.args[0])
+        self.assertTrue(raised is not None)
+        self.assertTrue("modelz" in raised.args[0])
+
+    def test_missing_app(self):
+        """
+        Test that repeated app loading doesn't succeed in case there is an
+        error. Refs #17667.
+        """
+        # AppCache is a Borg, so we can instantiate one and change its
+        # loaded to False to force the following code to actually try to
+        # populate the cache.
+        a = AppCache()
+        a.loaded = False
+        try:
+            with override_settings(INSTALLED_APPS=('notexists',)):
+                self.assertRaises(ImportError, get_model, 'notexists', 'nomodel', seed_cache=True)
+                self.assertRaises(ImportError, get_model, 'notexists', 'nomodel', seed_cache=True)
+        finally:
+            a.loaded = True
 
 
 class GetModelsTest(TestCase):