[2.2.x] Fixed #30351 -- Handled pre-existing permissions in proxy model permissions data migration.

Regression in 181fb60159.

Backport of 98296f86b3 from master

django/contrib/auth/migrations/0011_update_proxy_permissions.py

@@ -1,5 +1,18 @@
-from django.db import migrations
+import sys
+
+from django.core.management.color import color_style
+from django.db import migrations, transaction
 from django.db.models import Q
+from django.db.utils import IntegrityError
+
+WARNING = """
+    A problem arose migrating proxy model permissions for {old} to {new}.
+
+      Permission(s) for {new} already existed.
+      Codenames Q: {query}
+
+    Ensure to audit ALL permissions for {old} and {new}.
+"""
 
 
 def update_proxy_model_permissions(apps, schema_editor, reverse=False):
@@ -7,6 +20,7 @@ def update_proxy_model_permissions(apps, schema_editor, reverse=False):
     Update the content_type of proxy model permissions to use the ContentType
     of the proxy model.
     """
+    style = color_style()
     Permission = apps.get_model('auth', 'Permission')
     ContentType = apps.get_model('contenttypes', 'ContentType')
     for Model in apps.get_models():
@@ -24,10 +38,16 @@ def update_proxy_model_permissions(apps, schema_editor, reverse=False):
         proxy_content_type = ContentType.objects.get_for_model(Model, for_concrete_model=False)
         old_content_type = proxy_content_type if reverse else concrete_content_type
         new_content_type = concrete_content_type if reverse else proxy_content_type
+        try:
+            with transaction.atomic():
                 Permission.objects.filter(
                     permissions_query,
                     content_type=old_content_type,
                 ).update(content_type=new_content_type)
+        except IntegrityError:
+            old = '{}_{}'.format(old_content_type.app_label, old_content_type.model)
+            new = '{}_{}'.format(new_content_type.app_label, new_content_type.model)
+            sys.stdout.write(style.WARNING(WARNING.format(old=old, new=new, query=permissions_query)))
 
 
 def revert_proxy_model_permissions(apps, schema_editor):

docs/releases/2.2.1.txt

@@ -59,3 +59,8 @@ Bugfixes
 * Increased the default timeout when using ``Watchman`` to 5 seconds to prevent
   falling back to ``StatReloader`` on larger projects and made it customizable
   via the ``DJANGO_WATCHMAN_TIMEOUT`` environment variable (:ticket:`30361`).
+
+* Fixed a regression in Django 2.2 that caused a crash when migrating
+  permissions for proxy models if the target permissions already existed. For
+  example, when a permission had been created manually or a model had been
+  migrated from concrete to proxy (:ticket:`30351`).

tests/auth_tests/test_migrations.py

@@ -4,6 +4,7 @@ from django.apps import apps
 from django.contrib.auth.models import Permission, User
 from django.contrib.contenttypes.models import ContentType
 from django.test import TestCase
+from django.test.utils import captured_stdout
 
 from .models import Proxy, UserProxy
 
@@ -152,3 +153,27 @@ class ProxyModelWithSameAppLabelTests(TestCase):
         user = User._default_manager.get(pk=user.pk)
         for permission in [self.default_permission, self.custom_permission]:
             self.assertTrue(user.has_perm('auth_tests.' + permission.codename))
+
+    def test_migrate_with_existing_target_permission(self):
+        """
+        Permissions may already exist:
+
+        - Old workaround was to manually create permissions for proxy models.
+        - Model may have been concrete and then converted to proxy.
+
+        Output a reminder to audit relevant permissions.
+        """
+        proxy_model_content_type = ContentType.objects.get_for_model(Proxy, for_concrete_model=False)
+        Permission.objects.create(
+            content_type=proxy_model_content_type,
+            codename='add_proxy',
+            name='Can add proxy',
+        )
+        Permission.objects.create(
+            content_type=proxy_model_content_type,
+            codename='display_proxys',
+            name='May display proxys information',
+        )
+        with captured_stdout() as stdout:
+            update_proxy_permissions.update_proxy_model_permissions(apps, None)
+        self.assertIn('A problem arose migrating proxy model permissions', stdout.getvalue())