Fixed #24075 -- Prevented running post_migrate signals when unapplying initial migrations of contenttypes and auth

Thanks Florian Apolloner for the report and Claude Paroz and Tim Graham for the review and help on the patch.

6 files changed, 63 insertions, 3 deletions

diff --git a/django/contrib/auth/management/__init__.py b/django/contrib/auth/management/__init__.py
index 5440e597c1..cf00908cab 100644
--- a/django/contrib/auth/management/__init__.py
+++ b/django/contrib/auth/management/__init__.py
@@ -11,6 +11,7 @@ from django.contrib.auth import get_permission_codename
 from django.core import exceptions
 from django.core.management.base import CommandError
 from django.db import DEFAULT_DB_ALIAS, router
+from django.db.migrations.loader import is_latest_migration_applied
 from django.utils.encoding import DEFAULT_LOCALE_ENCODING
 from django.utils import six
 
@@ -58,6 +59,10 @@ def _check_permission_clashing(custom, builtin, ctype):
 
 
 def create_permissions(app_config, verbosity=2, interactive=True, using=DEFAULT_DB_ALIAS, **kwargs):
+    # TODO: Remove when migration plan / state is passed (#24100).
+    if not is_latest_migration_applied('auth'):
+        return
+
     if not app_config.models_module:
         return
 
diff --git a/django/contrib/auth/tests/test_management.py b/django/contrib/auth/tests/test_management.py
index 0c039bd0ec..10d7cb07f1 100644
--- a/django/contrib/auth/tests/test_management.py
+++ b/django/contrib/auth/tests/test_management.py
@@ -17,7 +17,7 @@ from django.core import checks
 from django.core import exceptions
 from django.core.management import call_command
 from django.core.management.base import CommandError
-from django.test import TestCase, override_settings, override_system_checks
+from django.test import TestCase, override_settings, override_system_checks, skipUnlessDBFeature
 from django.utils import six
 from django.utils.encoding import force_str
 
@@ -570,3 +570,21 @@ class PermissionTestCase(TestCase):
         six.assertRaisesRegex(self, exceptions.ValidationError,
             "The verbose_name of permission is longer than 244 characters",
             create_permissions, auth_app_config, verbosity=0)
+
+
+class MigrateTests(TestCase):
+
+    @skipUnlessDBFeature('can_rollback_ddl')
+    def test_unmigrating_first_migration_post_migrate_signal(self):
+        """
+        #24075 - When unmigrating an app before its first migration,
+        post_migrate signal handler must be aware of the missing tables.
+        """
+        try:
+            with override_settings(
+                INSTALLED_APPS=["django.contrib.auth", "django.contrib.contenttypes"],
+                MIGRATION_MODULES={'auth': 'django.contrib.auth.migrations'},
+            ):
+                call_command("migrate", "auth", "zero", stdout=six.StringIO())
+        finally:
+            call_command("migrate", stdout=six.StringIO())
diff --git a/django/contrib/contenttypes/management.py b/django/contrib/contenttypes/management.py
index abcc5b836f..3617dbbcf6 100644
--- a/django/contrib/contenttypes/management.py
+++ b/django/contrib/contenttypes/management.py
@@ -1,5 +1,6 @@
 from django.apps import apps
 from django.db import DEFAULT_DB_ALIAS, router
+from django.db.migrations.loader import is_latest_migration_applied
 from django.utils.encoding import smart_text
 from django.utils import six
 from django.utils.six.moves import input
@@ -10,6 +11,10 @@ def update_contenttypes(app_config, verbosity=2, interactive=True, using=DEFAULT
     Creates content types for models in the given app, removing any model
     entries that no longer have a matching model class.
     """
+    # TODO: Remove when migration plan / state is passed (#24100).
+    if not is_latest_migration_applied('contenttypes'):
+        return
+
     if not app_config.models_module:
         return
 
diff --git a/django/contrib/contenttypes/tests/tests.py b/django/contrib/contenttypes/tests/tests.py
index 924ffdca46..5360b2a59a 100644
--- a/django/contrib/contenttypes/tests/tests.py
+++ b/django/contrib/contenttypes/tests/tests.py
@@ -3,8 +3,9 @@ from __future__ import unicode_literals
 from django.contrib.contenttypes.models import ContentType
 from django.contrib.contenttypes.views import shortcut
 from django.contrib.sites.shortcuts import get_current_site
+from django.core.management import call_command
 from django.http import HttpRequest, Http404
-from django.test import TestCase, override_settings
+from django.test import TestCase, override_settings, skipUnlessDBFeature
 from django.utils import six
 
 from .models import ConcreteModel, ProxyModel, FooWithoutUrl, FooWithUrl, FooWithBrokenAbsoluteUrl
@@ -241,3 +242,21 @@ class ContentTypesTests(TestCase):
         # Instead, just return the ContentType object and let the app detect stale states.
         ct_fetched = ContentType.objects.get_for_id(ct.pk)
         self.assertIsNone(ct_fetched.model_class())
+
+
+class MigrateTests(TestCase):
+
+    @skipUnlessDBFeature('can_rollback_ddl')
+    def test_unmigrating_first_migration_post_migrate_signal(self):
+        """
+        #24075 - When unmigrating an app before its first migration,
+        post_migrate signal handler must be aware of the missing tables.
+        """
+        try:
+            with override_settings(
+                INSTALLED_APPS=["django.contrib.contenttypes"],
+                MIGRATION_MODULES={'contenttypes': 'django.contrib.contenttypes.migrations'},
+            ):
+                call_command("migrate", "contenttypes", "zero", stdout=six.StringIO())
+        finally:
+            call_command("migrate", stdout=six.StringIO())
diff --git a/django/db/migrations/loader.py b/django/db/migrations/loader.py
index 9cc428e162..51fb2783eb 100644
--- a/django/db/migrations/loader.py
+++ b/django/db/migrations/loader.py
@@ -5,6 +5,7 @@ import os
 import sys
 
 from django.apps import apps
+from django.db import connection
 from django.db.migrations.recorder import MigrationRecorder
 from django.db.migrations.graph import MigrationGraph, NodeNotFoundError
 from django.utils import six
@@ -339,3 +340,14 @@ class AmbiguityError(Exception):
     Raised when more than one migration matches a name prefix
     """
     pass
+
+
+def is_latest_migration_applied(app_label):
+    # TODO: Remove when migration plan / state is passed (#24100).
+    loader = MigrationLoader(connection)
+    loader.load_disk()
+    leaf_nodes = loader.graph.leaf_nodes(app=app_label)
+    return (
+        leaf_nodes and leaf_nodes[0] in loader.applied_migrations or
+        app_label in loader.unmigrated_apps
+    )
diff --git a/docs/releases/1.7.4.txt b/docs/releases/1.7.4.txt
index 379b148488..486baa6e07 100644
--- a/docs/releases/1.7.4.txt
+++ b/docs/releases/1.7.4.txt
@@ -9,4 +9,5 @@ Django 1.7.4 fixes several bugs in 1.7.3.
 Bugfixes
 ========
 
-* ...
+* Fixed a migration crash when unapplying ``contrib.contenttypes``’s or
+  ``contrib.auth``’s first migration (:ticket:`24075`).