[1.7.x] 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.

Backport of 737d24923ac69bb8b89af1bb2f3f4c4c744349e8 from master.

6 files changed, 65 insertions, 3 deletions

diff --git a/django/contrib/auth/management/__init__.py b/django/contrib/auth/management/__init__.py
index 18cdcce598..683a8d5903 100644
--- a/django/contrib/auth/management/__init__.py
+++ b/django/contrib/auth/management/__init__.py
@@ -11,6 +11,7 @@ from django.contrib.auth import models as auth_app, 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.db.models import signals
 from django.utils.encoding import DEFAULT_LOCALE_ENCODING
 from django.utils import six
@@ -59,6 +60,10 @@ 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 7fd41b9e8b..48c98a1e13 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
 
@@ -506,3 +506,21 @@ class PermissionTestCase(TestCase):
         six.assertRaisesRegex(self, exceptions.ValidationError,
             "The verbose_name of permission is longer than 39 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 f705330137..91984a88df 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.db.models import signals
 from django.utils.encoding import smart_text
 from django.utils import six
@@ -11,6 +12,10 @@ 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..d8d967fb80 100644
--- a/django/contrib/contenttypes/tests/tests.py
+++ b/django/contrib/contenttypes/tests/tests.py
@@ -3,8 +3,10 @@ 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.test.utils import override_system_checks
 from django.utils import six
 
 from .models import ConcreteModel, ProxyModel, FooWithoutUrl, FooWithUrl, FooWithBrokenAbsoluteUrl
@@ -241,3 +243,22 @@ 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')
+    @override_system_checks([])
+    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 923202a777..b700616746 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
 from django.utils import six
@@ -284,3 +285,14 @@ 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`).