[1.10.x] Fixed #27054 -- Fixed makemigrations crash with a read-only database.

Backport of 76ab885118 from master

django/db/migrations/loader.py

@@ -6,6 +6,7 @@ from importlib import import_module
 
 from django.apps import apps
 from django.conf import settings
+from django.db.migrations.exceptions import MigrationSchemaMissing
 from django.db.migrations.graph import MigrationGraph
 from django.db.migrations.recorder import MigrationRecorder
 from django.utils import six
@@ -273,7 +274,12 @@ class MigrationLoader(object):
         unapplied dependencies.
         """
         recorder = MigrationRecorder(connection)
+        try:
             applied = recorder.applied_migrations()
+        except MigrationSchemaMissing:
+            # Skip check if the django_migrations table is missing and can't be
+            # created.
+            return
         for migration in applied:
             # If the migration is unknown, skip it.
             if migration not in self.graph.nodes:

docs/releases/1.10.1.txt

@@ -54,3 +54,5 @@ Bugfixes
   PostGIS (:ticket:`27014`).
 
 * Reallowed the ``{% for %}`` tag to unpack any iterable (:ticket:`27058`).
+
+* Fixed ``makemigrations`` crash if a database is read-only (:ticket:`27054`).

tests/migrations/test_commands.py

@@ -12,7 +12,9 @@ from django.core.management import CommandError, call_command
 from django.db import (
     ConnectionHandler, DatabaseError, connection, connections, models,
 )
-from django.db.migrations.exceptions import InconsistentMigrationHistory
+from django.db.migrations.exceptions import (
+    InconsistentMigrationHistory, MigrationSchemaMissing,
+)
 from django.db.migrations.recorder import MigrationRecorder
 from django.test import ignore_warnings, mock, override_settings
 from django.utils import six
@@ -595,6 +597,27 @@ class MakeMigrationsTests(MigrationTestBase):
                 init_file = os.path.join(migration_dir, '__init__.py')
                 self.assertTrue(os.path.exists(init_file))
 
+    def test_makemigrations_other_datbase_is_readonly(self):
+        """
+        makemigrations ignores the non-default database if it's read-only.
+        """
+        def patched_ensure_schema(migration_recorder):
+            from django.db import connections
+            if migration_recorder.connection is connections['other']:
+                raise MigrationSchemaMissing()
+            else:
+                return mock.DEFAULT
+
+        self.assertTableNotExists('migrations_unicodemodel')
+        apps.register_model('migrations', UnicodeModel)
+        with mock.patch.object(
+                MigrationRecorder, 'ensure_schema',
+                autospec=True, side_effect=patched_ensure_schema):
+            with self.temporary_migration_module() as migration_dir:
+                call_command("makemigrations", "migrations", verbosity=0)
+                initial_file = os.path.join(migration_dir, "0001_initial.py")
+                self.assertTrue(os.path.exists(initial_file))
+
     def test_failing_migration(self):
         # If a migration fails to serialize, it shouldn't generate an empty file. #21280
         apps.register_model('migrations', UnserializableModel)

tests/migrations/test_loader.py

@@ -4,11 +4,12 @@ from unittest import skipIf
 
 from django.db import connection, connections
 from django.db.migrations.exceptions import (
-    AmbiguityError, InconsistentMigrationHistory, NodeNotFoundError,
+    AmbiguityError, InconsistentMigrationHistory, MigrationSchemaMissing,
+    NodeNotFoundError,
 )
 from django.db.migrations.loader import MigrationLoader
 from django.db.migrations.recorder import MigrationRecorder
-from django.test import TestCase, modify_settings, override_settings
+from django.test import TestCase, mock, modify_settings, override_settings
 from django.utils import six
 
 
@@ -464,3 +465,12 @@ class LoaderTests(TestCase):
             ('app1', '4_auto'),
         }
         self.assertEqual(plan, expected_plan)
+
+    def test_readonly_database(self):
+        """
+        check_consistent_history() ignores read-only databases, possibly
+        without a django_migrations table.
+        """
+        with mock.patch.object(MigrationRecorder, 'ensure_schema', side_effect=MigrationSchemaMissing()):
+            loader = MigrationLoader(connection=None)
+            loader.check_consistent_history(connection)