Fixed #36652 -- Increased determinism when loading migrations from disk.

Ordering still depends on pkgutil.iter_modules, which does not guarantee
order, but at least now Django is not introducing additional indeterminism,
causing CircularDependencyError to appear or not appear in some edge cases.

Co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

django/db/migrations/loader.py

@@ -109,11 +109,11 @@ class MigrationLoader:
                 if was_loaded:
                     reload(module)
             self.migrated_apps.add(app_config.label)
-            migration_names = {
+            migration_names = [
                 name
                 for _, name, is_pkg in pkgutil.iter_modules(module.__path__)
                 if not is_pkg and name[0] not in "_~"
-            }
+            ]
             # Load migrations
             for migration_name in migration_names:
                 migration_path = "%s.%s" % (module_name, migration_name)

tests/migrations/test_loader.py

@@ -1,7 +1,12 @@
 import compileall
 import os
+import subprocess
+import sys
+import tempfile
 from importlib import import_module
+from pathlib import Path
 
+from django.conf import settings
 from django.db import connection, connections
 from django.db.migrations.exceptions import (
     AmbiguityError,
@@ -649,6 +654,70 @@ class LoaderTests(TestCase):
             test_module.__spec__.origin = module_origin
             test_module.__spec__.has_location = module_has_location
 
+    def test_loading_order_does_not_create_circular_dependency(self):
+        """
+        Before, for these migrations:
+        app1
+        [ ] 0001_squashed_initial <- replaces app1.0001
+        [ ] 0002_squashed_initial <- replaces app1.0001
+            depends on app1.0001_squashed_initial & app2.0001_squashed_initial
+        app2
+        [ ] 0001_squashed_initial <- replaces app2.0001
+
+        When loading app1's migrations, if 0002_squashed_initial was first:
+        {'0002_squashed_initial', '0001_initial', '0001_squashed_initial'}
+        Then CircularDependencyError was raised, but it's resolvable as:
+        {'0001_initial', '0001_squashed_initial', '0002_squashed_initial'}
+        """
+        # Create a test settings file to provide to the subprocess.
+        MIGRATION_MODULES = {
+            "app1": "migrations.test_migrations_squashed_replaced_order.app1",
+            "app2": "migrations.test_migrations_squashed_replaced_order.app2",
+        }
+        INSTALLED_APPS = [
+            "migrations.test_migrations_squashed_replaced_order.app1",
+            "migrations.test_migrations_squashed_replaced_order.app2",
+        ]
+        tests_dir = Path(__file__).parent.parent
+        with tempfile.NamedTemporaryFile(
+            mode="w", encoding="utf-8", suffix=".py", dir=tests_dir, delete=False
+        ) as test_settings:
+            for attr, value in settings._wrapped.__dict__.items():
+                if attr.isupper():
+                    test_settings.write(f"{attr} = {value!r}\n")
+            # Provide overrides here, instead of via decorators.
+            test_settings.write(f"DATABASES = {settings.DATABASES}\n")
+            test_settings.write(f"MIGRATION_MODULES = {MIGRATION_MODULES}\n")
+            # Isolate away other test apps.
+            test_settings.write(
+                "INSTALLED_APPS=[a for a in INSTALLED_APPS if a.startswith('django')]\n"
+            )
+            test_settings.write(f"INSTALLED_APPS += {INSTALLED_APPS}\n")
+        test_settings_name = test_settings.name
+        self.addCleanup(os.remove, test_settings_name)
+
+        test_environ = os.environ.copy()
+        test_environ["PYTHONPATH"] = str(tests_dir)
+        # Ensure deterministic failures.
+        test_environ["PYTHONHASHSEED"] = "1"
+
+        args = [
+            sys.executable,
+            "-m",
+            "django",
+            "showmigrations",
+            "app1",
+            "--skip-checks",
+            "--settings",
+            Path(test_settings_name).stem,
+        ]
+        try:
+            subprocess.run(
+                args, capture_output=True, env=test_environ, check=True, text=True
+            )
+        except subprocess.CalledProcessError as err:
+            self.fail(err.stderr)
+
 
 class PycLoaderTests(MigrationTestBase):
     def test_valid(self):

tests/migrations/test_migrations_squashed_replaced_order/app1/0001_initial.py

@@ -0,0 +1,9 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    initial = True
+
+    dependencies = []
+
+    operations = []

tests/migrations/test_migrations_squashed_replaced_order/app1/0001_squashed_initial.py

@@ -0,0 +1,13 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    initial = True
+
+    replaces = [
+        ("app1", "0001_initial"),
+    ]
+
+    dependencies = []
+
+    operations = []

tests/migrations/test_migrations_squashed_replaced_order/app1/0002_squashed_initial.py

@@ -0,0 +1,16 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    initial = True
+
+    replaces = [
+        ("app1", "0001_initial"),
+    ]
+
+    dependencies = [
+        ("app1", "0001_squashed_initial"),
+        ("app2", "0001_squashed_initial"),
+    ]
+
+    operations = []

tests/migrations/test_migrations_squashed_replaced_order/app2/0001_initial.py

@@ -0,0 +1,11 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    initial = True
+
+    dependencies = [
+        ("app1", "0001_initial"),
+    ]
+
+    operations = []

tests/migrations/test_migrations_squashed_replaced_order/app2/0001_squashed_initial.py

@@ -0,0 +1,13 @@
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    initial = True
+
+    replaces = [
+        ("app2", "0001_initial"),
+    ]
+
+    dependencies = []
+
+    operations = []