Fixed #25280 -- Properly checked regex objects for equality to prevent infinite migrations

Thanks Sayid Munawar and Tim Graham for the report, investigation and
review.

django/db/migrations/autodetector.py

@@ -11,6 +11,7 @@ from django.db.migrations.migration import Migration
 from django.db.migrations.operations.models import AlterModelOptions
 from django.db.migrations.optimizer import MigrationOptimizer
 from django.db.migrations.questioner import MigrationQuestioner
+from django.db.migrations.utils import COMPILED_REGEX_TYPE, RegexObject
 from django.utils import six
 
 from .topological_sort import stable_topological_sort
@@ -62,6 +63,8 @@ class MigrationAutodetector(object):
                 key: self.deep_deconstruct(value)
                 for key, value in obj.items()
             }
+        elif isinstance(obj, COMPILED_REGEX_TYPE):
+            return RegexObject(obj)
         elif isinstance(obj, type):
             # If this is a type that implements 'deconstruct' as an instance method,
             # avoid treating this as being deconstructible itself - see #22951

django/db/migrations/utils.py

+import re
+
+COMPILED_REGEX_TYPE = type(re.compile(''))
+
+
+class RegexObject(object):
+    def __init__(self, obj):
+        self.pattern = obj.pattern
+        self.flags = obj.flags
+
+    def __eq__(self, other):
+        return self.pattern == other.pattern and self.flags == other.flags

django/db/migrations/writer.py

@@ -14,6 +14,7 @@ from django.apps import apps
 from django.db import migrations, models
 from django.db.migrations.loader import MigrationLoader
 from django.db.migrations.operations.base import Operation
+from django.db.migrations.utils import COMPILED_REGEX_TYPE, RegexObject
 from django.utils import datetime_safe, six
 from django.utils._os import upath
 from django.utils.encoding import force_text
@@ -23,8 +24,6 @@ from django.utils.module_loading import module_dir
 from django.utils.timezone import utc
 from django.utils.version import get_docs_version
 
-COMPILED_REGEX_TYPE = type(re.compile(''))
-
 
 class SettingsReference(str):
     """
@@ -506,7 +505,7 @@ class MigrationWriter(object):
             format = "(%s)" if len(strings) != 1 else "(%s,)"
             return format % (", ".join(strings)), imports
         # Compiled regex
-        elif isinstance(value, COMPILED_REGEX_TYPE):
+        elif isinstance(value, (COMPILED_REGEX_TYPE, RegexObject)):
             imports = {"import re"}
             regex_pattern, pattern_imports = cls.serialize(value.pattern)
             regex_flags, flag_imports = cls.serialize(value.flags)

tests/migrations/test_autodetector.py

 # -*- coding: utf-8 -*-
+import re
+
 from django.conf import settings
 from django.contrib.auth.models import AbstractBaseUser
+from django.core.validators import RegexValidator, validate_slug
 from django.db import connection, models
 from django.db.migrations.autodetector import MigrationAutodetector
 from django.db.migrations.graph import MigrationGraph
@@ -997,6 +1000,46 @@ class AutodetectorTests(TestCase):
         self.assertOperationAttributes(changes, "testapp", 0, 0, old_name="Author", new_name="NewAuthor")
         self.assertOperationAttributes(changes, "testapp", 0, 1, name="newauthor", table="author_three")
 
+    def test_identical_regex_doesnt_alter(self):
+        from_state = ModelState(
+            "testapp", "model", [("id", models.AutoField(primary_key=True, validators=[
+                RegexValidator(
+                    re.compile('^[-a-zA-Z0-9_]+\\Z'),
+                    "Enter a valid 'slug' consisting of letters, numbers, underscores or hyphens.",
+                    'invalid'
+                )
+            ]))]
+        )
+        to_state = ModelState(
+            "testapp", "model", [("id", models.AutoField(primary_key=True, validators=[validate_slug]))]
+        )
+        before = self.make_project_state([from_state])
+        after = self.make_project_state([to_state])
+        autodetector = MigrationAutodetector(before, after)
+        changes = autodetector._detect_changes()
+        # Right number/type of migrations?
+        self.assertNumberMigrations(changes, "testapp", 0)
+
+    def test_different_regex_does_alter(self):
+        from_state = ModelState(
+            "testapp", "model", [("id", models.AutoField(primary_key=True, validators=[
+                RegexValidator(
+                    re.compile('^[a-z]+\\Z', 32),
+                    "Enter a valid 'slug' consisting of letters, numbers, underscores or hyphens.",
+                    'invalid'
+                )
+            ]))]
+        )
+        to_state = ModelState(
+            "testapp", "model", [("id", models.AutoField(primary_key=True, validators=[validate_slug]))]
+        )
+        before = self.make_project_state([from_state])
+        after = self.make_project_state([to_state])
+        autodetector = MigrationAutodetector(before, after)
+        changes = autodetector._detect_changes()
+        self.assertNumberMigrations(changes, "testapp", 1)
+        self.assertOperationTypes(changes, "testapp", 0, ["AlterField"])
+
     def test_empty_foo_together(self):
         """
         #23452 - Empty unique/index_together shouldn't generate a migration.