Fixed #32503 -- Fixed altering BLOB/TEXT field to non-nullable with default on MySQL 8.0.13+.

MySQL 8.0.13+ supports defaults for BLOB/TEXT but not in the
ALTER COLUMN statement.

Regression in 6b16c91157.

Thanks Matt Westcott for the report.

django/db/backends/base/schema.py

@@ -266,6 +266,13 @@ class BaseDatabaseSchemaEditor:
         """
         return False
 
+    def skip_default_on_alter(self, field):
+        """
+        Some backends don't accept default values for certain columns types
+        (i.e. MySQL longtext and longblob) in the ALTER COLUMN statement.
+        """
+        return False
+
     def prepare_default(self, value):
         """
         Only used for backends which have requires_literal_defaults feature
@@ -721,7 +728,7 @@ class BaseDatabaseSchemaEditor:
             old_default = self.effective_default(old_field)
             new_default = self.effective_default(new_field)
             if (
-                not self.skip_default(new_field) and
+                not self.skip_default_on_alter(new_field) and
                 old_default != new_default and
                 new_default is not None
             ):

django/db/backends/mysql/schema.py

@@ -68,6 +68,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             return self._is_limited_data_type(field)
         return False
 
+    def skip_default_on_alter(self, field):
+        if self._is_limited_data_type(field) and not self.connection.mysql_is_mariadb:
+            # MySQL doesn't support defaults for BLOB and TEXT in the
+            # ALTER COLUMN statement.
+            return True
+        return False
+
     @property
     def _supports_limited_data_type_defaults(self):
         # MariaDB >= 10.2.1 and MySQL >= 8.0.13 supports defaults for BLOB

tests/schema/models.py

@@ -158,6 +158,7 @@ class IntegerPK(models.Model):
 
 class Note(models.Model):
     info = models.TextField()
+    address = models.TextField(null=True)
 
     class Meta:
         apps = new_apps

tests/schema/tests.py

@@ -778,6 +778,15 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor:
             editor.alter_field(Note, old_field, new_field, strict=True)
 
+    def test_alter_text_field_to_not_null_with_default_value(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Note)
+        old_field = Note._meta.get_field('address')
+        new_field = TextField(blank=True, default='', null=False)
+        new_field.set_attributes_from_name('address')
+        with connection.schema_editor() as editor:
+            editor.alter_field(Note, old_field, new_field, strict=True)
+
     @skipUnlessDBFeature('can_defer_constraint_checks', 'can_rollback_ddl')
     def test_alter_fk_checks_deferred_constraints(self):
         """