[1.7.x] Fixed #23920 -- Fixed MySQL crash when adding blank=True to TextField.

Thanks wkornewald for the report and Markus Holtermann for review.

Backport of 765fa36d57 from master

django/db/backends/schema.py

@@ -560,7 +560,7 @@ class BaseDatabaseSchemaEditor(object):
         # Default change?
         old_default = self.effective_default(old_field)
         new_default = self.effective_default(new_field)
-        if old_default != new_default:
+        if old_default != new_default and not self.skip_default(new_field):
             if new_default is None:
                 actions.append((
                     self.sql_alter_column_no_default % {

docs/releases/1.7.2.txt

@@ -107,4 +107,7 @@ Bugfixes
 
 * Prevented the
   :class:`~django.contrib.auth.middleware.SessionAuthenticationMiddleware` from
-  setting a ``"Vary: Cookie"`` header on all responses.
+  setting a ``"Vary: Cookie"`` header on all responses (:ticket:`23939`).
+
+* Fixed a crash when adding ``blank=True`` to ``TextField()`` on MySQL
+  (:ticket:`23920`).

tests/schema/models.py

@@ -171,3 +171,7 @@ class Thing(models.Model):
 
     def __str__(self):
         return self.when
+
+
+class Note(models.Model):
+    info = models.TextField()

tests/schema/tests.py

@@ -10,7 +10,7 @@ from django.db.transaction import atomic
 from .models import (Author, AuthorWithDefaultHeight, AuthorWithM2M, Book, BookWithLongName,
     BookWithSlug, BookWithM2M, Tag, TagIndexed, TagM2MTest, TagUniqueRename,
     UniqueTest, Thing, TagThrough, BookWithM2MThrough, AuthorTag, AuthorWithM2MThrough,
-    AuthorWithEvenLongerName, BookWeak)
+    AuthorWithEvenLongerName, BookWeak, Note)
 
 
 class SchemaTests(TransactionTestCase):
@@ -423,6 +423,19 @@ class SchemaTests(TransactionTestCase):
         self.assertEqual(columns['name'][0], "TextField")
         self.assertEqual(bool(columns['name'][1][6]), False)
 
+    def test_alter_text_field(self):
+        # Regression for "BLOB/TEXT column 'info' can't have a default value")
+        # on MySQL.
+        new_field = TextField(blank=True)
+        new_field.set_attributes_from_name("info")
+        with connection.schema_editor() as editor:
+            editor.alter_field(
+                Note,
+                Note._meta.get_field_by_name("info")[0],
+                new_field,
+                strict=True,
+            )
+
     def test_alter_null_to_not_null(self):
         """
         #23609 - Tests handling of default values when altering from NULL to NOT NULL.