Review from @morozov about nulls

lib/Doctrine/DBAL/Schema/MySqlSchemaManager.php

@@ -220,9 +220,15 @@ class MySqlSchemaManager extends AbstractSchemaManager
      * - CURRENT_TIMESTAMP, CURRENT_TIME, CURRENT_DATE are stored in information_schema
      *   as current_timestamp(), currdate(), currtime()
      * - Literal escaping is normalized in information schema (store "''" instead of "\'")
+     * - Note: columnDefault should only be null when column is not_nullable otherwise
+     *   a string 'NULL' should be given. Unfortunately, it's not 100% correct in case of
+     *   upgrade from previous versions: see https://jira.mariadb.org/browse/MDEV-14053.
+     *   Otherwise we could simplify the condition === 'NULL' || === null by adding the parameter
+     *   is_nullable.
      *
      * @link https://mariadb.com/kb/en/library/information-schema-columns-table/
      * @link https://jira.mariadb.org/browse/MDEV-13132
+     * @link https://jira.mariadb.org/browse/MDEV-14053
      *
      * @param null|string $columnDefault default value as stored in information_schema for MariaDB >= 10.2.7
      */

tests/Doctrine/Tests/DBAL/Functional/Schema/MySqlSchemaManagerTest.php

@@ -423,18 +423,18 @@ class MySqlSchemaManagerTest extends SchemaManagerFunctionalTestCase
         $sql = "
             CREATE TABLE test_column_defaults_with_create (
                 col1 VARCHAR(255) NULL DEFAULT 'O''Connor\'\"',              
-                col2 VARCHAR(255) NULL DEFAULT '''A'''
+                col2 VARCHAR(255) NOT NULL DEFAULT 'O''Connor\'\"'                                
                 );
         ";
         $this->_conn->query($sql);
 
         $onlineTable = $this->_sm->listTableDetails("test_column_defaults_with_create");
         self::assertSame("O'Connor'\"", $onlineTable->getColumn('col1')->getDefault());
-        self::assertSame("'A'", $onlineTable->getColumn('col2')->getDefault());
+        self::assertSame("O'Connor'\"", $onlineTable->getColumn('col2')->getDefault());
 
         $table = new Table("test_column_defaults_no_diff");
         $table->addColumn('col1', 'string', ['notnull' => false, 'default' => "O'Connor'\""]);
-        $table->addColumn('col2', 'string', ['notnull' => false, 'default' => "'A'"]);
+        $table->addColumn('col2', 'string', ['notnull' => true, 'default' => "O'Connor'\""]);
 
         $comparator = new Comparator();
         $diff = $comparator->diffTable($table, $onlineTable);