MDEV-11126: Crash while altering persistent virtual column

Problem was that if old virtual column is computed and stored there
was no check if new column is really virtual column.

mysql-test/r/alter_table.result

@@ -2021,3 +2021,58 @@ ALTER TABLE t1 ADD PRIMARY KEY IF NOT EXISTS event_id (event_id,market_id);
 Warnings:
 Note	1061	Multiple primary key defined
 DROP TABLE t1;
+#
+# MDEV-11126 Crash while altering persistent virtual column
+#
+CREATE TABLE `tab1` (
+`id` bigint(20) NOT NULL AUTO_INCREMENT,
+`field2` set('option1','option2','option3','option4') NOT NULL,
+`field3` set('option1','option2','option3','option4','option5') NOT NULL,
+`field4` set('option1','option2','option3','option4') NOT NULL,
+`field5` varchar(32) NOT NULL,
+`field6` varchar(32) NOT NULL,
+`field7` varchar(32) NOT NULL,
+`field8` varchar(32) NOT NULL,
+`field9` int(11) NOT NULL DEFAULT '1',
+`field10` varchar(16) NOT NULL,
+`field11` enum('option1','option2','option3') NOT NULL DEFAULT 'option1',
+`v_col` varchar(128) AS (IF(field11='option1',CONCAT_WS(":","field1",field2,field3,field4,field5,field6,field7,field8,field9,field10), CONCAT_WS(":","field1",field11,field2,field3,field4,field5,field6,field7,field8,field9,field10))) PERSISTENT,
+PRIMARY KEY (`id`)
+) DEFAULT CHARSET=latin1;
+ALTER TABLE `tab1` CHANGE COLUMN v_col `v_col` varchar(128);
+SHOW CREATE TABLE `tab1`;
+Table	Create Table
+tab1	CREATE TABLE `tab1` (
+  `id` bigint(20) NOT NULL AUTO_INCREMENT,
+  `field2` set('option1','option2','option3','option4') NOT NULL,
+  `field3` set('option1','option2','option3','option4','option5') NOT NULL,
+  `field4` set('option1','option2','option3','option4') NOT NULL,
+  `field5` varchar(32) NOT NULL,
+  `field6` varchar(32) NOT NULL,
+  `field7` varchar(32) NOT NULL,
+  `field8` varchar(32) NOT NULL,
+  `field9` int(11) NOT NULL DEFAULT '1',
+  `field10` varchar(16) NOT NULL,
+  `field11` enum('option1','option2','option3') NOT NULL DEFAULT 'option1',
+  `v_col` varchar(128) DEFAULT NULL,
+  PRIMARY KEY (`id`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+ALTER TABLE `tab1` CHANGE COLUMN v_col `v_col` varchar(128) AS (IF(field11='option1',CONCAT_WS(":","field1",field2,field3,field4,field5,field6,field7,field8,field9,field10), CONCAT_WS(":","field1",field11,field2,field3,field4,field5,field6,field7,field8,field9,field10))) PERSISTENT;
+SHOW CREATE TABLE `tab1`;
+Table	Create Table
+tab1	CREATE TABLE `tab1` (
+  `id` bigint(20) NOT NULL AUTO_INCREMENT,
+  `field2` set('option1','option2','option3','option4') NOT NULL,
+  `field3` set('option1','option2','option3','option4','option5') NOT NULL,
+  `field4` set('option1','option2','option3','option4') NOT NULL,
+  `field5` varchar(32) NOT NULL,
+  `field6` varchar(32) NOT NULL,
+  `field7` varchar(32) NOT NULL,
+  `field8` varchar(32) NOT NULL,
+  `field9` int(11) NOT NULL DEFAULT '1',
+  `field10` varchar(16) NOT NULL,
+  `field11` enum('option1','option2','option3') NOT NULL DEFAULT 'option1',
+  `v_col` varchar(128) AS (IF(field11='option1',CONCAT_WS(":","field1",field2,field3,field4,field5,field6,field7,field8,field9,field10), CONCAT_WS(":","field1",field11,field2,field3,field4,field5,field6,field7,field8,field9,field10))) PERSISTENT,
+  PRIMARY KEY (`id`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE `tab1`;

mysql-test/t/alter_table.test

@@ -1712,3 +1712,28 @@ CREATE TABLE t1 (
 ALTER TABLE t1 ADD PRIMARY KEY IF NOT EXISTS event_id (event_id,market_id);
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-11126 Crash while altering persistent virtual column
+--echo #
+
+CREATE TABLE `tab1` (
+  `id` bigint(20) NOT NULL AUTO_INCREMENT,
+  `field2` set('option1','option2','option3','option4') NOT NULL,
+  `field3` set('option1','option2','option3','option4','option5') NOT NULL,
+  `field4` set('option1','option2','option3','option4') NOT NULL,
+  `field5` varchar(32) NOT NULL,
+  `field6` varchar(32) NOT NULL,
+  `field7` varchar(32) NOT NULL,
+  `field8` varchar(32) NOT NULL,
+  `field9` int(11) NOT NULL DEFAULT '1',
+  `field10` varchar(16) NOT NULL,
+  `field11` enum('option1','option2','option3') NOT NULL DEFAULT 'option1',
+  `v_col` varchar(128) AS (IF(field11='option1',CONCAT_WS(":","field1",field2,field3,field4,field5,field6,field7,field8,field9,field10), CONCAT_WS(":","field1",field11,field2,field3,field4,field5,field6,field7,field8,field9,field10))) PERSISTENT,
+  PRIMARY KEY (`id`)
+) DEFAULT CHARSET=latin1;
+
+ALTER TABLE `tab1` CHANGE COLUMN v_col `v_col` varchar(128);
+SHOW CREATE TABLE `tab1`;
+ALTER TABLE `tab1` CHANGE COLUMN v_col `v_col` varchar(128) AS (IF(field11='option1',CONCAT_WS(":","field1",field2,field3,field4,field5,field6,field7,field8,field9,field10), CONCAT_WS(":","field1",field11,field2,field3,field4,field5,field6,field7,field8,field9,field10))) PERSISTENT;
+SHOW CREATE TABLE `tab1`;
+DROP TABLE `tab1`;

sql/sql_table.cc

@@ -6274,6 +6274,7 @@ static bool fill_alter_inplace_info(THD *thd,
           (field->stored_in_db || field->vcol_info->is_in_partitioning_expr()))
       {
         if (is_equal == IS_EQUAL_NO ||
+            !new_field->vcol_info ||
             !field->vcol_info->is_equal(new_field->vcol_info))
           ha_alter_info->handler_flags|= Alter_inplace_info::ALTER_COLUMN_VCOL;
         else