MDEV-21645 SIGSEGV in innobase_get_computed_value

ha_innobase::commit_inplace_alter_table(): After
ALTER_STORED_COLUMN_ORDER, ensure that the virtual column metadata
will be reloaded also when the table is not being rebuilt.

mysql-test/suite/innodb/r/instant_alter_bugs.result

@@ -358,4 +358,13 @@ t1	CREATE TABLE `t1` (
   `a` char(1) DEFAULT NULL
 ) ENGINE=InnoDB DEFAULT CHARSET=latin2 ROW_FORMAT=REDUNDANT
 DROP TABLE t1;
+#
+# MDEV-21645 SIGSEGV in innobase_get_computed_value
+#
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, va INTEGER GENERATED ALWAYS AS (a))
+ENGINE=InnoDB;
+INSERT INTO t1 SET a=1, b=NULL;
+ALTER TABLE t1 MODIFY COLUMN b INT FIRST;
+ALTER TABLE t1 ADD UNIQUE INDEX (va);
+DROP TABLE t1;
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

mysql-test/suite/innodb/t/instant_alter_bugs.test

@@ -374,4 +374,14 @@ ALTER TABLE t1 MODIFY a CHAR, ALGORITHM=INSTANT;
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-21645 SIGSEGV in innobase_get_computed_value
+--echo #
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, va INTEGER GENERATED ALWAYS AS (a))
+ENGINE=InnoDB;
+INSERT INTO t1 SET a=1, b=NULL;
+ALTER TABLE t1 MODIFY COLUMN b INT FIRST;
+ALTER TABLE t1 ADD UNIQUE INDEX (va);
+DROP TABLE t1;
+
 SET GLOBAL innodb_purge_rseg_truncate_frequency=@save_frequency;

storage/innobase/handler/handler0alter.cc

@@ -11172,7 +11172,10 @@ ha_innobase::commit_inplace_alter_table(
 	/* MDEV-17468: Avoid this at least when ctx->is_instant().
 	Currently dict_load_column_low() is the only place where
 	num_base for virtual columns is assigned to nonzero. */
-	if (ctx0->num_to_drop_vcol || ctx0->num_to_add_vcol) {
+	if (ctx0->num_to_drop_vcol || ctx0->num_to_add_vcol
+	    || (ctx0->is_instant()
+		&& m_prebuilt->table->n_v_cols
+		&& ha_alter_info->handler_flags & ALTER_STORED_COLUMN_ORDER)) {
 		DBUG_ASSERT(ctx0->old_table->get_ref_count() == 1);
 
 		trx_commit_for_mysql(m_prebuilt->trx);