MDEV-18775 Fix ALTER TABLE error handling for DROP INDEX

On an error (such as when an index cannot be dropped due to
FOREIGN KEY constraints), the field dict_index_t::to_be_dropped
was only being cleared in debug builds, even though the field
is available and being used also in non-debug builds.

This was a regression that was introduced by myself originally
in MySQL 5.7.6 and later merged to MariaDB 10.2.2, in
mysql/mysql-server@d39898d

An error manifested itself in the MariaDB Server 10.4 non-debug build,
involving instant ADD or DROP column. Because an earlier failed
ALTER TABLE operation incorrectly left the dict_index_t::to_be_dropped
flag set, the column pointers of the index fields would fail to be
adjusted for instant ADD or DROP column (MDEV-15562). The instant
ADD COLUMN in MariaDB Server 10.3 is unlikely to be affected by a
similar scenario, because dict_table_t::instant_add_column() in 10.3
is applying the transformations to all indexes, not skipping
to-be-dropped ones.

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

@@ -38,3 +38,18 @@ Warnings:
 Warning	1105	ORDER BY ignored as there is a user-defined clustered index in the table 't1'
 DROP TABLE t1;
 SET sql_mode=DEFAULT;
+#
+# MDEV-18775 Server crashes in dict_table_t::instant_column
+# upon ADD COLUMN
+#
+CREATE TABLE tx (pk INT PRIMARY KEY) ENGINE=InnoDB;
+CREATE TABLE t1 (pk INT, a INT, PRIMARY KEY (pk), KEY (a), FOREIGN KEY (a) REFERENCES tx (pk)) ENGINE=InnoDB;
+SET FOREIGN_KEY_CHECKS=OFF;
+ALTER TABLE t1 DROP a;
+ERROR HY000: Cannot drop column 'a': needed in a foreign key constraint 'test/t1_ibfk_1'
+SET FOREIGN_KEY_CHECKS=ON;
+ALTER TABLE t1 ADD b INT;
+ALTER TABLE t1 DROP a;
+ERROR HY000: Cannot drop index 'a': needed in a foreign key constraint
+ALTER TABLE t1 ADD c INT;
+DROP TABLE t1, tx;

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

@@ -38,3 +38,24 @@ ALTER TABLE t1 ORDER BY a;
 DROP TABLE t1;
 
 SET sql_mode=DEFAULT;
+
+--echo #
+--echo # MDEV-18775 Server crashes in dict_table_t::instant_column
+--echo # upon ADD COLUMN
+--echo #
+
+CREATE TABLE tx (pk INT PRIMARY KEY) ENGINE=InnoDB;
+CREATE TABLE t1 (pk INT, a INT, PRIMARY KEY (pk), KEY (a), FOREIGN KEY (a) REFERENCES tx (pk)) ENGINE=InnoDB;
+
+SET FOREIGN_KEY_CHECKS=OFF;
+
+--error ER_FK_COLUMN_CANNOT_DROP
+ALTER TABLE t1 DROP a;
+
+SET FOREIGN_KEY_CHECKS=ON;
+
+ALTER TABLE t1 ADD b INT;
+--error ER_DROP_INDEX_FK
+ALTER TABLE t1 DROP a;
+ALTER TABLE t1 ADD c INT;
+DROP TABLE t1, tx;

storage/innobase/handler/handler0alter.cc

@@ -5142,14 +5142,12 @@ prepare_inplace_alter_table_dict(
 	ut_ad(!user_table->drop_aborted);
 
 err_exit:
-#ifdef UNIV_DEBUG
 	/* Clear the to_be_dropped flag in the data dictionary cache. */
 	for (ulint i = 0; i < ctx->num_to_drop_index; i++) {
 		DBUG_ASSERT(ctx->drop_index[i]->is_committed());
 		DBUG_ASSERT(ctx->drop_index[i]->to_be_dropped);
 		ctx->drop_index[i]->to_be_dropped = 0;
 	}
-#endif /* UNIV_DEBUG */
 
 	row_mysql_unlock_data_dictionary(ctx->trx);