MDEV-32638 MariaDB crashes with foreign_key_checks=0 when changing a column and adding a foreign key at the same time

Problem:
=======
 - InnoDB fails to find the foreign key index for the newly
added foreign key relation. This is caused by commit
5f09b53 (MDEV-31086).

FIX:
===
In check_col_is_in_fk_indexes(), while iterating through
the newly added foreign key relationship, InnoDB should
consider that foreign key relation may not have foreign index
when foreign key check is disabled.

Author: Thirunarayanan

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

@@ -118,4 +118,17 @@ ALTER TABLE t2 DROP INDEX idx;
 ALTER TABLE t2 MODIFY f2 VARCHAR(1023);
 SET SESSION FOREIGN_KEY_CHECKS = ON;
 DROP TABLE t2, t1;
+#
+#  MDEV-32638 MariaDB crashes with foreign_key_checks=0
+#	when changing a column and adding a foreign
+#		key at the same time
+#
+CREATE TABLE t1(f1 VARCHAR(2) NOT NULL, PRIMARY KEY(f1))ENGINE=InnoDB;
+CREATE TABLE t2(f1 INT NOT NULL PRIMARY KEY,
+f2 VARCHAR(10) NOT NULL DEFAULT '')ENGINE=InnoDB;
+SET SESSION FOREIGN_KEY_CHECKS = OFF;
+ALTER TABLE t2 CHANGE COLUMN f2 f3 VARCHAR(20) NOT NULL,
+ADD CONSTRAINT t2_fk FOREIGN KEY(f3) REFERENCES t1(f1);
+DROP TABLE t2, t1;
+SET SESSION FOREIGN_KEY_CHECKS = ON;
 # End of 10.4 tests

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

@@ -153,4 +153,18 @@ ALTER TABLE t2 DROP INDEX idx;
 ALTER TABLE t2 MODIFY f2 VARCHAR(1023);
 SET SESSION FOREIGN_KEY_CHECKS = ON;
 DROP TABLE t2, t1;
+
+--echo #
+--echo #  MDEV-32638 MariaDB crashes with foreign_key_checks=0
+--echo #	when changing a column and adding a foreign
+--echo #		key at the same time
+--echo #
+CREATE TABLE t1(f1 VARCHAR(2) NOT NULL, PRIMARY KEY(f1))ENGINE=InnoDB;
+CREATE TABLE t2(f1 INT NOT NULL PRIMARY KEY,
+                f2 VARCHAR(10) NOT NULL DEFAULT '')ENGINE=InnoDB;
+SET SESSION FOREIGN_KEY_CHECKS = OFF;
+ALTER TABLE t2 CHANGE COLUMN f2 f3 VARCHAR(20) NOT NULL,
+        ADD CONSTRAINT t2_fk FOREIGN KEY(f3) REFERENCES t1(f1);
+DROP TABLE t2, t1;
+SET SESSION FOREIGN_KEY_CHECKS = ON;
 --echo # End of 10.4 tests

storage/innobase/handler/handler0alter.cc

@@ -7695,6 +7695,7 @@ bool check_col_is_in_fk_indexes(
 
   for (const auto &a : add_fk)
   {
+    if (!a->foreign_index) continue;
     for (ulint i= 0; i < a->n_fields; i++)
     {
       if (a->foreign_index->fields[i].col == col)