If a lock wait occurs during the processing of a FOREIGN KEY constraint, InnoDB will fail to resume the FOREIGN KEY operation once the conflicting lock has been released.

The problem was introduced in MDEV-13331, in the function row_ins_check_foreign_constraint():

		err = check_table->to_be_dropped

			? DB_LOCK_WAIT_TIMEOUT

			: trx->error_state;

We should not assign other values than DB_LOCK_WAIT_TIMEOUT to err here. The following code works both for innodb.foreign_keys and the MySQL 5.7 test innodb.update-cascade which had been missing from MariaDB:

		if (check_table->to_be_dropped

		    || trx->error_state == DB_LOCK_WAIT_TIMEOUT) {

			err = DB_LOCK_WAIT_TIMEOUT;

		}