mysql递归

[SuperCustomer]

依赖关系表：t_task_link(task_id int, task_pid int);
依赖关系为一个有向无环图，例如：


当添加依赖关系时需要检测是否有回路，下面是我写的存储过程，有的能能正确检测，有的却不行，希望高手指点

-- 检查任务依赖是否有回路
DROP PROCEDURE IF EXISTS check_loop;
CREATE PROCEDURE check_loop (in_task_id INT, in_task_pid INT, OUT out_status TINYINT)
BEGIN
	DECLARE _task_id INT;
	DECLARE done INT DEFAULT FALSE;
	DECLARE cur CURSOR FOR SELECT task_pid FROM t_task_link WHERE task_id = in_task_pid;
	DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

	OPEN cur;
	read_loop : LOOP
		FETCH cur INTO _task_id;
		IF in_task_id = _task_id THEN
			SET out_status = 1;
			SET done = TRUE;
		END IF;

		IF done THEN
			LEAVE read_loop;
		END IF;

		CALL check_loop (in_task_id, _task_id, @out_status);
	END LOOP;
	CLOSE cur;
END;

DROP PROCEDURE IF EXISTS exists_loop;
CREATE PROCEDURE exists_loop (in_task_id INT, in_task_pid INT)
BEGIN
	SET @@max_sp_recursion_depth = 255;
	CALL check_loop (in_task_id, in_task_pid, @out_status);
	SELECT IFNULL(@out_status, 0);
END;