MySQL - detecting and handling Lock Wait Timeout inside Stored Procedure - mysql

Question

The Question
purely for academic reasons, I'm wondering if you could add a handler to a mysql stored procedure that is able to recover from a lock wait timeout error if one of its queries locks up (such as a SELECT ... FOR UPDATE or UPDATE) query.
The Example
This is assuming an innoDB database, set to issolation level Repeatable read, with an empty users table defined.
1. Example Procedure:
DROP PROCEDURE IF EXISTS `lock_test`;
DELIMITER ;;
CREATE PROCEDURE `lock_test`(OUT status ENUM('success','timeout'))
MODIFIES SQL DATA
BEGIN
START TRANSACTION;
SELECT * FROM `users` FOR UPDATE;
SET status := 'success';
COMMIT;
END;;
DELIMITER ;
2. Run code in mysql terminal 1:
START TRANSACTION;
SELECT * FROM `users` FOR UPDATE;
the contents of users will be displayed, but the transaction will remain open.
3. Run code in mysql terminal 2:
CALL `lock_test`(#out);
SELECT #out;
the transaction will run until it times out (default value of innodb_lock_wait_timeout is 50 seconds)
Is it possible to add a handler inside the lock_test() procedure, so that we can have #out hold 'timeout'?

Answer 1

After spending some time reading through the MySQL Handler Documentation I was able to get what I was looking for:
DROP PROCEDURE IF EXISTS `lock_test`;
DELIMITER ;;
CREATE PROCEDURE `lock_test`(OUT status_out VARCHAR(255))
MODIFIES SQL DATA
BEGIN
DECLARE procedure_attempts INT DEFAULT 5;
DECLARE query_timeout INT DEFAULT FALSE;
SET status_out := 'start';
procedure_loop:
REPEAT
BEGIN
DECLARE CONTINUE HANDLER FOR 1205
-- Error: 1205 SQLSTATE: HY000 (ER_LOCK_WAIT_TIMEOUT)
BEGIN
SET query_timeout := TRUE;
SET status_out := CONCAT(status_out,'-timeout');
END;
IF ( procedure_attempts < 1) THEN
LEAVE procedure_loop;
END IF;
START TRANSACTION;
SELECT * FROM `users` FOR UPDATE;
IF (query_timeout) THEN
SET query_timeout := FALSE;
ELSE
SET status_out := CONCAT(status_out,'-success');
SET procedure_attempts := 0;
END IF;
COMMIT;
SET procedure_attempts := procedure_attempts - 1;
END;
UNTIL FALSE END REPEAT;
-- loop
SET status_out := CONCAT(status_out,'-end');
END;;
DELIMITER ;
When run as follows:
SET ##innodb_lock_wait_timeout:=1;
CALL `lock_test`(#out);
SELECT #out;
The output will be start-timeout-timeout-timeout-timeout-timeout-end after about 10 seconds of running time (which would be much longer if run without setting the timeout to 1 second.
While probably not too practical (or advisable) in most projects, could potentially be useful when debugging timeout issues when running a query from inside another query - I hope it might help someone else in the future.