[MDEV-30658] lock_row_lock_current_waits counter in information_schema.innodb_metrics may become negative - Jira

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Consider the following test case involving 3 sessions. In the first one run this:

MariaDB [test]> CREATE TABLE `test` (`col` varchar(10) ) engine=InnoDB;

Query OK, 0 rows affected (0.106 sec)

MariaDB [test]> insert into test values ('a');

Query OK, 1 row affected (0.038 sec)

MariaDB [test]> select name, count,time_enabled from information_schema.innodb_metrics where name ='lock_row_lock_current_waits';

+-----------------------------+-------+---------------------+

| name                        | count | time_enabled        |

+-----------------------------+-------+---------------------+

| lock_row_lock_current_waits |     0 | 2023-02-14 18:31:50 |

+-----------------------------+-------+---------------------+

1 row in set (0.001 sec)

Now, in the second one update the only row in the InnoDB table created:

MariaDB [test]> begin;update test set col='c' where col='a';

Query OK, 0 rows affected (0.000 sec)

Query OK, 1 row affected (0.003 sec)

Rows matched: 1  Changed: 1  Warnings: 0

In the third one run similar update that will hang waiting for 50 seconds (with default settings):

MariaDB [test]> begin;update test set col='b' where col='a';

Now in the first session we can see counter increased, reset it and wait a bit:

MariaDB [test]> select name, count,time_enabled from information_schema.innodb_metrics where name ='lock_row_lock_current_waits';

+-----------------------------+-------+---------------------+

| name                        | count | time_enabled        |

+-----------------------------+-------+---------------------+

| lock_row_lock_current_waits |     1 | 2023-02-14 18:31:50 |

+-----------------------------+-------+---------------------+

1 row in set (0.001 sec)

MariaDB [test]> set global innodb_monitor_disable='lock_row_lock_current_waits';

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> set global innodb_monitor_reset_all='lock_row_lock_current_waits';

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> set global innodb_monitor_enable='lock_row_lock_current_waits';

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> select name, count,time_enabled from information_schema.innodb_metrics where name ='lock_row_lock_current_waits';

+-----------------------------+-------+---------------------+

| name                        | count | time_enabled        |

+-----------------------------+-------+---------------------+

| lock_row_lock_current_waits |     0 | 2023-02-15 18:19:42 |

+-----------------------------+-------+---------------------+

1 row in set (0.001 sec)

When we get this error message in the third session:

Query OK, 0 rows affected (0.000 sec)

ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

MariaDB [test]>

in the first one we get a negative value of the counter:

MariaDB [test]> select name, count,time_enabled from information_schema.innodb_metrics where name ='lock_row_lock_current_waits';

+-----------------------------+-------+---------------------+

| name                        | count | time_enabled        |

+-----------------------------+-------+---------------------+

| lock_row_lock_current_waits |    -1 | 2023-02-15 18:19:42 |

+-----------------------------+-------+---------------------+

1 row in set (0.001 sec)

This is a bug and similar bugs were reported (and some fixed) many times for MySQL:

causes

MDEV-32554 mon_lock_wait_current_count test causes innodb.monitor failure

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