After all, it can be useful to keep the concept of ‘dictionary transaction’, so that such transactions can be rolled back during an early phase of startup. Other transactions that do not hold locks on dictionary tables can be rolled back in the background while the server is already accepting connections.

What really needs to change is that the current thread must not hold any dict_sys latch when attempting to acquire an InnoDB table or record lock. We must remove any table lookup from pars_sql() and always pass the table names via pars_bound_id_t.

As part of this fix, we may reduce the use of trx_t::dict_operation_lock_mode. Also MDEV-23484 would reduce the use of the field. Maybe the field can be removed altogether.

Random failures of the test parts.partition_special_innodb were causing some headache until I realized that

--error ER_LOCK_WAIT_TIMEOUT

ALTER TABLE t1 ADD PARTITION PARTITIONS 2;

will invoke ha_innobase::delete_table() without holding MDL_EXCLUSIVE. Therefore, background tasks that might access the newly created partitions must explicitly be disabled in this case; we cannot rely on MDL.

We also removed dict_table_t::stats_bg_flag with proper use of MDL. The background tasks that would update persistent statistics for InnoDB tables or defragment them will acquire MDL on the table name, which will ensure that no DDL may be executed concurrently on the tables.

The refactored waiting logic should essentially fix MDEV-15020.

For the record, I just got this test to fail once on a 10.5-based branch. I had added the test into my local development tree months ago, and only now it failed for the first time:

--source include/have_innodb.inc

CREATE TABLE t (a INT) ENGINE=InnoDB STATS_PERSISTENT=1;

--source include/restart_mysqld.inc

connect (con,localhost,root);

send ALTER TABLE mysql.innodb_index_stats FORCE, ALGORITHM=INPLACE;

connection default;

SELECT * FROM t;

connection con;

reap;

connection default;

DROP TABLE t;

The failure looked like this:

mysqltest: At line 8: query 'SELECT * FROM t' failed: 2013: Lost connection to MySQL server during query

…

mariadbd: /mariadb/10.5m/storage/innobase/handler/handler0alter.cc:10008: bool commit_try_rebuild(Alter_inplace_info *, ha_innobase_inplace_ctx *, TABLE *, const TABLE *, trx_t *, const char *): Assertion `user_table->get_ref_count() == 1' failed.