MDEV-39548 Cleanup MDL_request boilerplate with ACQUIRE_LOCK macro

[longjinvan]

Description

MDL (Metadata Lock) is MariaDB's locking subsystem that protects database objects (tables, schemas, stored procedures, etc.) from concurrent DDL — for example, preventing a table from being dropped while another thread is reading from it. MDL_request is the "lock request form" that callers must create, initialize, submit, and then extract the resulting MDL_ticket from.

Currently, every MDL lock acquisition requires repetitive boilerplate:

> MDL_request mdl_request;
> MDL_REQUEST_INIT(&mdl_request, namespace, db, object, lock_type, duration);
> if (mdl_context.acquire_lock(&mdl_request, lock_wait_timeout))
>   /* error */;
> ticket = mdl_request.ticket;

This patch introduces MDL_context::ACQUIRE_LOCK() — a set of overloaded methods (wrapped in a macro for FILE/LINE tracking) that combine the above steps into a single call returning MDL_ticket* directly:

> if (!(ticket = mdl_context.ACQUIRE_LOCK(namespace, db, object, lock_type, lock_wait_timeout)))
>   /* error */;
> 

This is a step toward making MDL_request a private implementation detail of the MDL subsystem, reducing its exposure to callers and preventing the class of bugs where stale or misused MDL_request objects cause issues (MDEV-39241, MDEV-39184).

Release Notes

N/A

How can this PR be tested?

• All MTR tests pass, confirming no regressions introduced by this refactoring.

Basing the PR against the correct MariaDB version

• This is a refactoring change, and the PR is based against the latest MariaDB development branch.

Copyright

All new code of the whole pull request, including one or several files that are either new files or modified ones, are contributed under the BSD-new license. I am contributing on behalf of my employer Amazon Web Services, Inc.

[gemini-code-assist]

Code Review

This pull request streamlines the process of acquiring Metadata Locks (MDL) by adding new acquire_lock methods to the MDL_context class and introducing ACQUIRE_LOCK and ACQUIRE_LOCK_BY_KEY macros. These additions allow for more concise code by removing the need to manually declare and initialize MDL_request objects in many parts of the server, such as in backup operations, DDL logging, and table handling. The reviewer recommended adding an MDL_ prefix to the new macros to ensure consistency with existing naming patterns and to prevent potential namespace conflicts.

[gkodinov]

Thank you for your contribution! This is a preliminary review.

Nice cleanup. Some more changes and it will be perfect.

[svoj]

Looks very nice, a few comments inline.

[svoj]

Dunno, I'd avoid this change as we can't trivially verify that subsequently executed code doesn't rely on initialized tl->mdl_request. Or did you verify this?

[longjinvan]

Reverted — keeping the original MDL_REQUEST_INIT + acquire_lock pattern here since subsequent code may rely on tl->mdl_request being initialized.

[svoj]

I don't see this reverted. Please double check.

[longjinvan]

Sorry about that — I forgot to copy the revert from my test workspace to the submission package. Fixed now.

[gkodinov]

LGTM. Please keep working on it for the final review.

[svoj]

There're a few more cases that can be updated:

1. dict_acquire_mdl_share() - replace both acquire and try_acquire
2. backup_lock() it is not right that sql_yacc.yy makes decision which lock type to use, change to MDL_ACQIRE_LOCK() please
3. Item_func_get_lock::val_int() - pre-initialize ull_key instead of ull_request like in Item_func_release_lock::val_int()
4. open_and_lock_for_insert_delayed() - protection_request can be replaced
5. trx_purge_table_acquire()
6. lock_table_names() - global_request can be moved

[svoj]

You don’t need to use mdl_ticket here. Checking the result of MDL_ACQUIRE_LOCK() is sufficient, as in the original code.

[longjinvan]

Fixed. Now just checks the return value without assigning to mdl_ticket.

[longjinvan]

There're a few more cases that can be updated:

1. dict_acquire_mdl_share() - replace both acquire and try_acquire
2. backup_lock() it is not right that sql_yacc.yy makes decision which lock type to use, change to MDL_ACQIRE_LOCK() please
3. Item_func_get_lock::val_int() - pre-initialize ull_key instead of ull_request like in Item_func_release_lock::val_int()
4. open_and_lock_for_insert_delayed() - protection_request can be replaced
5. trx_purge_table_acquire()
6. lock_table_names() - global_request can be moved

Done. Updated in the latest revision:

• dict_acquire_mdl_shared() — split trylock/blocking paths; trylock uses a new try_acquire_lock() overload, blocking uses MDL_ACQUIRE_LOCK().
• backup_lock() — now uses MDL_ACQUIRE_LOCK() directly with MDL_SHARED_HIGH_PRIO, no longer relies on the lock type set in sql_yacc.yy.
• Item_func_get_lock::val_int() — pre-initializes MDL_key ull_key (same pattern as Item_func_release_lock::val_int()), then uses MDL_ACQUIRE_LOCK_BY_KEY().
• open_and_lock_for_insert_delayed() — protection_request replaced with MDL_ACQUIRE_LOCK(); delayed_get_table() now takes MDL_ticket* directly.
• trx_purge_table_acquire() — uses the new try_acquire_lock() overload, MDL_request removed.
• lock_table_names() — global_request removed; loop now uses try_acquire_lock() overload and MDL_ACQUIRE_LOCK() inline.