Update documentation about MariaDB and MySQL support for nowait and skip_locked arguments.

[Par Andersson]

The documentation incorrectly claims that NOWAIT & SKIP LOCKED is not supported by MySQL:

​https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-for-update

Currently, the postgresql, oracle, and mysql database backends support select_for_update(). However, MySQL doesn’t support the nowait, skip_locked, and of arguments.

Passing nowait=True, skip_locked=True, or of to select_for_update() using database backends that do not support these options, such as MySQL, raises a NotSupportedError. This prevents code from unexpectedly blocking.

However MySQL added support for nowait and skip locked since version 8, as documented here:
​https://dev.mysql.com/doc/refman/8.0/en/innodb-locking-reads.html

[Mariusz Felisiak]

Thanks, good catch. We should also update ​row-locking-with-queryset-select-for-update and it will be good to mention MariaDB in the select_for_update() ​documentation. Please remember that both don't support the of argument.

[Kolawole Nafiu Erinoso]

The changes for the fix reside in this branch ​https://github.com/kola-er/django/tree/ticket_30928_2_2.

Note that the changes do not include MariaDB because the documentation as a whole specifically listed the supported database backends (PostgreSQL, MySQL, Oracle and SQLite) without the mention of it even though it's a variant of MySQL. In addition, there's an explicit omission of MariaDB backend done within the codebase here ​https://github.com/django/django/blob/stable/2.2.x/django/db/backends/mysql/features.py#L94 for the options.

[Kolawole Nafiu Erinoso]

Here's the PR ​https://github.com/django/django/pull/12015 ready for review.

[Carlos_Mir_de_Souza]

The PR is closed, please update the PR to an open one.

[Kolawole Nafiu Erinoso]

Here's the new PR ​https://github.com/django/django/pull/12018 ready for review.

[Mariusz Felisiak <felisiak.mariusz@…>]

In a5b4a909:

[3.0.x] Fixed #30928 -- Clarified MySQL/MariaDB support of QuerySet.select_for_update() options.

Thanks Par Andersson for reporting the issue.

Backport of d94d7b113c21d7a0fd802fadb1adbbec8155e880 from master

[Mariusz Felisiak <felisiak.mariusz@…>]

In d94d7b11:

Fixed #30928 -- Clarified MySQL/MariaDB support of QuerySet.select_for_update() options.

Thanks Par Andersson for reporting the issue.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 22466f91:

[2.2.x] Fixed #30928 -- Clarified MySQL/MariaDB support of QuerySet.select_for_update() options.

Thanks Par Andersson for reporting the issue.

Backport of d94d7b113c21d7a0fd802fadb1adbbec8155e880 from master