KX style second SQL-encapsulated increment column for board id

[ctrlcctrlv]

Much to @jaw-sh's disapopintment, it is a definite requirement to have a board_id that autoincrements along with a global ID that auto increments.

Kusaba X, one of the original chan engine and the one that Tinyboard rewrote provides us with this schema:

CREATE TABLE `PREFIX_posts` (
  `id` int(10) unsigned NOT NULL auto_increment,
  `boardid` smallint(5) unsigned NOT NULL,
  -- snip
  PRIMARY KEY  (`boardid`,`id`),
  -- snip
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;

Psql supports it as well. SQLite does with a CREATE TRIGGER. Unfortuantely, in MySQL in only one engine InnoDB this is impossible.

Looping forever like (psuedocode):

while ($error && $error->isDuplicateIdError) try_again();

hits the database far too often and is assuredly bad practice. In very active threads this could potentially loop ten times with a busy DB. I've done tests before of this on live software for a project and it was a disaster, but it 'worked' and I got paid. Let's try to do better than just get paid and do a good job as well.

We cannot just use global IDs and we cannot just use IDs based on position in the thread (>>1, >>2 etc) due to familiarity reasons. We need to keep this hack from KX, without just devolving into a million post tables like Tinyboard/vichan/infinity stable. I will write a migration for it.

[jaw-sh]

[jaw-sh]

Concurrency testing indicates that the database is now able to properly deal with many posts at one time using transactions.

[ctrlcctrlv]

Yup, I wrote a quick flooder by disabling CAPTCHA and CSRF token and then bombing it with posts from one hundred threads. I got up to 30 posts/second and no rejections/errors. Nice one.