wp_get_post_autosave() uses a non-limited SQL query

[dd32]

In [59715] via #62658 wp_get_post_autosave() was switched from a raw SQL (with a LIMIT 1) to a cached WP_Query instance with posts_per_page=1

However, as name=... is specified, WP_Query doesn't apply the posts_per_page limitation.

This is because a name=... sets is_singular, which then causes the post pagination to be skipped:
https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-query.php?rev=60697&marks=2797-2798,2811#L2797

This is not a problem at present, as the name being queried should only have a singular result.

For example (Returns false, as there's no autosave, but it's the query we're interested in, noting the lack of limit)

wp> wp_get_post_autosave( 123 );
bool(false)

wp> $wpdb->last_query;
SELECT   wp_posts.ID
FROM wp_posts
WHERE 1=1  AND wp_posts.post_name = '123-autosave-v1' AND wp_posts.post_parent = 123  AND wp_posts.post_type = 'revision' AND ((wp_posts.post_status = 'inherit'))

ORDER BY wp_posts.post_date DESC

This could have other side effects for plugin code, where instead of using the core wp_get_post_revisions() a WP_Query similar to post_type=revision&post_name=$ID-revision-v1&posts_per_page=1&order=DESC is made, expecting a singular result but getting all revisions. (Querying by name=... is not expected here, and should be post_type=revision&post_parent=$ID, but I can imagine a world where someone queries by name to avoid image attachments or something that wouldn't ever be returned anyway)

Two options as I see it:

1. LIMIT 1 for all singular queries (Although, as noted, revisions break the notion of a singular post_name per post_type)
2. Respect the posts_per_page given for singular queries, if provided. This may mean having to set a default of 1 when name is set.

Thanks to @vortfu for raising this, fyi @spacedmonkey since you were the committer of above

This PR fixes a bug in WP_Query::get_posts() where the pagination logic incorrectly skips adding LIMIT clauses for singular queries, even when posts_per_page is explicitly set.

Trac ticket: https://core.trac.wordpress.org/ticket/63902