Allow global and non-persistent groups to be filtered

[tillkruss]

See https://core.trac.wordpress.org/ticket/54303 and https://core.trac.wordpress.org/ticket/47884

This is a rarely requested feature, but given the complete lack of flexibility should be addressed.

Core calls wp_cache_add_global_groups() very early and using apply_filters() is unlikely an option.

Adding and removing groups mid-request could be catastrophic, so having an ignore-list may be the best approach:

define( 'WP_OBJECT_CACHE_NON_GLOBAL_GROUPS' => [ 'users', 'usermeta' ] );
define( 'WP_OBJECT_CACHE_NON_GLOBAL_GROUPS' => 'users, usermeta' ); // support strings as well?

We should do the exact same for the wp_cache_add_non_persistent_groups() call.

[tillkruss]

@JanThiel is taking a go at this.

[tillkruss]

See pantheon-systems/wp-redis#331 and pantheon-systems/wp-redis#333

[JanThiel]

Trac Ticket ref: https://core.trac.wordpress.org/ticket/54303

[JanThiel]

WordPress PR (currently still without tests): WordPress/wordpress-develop#2149

[peterwilsoncc]

Just some notes of a discussion Till and I had on the PR.

It occurred to me that it might be possible to use a filter for the global groups & require site owners wishing to make use of it to require_once wp-includes/plugin.php, this is similar to how enable_loading_object_cache_dropin works. WordPress/wordpress-develop#2149 (comment)

@tillkruss asked some follow up questions, see the following few comments in the thread.

[joemcgill]

This is not something we're currently focused on, so I'm going to close this so the any related discussion about this issue can remain in the related Trac ticket: https://core.trac.wordpress.org/ticket/54303