Remove unused 'usermeta' global cache group

[SergeyBiryukov]

The usermeta cache group was introduced in [3011], but was never actually used in core.

The user_meta group is used instead, see [15482], [16562], and [54940].

To avoid confusion, I think the former should be removed from wp_cache_add_global_groups() calls in:

• wp-includes/load.php
• wp-includes/ms-blogs.php
• tests/phpunit/includes/abstract-testcase.php

See [40347] / #38725 for a precedent of removing another unused cache group.

Removing unused 'usermeta' global cache group, as it was never acutally used in core, instead the 'user_meta' group is used instead.

Trac ticket: https://core.trac.wordpress.org/ticket/58175

[ignatggeorgiev]

Added the patch above, should be taking care of the issue.

[oglekler]

I am adding needs-testing and needs-testing-info because from my point of view, proper testing looks a bit tricky for the good-first-bug.

[peterwilsoncc]

Test Report

Testing the absence of something is quite challenging, so I validated the following:

• usermeta is not used by WP Core Object Cache
• user_meta is used by WP Core's Object Cache

Method

1. Add xdebug conditional break points to wp_cache_add(), wp_cache_add_multiple(), wp_cache_replace() and wp_cache_set()
2. For test run one, the condition is $group === 'usermeta'
3. For test run two, the condition is $group === 'user_meta'

For each test, I clicked on all admin pages, and the following front-end pages:

• home
• singular post
• author
• term archive

Environment

• OS: macOS 12.6.6
• Web Server: nginx/1.18.0
• PHP: 7.4.33
• WordPress: 6.3-alpha-55505-src
• Browser: Firefox 114
• Theme: Twenty Twenty-Two
• Active Plugins: none

Actual Results

• ✅ No breakpoints hit for the usermeta condition
• ✅ All pages hit breakpoints for the user_meta condition

I agree with Jonny above, this is ready for commit.

[SergeyBiryukov]

In 55940:

Cache API: Remove unused usermeta global cache group.

The usermeta group was introduced in WP 2.0, but was never actually used in WordPress core.

The user_meta group is used instead.

Follow-up to [3011], [15482], [16562], [53823], [53832], [54940].

Props ignatggeorgiev, peterwilsoncc, oglekler, pamprn09, spacedmonkey, SergeyBiryukov.
Fixes #58175.

Thanks for the PR! Merged in r55940.