key/group naming confusion in _get_non_cached_ids()

[johnjamesjacoby]

The _get_non_cached_ids() function uses a parameter named $cache_key that actually represents a cache group everywhere else within WordPress.

This means when looking at the block or inline documentations, a developer may accidentally be guided down the path of passing in an actual cache-key instead of the cache-group they're checking against.

My recommendation would be to clarify this bit of code to use $group or $cache_group rather than $cache_key, and update any surrounding code documentation to be explicit if it needs it. It's a non-functional change, but will make the code more clear for people.

(Related: BuddyPress comes packaged with it's own similar helper function, and opted to use $cache_group.)

[johnjamesjacoby]

40420.patch paints the bikeshed in the following ways:

• Uses $cache_group instead of $cache_key
• Updates "cache bucket" documentation to "cache group"

Notes:

• Core mostly $group instead of $cache_group everywhere else but 1 place (the customizer) which was newly introduced in 4.7.0. (See: WP_Customize_Manager->find_changeset_post_id())
• My personal preference would be to use $cache_group everywhere, but that's a lot of code-churn
• $group reads too ambiguously to me, especially given the context of BuddyPress Groups

Additional findings:

• We do !wp_cache_get() here, which might actually be a logic error. It's plausible this should be a strict false comparison, as the intended functionality is to identify totally uncached keys, and not a value that might be an empty string or array. (This type of problem exists through-out WordPress already though.)
• See #22174 & #20875 for other nice improvements to this function

[spacedmonkey]

@SergeyBiryukov Pinging as this is related to renaming variables.

[SergeyBiryukov]

Thanks for the ping!

This makes sense to me, and I agree that $cache_group would be the preferable name here.

[SergeyBiryukov]

In 55566:

Cache API: Correct the second parameter's name in _get_non_cached_ids().

The parameter represents the cache group, not the cache key.

Follow-up to [19918].

Props johnjamesjacoby, spacedmonkey.
Fixes #40420.