Global styles: cache post-processed style variations

[oandregal]

Trac ticket https://core.trac.wordpress.org/ticket/61451
Follow-up to #6843
Backports WordPress/gutenberg#62610

What

This PR caches the post-processed style variation files (theme.json within the styles/ folder).

Why

To improve performance, so we don't have to process them more than once.

How

Introduces a static $style_variations_cache variable.

Test

How to test

1. Verify the theme style variations are working as expected.

Go to "Site Editor > Styles" and apply one of them. Verify the changes are reflected in the front end. Do the same from the global styles sidebar in the site editor.

2. Verify the block style variations defined via theme.json are working as expected.

Create a partial.json file within the styles/ folder with the following contents:

{
        "$schema": "https://schemas.wp.org/trunk/theme.json",
        "version": 2,
        "title": "Partial",
        "blockTypes": [ "core/group" ],
        "styles": {
                "color": {
                        "background": "aliceblue"
                }
        }
}

Go to any editor, add a group block, and verify there is a "Partial" style variation (Block Settings > Styles). Apply the variation and save the changes. Verify the contents are the expected (background color is aliceblue) — also in the front-end.

Commit

Proposal for commit message, to make it easier for committers:

Global styles: cache post-processed style variations.

Props aaronrobertshaw, ramonopoly, oandregal.
See #61451.

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Props oandregal, joemcgill.

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[oandregal]

@joemcgill what are your thoughts on doing this?

I was looking at the performance results for #6843 (cache reading the contents from the filesystem), and I'm a bit confused.

I'm not familiar with what we are testing for the "Admin" test, so I don't know if it should have been impacted. However, for the "Front End › Theme: twentytwentyfour" I would have expected to be more impacted. Instead, I see timid or conflicting results:

• wpMemory: increased, which makes sense given we now cache those files.
• wpTotal: it has been reduced minimally and increased (also minimally) in others.
• timeToFirstByte: increased minimally

The results for this PR are similarly confusing:

• wpMemory: increased, as expected.
• wpTotal & timeToFirstByte are both higher (I'd expect the contrary).

This is my current understanding of the new feature:

• In the front-end, we may or may not need to enqueue new styles related to block style variations defined by the theme.
• This requires inspecting all the files within styles/ which is done in the get_style_variations method at some point.

Unless the tests are not triggering the file reads from the filesystem for some reason, I'd have expected the numbers to have improved with the cache (previous PR & this one).

[joemcgill]

In theory, adding a cache here should be more performant. However, it seem that the refactor of WP_Theme_JSON_Resolver::get_style_variations has also added a lot more processing to that method, and the cache (if it's even being used) is not offsetting the additional processing.

Trunk

This PR

It seems odd to me that in trunk, none of the WP_Theme_JSON construction calls were actually running. I've investigated this by setting a breakpoint in this method to see the code execution via XDebug, and it seems like in each of the 4 times this method is called on the TT4 homepage, it's always passing 'block' as the scope, but all of the variations are from the 'theme' scope, so in trunk this is all being skipped and is returning an empty array, whereas now this PR is just checking to see whether there is a scope at all, not just that it matches the scope being passed to the method.

This seems like a likely bug.

[oandregal]

This seems like a likely bug.

As far as I tested, the feature is working correctly in trunk (instructions, though I did more random things as well). I also tested the related one (theme style variations).

I haven't had the time to look at this using xdebug today, as I was investigating a proof of concept to optimize the data flow.

[joemcgill]

Thanks, I'll do some more testing. I'm just surprised that currently no variations are being processed while running TT4 in trunk, but perhaps it's because those code paths aren't being triggered without adding some variations manually?