Improve logic in render_block_core_template_part.

[spacedmonkey]

What?

Improve logic in render_block_core_template_part. These improves include.

• Skipping looking for files if slug is invalid.
• Check to see if theme if the theme has a parent theme and not do anything.
• Save repeated calls to get_stylesheet.

Why?

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[felixarntz]

@spacedmonkey The logic here looks good, though it would be great to do a performance benchmark first to assess the impact.

It would be great to do two benchmarks, one for a regular block theme, and another one for a block theme with a child theme being used.

Based on the logic changes, it seems this change would be most beneficial for sites not using a child theme (which is probably the majority of sites).

[spacedmonkey]

@felixarntz I applied these changes manaully to WordPress trunk and run benchmarks. 500 runs again, TT3 and TT3 child theme.

	Trunk - TT3	PR - TT3	Trunk - TT3 Child	PR - TT3 Child
Response Time (median)	109.4	105.55	114.65	109.1
wp-load-alloptions-query	0.64	0.64	0.64	0.64
wp-before-template (median)	52.23	53.1	57.54	56.82
wp-before-template-db-queries (median)	4.43	4.52	4.85	4.81
wp-template (median)	50.56	45.77	50.89	45.93
wp-total (median)	103.76	99.99	108.99	103.25
wp-template-db-queries (median)	2.58	2.03	2.55	1.96

This change seems to have a nice benefit for both types of site. 👍

[swissspidy]

I really wish Gutenberg would have PHP unit tests for all the block render callbacks 😞 That would make me much more confident in reviewing this change here. Maybe we could start adding some tests?

[spacedmonkey]

Maybe we could start adding some tests?

@swissspidy I agree that the lack of a unit testing for callbacks is here is a real problem. But not one in the scope of this change. However we test these callbacks, how that is backported, it is another ticket.

I could maybe add tests once this is backported, that would be much easier.

[Mamaduka]

I agree that unit testing is needed for essential logic in rendering callbacks like this.

It is also beneficial to provide testing instructions; they help reviewers and can later be adopted in tests. The original PR(#36910) was also missing the instructions, so apologies if I missed something.

Testing instructions

• Create a child theme - wp scaffold child-theme tt3-child --parent_theme=twentytwentythree --theme_name="TT3 Child"
• Override the footer template part using the legacy directory - tt3-child/block-template-parts/footer.html.
• Confirm override works as expected - ✅
• Rename the legacy template-parts directory - tt3-child/parts/footer.html.
• Confirm override works as expected - ✅
• Edit the template in Site Editor.
• Confirm that editor content is rendered - ✅

[github-actions]

Flaky tests detected in 9d55418.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/5312394222
📝 Reported issues:

• [Flaky Test] ensures paragraph block toolbar uses roving tabindex #49733 in /test/e2e/specs/editor/various/toolbar-roving-tabindex.spec.js
• [Flaky Test] should not allow blocks to be inserted in inner blocks #44341 in specs/editor/plugins/cpt-locking.test.js