Block Theme Previews does not work if we call the get_template or get_stylesheet functions early

[okmttdhr]

Description

Block Theme Previews does not work if we call the get_template or get_stylesheet functions before the pluggable functions are not ready. So, if a 3rd party plugin (e.g. CoBlocks, Elementor, Classic Editor, and WooCommerce) needs to access the template or stylesheet before the pluggable functions are ready, then the site with such a plugin will break (see the reproduction instruction below).

The reason this error happens is;

• Block Theme Previews adds hooks to stylesheet and template. https://github.com/WordPress/wordpress-develop/blob/4417367/src/wp-includes/theme-previews.php#L80-L81
• It checks current_user_can( 'switch_themes' ), which uses wp_get_current_user inside of it. https://github.com/WordPress/wordpress-develop/blob/4417367/src/wp-includes/theme-previews.php#L17-L19
• wp_get_current_user is initialized after pluggable.php is called. https://github.com/WordPress/wordpress-develop/blob/1d7554d/src/wp-settings.php#L482-L484

A possible solution would be one of the following;

• Load the pluggable functions early
• Load theme-previews.php after the pluggable functions
• Load the pluggable functions on demand in theme-previews.php
• Tell developers for the 3rd party plugins not to access the template or stylesheet directly when loading plugins (a developer guide should be covered, maybe)
• Something else

Step-by-step reproduction instructions

1. Add a file in wp-content/plugins/sample-plugin/sample-plugin.php on your site.

<?php
/*
Plugin Name: Sample plugin
*/

$wp_theme = wp_get_theme();
$wp_theme->get_stylesheet();

2. Go to /wp-admin/themes.php and click Live Preview button on your installed Block theme.
3. See the error: Uncaught Error: Call to undefined function wp_get_current_user()

Screenshots, screen recording, code snippet

Environment info

• WordPress: 6.3-RC3-56344
• Gutenberg: 16.3.0

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[scruffian]

I had an idea for how to solve this here: WordPress/wordpress-develop#4959

[okmttdhr]

So, if a 3rd party plugin needs to access the template or stylesheet before the pluggable functions are ready, then the site with such a plugin will break

For example, it breaks if a user has the CoBlocks plugin activated.
https://github.com/godaddy-wordpress/coblocks/blob/1bf29ec/class-coblocks.php#L225

Update: This issue happens on the site with a plugin like Elementor, and WooCommerce.

[okmttdhr]

I had an idea for how to solve this here: WordPress/wordpress-develop#4959

Thanks for trying this out, @scruffian!
However, I've noticed some comments on https://core.trac.wordpress.org/ticket/59000 expressing concerns about its reliability.

Load theme-previews.php after the pluggable functions

This alternative seems more fitting to me, as I feel that performing capability checks before defining pluggable functions itself might not be the optimal approach. What are your thoughts?

[fushar]

@okmttdhr @scruffian, this has been shipped and backported to WP 6.3.2, should we close this issue?

[okmttdhr]

Closing, as this issue has been solved at https://core.trac.wordpress.org/ticket/59000!