Add Upgrade Notice for Themes

[afragen]

There is upgrade notice for plugins that displays on the core update page if $transient->upgrade_notice is set in the pre_set_site_transient_update_{plugins|themes} filter, but there is no corresponding upgrade notice used or available for themes.

I propose adding a similar usage for themes as currently exists for plugins.

Use case, when theme upgrades are pending a notice of what changes, etc. will be available from the main update page.

[afragen]

Any comments?

[costdev]

@afragen Can you drop some testing info so that the patch can be tested? Thanks!

Additional props: @pbiron

Updated PR for adding an upgrade notice to theme updates in update-core.php as there is for plugin updates.

Trac ticket: https://core.trac.wordpress.org/ticket/34986

[afragen]

You can test the PR with the following plugin code. Obviously this would need to be added by the theme dev.

<?php
/**
 * Plugin Name: Test #34986
 * Description: Testing Trac Ticket #34986
 * Author: WP Core Contributors
 * Version: 0.1
 */

add_filter( 'site_transient_update_themes', 'add_upgrade_notice_34986', 20, 1 );

function add_upgrade_notice_34986($transient){
    $theme_slug = 'twentytwentythree';
    if ( isset($transient->response[$theme_slug])){
        $transient->response[$theme_slug]['upgrade_notice'] = 'Test of upgrade notice, #34986.';
    }

    return $transient;
}

To test, decrease the local version number of the Twenty Twenty Three theme so that it shows an update.

[afragen]

Above is for testing. I'm pretty sure all the dev needs to do is add a section == Upgrade Notice == to their readme.txt for the specific update and the readme parser in dot org will add the data to the update transient.