[wp-trac] [WordPress Trac] #48610: Fatal Error during upgrade (upgrade.php) caused by theme unavailability

[WordPress Trac]

#48610: Fatal Error during upgrade (upgrade.php) caused by theme unavailability
-----------------------------+-----------------------------
 Reporter:  thomasfw         |      Owner:  (none)
     Type:  defect (bug)     |     Status:  new
 Priority:  normal           |  Milestone:  Awaiting Review
Component:  Upgrade/Install  |    Version:  5.3
 Severity:  normal           |   Keywords:
  Focuses:                   |
-----------------------------+-----------------------------
 **Some Context**

 We have a number of mu-plugins that handle core functionality in our
 client's website. Within one of these mu-plugins, we're using a class
 that's defined in the active theme in order to load a template part. I'm
 aware that this wouldn't be advised for either a distributed plugin or
 theme, but our project utilises Bedrock, Composer etc. to lock everything
 down, so the active theme and (mu-)plugins cannot be changed.

 The mu-plugin in question registers a shortcode with `add_shortcode()`,
 and we're using the theme's helper class in the callback to output some
 pagination within the shortcode.

 **The Issue**

 After upgrading WP, the dashboard redirect to "mywebsite.test/wp-
 admin/upgrade.php" results in a blank screen, and i'm not able to upgrade
 the database or view the dashboard.

 Because `upgrade.php` is seemingly ''not'' loading the theme (whilst all
 mu-plugins ''are'' loaded), a fatal error occurs because the class
 dependency inside our mu-plugin can't be found.

 In an attempt to resolve this I tried registering the shortcode only
 ''after'' the theme was initialised, by hooking on the `after_setup_theme`
 action, instead of on `init`.

 The `after_setup_theme` action as described in the WP docs - "This is the
 first action hook available to themes, triggered immediately after the
 active theme's functions.php file is loaded.".

 So one would assume that this action would only be fired when the theme
 was available, making it a suitable candidate, but the fatal error still
 occurs. Why does this action still fire when it appears that the theme is
 not loaded during the upgrade process?

 What does fix this for me is using an even later hook
 (`template_redirect`) to register the shortcode. In this case, the
 upgrade.php page loads without a fatal error and I am able to upgrade the
 site, but i'm unsure of any potential side effects of registering a
 shortcode so late on, and i'm hoping there's a potential fix available in
 the core to prevent this issue reoccurring.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/48610>
WordPress Trac <https://core.trac.wordpress.org/>
WordPress publishing platform