[wp-trac] [WordPress Trac] #31392: Undefined function error on mu-plugin during update/upgrade

[WordPress Trac]

#31392: Undefined function error on mu-plugin during update/upgrade
-----------------------------+-----------------------------
 Reporter:  jadpm            |      Owner:
     Type:  defect (bug)     |     Status:  new
 Priority:  normal           |  Milestone:  Awaiting Review
Component:  Upgrade/Install  |    Version:  trunk
 Severity:  normal           |   Keywords:
  Focuses:                   |
-----------------------------+-----------------------------
 It seems that there is a weird issue on callbacks added to proper action
 hooks inside mu-plugins. Somehow, it seems that theme and plugin functions
 are not available during some actions while performing an update/upgrade.

 Steps to reproduce:

 Create a dummy function inside your active theme functions.php file or
 create a generic plugin containing a dummy function definition:
 {{{
 function prefix_mu_init_callback() {
         return;
 }
 }}}

 Create a dummy mu-plugin containing a callback hooked into init and
 calling that dummy function:
 {{{
 add_action( 'init', 'prefix_mu_init_test' );
 function prefix_mu_init_test() {
         prefix_mu_init_callback();
 }
 }}}

 All in all, everything works fine. Now, if you perform an update/upgrade,
 you will get the following error:
 {{{
 Fatal error:  Call to undefined function prefix_mu_init_callback() in
 C:\wamp\www\wp_dev\wp-content\mu-plugins\prefix_mu_plugin.php on line 14
 PHP Stack trace:
 PHP   1. {main}() C:\wamp\www\wp_dev\wp-admin\upgrade.php:0
 PHP   2. require() C:\wamp\www\wp_dev\wp-admin\upgrade.php:18
 PHP   3. require_once() C:\wamp\www\wp_dev\wp-load.php:37
 PHP   4. require_once() C:\wamp\www\wp_dev\wp-config.php:97
 PHP   5. do_action() C:\wamp\www\wp_dev\wp-settings.php:353
 PHP   6. call_user_func_array() C:\wamp\www\wp_dev\wp-
 includes\plugin.php:496
 PHP   7. prefix_mu_init_test() C:\wamp\www\wp_dev\wp-
 includes\plugin.php:496
 }}}

 I managed to reproduce this issue on two different scenarios. First, doing
 a manual upgrade from 3.X to 4.1.1. Also, you can easily reproduce it by
 using the WordPress Beta Tester plugin, as this happens every time an
 update is installed.

 I am not sure whether this is indeed a bug or not, since mu-plugins should
 not depend on things defined on the active theme or a third-party plugin,
 so I'm using function_exists() to workaround it. But it feels like things
 hooked on init should be ready to use on //real// init, that is, after
 plugins_loaded and after_setup_theme.

 Possible related: #28668

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Ticket URL: <https://core.trac.wordpress.org/ticket/31392>
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