Fatal errors possible when 3.9.1 runs alongside 3.9.0 or earlier

[barryhughes]

Some recent changes touched the ActionScheduler_Versions class, as part of an effort to make it easier to determine which plugin is loading Action Scheduler (see #1218 and #1219).

The problem is that, even if 3.9.1 is the active version of Action Scheduler, the ActionScheduler_Versions class itself may come from an older version and therefore may not contain the new methods introduced in those PRs.

• This problem can be demonstrated if you have a plugin named aaa (named as such so that it loads earlier than most other plugins) containing Action Scheduler 3.8.2, and also Action Scheduler 3.9.1.
• Activate both and try visiting the Tools ‣ Scheduled Actions screen: the page will be broken. Via your logs, or possibly on screen if display_errors is enabled, you should be able to see that a fatal error has taken place ("call to undefined method ActionScheduler_Versions::active_source()").
• The same problem also surfaces via a limited number of WP CLI paths, for example: wp action-scheduler source.
• Core functionality, by which I mean the ability for plugins to add actions and have them processed asynchronously, is not impacted.

Steps to replicate

On a clean WordPress site, add two plugins. One should be Action Scheduler 3.9.1, the other should be a custom plugin named aaa/aaa.php:

<?php
/**
 * Plugin name: Test plugin
 */

require __DIR__ . '/vendor/woocommerce/action-scheduler/action-scheduler.php';

It should contain a composer.json file as follows:

{
    "require": {
        "woocommerce/action-scheduler": "3.8.2"
    }
}

Go ahead and run composer install for that plugin, then activate both of them. Next, navigate to Tools ‣ Scheduled Actions and observe the problem.