1. Objective

In a race-condition where Omise Webhook is fired at the same time as buyer returns to the Callback URI (return_uri). A particular WooCommerce Order object will get updated its status twice which creates a consequence of a confirmation email is being sent to the recipient twice causing a confusion in a business flow that rely on this particular email.

This happens because at the moment where Omise Webhook and Callback URI are triggered asynchronously at the same time, they both retrieve WC Order object with the pending status and update it to processing using WC_Order::payment_complete(), which triggers an email function to be executed twice.

This pull request is providing a solution to prevent the async request from creating a race-condition, by refactoring Event Handlers to make the code support for the Queue worker system in case if the race-condition happened.

Related information:
Related issue(s): T22125 (internal ticket)

2. Description of change

3. Quality assurance

🔧 Environments:

• WooCommerce: v4.2.2.
• WordPress: v5.4.2
• PHP version: 7.3.3.

✏️ Details:

4. Impact of the change

There is a limitation of concurrent that Action Scheduler library can handle.
Please check the following document as reference: https://actionscheduler.org/perf

Action Scheduler will only process actions in a request until:
• 90% of available memory is used
• processing another 3 actions would exceed 30 seconds of total request time, based on the average processing time for the current batch
• in a single concurrent queue

Also

By default, Action Scheduler will only process actions for a maximum of 30 seconds in each request. This time limit minimises the risk of a script timeout on unknown hosting environments, some of which enforce 30 second timeouts.

By default, Action Scheduler will claim a batch of 25 actions. This small batch size is because the default time limit is only 30 seconds; however, if you know your actions are processing very quickly, e.g. taking microseconds not seconds, or that you have more than 30 second available to process each batch, increasing the batch size can slightly improve performance.

By default, Action Scheduler will run only one concurrent batches of actions. This is to prevent consuming a lot of available connections or processes on your webserver.

However, it shall not effect to its capability to handle the queue. Just for some high-volume transaction per minute website, it may take some seconds or minutes until all the queue get executed.

As stated in WP-Cron: https://developer.wordpress.org/plugins/cron/#what-is-wp-cron

5. Priority of change

Immediate.

6. Additional Notes

Just to clarify the relationship between 3 names.

• WooCommerce integrated Action Scheduler library in the project
• Action Scheduler is designed to work with WP-Cron
• WP-Cron is a default built-in Cron worker that WordPress provided.

However, there is one point that we should understand of WP-Cron behaviour, to understand how the WC_Queue work.
As stated in WP-Cron document: https://developer.wordpress.org/plugins/cron/#what-is-wp-cron

WP-Cron works by checking, on every page load, a list of scheduled tasks to see what needs to be run. Any tasks due to run will be called during that page load.

WP-Cron does not run constantly as the system cron does; it is only triggered on page load.

With the system scheduler, if the time passes and the task did not run, it will not be re-attempted. With WP-Cron, all scheduled tasks are put into a queue and will run at the next opportunity (meaning the next page load). So while you can’t be 100% sure when your task will run, you can be 100% sure that it will run eventually.

The WP-Cron is only working when "any" of page get loaded. It's not time-based as a typical system scheduler.
Meaning that, even though we set some schedule to be executed after "5" mins, but if there is no one open "any" page of the WordPress website, then that schedule won't be triggered.