Event subscribers loaded twice with Rails 6/Zeitwerk

[spaghetticode]

Solidus Version:
master

Description

Custom defined event subscriber modules may end up being subscribed twice (or more!) on Solidus apps based on Rails 6.x. I was not able to reproduce this issue on Rails 5.2.

The problem happens only in development (or more generally when classes are not cached) and when event subscribers are loaded using initializers, as we used to suggest in the Event section of Solidus documentation.

Apparently, the issue is related to the new Zeitwerk autoload.

The simplest fix for this is to simply remove the offending initializers. Subscribers will be loaded automatically anyways thanks to #3571.

If for any reason somebody does not want to remove the initializers or wants to experiment with the issue, by explicitly requiring the subscriber file in the initializer will fix it:

require Rails.root.join('app/subscribers/custom_subscriber')

CustomSubscriber.subscribe!

I'm working on a more robust solution (a centralized way to track event subscribers subscriptions) that will completely remove the issue.