Use Omnes for pub/sub

[waiting-for-dev]

Description

This PR removes the new event bus adapter and delegates the pub/sub backbone to an external dependency: omnes. Omnes is, in fact, an extraction of that code into a standalone library, which has given us the possibility to add more features from which Solidus can benefit.

The changes can be more easily reviewed commit by commit:

• To begin with, we revert all the changes we've been doing so far regarding the new event bus system. That undoes:
  • Introduce new EventBus adapter #4130
  • Adds observability to the new event bus #4203
  • Introduce Spree::Event's test interface to run only selected listeners #4204
  • Enforce event registration #4226
  • Centralize legacy event bus deprecation and test legacy on CI #4176
  • Introduce new EventBus adapter #4130
  • Cosmetic changes to the unknown event message #4246
  • Add public interface to fetch registered events #4252
  • Update warning with copy/pastable code #4215
  • Use default spree event adapter #4216
• Add add omnes as a dependency. As said, that contains all the reverted code and new features.
• Use omnes internally by default from Solidus 3.2.0 while still supporting the legacy system through a configuration setting. Creates an omnes subscriber to replace the legacy subscriber module.
• Update test helpers to stub the new Spree::Bus instance. That is something that won't be on Omnes, as it's a specific stub for our global bus instance.
• Warn when the legacy system is used. We'll update the deprecation message with a link to an upgrade link once that's ready.

How will Solidus benefit from it?

Being free from the need to support two completely different adapters through the same API enormously simplifies our code. We've also been able to add new features without constraints. Omnes has better support for debugging, testing, and async (see its README for details). We can potentially benefit from contributors using it on other kinds of projects.

There's still a place to take more advantage of omnes in future work:

• Create an internal configurable adapter to allow users to run Solidus events in their preferred async adapter.
• Use instance events and add an option to store their serialization in the database.

How will people upgrade from the legacy system to omnes?

As pointed out, we'll write an upgrade guide and add a link to it in the deprecation message. As heads-up:

• Replace references of Spree::Event to Spree::Bus.
• Rename #fire to #publish.
• Register custom events in an initializer.
• Transform module subscribers into Omnes subscribers and load them in an initializer.
• Don't provide a block to #publish. Instead, run whatever you want before it.
• Don't use regular expression subscriptions as they are an anti-pattern (they can catch undesired events if they're added in the future). Probably, what most people will need instead is omnes' #subscribe_to_all. If they really want it, they can still create a custom matcher on omnes.

How will people upgrade from master to omnes?

All the work on the new event bus adapter was on master, but it hadn't been released yet.

Anyway, the migration from master will be pretty straightforward. We'll happily support it, and once this gets merged, we'll post a message on Slack to let people know they can come to us (more explicitly to @waiting-for-dev) if they have any issues.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[waiting-for-dev]

cc @forkata

[kennyadsl]

Nicely implemented, thanks @waiting-for-dev.

🚌 🚌 🚌

[jarednorman]

Not a strong opinion, but I know tap can be a stumbling block for people not familiar with it. I like tap, but only use it when it actually makes the code cleaner or avoids polluting a scope with too many local variables. The use here doesn't really seem to add much to me, since we could write it like this instead and I think it's just as good, but doesn't require you to be familiar with tap:

Suggested change

	Bus = Omnes::Bus.new.tap do |bus|
	bus.define_singleton_method(:publish) do |*args, **kwargs, &block|
	if Spree::Config.use_legacy_events
	Spree::Event.fire(*args, **kwargs, &block)
	else
	super(*args, **kwargs, caller_location: caller_locations(1)[0], &block)
	end
	end
	end
	Bus = Omnes::Bus.new
	Bus.define_singleton_method(:publish) do |*args, **kwargs, &block|
	if Spree::Config.use_legacy_events
	Spree::Event.fire(*args, **kwargs, &block)
	else
	super(*args, **kwargs, caller_location: caller_locations(1)[0], &block)
	end
	end

[waiting-for-dev]

Fair enough. Changed. Thanks, @jarednorman.

[waiting-for-dev]

Updated with a fix for autoloading subscribers (see the addition of Spree::Bus.clear on the engine file). It required bumping omnes to v0.2.1.

[forkata]

Nice work here @waiting-for-dev!

[forkata]

I was a bit puzzled as to why we are overriding the caller_location here. Would it be worth leaving a comment that this is intended to omit this from the stack trace, or just dropping that in order to ease debugging for anyone who is getting the new bus implementation and trying to track down why?

[waiting-for-dev]

I went with the addition of a comment. We do need to go one level higher in the stack. Otherwise, it'll be useless for debugging, as all events will look as published from this line (see https://github.com/nebulab/omnes/blob/bb73fce75414816e304a7402efbea655b82c857b/lib/omnes/bus.rb#L187)

[forkata]

Thanks for clarifying this @waiting-for-dev, your decision here makes a lot more sense now.

[forkata]

Thanks for clarifying this @waiting-for-dev, your decision here makes a lot more sense now.

[forkata]

❓ Does this change anything with regards to where applications or Solidus extensions should be registering any custom events they want to introduce that use the Spree::Bus 🤔

[waiting-for-dev]

That's a great question. More than a change, it's a fix. To survive code reloading, events need to be registered, and subscriptions must be added within a #to_prepare block (which runs one on production but at every reload on development). Solidus owns Spree::Bus, so it clears it at every reload. Then, solidus itself, extensions, and applications can just call #register & #subscribe_to within their own #to_prepare blocks, which will run later in time.

[waiting-for-dev]

Rebasing from master, and updating Omnes version to 0.2.2 to support Ruby 2.5 & Ruby 2.6, which are still supported on Solidus.

[spaghetticode]

This looks good to me, thanks Marc for the usual great work! What about removing the spurious fixup commit that updates omnes? Also, there's another commit that changes its version after the commit that introduces the gem.

[waiting-for-dev]

This looks good to me, thanks Marc for the usual great work! What about removing the spurious fixup commit that updates omnes? Also, there's another commit that changes its version after the commit that introduces the gem.

Thanks, @spaghetticode. Oh, sorry, I missed that (forgot the rebase). I'll fix it 🙂

[waiting-for-dev]

This looks good to me, thanks Marc for the usual great work! What about removing the spurious fixup commit that updates omnes? Also, there's another commit that changes its version after the commit that introduces the gem.

Thanks for spotting it, @spaghetticode. Fixed!

[spaghetticode]

Thanks for spotting it, @spaghetticode. Fixed!

I still see the fixup commit: cb0e910

To be super explicit, here's the three commits related to the omnes dependency:

• introduces 0.20: 690a198

• introduces 0.2.1: 0751249

• introduces 0.2.2: cb0e910

[waiting-for-dev]

Thanks for spotting it, @spaghetticode. Fixed!

I still see the fixup commit: cb0e910

To be super explicit, here's the three commits related to the omnes dependency:

* introduces 0.20: [690a198](https://github.com/solidusio/solidus/commit/690a19813f57071e160cdbd5f8c5e1b37ab2142d)

* introduces 0.2.1: [0751249](https://github.com/solidusio/solidus/commit/07512498380113099f198067b3a492bd64a4be19)

* introduces 0.2.2: [cb0e910](https://github.com/solidusio/solidus/commit/cb0e9101bb8fb9282f770e74ffc6908000740c55)

My bad @spaghetticode. I forced the push to the wrong repo. Everything should be fine now.

[spaghetticode]

thank you Marc!

[waiting-for-dev]

Rebased from master and made a minor change to adopt the new way to define versioned preferences (see #4375).

[waiting-for-dev]

See #4380