Add EE only version of EventStore configuration

We want to be able to subscribe to events with workers that are only
declared in the EE version of GitLab. Add an EE version of the
EventStore class that we can declare these subscribers in. Also ensure
that the publish event doesn't raise an error when there are not
subscribers for that event. This stops the CE code from raising an error
when a subscriber is only declared in EE.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/351423
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79291

doc/development/event_store.md

@@ -252,19 +252,20 @@ add a line like this to the `Gitlab::EventStore.configure!` method:
 ```ruby
 module Gitlab
   module EventStore
-    def self.configure!
-      Store.new.tap do |store|
-        # ...
+    def self.configure!(store)
+      # ...
 
-        store.subscribe ::MergeRequests::UpdateHeadPipelineWorker, to: ::Ci::PipelineCreatedEvent
+      store.subscribe ::MergeRequests::UpdateHeadPipelineWorker, to: ::Ci::PipelineCreatedEvent
 
-        # ...
-      end
+      # ...
     end
   end
 end
 ```
 
+A worker that is only defined in the EE codebase can subscribe to an event in the same way by 
+declaring the subscription in `ee/lib/ee/gitlab/event_store.rb`.
+
 Subscriptions are stored in memory when the Rails app is loaded and they are immediately frozen.
 It's not possible to modify subscriptions at runtime.
 

ee/lib/ee/gitlab/event_store.rb

+# frozen_string_literal: true
+
+module EE
+  module Gitlab
+    module EventStore
+      extend ActiveSupport::Concern
+
+      class_methods do
+        extend ::Gitlab::Utils::Override
+        # Define event subscriptions using:
+        #
+        #   store.subscribe(DomainA::SomeWorker, to: DomainB::SomeEvent)
+        #
+        # It is possible to subscribe to a subset of events matching a condition:
+        #
+        #   store.subscribe(DomainA::SomeWorker, to: DomainB::SomeEvent), if: ->(event) { event.data == :some_value }
+        #
+        # Only EE subscriptions should be declared in this module.
+        override :configure!
+        def configure!(store)
+          super(store)
+
+          ###
+          # Add EE only subscriptions here:
+        end
+      end
+    end
+  end
+end

lib/gitlab/event_store.rb

@@ -18,7 +18,7 @@ module Gitlab
     end
 
     def self.instance
-      @instance ||= configure!
+      @instance ||= Store.new { |store| configure!(store) }
     end
 
     # Define all event subscriptions using:
@@ -29,14 +29,14 @@ module Gitlab
     #
     #   store.subscribe(DomainA::SomeWorker, to: DomainB::SomeEvent), if: ->(event) { event.data == :some_value }
     #
-    def self.configure!
-      Store.new do |store|
-        ###
-        # Add subscriptions here:
+    def self.configure!(store)
+      ###
+      # Add subscriptions here:
 
-        store.subscribe ::MergeRequests::UpdateHeadPipelineWorker, to: ::Ci::PipelineCreatedEvent
-      end
+      store.subscribe ::MergeRequests::UpdateHeadPipelineWorker, to: ::Ci::PipelineCreatedEvent
     end
     private_class_method :configure!
   end
 end
+
+Gitlab::EventStore.prepend_mod_with('Gitlab::EventStore')

lib/gitlab/event_store/store.rb

@@ -29,7 +29,7 @@ module Gitlab
           raise InvalidEvent, "Event being published is not an instance of Gitlab::EventStore::Event: got #{event.inspect}"
         end
 
-        subscriptions[event.class].each do |subscription|
+        subscriptions.fetch(event.class, []).each do |subscription|
           subscription.consume_event(event)
         end
       end

spec/lib/gitlab/event_store/store_spec.rb

@@ -224,6 +224,26 @@ RSpec.describe Gitlab::EventStore::Store do
         store.publish(event)
       end
     end
+
+    context 'when the event does not have any subscribers' do
+      let(:store) do
+        described_class.new do |s|
+          s.subscribe unrelated_worker, to: another_event_klass
+        end
+      end
+
+      let(:event) { event_klass.new(data: data) }
+
+      it 'returns successfully' do
+        expect { store.publish(event) }.not_to raise_error
+      end
+
+      it 'does not dispatch the event to another subscription' do
+        expect(unrelated_worker).not_to receive(:perform_async)
+
+        store.publish(event)
+      end
+    end
   end
 
   describe 'subscriber' do