Update MR head pipeline when pipeline event is published

Introduce Gitlab::EventStore to publish and subscribe domain events.
When a pipeline is created an event is published and the MR
domain reacts as subscriber to this event by updating the head
pipeline.

Changelog: changed

app/services/ci/create_pipeline_service.rb

@@ -95,13 +95,9 @@ module Ci
         .build!
 
       if pipeline.persisted?
-        if Feature.enabled?(:ci_publish_pipeline_events, pipeline.project, default_enabled: :yaml)
-          Gitlab::EventStore.publish(
-            Ci::PipelineCreatedEvent.new(data: { pipeline_id: pipeline.id })
-          )
-        else
-          schedule_head_pipeline_update
-        end
+        Gitlab::EventStore.publish(
+          Ci::PipelineCreatedEvent.new(data: { pipeline_id: pipeline.id })
+        )
 
         create_namespace_onboarding_action
       else
@@ -141,12 +137,6 @@ module Ci
       commit.try(:id)
     end
 
-    def schedule_head_pipeline_update
-      pipeline.all_merge_requests.opened.each do |merge_request|
-        UpdateHeadPipelineForMergeRequestWorker.perform_async(merge_request.id)
-      end
-    end
-
     def create_namespace_onboarding_action
       Namespaces::OnboardingPipelineCreatedWorker.perform_async(project.namespace_id)
     end

config/feature_flags/development/ci_publish_pipeline_events.yml

----
-name: ci_publish_pipeline_events
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34042
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/336752
-milestone: '14.3'
-type: development
-group: group::pipeline execution
-default_enabled: false

spec/services/ci/create_pipeline_service_spec.rb

@@ -161,30 +161,6 @@ RSpec.describe Ci::CreatePipelineService do
             expect(merge_request_1.reload.head_pipeline).to eq(head_pipeline)
             expect(merge_request_2.reload.head_pipeline).to eq(head_pipeline)
           end
-
-          # TODO: remove after ci_publish_pipeline_events FF is removed
-          # https://gitlab.com/gitlab-org/gitlab/-/issues/336752
-          it 'does not schedule sync update for the head pipeline of the merge request' do
-            expect(UpdateHeadPipelineForMergeRequestWorker)
-              .not_to receive(:perform_async)
-
-            execute_service(ref: 'feature', after: nil)
-          end
-        end
-
-        context 'when feature flag ci_publish_pipeline_events is disabled' do
-          before do
-            stub_feature_flags(ci_publish_pipeline_events: false)
-          end
-
-          it 'schedules update for the head pipeline of the merge request' do
-            expect(UpdateHeadPipelineForMergeRequestWorker)
-              .to receive(:perform_async).with(merge_request_1.id)
-            expect(UpdateHeadPipelineForMergeRequestWorker)
-              .to receive(:perform_async).with(merge_request_2.id)
-
-            execute_service(ref: 'feature', after: nil)
-          end
         end
       end