Move NewMergeRequestWorker methods to a service

This makes it easier to refactor in future commits.

app/services/merge_requests/after_create_service.rb

+# frozen_string_literal: true
+
+module MergeRequests
+  class AfterCreateService < MergeRequests::BaseService
+    def execute(merge_request)
+      event_service.open_mr(merge_request, current_user)
+      notification_service.new_merge_request(merge_request, current_user)
+
+      merge_request.diffs(include_stats: false).write_cache
+      merge_request.create_cross_references!(current_user)
+    end
+  end
+end

app/services/merge_requests/create_service.rb

@@ -13,15 +13,11 @@ module MergeRequests
       create(merge_request)
     end
 
-    def before_create(merge_request)
-      # current_user (defined in BaseService) is not available within run_after_commit block
-      user = current_user
-      merge_request.run_after_commit do
-        NewMergeRequestWorker.perform_async(merge_request.id, user.id)
-      end
-    end
-
     def after_create(issuable)
+      # Add new items to MergeRequests::AfterCreateService if they can
+      # be performed in Sidekiq
+      NewMergeRequestWorker.perform_async(issuable.id, current_user.id)
+
       todo_service.new_merge_request(issuable, current_user)
       issuable.cache_merge_request_closes_issues!(current_user)
       create_pipeline_for(issuable, current_user)

app/workers/new_merge_request_worker.rb

@@ -12,11 +12,9 @@ class NewMergeRequestWorker # rubocop:disable Scalability/IdempotentWorker
   def perform(merge_request_id, user_id)
     return unless objects_found?(merge_request_id, user_id)
 
-    EventCreateService.new.open_mr(issuable, user)
-    NotificationService.new.new_merge_request(issuable, user)
-
-    issuable.diffs(include_stats: false).write_cache
-    issuable.create_cross_references!(user)
+    MergeRequests::AfterCreateService
+      .new(issuable.target_project, user)
+      .execute(issuable)
   end
 
   def issuable_class

spec/services/merge_requests/after_create_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe MergeRequests::AfterCreateService do
+  let_it_be(:merge_request) { create(:merge_request) }
+
+  subject(:after_create_service) do
+    described_class.new(merge_request.target_project, merge_request.author)
+  end
+
+  describe '#execute' do
+    let(:event_service) { instance_double('EventCreateService', open_mr: true) }
+    let(:notification_service) { instance_double('NotificationService', new_merge_request: true) }
+
+    before do
+      allow(after_create_service).to receive(:event_service).and_return(event_service)
+      allow(after_create_service).to receive(:notification_service).and_return(notification_service)
+    end
+
+    it 'creates a merge request open event' do
+      expect(event_service).to receive(:open_mr).with(merge_request, merge_request.author)
+
+      after_create_service.execute(merge_request)
+    end
+
+    it 'creates a new merge request notification' do
+      expect(notification_service).to receive(:new_merge_request).with(merge_request, merge_request.author)
+
+      after_create_service.execute(merge_request)
+    end
+
+    it 'writes diffs to the cache' do
+      expect(merge_request).to receive_message_chain(:diffs, :write_cache)
+
+      after_create_service.execute(merge_request)
+    end
+
+    it 'creates cross references' do
+      expect(merge_request).to receive(:create_cross_references!).with(merge_request.author)
+
+      after_create_service.execute(merge_request)
+    end
+  end
+end