Process MWBS in successful pipeline asynchronously

app/models/ci/pipeline.rb

@@ -71,7 +71,7 @@ module Ci
       end
 
       after_transition [:created, :pending, :running] => :success do |pipeline|
-        MergeRequests::MergeWhenBuildSucceedsService.new(pipeline.project, nil).trigger(pipeline)
+        PipelineSuccessWorker.perform_async(pipeline.id)
       end
 
       after_transition do |pipeline, transition|

app/workers/pipeline_success_worker.rb

+class PipelineSuccessWorker
+  include Sidekiq::Worker
+  sidekiq_options queue: :default
+
+  def perform(pipeline_id)
+    Ci::Pipeline.find_by(id: pipeline_id).try do |pipeline|
+      MergeRequests::MergeWhenBuildSucceedsService
+        .new(pipeline.project, nil)
+        .trigger(pipeline)
+    end
+  end
+end

spec/workers/pipeline_success_worker_spec.rb

+require 'spec_helper'
+
+describe PipelineSuccessWorker do
+  describe '#perform' do
+    context 'when pipeline exists' do
+      let(:pipeline) { create(:ci_pipeline, status: 'success') }
+
+      it 'performs "merge when pipeline succeeds"' do
+        expect_any_instance_of(
+          MergeRequests::MergeWhenBuildSucceedsService
+        ).to receive(:trigger)
+
+        described_class.new.perform(pipeline.id)
+      end
+    end
+
+    context 'when pipeline does not exist' do
+      it 'does not raise exception' do
+        expect { described_class.new.perform(123) }
+          .not_to raise_error
+      end
+    end
+  end
+end