Introduce sidekiq worker for auto merge process

As we have a central domain for auto merge process today,
we should use a single worker for any auto merge process.

app/models/ci/pipeline.rb

@@ -166,6 +166,16 @@ module Ci
         end
       end
+      after_transition any => ::Ci::Pipeline.completed_statuses do |pipeline|
+        pipeline.run_after_commit do
+          pipeline.all_merge_requests.each do |merge_request|
+            next unless merge_request.auto_merge_enabled?
+
+            AutoMergeProcessWorker.perform_async(merge_request.id)
+          end
+        end
+      end
+
       after_transition any => [:success, :failed] do |pipeline|
         pipeline.run_after_commit do
           PipelineNotificationWorker.perform_async(pipeline.id)

app/workers/all_queues.yml

 ---
 - auto_devops:auto_devops_disable
+- auto_merge:auto_merge_process
+
 - cronjob:admin_email
 - cronjob:expire_build_artifacts
 - cronjob:gitlab_usage_ping

app/workers/auto_merge_process_worker.rb

+# frozen_string_literal: true
+
+class AutoMergeProcessWorker
+  include ApplicationWorker
+
+  queue_namespace :auto_merge
+
+  def perform(merge_request_id)
+    MergeRequest.find_by_id(merge_request_id).try do |merge_request|
+      AutoMergeService.new(merge_request.project, merge_request.merge_user)
+                      .process(merge_request)
+    end
+  end
+end

app/workers/pipeline_success_worker.rb

@@ -6,14 +6,7 @@ class PipelineSuccessWorker
   queue_namespace :pipeline_processing
-  # rubocop: disable CodeReuse/ActiveRecord
   def perform(pipeline_id)
-    Ci::Pipeline.find_by(id: pipeline_id).try do |pipeline|
-      pipeline.all_merge_requests.preload(:merge_user).each do |merge_request|
-        AutoMergeService.new(pipeline.project, merge_request.merge_user)
-                        .process(merge_request)
-      end
-    end
+    # no-op
   end
-  # rubocop: enable CodeReuse/ActiveRecord
 end

config/sidekiq_queues.yml

@@ -30,6 +30,7 @@
   - [pipeline_default, 3]
   - [pipeline_cache, 3]
   - [deployment, 3]
+  - [auto_merge, 3]
   - [pipeline_hooks, 2]
   - [gitlab_shell, 2]
   - [email_receiver, 2]

spec/models/ci/pipeline_spec.rb

@@ -1381,6 +1381,40 @@ describe Ci::Pipeline, :mailer do
       end
     end
+    describe 'auto merge' do
+      let(:merge_request) { create(:merge_request, :merge_when_pipeline_succeeds) }
+
+      let(:pipeline) do
+        create(:ci_pipeline, :running, project: merge_request.source_project,
+                                       ref: merge_request.source_branch,
+                                       sha: merge_request.diff_head_sha)
+      end
+
+      before do
+        merge_request.update_head_pipeline
+      end
+
+      %w[succeed! drop! cancel! skip!].each do |action|
+        context "when the pipeline recieved #{action} event" do
+          it 'performs AutoMergeProcessWorker' do
+            expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request.id)
+
+            pipeline.public_send(action)
+          end
+        end
+      end
+
+      context 'when auto merge is not enabled in the merge request' do
+        let(:merge_request) { create(:merge_request) }
+
+        it 'performs AutoMergeProcessWorker' do
+          expect(AutoMergeProcessWorker).not_to receive(:perform_async)
+
+          pipeline.succeed!
+        end
+      end
+    end
+
     def create_build(name, *traits, queued_at: current, started_from: 0, **opts)
       create(:ci_build, *traits,
              name: name,

spec/workers/pipeline_success_worker_spec.rb → spec/workers/auto_merge_process_worker_spec.rb

@@ -2,25 +2,29 @@
 require 'spec_helper'
-describe PipelineSuccessWorker do
+describe AutoMergeProcessWorker do
   describe '#perform' do
-    context 'when pipeline exists' do
-      let(:pipeline) { create(:ci_pipeline, status: 'success', ref: merge_request.source_branch, project: merge_request.source_project) }
+    subject { described_class.new.perform(merge_request&.id) }
+
+    context 'when merge request is found' do
       let(:merge_request) { create(:merge_request) }
-      it 'performs "merge when pipeline succeeds"' do
+      it 'executes AutoMergeService' do
         expect_next_instance_of(AutoMergeService) do |auto_merge|
           expect(auto_merge).to receive(:process)
         end
-        described_class.new.perform(pipeline.id)
+        subject
       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
+    context 'when merge request is not found' do
+      let(:merge_request) { nil }
+
+      it 'does not execute AutoMergeService' do
+        expect(AutoMergeService).not_to receive(:new)
+
+        subject
       end
     end
   end