Add To Merge Train When Pipeline Succeeds auto merge strategy

This commit adds the new auto merge strategy "Add to Merge Train When
Pipeline succeeds". It works similarily with MWPS, but it adds to
merge train instead of merging.

ee/app/services/auto_merge/add_to_merge_train_when_pipeline_succeeds_service.rb

+# frozen_string_literal: true
+
+module AutoMerge
+  class AddToMergeTrainWhenPipelineSucceedsService < AutoMerge::BaseService
+    def execute(merge_request)
+      super do
+        SystemNoteService.add_to_merge_train_when_pipeline_succeeds(merge_request, project, current_user, merge_request.diff_head_commit)
+      end
+    end
+
+    def process(merge_request)
+      return unless merge_request.actual_head_pipeline&.success?
+
+      merge_train_service = AutoMerge::MergeTrainService.new(project, merge_request.merge_user)
+
+      ##
+      # We are currently abusing `#cancel` method to cancel the auto merge when
+      # a system failure happens. We should split the interfaces into two
+      # for explicitly telling that the cancel action is not triggered by the merge user directly.
+      # https://gitlab.com/gitlab-org/gitlab-ee/issues/12134
+      return cancel(merge_request) unless merge_train_service.available_for?(merge_request)
+
+      merge_train_service.execute(merge_request)
+    end
+
+    def cancel(merge_request)
+      super(merge_request) do
+        SystemNoteService.cancel_add_to_merge_train_when_pipeline_succeeds(merge_request, project, current_user)
+      end
+    end
+
+    def available_for?(merge_request)
+      merge_request.project.merge_trains_enabled? &&
+        !merge_request.for_fork? &&
+        merge_request.actual_head_pipeline&.active? &&
+        merge_request.mergeable_state?(skip_ci_check: true)
+    end
+  end
+end

ee/app/services/ee/auto_merge_service.rb

@@ -5,7 +5,8 @@ module EE
     extend ActiveSupport::Concern
 
     STRATEGY_MERGE_TRAIN = 'merge_train'.freeze
-    EE_STRATEGIES = [STRATEGY_MERGE_TRAIN].freeze
+    STRATEGY_ADD_TO_MERGE_TRAIN_WHEN_PIPELINE_SUCCEEDS = 'add_to_merge_train_when_pipeline_succeeds'.freeze
+    EE_STRATEGIES = [STRATEGY_MERGE_TRAIN, STRATEGY_ADD_TO_MERGE_TRAIN_WHEN_PIPELINE_SUCCEEDS].freeze
 
     class_methods do
       extend ::Gitlab::Utils::Override

ee/app/services/ee/system_note_service.rb

@@ -210,5 +210,19 @@ module EE
 
       create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
     end
+
+    # Called when 'add to merge train when pipeline succeeds' is executed
+    def add_to_merge_train_when_pipeline_succeeds(noteable, project, author, last_commit)
+      body = "enabled automatic add to merge train when the pipeline for #{last_commit.to_reference(project)} succeeds"
+
+      create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+    end
+
+    # Called when 'add to merge train when pipeline succeeds' is canceled
+    def cancel_add_to_merge_train_when_pipeline_succeeds(noteable, project, author)
+      body = 'cancelled automatic add to merge train'
+
+      create_note(NoteSummary.new(noteable, project, author, body, action: 'merge'))
+    end
   end
 end

ee/changelogs/unreleased/add-add-to-merge-train-when-pipeline-succeeds-strategy.yml

+---
+title: "[New Auto Merge Strategy] Add To Merge Train When Pipeline Succeeds"
+merge_request: 13767
+author:
+type: added

ee/spec/factories/merge_requests.rb

@@ -24,6 +24,12 @@ FactoryBot.modify do
       end
     end
 
+    trait :add_to_merge_train_when_pipeline_succeeds do
+      auto_merge_enabled true
+      auto_merge_strategy AutoMergeService::STRATEGY_ADD_TO_MERGE_TRAIN_WHEN_PIPELINE_SUCCEEDS
+      merge_user { author }
+    end
+
     transient do
       approval_groups []
       approval_users []

ee/spec/services/auto_merge/add_to_merge_train_when_pipeline_succeeds_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe AutoMerge::AddToMergeTrainWhenPipelineSucceedsService do
+  set(:project) { create(:project, :repository) }
+  set(:user) { create(:user) }
+  let(:service) { described_class.new(project, user) }
+
+  let(:merge_request) do
+    create(:merge_request, :with_merge_request_pipeline,
+      source_project: project, source_branch: 'feature',
+      target_project: project, target_branch: 'master')
+  end
+
+  let(:pipeline) { merge_request.reload.all_pipelines.first }
+
+  before do
+    stub_licensed_features(merge_trains: true, merge_pipelines: true)
+    project.add_maintainer(user)
+    project.update!(merge_trains_enabled: true, merge_pipelines_enabled: true)
+    allow(AutoMergeProcessWorker).to receive(:perform_async) { }
+    merge_request.update_head_pipeline
+  end
+
+  describe '#execute' do
+    subject { service.execute(merge_request) }
+
+    it 'enables auto merge' do
+      expect(SystemNoteService)
+        .to receive(:add_to_merge_train_when_pipeline_succeeds)
+        .with(merge_request, project, user, merge_request.diff_head_commit)
+
+      subject
+
+      expect(merge_request).to be_auto_merge_enabled
+    end
+  end
+
+  describe '#process' do
+    subject { service.process(merge_request) }
+
+    context 'when the latest pipeline in the merge request has succeeded' do
+      before do
+        pipeline.succeed!
+      end
+
+      it 'executes MergeTrainService' do
+        expect_next_instance_of(AutoMerge::MergeTrainService) do |train_service|
+          expect(train_service).to receive(:execute).with(merge_request)
+        end
+
+        subject
+      end
+
+      context 'when merge train strategy is not available for the merge request' do
+        before do
+          train_service = double
+          allow(train_service).to receive(:available_for?) { false }
+          allow(AutoMerge::MergeTrainService).to receive(:new) { train_service }
+        end
+
+        it 'cancels auto merge' do
+          expect(service).to receive(:cancel).once
+
+          subject
+        end
+      end
+    end
+
+    context 'when the latest pipeline in the merge request is running' do
+      it 'does not initialize MergeTrainService' do
+        expect(AutoMerge::MergeTrainService).not_to receive(:new)
+
+        subject
+      end
+    end
+  end
+
+  describe '#cancel' do
+    subject { service.cancel(merge_request) }
+
+    let(:merge_request) { create(:merge_request, :add_to_merge_train_when_pipeline_succeeds, merge_user: user) }
+
+    it 'cancels auto merge' do
+      expect(SystemNoteService)
+        .to receive(:cancel_add_to_merge_train_when_pipeline_succeeds)
+        .with(merge_request, project, user)
+
+      subject
+
+      expect(merge_request).not_to be_auto_merge_enabled
+    end
+  end
+
+  describe '#available_for?' do
+    subject { service.available_for?(merge_request) }
+
+    it { is_expected.to eq(true) }
+
+    context 'when merge trains option is disabled' do
+      before do
+        expect(merge_request.project).to receive(:merge_trains_enabled?) { false }
+      end
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when merge request is submitted from a forked project' do
+      before do
+        allow(merge_request).to receive(:for_fork?) { true }
+      end
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when the latest pipeline in the merge request is not active' do
+      before do
+        pipeline.succeed!
+      end
+
+      it { is_expected.to eq(false) }
+    end
+
+    context 'when merge request is not mergeable' do
+      before do
+        merge_request.update!(title: merge_request.wip_title)
+      end
+
+      it { is_expected.to eq(false) }
+    end
+  end
+end

ee/spec/services/ee/auto_merge_service_spec.rb

@@ -7,7 +7,8 @@ describe AutoMergeService do
     subject { described_class.all_strategies }
 
     it 'includes all strategies' do
-      is_expected.to include(AutoMergeService::STRATEGY_MERGE_TRAIN)
+      is_expected.to include(AutoMergeService::STRATEGY_MERGE_TRAIN,
+                             AutoMergeService::STRATEGY_ADD_TO_MERGE_TRAIN_WHEN_PIPELINE_SUCCEEDS)
     end
   end
 end

ee/spec/services/system_note_service_spec.rb

@@ -370,4 +370,38 @@ describe SystemNoteService do
       end
     end
   end
+
+  describe '.add_to_merge_train_when_pipeline_succeeds' do
+    subject { described_class.add_to_merge_train_when_pipeline_succeeds(noteable, project, author, noteable.diff_head_commit) }
+
+    let(:pipeline) { build(:ci_pipeline_without_jobs) }
+
+    let(:noteable) do
+      create(:merge_request, source_project: project, target_project: project)
+    end
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'merge' }
+    end
+
+    it "posts the 'add to merge train when pipeline succeeds' system note" do
+      expect(subject.note).to match(%r{enabled automatic add to merge train when the pipeline for (\w+/\w+@)?\h{40} succeeds})
+    end
+  end
+
+  describe '.cancel_add_to_merge_train_when_pipeline_succeeds' do
+    subject { described_class.cancel_add_to_merge_train_when_pipeline_succeeds(noteable, project, author) }
+
+    let(:noteable) do
+      create(:merge_request, source_project: project, target_project: project)
+    end
+
+    it_behaves_like 'a system note' do
+      let(:action) { 'merge' }
+    end
+
+    it "posts the 'add to merge train when pipeline succeeds' system note" do
+      expect(subject.note).to eq 'cancelled automatic add to merge train'
+    end
+  end
 end