Disabling setting drops merge requests

When the project-level setting is disabled,
merge requests will be immediately dropped.

Closes https://gitlab.com/gitlab-org/gitlab/issues/14959

ee/app/models/ee/project.rb

@@ -148,7 +148,7 @@ module EE
 
       delegate :log_jira_dvcs_integration_usage, :jira_dvcs_server_last_sync_at, :jira_dvcs_cloud_last_sync_at, to: :feature_usage
 
-      delegate :merge_pipelines_enabled, :merge_pipelines_enabled=, :merge_pipelines_enabled?, to: :ci_cd_settings
+      delegate :merge_pipelines_enabled, :merge_pipelines_enabled=, :merge_pipelines_enabled?, :merge_pipelines_were_disabled?, to: :ci_cd_settings
       delegate :merge_trains_enabled, :merge_trains_enabled=, :merge_trains_enabled?, to: :ci_cd_settings
 
       validates :repository_size_limit,

ee/app/models/ee/project_ci_cd_setting.rb

@@ -15,5 +15,9 @@ module EE
       merge_pipelines_enabled? && project.feature_available?(:merge_trains) &&
         ::Feature.enabled?(:merge_trains_enabled, project, default_enabled: true)
     end
+
+    def merge_pipelines_were_disabled?
+      saved_change_to_attribute?(:merge_pipelines_enabled, from: true, to: false)
+    end
   end
 end

ee/app/models/merge_train.rb

@@ -21,6 +21,10 @@ class MergeTrain < ApplicationRecord
       all_in_train(merge_request).first
     end
 
+    def first_in_trains(project)
+      MergeRequest.preload(:target_project).where(id: first_merge_request_ids(project))
+    end
+
     def first_in_train_from(merge_request_ids)
       merge_request = MergeRequest.find(merge_request_ids.first)
       all_in_train(merge_request).where(id: merge_request_ids).first
@@ -35,6 +39,14 @@ class MergeTrain < ApplicationRecord
         .where('merge_requests.target_project_id = ?', merge_request.target_project_id)
         .where('merge_requests.target_branch = ?', merge_request.target_branch)
     end
+
+    private
+
+    def first_merge_request_ids(project)
+      MergeTrain.where(target_project: project)
+        .select('DISTINCT ON (target_branch) merge_request_id')
+        .order(:target_branch, :id)
+    end
   end
 
   def all_next

ee/app/services/ee/projects/update_service.rb

@@ -29,6 +29,7 @@ module EE
 
         if result[:status] == :success
           cleanup_approvers(project) if should_remove_old_approvers
+          refresh_merge_trains(project)
 
           log_audit_events
 
@@ -65,6 +66,14 @@ module EE
       def sync_wiki_on_enable
         ::Geo::RepositoryUpdatedService.new(project.wiki.repository).execute
       end
+
+      def refresh_merge_trains(project)
+        return unless project.merge_pipelines_were_disabled?
+
+        MergeTrain.first_in_trains(project).each do |merge_request|
+          AutoMergeProcessWorker.perform_async(merge_request.id)
+        end
+      end
     end
   end
 end

ee/changelogs/unreleased/14959-drop-all-merge-requests-from-merge-trains-when-the-project-level.yml

+---
+title: Drop all merge requests from merge trains when the project-level setting is disabled
+merge_request: 17774
+author:
+type: added

ee/spec/models/merge_train_spec.rb

@@ -67,6 +67,20 @@ describe MergeTrain do
     end
   end
 
+  describe '.first_in_trains' do
+    let!(:first_on_master) { create_merge_request_on_train(target_branch: 'master', source_branch: 'feature-1') }
+    let!(:second_on_master) { create_merge_request_on_train(target_branch: 'master', source_branch: 'feature-2') }
+
+    let!(:first_on_stable) { create_merge_request_on_train(target_branch: 'stable', source_branch: 'feature-1-backport') }
+    let!(:second_on_stable) { create_merge_request_on_train(target_branch: 'stable', source_branch: 'feature-2-backport') }
+
+    subject { described_class.first_in_trains(project) }
+
+    it 'returns only first merge requests per merge train' do
+      is_expected.to contain_exactly(first_on_master, first_on_stable)
+    end
+  end
+
   describe '.first_in_train_from' do
     subject { described_class.first_in_train_from(merge_request_ids) }
 

ee/spec/models/project_ci_cd_setting_spec.rb

@@ -64,4 +64,27 @@ describe ProjectCiCdSetting do
       it { is_expected.to be_falsy }
     end
   end
+
+  describe '#merge_pipelines_were_disabled?' do
+    subject { project.merge_pipelines_were_disabled? }
+
+    let(:project) { create(:project) }
+
+    before do
+      stub_licensed_features(merge_pipelines: true, merge_trains: true)
+      project.update(merge_pipelines_enabled: true)
+    end
+
+    context 'when merge pipelines option is disabled' do
+      before do
+        project.update(merge_pipelines_enabled: false)
+      end
+
+      it { is_expected.to be true }
+    end
+
+    context 'when merge pipelines option is intact' do
+      it { is_expected.to be false }
+    end
+  end
 end

ee/spec/services/projects/update_service_spec.rb

@@ -237,6 +237,37 @@ describe Projects::UpdateService, '#execute' do
     end
   end
 
+  context 'when there are merge requests in merge train' do
+    before do
+      stub_licensed_features(merge_pipelines: true, merge_trains: true)
+      project.update(merge_pipelines_enabled: true)
+    end
+
+    let!(:first_merge_request) do
+      create(:merge_request, :on_train, target_project: project, source_project: project)
+    end
+
+    let!(:second_merge_request) do
+      create(:merge_request, :on_train, target_project: project, source_project: project, source_branch: 'feature-1')
+    end
+
+    context 'when merge pipelines option is disabled' do
+      it 'drops all merge request in the train' do
+        expect do
+          update_project(project, user, merge_pipelines_enabled: false)
+        end.to change { MergeTrain.count }.from(2).to(0)
+      end
+    end
+
+    context 'when merge pipelines option stays enabled' do
+      it 'does not drop all merge request in the train' do
+        expect do
+          update_project(project, user, merge_pipelines_enabled: true)
+        end.not_to change { MergeTrain.count }
+      end
+    end
+  end
+
   it 'returns an error result when record cannot be updated' do
     admin = create(:admin)