Add "Allow merge trains" setting

This commit adds the "Allow merge trains" options to the project merge
request settings page.  This option is only visible in EE.  This option
is also hidden behind the "merge_trains" feature flag.

ee/app/assets/javascripts/pages/projects/edit/index.js

@@ -7,6 +7,7 @@ import groupsSelect from '~/groups_select';
 import ApproversSelect from 'ee/approvers_select';
 import mountApprovals from 'ee/approvals/mount_project_settings';
 import initServiceDesk from 'ee/projects/settings_service_desk';
+import initMergeOptionsCheckboxSync from './init_merge_options_checkbox_sync';
 
 document.addEventListener('DOMContentLoaded', () => {
   new UsersSelect();
@@ -17,4 +18,6 @@ document.addEventListener('DOMContentLoaded', () => {
   new ApproversSelect();
   initServiceDesk();
   mountApprovals(document.getElementById('js-mr-approvals-settings'));
+
+  initMergeOptionsCheckboxSync();
 });

ee/app/assets/javascripts/pages/projects/edit/init_merge_options_checkbox_sync.js

+export default () => {
+  const mergePipelinesCheckbox = document.querySelector('.js-merge-options-merge-pipelines');
+  const mergeTrainsCheckbox = document.querySelector('.js-merge-options-merge-trains');
+
+  if (mergePipelinesCheckbox && mergeTrainsCheckbox) {
+    mergePipelinesCheckbox.addEventListener('change', event => {
+      if (!event.target.checked && mergeTrainsCheckbox.checked) {
+        mergeTrainsCheckbox.click();
+      }
+    });
+
+    mergeTrainsCheckbox.addEventListener('change', event => {
+      if (event.target.checked && !mergePipelinesCheckbox.checked) {
+        mergePipelinesCheckbox.click();
+      }
+    });
+  }
+};

ee/app/views/projects/_merge_pipelines_settings.html.haml

 - return unless merge_pipelines_available?
 
 .form-check.builds-feature
-  = form.check_box :merge_pipelines_enabled, class: 'form-check-input'
+  = form.check_box :merge_pipelines_enabled, class: 'form-check-input js-merge-options-merge-pipelines'
   = form.label :merge_pipelines_enabled, class: 'form-check-label' do
     = s_('ProjectSettings|Merge pipelines will try to validate the post-merge result prior to merging')
     .descr.text-secondary.mb-2
@@ -10,3 +10,8 @@
         help_page_path('ci/merge_request_pipelines/index.md',
         anchor: 'pipelines-for-merge-requests'),
         target: '_blank'
+    - if merge_trains_available?
+      .form-check.mb-2
+        = form.check_box :merge_trains_enabled, class: 'form-check-input js-merge-options-merge-trains'
+        = form.label :merge_trains_enabled, class: 'form-check-label' do
+          = s_('ProjectSettings|Allow merge trains')

ee/changelogs/unreleased/9186-add-merge-train-setting.yml

+---
+title: Add "Allow merge trains" option to project settings page
+merge_request: 10803
+author:
+type: added

ee/spec/features/projects/settings/merge_requests_settings_spec.rb

+# frozen_string_literal: true
 require 'spec_helper'
 
 describe 'Project settings > [EE] Merge Requests', :js do
@@ -109,4 +110,79 @@ describe 'Project settings > [EE] Merge Requests', :js do
       expect(page).to have_selector('#project_merge_requests_template')
     end
   end
+
+  shared_examples 'the merge train feature is not available' do
+    it 'does not render the merge trains checkbox' do
+      visit edit_project_path(project)
+
+      expect(page).not_to have_selector('#project_merge_trains_enabled')
+    end
+  end
+
+  context 'when merge_pipelines and merge_trains are disabled' do
+    before do
+      stub_licensed_features(merge_pipelines: false, merge_trains: false)
+    end
+
+    it_behaves_like 'the merge train feature is not available'
+  end
+
+  context 'when merge_pipelines is disabled and merge_trains is enabled' do
+    before do
+      stub_licensed_features(merge_pipelines: false, merge_trains: true)
+    end
+
+    it_behaves_like 'the merge train feature is not available'
+  end
+
+  context 'when merge_trains is disabled and merge_pipelines is enabled' do
+    before do
+      stub_licensed_features(merge_pipelines: true, merge_trains: false)
+    end
+
+    it_behaves_like 'the merge train feature is not available'
+  end
+
+  context 'when merge_pipelines and merge_trains are enabled' do
+    before do
+      stub_licensed_features(merge_pipelines: true, merge_trains: true)
+    end
+
+    context 'when both the merge pipelines and merge trains checkboxes are unchecked' do
+      before do
+        visit edit_project_path(project)
+      end
+
+      it 'automatically checks the merge pipelines checkbox when the merge trains checkbox is checked' do
+        expect(page.find('#project_merge_trains_enabled').checked?).to be false
+
+        expect(page.find('#project_merge_pipelines_enabled').checked?).to be false
+
+        check('project_merge_trains_enabled')
+
+        expect(page.find('#project_merge_trains_enabled').checked?).to be true
+
+        expect(page.find('#project_merge_pipelines_enabled').checked?).to be true
+      end
+    end
+
+    context 'when both the merge pipelines and merge trains checkboxes are checked' do
+      before do
+        project.update(merge_pipelines_enabled: true, merge_trains_enabled: true)
+        visit edit_project_path(project)
+      end
+
+      it 'automatically unchecks the merge trains checkbox when the merge pipelines checkbox is unchecked' do
+        expect(page.find('#project_merge_trains_enabled').checked?).to be true
+
+        expect(page.find('#project_merge_pipelines_enabled').checked?).to be true
+
+        uncheck('project_merge_pipelines_enabled')
+
+        expect(page.find('#project_merge_trains_enabled').checked?).to be false
+
+        expect(page.find('#project_merge_pipelines_enabled').checked?).to be false
+      end
+    end
+  end
 end

locale/gitlab.pot

@@ -9828,6 +9828,9 @@ msgstr ""
 msgid "ProjectSettings|All discussions must be resolved"
 msgstr ""
 
+msgid "ProjectSettings|Allow merge trains"
+msgstr ""
+
 msgid "ProjectSettings|Automatically resolve merge request diff discussions when they become outdated"
 msgstr ""