Add basic E2E test of merge trains

New test added to the pipelines for merged results spec file because
it uses the the same setup, including a runner (which is not cheap
to set up)

qa/qa/ee/page/merge_request/show.rb

@@ -161,6 +161,12 @@ module QA
             approvals_content.match(/Requires (\d+) more approvals/)[1].to_i
           end
 
+          def merge_via_merge_train
+            raise ElementNotFound, "Not ready to merge" unless ready_to_merge?
+
+            click_element(:merge_button, text: "Start merge train")
+          end
+
           private
 
           def approvals_content

qa/qa/page/merge_request/show.rb

@@ -129,17 +129,7 @@ module QA
         end
 
         def try_to_merge!
-          # The merge button is disabled on load
-          wait do
-            has_element?(:merge_button)
-          end
-
-          # The merge button is enabled via JS
-          wait(reload: false) do
-            !find_element(:merge_button).disabled?
-          end
-
-          merge_immediately
+          merge_immediately if ready_to_merge?
         end
 
         def merge!
@@ -187,6 +177,18 @@ module QA
           click_element :edit_button
         end
 
+        def ready_to_merge?
+          # The merge button is disabled on load
+          wait do
+            has_element?(:merge_button)
+          end
+
+          # The merge button is enabled via JS
+          wait(reload: false) do
+            !find_element(:merge_button).disabled?
+          end
+        end
+
         def view_email_patches
           click_element :dropdown_toggle
           visit_link_in_element(:download_email_patches)

qa/qa/specs/features/ee/browser_ui/6_release/pipelines_for_merged_results_spec.rb → qa/qa/specs/features/ee/browser_ui/6_release/pipelines_for_merged_results_and_merge_trains_spec.rb

@@ -2,23 +2,21 @@
 
 module QA
   context 'Release', :docker do
-    describe 'Pipelines for merged results' do
-      let(:project) do
-        Resource::Project.fabricate_via_api! do |project|
-          project.name = 'pipelines-for-merged-results'
+    describe 'Pipelines for merged results and merge trains' do
+      before(:context) do
+        @project = Resource::Project.fabricate_via_api! do |project|
+          project.name = 'pipelines-for-merged-results-and-merge-trains'
         end
-      end
-      let(:executor) { "qa-runner-#{Time.now.to_i}" }
+        @executor = "qa-runner-#{Time.now.to_i}"
 
-      before do
         Resource::Runner.fabricate_via_api! do |runner|
-          runner.project = project
-          runner.name = executor
+          runner.project = @project
+          runner.name = @executor
           runner.tags = %w[qa test]
         end
 
         Resource::Repository::ProjectPush.fabricate! do |project_push|
-          project_push.project = project
+          project_push.project = @project
           project_push.file_name = '.gitlab-ci.yml'
           project_push.commit_message = 'Add .gitlab-ci.yml'
           project_push.file_content = <<~EOF
@@ -28,42 +26,88 @@ module QA
               - merge_requests
           EOF
         end
+      end
 
+      before do
         Runtime::Browser.visit(:gitlab, Page::Main::Login)
         Page::Main::Login.perform(&:sign_in_using_credentials)
-      end
-
-      after do
-        Service::Runner.new(executor).remove!
-      end
 
-      it 'creates a pipeline with merged results' do
-        project.visit!
+        @project.visit!
 
         Page::Project::Menu.perform(&:go_to_general_settings)
-
         Page::Project::Settings::Main.perform do |main|
           main.expand_merge_requests_settings do |settings|
             settings.click_pipelines_for_merged_results_checkbox
             settings.click_save_changes
           end
         end
+      end
+
+      after(:context) do
+        Service::Runner.new(@executor).remove!
+      end
+
+      it 'creates a pipeline with merged results' do
+        # Create a branch that will be merged into master
+        Resource::Repository::ProjectPush.fabricate! do |project_push|
+          project_push.project = @project
+          project_push.new_branch = true
+          project_push.branch_name = 'merged-results'
+        end
 
+        # Create a merge request to merge the branch we just created
         Resource::MergeRequest.fabricate_via_api! do |merge_request|
-          merge_request.project = project
-          merge_request.target_new_branch = false
+          merge_request.project = @project
+          merge_request.source_branch = 'merged-results'
+          merge_request.no_preparation = true
         end.visit!
 
-        Page::MergeRequest::Show.perform do |merge_request|
-          expect(merge_request).to have_pipeline_status(/Merged result pipeline #\d+ passed/)
+        Page::MergeRequest::Show.perform do |show|
+          expect(show).to have_pipeline_status(/Merged result pipeline #\d+ passed/)
 
           # The default option is to merge via merge train,
           # but that will be covered by another test
-          merge_request.merge_immediately
+          show.merge_immediately
         end
 
         expect(page).to have_content('The changes were merged')
       end
+
+      it 'merges via a merge train' do
+        # Create a branch that will be merged into master
+        Resource::Repository::ProjectPush.fabricate! do |project_push|
+          project_push.project = @project
+          project_push.new_branch = true
+          project_push.branch_name = 'merge-train'
+          project_push.file_name = "another_file.txt"
+          project_push.file_content = "merge me"
+        end
+
+        # Create a merge request to merge the branch we just created
+        Resource::MergeRequest.fabricate_via_api! do |merge_request|
+          merge_request.project = @project
+          merge_request.source_branch = 'merge-train'
+          merge_request.no_preparation = true
+        end.visit!
+
+        Page::MergeRequest::Show.perform do |show|
+          expect(show).to have_pipeline_status(/Merged result pipeline #\d+ passed/)
+
+          show.merge_via_merge_train
+        end
+
+        expect(page).to have_content('Added to the merge train')
+        expect(page).to have_content('The changes will be merged into master')
+
+        # It's faster to refresh the page than to wait for the UI to
+        # automatically refresh, so we reload if the merge status
+        # doesn't update quickly.
+        merged = Support::Retrier.retry_until(reload_page: page) do
+          page.has_content?('The changes were merged')
+        end
+
+        expect(merged).to be_truthy, "Expected content 'The changes were merged' but it did not appear."
+      end
     end
   end
 end