Add stages_ajax endpoint to serve old HTML

app/controllers/projects/pipelines_controller.rb

@@ -109,6 +109,15 @@ class Projects::PipelinesController < Projects::ApplicationController
       .represent(@stage, details: true)
   end
+  # TODO: This endpoint is used by mini-pipeline-graph
+  # TODO: This endpoint should be migrated to `stage.json`
+  def stage_ajax
+    @stage = pipeline.legacy_stage(params[:stage])
+    return not_found unless @stage
+
+    render json: { html: view_to_html_string('projects/pipelines/_stage')) }
+  end
+
   def retry
     pipeline.retry_failed(current_user)

app/views/shared/_mini_pipeline_graph.html.haml

@@ -6,7 +6,7 @@
       - status_klass = "ci-status-icon ci-status-icon-#{detailed_status.group}"
       .stage-container.dropdown{ class: klass }
-        %button.mini-pipeline-graph-dropdown-toggle.has-tooltip.js-builds-dropdown-button{ class: "ci-status-icon-#{detailed_status.group}", type: 'button', data: { toggle: 'dropdown', title: "#{stage.name}: #{detailed_status.label}", placement: 'top', "stage-endpoint" => stage_project_pipeline_path(pipeline.project, pipeline, stage: stage.name) } }
+        %button.mini-pipeline-graph-dropdown-toggle.has-tooltip.js-builds-dropdown-button{ class: "ci-status-icon-#{detailed_status.group}", type: 'button', data: { toggle: 'dropdown', title: "#{stage.name}: #{detailed_status.label}", placement: 'top', "stage-endpoint" => stage_ajax_project_pipeline_path(pipeline.project, pipeline, stage: stage.name) } }
           = sprite_icon(icon_status)
           = icon('caret-down')

config/routes/project.rb

@@ -183,6 +183,7 @@ constraints(::Constraints::ProjectUrlConstrainer.new) do
         member do
           get :stage
+          get :stage_ajax
           post :cancel
           post :retry
           get :builds

spec/controllers/projects/pipelines_controller_spec.rb

@@ -109,8 +109,7 @@ describe Projects::PipelinesController do
       it 'returns html source for stage dropdown' do
         expect(response).to have_gitlab_http_status(:ok)
-        expect(response).to render_template('projects/pipelines/_stage')
-        expect(json_response).to include('html')
+        expect(response).to match_response_schema('pipeline_stage')
       end
     end
@@ -133,6 +132,42 @@ describe Projects::PipelinesController do
     end
   end
+  describe 'GET stages_ajax.json' do
+    let(:pipeline) { create(:ci_pipeline, project: project) }
+
+    context 'when accessing existing stage' do
+      before do
+        create(:ci_build, pipeline: pipeline, stage: 'build')
+
+        get_stage_ajax('build')
+      end
+
+      it 'returns html source for stage dropdown' do
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(response).to render_template('projects/pipelines/_stage')
+        expect(json_response).to include('html')
+      end
+    end
+
+    context 'when accessing unknown stage' do
+      before do
+        get_stage_ajax('test')
+      end
+
+      it 'responds with not found' do
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    def get_stage_ajax(name)
+      get :stage_ajax, namespace_id: project.namespace,
+                  project_id: project,
+                  id: pipeline.id,
+                  stage: name,
+                  format: :json
+    end
+  end
+
   describe 'GET status.json' do
     let(:pipeline) { create(:ci_pipeline, project: project) }
     let(:status) { pipeline.detailed_status(double('user')) }