Move async mergeability check into widget

Currently Projects::MergeRequestsController#show goes
into primary sometimes due to the mergeability check

Let's move it to the widget and check it async when
we load the merge request the first time and sync
when it's auto refreshed

Visually we have the same behaviour, but merge_requests#show
action no longer goes to primary

app/assets/javascripts/vue_merge_request_widget/stores/mr_widget_store.js

@@ -60,6 +60,7 @@ export default class MergeRequestStore {
     this.rebaseInProgress = data.rebase_in_progress;
     this.mergeRequestDiffsPath = data.diffs_path;
     this.approvalsWidgetType = data.approvals_widget_type;
+    this.mergeRequestWidgetPath = data.merge_request_widget_path;
 
     if (data.issues_links) {
       const links = data.issues_links;

app/controllers/projects/merge_requests/content_controller.rb

@@ -14,6 +14,8 @@ class Projects::MergeRequests::ContentController < Projects::MergeRequests::Appl
   SLOW_POLLING_INTERVAL = 5.minutes.in_milliseconds
 
   def widget
+    check_mergeability_async!
+
     respond_to do |format|
       format.json do
         render json: serializer(MergeRequestPollWidgetEntity)
@@ -38,6 +40,13 @@ class Projects::MergeRequests::ContentController < Projects::MergeRequests::Appl
 
   def serializer(entity)
     serializer = MergeRequestSerializer.new(current_user: current_user, project: merge_request.project)
-    serializer.represent(merge_request, {}, entity)
+    serializer.represent(merge_request, { async_mergeability_check: params[:async_mergeability_check] }, entity)
+  end
+
+  def check_mergeability_async!
+    return unless Feature.enabled?(:check_mergeability_async_in_widget, merge_request.project, default_enabled: :yaml)
+    return if params[:async_mergeability_check].blank?
+
+    merge_request.check_mergeability(async: true)
   end
 end

app/controllers/projects/merge_requests_controller.rb

@@ -92,7 +92,10 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
   def show
     close_merge_request_if_no_source_project
-    @merge_request.check_mergeability(async: true)
+
+    if Feature.disabled?(:check_mergeability_async_in_widget, @project, default_enabled: :yaml)
+      @merge_request.check_mergeability(async: true)
+    end
 
     respond_to do |format|
       format.html do

app/serializers/merge_request_poll_cached_widget_entity.rb

@@ -64,6 +64,10 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
     presenter(merge_request).target_branch_commits_path
   end
 
+  expose :merge_request_widget_path do |merge_request|
+    widget_project_json_merge_request_path(merge_request.target_project, merge_request, format: :json)
+  end
+
   expose :target_branch_tree_path do |merge_request|
     presenter(merge_request).target_branch_tree_path
   end

app/serializers/merge_request_poll_widget_entity.rb

@@ -29,7 +29,12 @@ class MergeRequestPollWidgetEntity < Grape::Entity
 
   expose :default_merge_commit_message
 
-  expose :mergeable?, as: :mergeable
+  expose :mergeable do |merge_request, options|
+    next merge_request.mergeable? if Feature.disabled?(:check_mergeability_async_in_widget, merge_request.project, default_enabled: :yaml)
+    next false if options[:async_mergeability_check].present? && merge_request.checking?
+
+    merge_request.mergeable?
+  end
 
   expose :default_merge_commit_message_with_description do |merge_request|
     merge_request.default_merge_commit_message(include_description: true)

app/serializers/merge_request_widget_entity.rb

@@ -36,7 +36,7 @@ class MergeRequestWidgetEntity < Grape::Entity
   end
 
   expose :merge_request_widget_path do |merge_request|
-    widget_project_json_merge_request_path(merge_request.target_project, merge_request, format: :json)
+    widget_project_json_merge_request_path(merge_request.target_project, merge_request, async_mergeability_check: true, format: :json)
   end
 
   expose :merge_request_cached_widget_path do |merge_request|

app/views/projects/merge_requests/show.html.haml

@@ -62,7 +62,7 @@
                   - add_page_startup_api_call notes_url
                 - else
                   - add_page_startup_api_call discussions_path(@merge_request)
-                - add_page_startup_api_call widget_project_json_merge_request_path(@project, @merge_request, format: :json)
+                - add_page_startup_api_call widget_project_json_merge_request_path(@project, @merge_request, async_mergeability_check: true, format: :json)
                 - add_page_startup_api_call cached_widget_project_json_merge_request_path(@project, @merge_request, format: :json)
               #js-vue-mr-discussions{ data: { notes_data: notes_data(@merge_request, Feature.enabled?(:paginated_notes, @project)).to_json,
                 endpoint_metadata: @endpoint_metadata_url,

config/feature_flags/development/check_mergeability_async_in_widget.yml

+---
+name: check_mergeability_async_in_widget
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/58178
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326567
+milestone: '13.11'
+type: development
+group: group::source code
+default_enabled: false

spec/controllers/projects/merge_requests/content_controller_spec.rb

@@ -11,13 +11,13 @@ RSpec.describe Projects::MergeRequests::ContentController do
     sign_in(user)
   end
 
-  def do_request(action = :cached_widget)
+  def do_request(action = :cached_widget, params = {})
     get action, params: {
       namespace_id: project.namespace.to_param,
       project_id: project,
       id: merge_request.iid,
       format: :json
-    }
+    }.merge(params)
   end
 
   context 'user has access to the project' do
@@ -42,6 +42,10 @@ RSpec.describe Projects::MergeRequests::ContentController do
     end
 
     describe 'GET widget' do
+      before do
+        merge_request.mark_as_unchecked!
+      end
+
       it 'checks whether the MR can be merged' do
         controller.instance_variable_set(:@merge_request, merge_request)
 
@@ -53,6 +57,17 @@ RSpec.describe Projects::MergeRequests::ContentController do
         expect(response.headers['Poll-Interval']).to eq('10000')
       end
 
+      context 'when async_mergeability_check param is passed' do
+        it 'checks mergeability asynchronously' do
+          expect_next_instance_of(MergeRequests::MergeabilityCheckService) do |service|
+            expect(service).not_to receive(:execute)
+            expect(service).to receive(:async_execute).and_call_original
+          end
+
+          do_request(:widget, { async_mergeability_check: true })
+        end
+      end
+
       context 'merged merge request' do
         let(:merge_request) do
           create(:merged_merge_request, :with_test_reports, target_project: project, source_project: project)

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -82,13 +82,19 @@ RSpec.describe Projects::MergeRequestsController do
         merge_request.mark_as_unchecked!
       end
 
-      it 'checks mergeability asynchronously' do
-        expect_next_instance_of(MergeRequests::MergeabilityCheckService) do |service|
-          expect(service).not_to receive(:execute)
-          expect(service).to receive(:async_execute)
+      context 'check_mergeability_async_in_widget feature flag is disabled' do
+        before do
+          stub_feature_flags(check_mergeability_async_in_widget: false)
         end
 
-        go
+        it 'checks mergeability asynchronously' do
+          expect_next_instance_of(MergeRequests::MergeabilityCheckService) do |service|
+            expect(service).not_to receive(:execute)
+            expect(service).to receive(:async_execute)
+          end
+
+          go
+        end
       end
     end
 

spec/serializers/merge_request_poll_widget_entity_spec.rb

@@ -11,9 +11,10 @@ RSpec.describe MergeRequestPollWidgetEntity do
   let_it_be(:user)     { create(:user) }
 
   let(:request) { double('request', current_user: user, project: project) }
+  let(:options) { {} }
 
   subject do
-    described_class.new(resource, request: request).as_json
+    described_class.new(resource, { request: request }.merge(options)).as_json
   end
 
   it 'has default_merge_commit_message_with_description' do
@@ -278,4 +279,39 @@ RSpec.describe MergeRequestPollWidgetEntity do
       ])
     end
   end
+
+  describe '#mergeable' do
+    it 'shows whether a merge request is mergeable' do
+      expect(subject[:mergeable]).to eq(true)
+    end
+
+    context 'when merge request is in checking state' do
+      before do
+        resource.mark_as_unchecked!
+        resource.mark_as_checking!
+      end
+
+      it 'calculates mergeability and returns true' do
+        expect(subject[:mergeable]).to eq(true)
+      end
+
+      context 'when async_mergeability_check is passed' do
+        let(:options) { { async_mergeability_check: true } }
+
+        it 'returns false' do
+          expect(subject[:mergeable]).to eq(false)
+        end
+
+        context 'when check_mergeability_async_in_widget is disabled' do
+          before do
+            stub_feature_flags(check_mergeability_async_in_widget: false)
+          end
+
+          it 'calculates mergeability and returns true' do
+            expect(subject[:mergeable]).to eq(true)
+          end
+        end
+      end
+    end
+  end
 end