Move merge-request merge_pipeline to cached_widget

Since we already have expiration mechanism for pipelines
What about moving them into cached_widget.json

app/serializers/merge_request_poll_cached_widget_entity.rb

@@ -50,6 +50,14 @@ class MergeRequestPollCachedWidgetEntity < IssuableEntity
     MergeRequests::PipelineEntity.represent(merge_request.actual_head_pipeline, options)
   end
 
+  expose :merge_pipeline, if: ->(mr, _) {
+    Feature.enabled?(:merge_request_cached_merge_pipeline_serializer, mr.project, default_enabled: :yaml) &&
+      mr.merged? &&
+      can?(request.current_user, :read_pipeline, mr.target_project)
+  } do |merge_request, options|
+    MergeRequests::PipelineEntity.represent(merge_request.merge_pipeline, options)
+  end
+
   # Paths
   #
   expose :target_branch_commits_path do |merge_request|

app/serializers/merge_request_poll_widget_entity.rb

@@ -19,7 +19,11 @@ class MergeRequestPollWidgetEntity < Grape::Entity
   # User entities
   expose :merge_user, using: UserEntity
 
-  expose :merge_pipeline, if: ->(mr, _) { mr.merged? && can?(request.current_user, :read_pipeline, mr.target_project)} do |merge_request, options|
+  expose :merge_pipeline, if: ->(mr, _) {
+    Feature.disabled?(:merge_request_cached_merge_pipeline_serializer, mr.project, default_enabled: :yaml) &&
+      mr.merged? &&
+      can?(request.current_user, :read_pipeline, mr.target_project)
+  } do |merge_request, options|
     MergeRequests::PipelineEntity.represent(merge_request.merge_pipeline, options)
   end
 

config/feature_flags/development/merge_request_cached_merge_pipeline_serializer.yml

+---
+name: merge_request_cached_merge_pipeline_serializer
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57827
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326317
+milestone: '13.11'
+type: development
+group: group::source code
+default_enabled: false

spec/serializers/merge_request_poll_cached_widget_entity_spec.rb

@@ -268,4 +268,49 @@ RSpec.describe MergeRequestPollCachedWidgetEntity do
       end
     end
   end
+
+  describe 'merge_pipeline' do
+    it 'returns nil' do
+      expect(subject[:merge_pipeline]).to be_nil
+    end
+
+    context 'when is merged' do
+      let(:resource) { create(:merged_merge_request, source_project: project, merge_commit_sha: project.commit.id) }
+      let(:pipeline) { create(:ci_empty_pipeline, project: project, ref: resource.target_branch, sha: resource.merge_commit_sha) }
+
+      before do
+        project.add_maintainer(user)
+      end
+
+      it 'returns merge_pipeline' do
+        pipeline.reload
+        pipeline_payload =
+          MergeRequests::PipelineEntity
+            .represent(pipeline, request: request)
+            .as_json
+
+        expect(subject[:merge_pipeline]).to eq(pipeline_payload)
+      end
+
+      context 'when user cannot read pipelines on target project' do
+        before do
+          project.add_guest(user)
+        end
+
+        it 'returns nil' do
+          expect(subject[:merge_pipeline]).to be_nil
+        end
+      end
+
+      context 'when merge_request_cached_merge_pipeline_serializer is disabled' do
+        before do
+          stub_feature_flags(merge_request_cached_merge_pipeline_serializer: false)
+        end
+
+        it 'returns nil' do
+          expect(subject[:merge_pipeline]).to be_nil
+        end
+      end
+    end
+  end
 end

spec/serializers/merge_request_poll_widget_entity_spec.rb

@@ -6,9 +6,9 @@ RSpec.describe MergeRequestPollWidgetEntity do
   include ProjectForksHelper
   using RSpec::Parameterized::TableSyntax
 
-  let(:project)  { create :project, :repository }
-  let(:resource) { create(:merge_request, source_project: project, target_project: project) }
-  let(:user)     { create(:user) }
+  let_it_be(:project)  { create :project, :repository }
+  let_it_be(:resource) { create(:merge_request, source_project: project, target_project: project) }
+  let_it_be(:user)     { create(:user) }
 
   let(:request) { double('request', current_user: user, project: project) }
 
@@ -22,20 +22,33 @@ RSpec.describe MergeRequestPollWidgetEntity do
   end
 
   describe 'merge_pipeline' do
+    before do
+      stub_feature_flags(merge_request_cached_merge_pipeline_serializer: false)
+    end
+
     it 'returns nil' do
       expect(subject[:merge_pipeline]).to be_nil
     end
 
     context 'when is merged' do
-      let(:resource) { create(:merged_merge_request, source_project: project, merge_commit_sha: project.commit.id) }
-      let(:pipeline) { create(:ci_empty_pipeline, project: project, ref: resource.target_branch, sha: resource.merge_commit_sha) }
+      let_it_be(:resource) { create(:merged_merge_request, source_project: project, merge_commit_sha: project.commit.id) }
+      let_it_be(:pipeline) { create(:ci_empty_pipeline, project: project, ref: resource.target_branch, sha: resource.merge_commit_sha) }
 
       before do
         project.add_maintainer(user)
       end
 
+      context 'when user cannot read pipelines on target project' do
+        before do
+          project.team.truncate
+        end
+
+        it 'returns nil' do
+          expect(subject[:merge_pipeline]).to be_nil
+        end
+      end
+
       it 'returns merge_pipeline' do
-        pipeline.reload
         pipeline_payload =
           MergeRequests::PipelineEntity
             .represent(pipeline, request: request)
@@ -44,9 +57,9 @@ RSpec.describe MergeRequestPollWidgetEntity do
         expect(subject[:merge_pipeline]).to eq(pipeline_payload)
       end
 
-      context 'when user cannot read pipelines on target project' do
+      context 'when merge_request_cached_merge_pipeline_serializer is enabled' do
         before do
-          project.add_guest(user)
+          stub_feature_flags(merge_request_cached_merge_pipeline_serializer: true)
         end
 
         it 'returns nil' do