Omit issues links in merge request entity API response

The merge request widget has a section that includes which issues may be
closed or mentioned based on the merge request description. The problem
is that rendering and redacting Markdown can be expensive, especially
since the browser polls for the data every 10 seconds.

Since these links don't change much and are just nice to have, we only
load them on first page load. The frontend will use the existing data if
the data doesn't appear on subsequent requests.

This saves about 30% of the rendering time of this endpoint, which adds
up to significant savings considering that
`MergeRequestsController#show.json` is called over a million times a day
on GitLab.com.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63546

app/helpers/issuables_helper.rb

@@ -74,7 +74,7 @@ module IssuablesHelper
     end
   end
 
-  def serialize_issuable(issuable, serializer: nil)
+  def serialize_issuable(issuable, opts = {})
     serializer_klass = case issuable
                        when Issue
                          IssueSerializer
@@ -84,7 +84,7 @@ module IssuablesHelper
 
     serializer_klass
       .new(current_user: current_user, project: issuable.project)
-      .represent(issuable, serializer: serializer)
+      .represent(issuable, opts)
       .to_json
   end
 

app/serializers/merge_request_widget_entity.rb

@@ -114,7 +114,10 @@ class MergeRequestWidgetEntity < IssuableEntity
     presenter(merge_request).ci_status
   end
 
-  expose :issues_links do
+  # Rendering and redacting Markdown can be expensive. These links are
+  # just nice to have in the merge request widget, so only
+  # include them if they are explicitly requested on first load.
+  expose :issues_links, if: -> (_, opts) { opts[:issues_links] } do
     expose :assign_to_closing do |merge_request|
       presenter(merge_request).assign_to_closing_issues_link
     end

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

@@ -19,7 +19,7 @@
     -# haml-lint:disable InlineJavaScript
     :javascript
       window.gl = window.gl || {};
-      window.gl.mrWidgetData = #{serialize_issuable(@merge_request, serializer: 'widget')}
+      window.gl.mrWidgetData = #{serialize_issuable(@merge_request, serializer: 'widget', issues_links: true)}
 
       window.gl.mrWidgetData.squash_before_merge_help_path = '#{help_page_path("user/project/merge_requests/squash_and_merge")}';
       window.gl.mrWidgetData.troubleshooting_docs_path = '#{help_page_path('user/project/merge_requests/index.md', anchor: 'troubleshooting')}';

changelogs/unreleased/sh-omit-issues-links-on-poll.yml

+---
+title: Omit issues links in merge request entity API response
+merge_request: 29917
+author:
+type: performance

spec/serializers/merge_request_widget_entity_spec.rb

@@ -32,6 +32,19 @@ describe MergeRequestWidgetEntity do
     end
   end
 
+  describe 'issues links' do
+    it 'includes issues links when requested' do
+      data = described_class.new(resource, request: request, issues_links: true).as_json
+
+      expect(data).to include(:issues_links)
+      expect(data[:issues_links]).to include(:assign_to_closing, :closing, :mentioned_but_not_closing)
+    end
+
+    it 'omits issue links by default' do
+      expect(subject).not_to include(:issues_links)
+    end
+  end
+
   describe 'pipeline' do
     let(:pipeline) { create(:ci_empty_pipeline, project: project, ref: resource.source_branch, sha: resource.source_branch_sha, head_pipeline_of: resource) }