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

1 files changed, 4 insertions, 1 deletions

diff --git a/app/serializers/merge_request_widget_entity.rb b/app/serializers/merge_request_widget_entity.rb
index a428930dbbf..43aced598a9 100644
--- a/app/serializers/merge_request_widget_entity.rb
+++ b/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