Merge branch 'retrieve-mr-closes-issues-just-when-required' into 'master'

Avoid calculation of closes_issues.

## What does this MR do?

Avoid unneeded calls to MR closes issues

## Are there points in the code the reviewer needs to double check?

I'm not sure if calling this method from a view is a good practice, but I cannot see another simple way of avoiding this problem. In case we want to avoid this in the controller we need to specify the action, format and status of the merge request, because in that case we know that the `_open` partial will render. We could add some lazy evaluation but it not a thing I see in use along the app but feedback is welcome

## What are the relevant issue numbers?

#14202 , #19490

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- ~~[ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- ~~[ ] API support added~~
- Tests
  - ~~[ ] Added for this feature/bug~~
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5140

CHANGELOG

@@ -39,6 +39,7 @@ v 8.10.0 (unreleased)
   - Bump Rinku to 2.0.0
   - Remove unused front-end variable -> default_issues_tracker
   - Better caching of git calls on ProjectsController#show.
+  - Avoid to retrieve MR closes_issues as much as possible.
   - Add API endpoint for a group issues !4520 (mahcsig)
   - Add Bugzilla integration !4930 (iamtjg)
   - Instrument Rinku usage

app/controllers/projects/merge_requests_controller.rb

@@ -9,7 +9,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     :edit, :update, :show, :diffs, :commits, :builds, :merge, :merge_check,
     :ci_status, :toggle_subscription, :cancel_merge_when_build_succeeds, :remove_wip
   ]
-  before_action :closes_issues, only: [:edit, :update, :show, :diffs, :commits, :builds]
   before_action :validates_merge_request, only: [:show, :diffs, :commits, :builds]
   before_action :define_show_vars, only: [:show, :diffs, :commits, :builds]
   before_action :define_widget_vars, only: [:merge, :cancel_merge_when_build_succeeds, :merge_check]
@@ -315,10 +314,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   alias_method :issuable, :merge_request
   alias_method :awardable, :merge_request
 
-  def closes_issues
-    @closes_issues ||= @merge_request.closes_issues
-  end
-
   def authorize_update_merge_request!
     return render_404 unless can?(current_user, :update_merge_request, @merge_request)
   end
@@ -387,7 +382,6 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   def define_widget_vars
     @pipeline = @merge_request.pipeline
     @pipelines = [@pipeline].compact
-    closes_issues
   end
 
   def invalid_mr

app/helpers/merge_requests_helper.rb

@@ -55,6 +55,10 @@ module MergeRequestsHelper
     end.sort.to_sentence
   end
 
+  def mr_closes_issues
+    @mr_closes_issues ||= @merge_request.closes_issues
+  end
+
   def mr_change_branches_path(merge_request)
     new_namespace_project_merge_request_path(
       @project.namespace, @project,

app/views/projects/merge_requests/widget/_open.html.haml

@@ -22,10 +22,10 @@
     - elsif @merge_request.can_be_merged?
       = render 'projects/merge_requests/widget/open/accept'
 
-  - if @closes_issues.present?
+  - if mr_closes_issues.present?
     .mr-widget-footer
       %span
         %i.fa.fa-check
-        Accepting this merge request will close #{"issue".pluralize(@closes_issues.size)}
+        Accepting this merge request will close #{"issue".pluralize(mr_closes_issues.size)}
         = succeed '.' do
-          != markdown issues_sentence(@closes_issues), pipeline: :gfm, author: @merge_request.author
+          != markdown issues_sentence(mr_closes_issues), pipeline: :gfm, author: @merge_request.author