Merge branch 'optimize-cross-ref-system-notes-check' into 'master'

Optimize cross ref system notes check

## What does this MR do?

This MR optimizes system note visibility checking by memoizing the visible reference count, reducing the overhead of calling `Note#cross_reference_not_visible_for?`.

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

Note that since a cross reference message contains, "Mentioned in XYZ", we EXPECT that there is at least one reference. That's why using the ref count > 0 works. 

## Why was this MR needed?

The previous implementation relied on `Note#cross_reference_not_visible_for?`, which essentially tries to render all the Markdown references in a system note
and only displays the note if the user can see the referring project. But this duplicated the work that Banzai::NotesRenderer was doing already. This shaves about 0.8 s
from the load time from https://gitlab.com/gitlab-com/operations/issues/42.

## What are the relevant issue numbers?

#19273

## Screenshots (if relevant)

## 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
  - [x] 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 !5070

1 files changed, 22 insertions, 0 deletions

diff --git a/spec/features/notes_on_merge_requests_spec.rb b/spec/features/notes_on_merge_requests_spec.rb
index dd016a36d1e..0b38c413f44 100644
--- a/spec/features/notes_on_merge_requests_spec.rb
+++ b/spec/features/notes_on_merge_requests_spec.rb
@@ -135,6 +135,28 @@ describe 'Comments', feature: true do
     end
   end
 
+  describe 'Handles cross-project system notes', js: true, feature: true do
+    let(:user) { create(:user) }
+    let(:project) { create(:project, :public) }
+    let(:project2) { create(:project, :private) }
+    let(:issue) { create(:issue, project: project2) }
+    let(:merge_request) { create(:merge_request, source_project: project, source_branch: 'markdown') }
+    let!(:note) { create(:note_on_merge_request, :system, noteable: merge_request, project: project, note: "mentioned in #{issue.to_reference(project)}") }
+
+    it 'shows the system note' do
+      login_as :admin
+      visit namespace_project_merge_request_path(project.namespace, project, merge_request)
+
+      expect(page).to have_css('.system-note')
+    end
+
+    it 'hides redacted system note' do
+      visit namespace_project_merge_request_path(project.namespace, project, merge_request)
+
+      expect(page).not_to have_css('.system-note')
+    end
+  end
+
   describe 'On a merge request diff', js: true, feature: true do
     let(:merge_request) { create(:merge_request) }
     let(:project) { merge_request.source_project }