Improve performance of filtering notes in NotesController

Reduces the number of queries needed to redact notes to which
the user does not have access. Also includes an N+1 query test
as a guard against future issues.

This is a follow-up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14327#note_40976854.

app/controllers/concerns/notes_actions.rb

@@ -15,9 +15,9 @@ module NotesActions
     notes = notes_finder.execute
       .inc_relations_for_view
-      .reject { |n| n.cross_reference_not_visible_for?(current_user) }
     notes = prepare_notes_for_rendering(notes)
+    notes = notes.reject { |n| n.cross_reference_not_visible_for?(current_user) }
     notes_json[:notes] =
       if noteable.discussions_rendered_on_frontend?

spec/controllers/projects/notes_controller_spec.rb

@@ -120,6 +120,40 @@ describe Projects::NotesController do
         expect(note_json[:diff_discussion_html]).to be_nil
       end
     end
+
+    context 'with cross-reference system note', :request_store do
+      let(:new_issue) { create(:issue) }
+      let(:cross_reference) { "mentioned in #{new_issue.to_reference(issue.project)}" }
+
+      before do
+        note
+        create(:discussion_note_on_issue, :system, noteable: issue, project: issue.project, note: cross_reference)
+      end
+
+      it 'filters notes that the user should not see' do
+        get :index, request_params
+
+        expect(parsed_response[:notes].count).to eq(1)
+        expect(note_json[:id]).to eq(note.id)
+      end
+
+      it 'does not result in N+1 queries' do
+        # Instantiate the controller variables to ensure QueryRecorder has an accurate base count
+        get :index, request_params
+
+        RequestStore.clear!
+
+        control_count = ActiveRecord::QueryRecorder.new do
+          get :index, request_params
+        end.count
+
+        RequestStore.clear!
+
+        create_list(:discussion_note_on_issue, 2, :system, noteable: issue, project: issue.project, note: cross_reference)
+
+        expect { get :index, request_params }.not_to exceed_query_limit(control_count)
+      end
+    end
   end
   describe 'POST create' do