Cache diff highlight in discussions

This commit handles note diffs caching, which considerably improves
the performance on merge requests with lots of comments.
Important to note that the caching approach taken here is different
from `Gitlab::Diff::HighlightCache`. We do not reset the whole cache
when a new push is sent or anything else. That's because discussions
diffs are persisted and do not change.

1 files changed, 64 insertions, 0 deletions

diff --git a/spec/controllers/projects/merge_requests_controller_spec.rb b/spec/controllers/projects/merge_requests_controller_spec.rb
index 1ab227bde39..7c167420e1f 100644
--- a/spec/controllers/projects/merge_requests_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests_controller_spec.rb
@@ -957,6 +957,70 @@ describe Projects::MergeRequestsController do
     end
   end
 
+  describe 'GET discussions' do
+    context 'when authenticated' do
+      before do
+        project.add_developer(user)
+        sign_in(user)
+      end
+
+      it 'returns 200' do
+        get :discussions, namespace_id: project.namespace, project_id: project, id: merge_request.iid
+
+        expect(response.status).to eq(200)
+      end
+
+      context 'highlight preloading' do
+        context 'with commit diff notes' do
+          let!(:commit_diff_note) do
+            create(:diff_note_on_commit, project: merge_request.project)
+          end
+
+          it 'preloads notes diffs highlights' do
+            expect_next_instance_of(Gitlab::DiscussionsDiff::FileCollection) do |collection|
+              note_diff_file = commit_diff_note.note_diff_file
+
+              expect(collection).to receive(:load_highlight).with([note_diff_file.id]).and_call_original
+              expect(collection).to receive(:find_by_id).with(note_diff_file.id).and_call_original
+            end
+
+            get :discussions, namespace_id: project.namespace, project_id: project, id: merge_request.iid
+          end
+        end
+
+        context 'with diff notes' do
+          let!(:diff_note) do
+            create(:diff_note_on_merge_request, noteable: merge_request, project: merge_request.project)
+          end
+
+          it 'preloads notes diffs highlights' do
+            expect_next_instance_of(Gitlab::DiscussionsDiff::FileCollection) do |collection|
+              note_diff_file = diff_note.note_diff_file
+
+              expect(collection).to receive(:load_highlight).with([note_diff_file.id]).and_call_original
+              expect(collection).to receive(:find_by_id).with(note_diff_file.id).and_call_original
+            end
+
+            get :discussions, namespace_id: project.namespace, project_id: project, id: merge_request.iid
+          end
+
+          it 'does not preload highlights when diff note is resolved' do
+            Notes::ResolveService.new(diff_note.project, user).execute(diff_note)
+
+            expect_next_instance_of(Gitlab::DiscussionsDiff::FileCollection) do |collection|
+              note_diff_file = diff_note.note_diff_file
+
+              expect(collection).to receive(:load_highlight).with([]).and_call_original
+              expect(collection).to receive(:find_by_id).with(note_diff_file.id).and_call_original
+            end
+
+            get :discussions, namespace_id: project.namespace, project_id: project, id: merge_request.iid
+          end
+        end
+      end
+    end
+  end
+
   describe 'GET edit' do
     it 'responds successfully' do
       get :edit, params: { namespace_id: project.namespace, project_id: project, id: merge_request }