Eliminate many Gitaly calls in discussions API

Previously, the API to retrieve discussions from merge requests often
generated hundreds of Gitaly calls to determine whether a system note
should be shown to the user. It did this by:

1. Rendering the Markdown
2. Extracting cross-references from the Markdown
3. For cross-references that were commits, a Gitaly FindCommit RPC
   would be issued to validate that the commit exists.

The last step is unnecessary because we don't need to display a commit
if the user doesn't have access to the project in the first place.

`RendersNotes#prepare_notes_for_rendering` is already used in
`MergeRequestsController`, which is why we don't see N+1 Gitaly calls
there. We use it here to optimize the note redaction process.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65957

1 files changed, 53 insertions, 0 deletions

diff --git a/spec/requests/api/discussions_spec.rb b/spec/requests/api/discussions_spec.rb
index ca1ffe3c524..daf75eb82bb 100644
--- a/spec/requests/api/discussions_spec.rb
+++ b/spec/requests/api/discussions_spec.rb
@@ -9,6 +9,59 @@ describe API::Discussions do
     project.add_developer(user)
   end
 
+  context 'with cross-reference system notes', :request_store do
+    let(:merge_request) { create(:merge_request) }
+    let(:project) { merge_request.project }
+    let(:new_merge_request) { create(:merge_request) }
+    let(:commit) { new_merge_request.project.commit }
+    let!(:note) { create(:system_note, noteable: merge_request, project: project, note: cross_reference) }
+    let!(:note_metadata) { create(:system_note_metadata, note: note, action: 'cross_reference') }
+    let(:cross_reference) { "test commit #{commit.to_reference(project)}" }
+    let(:url) { "/projects/#{project.id}/merge_requests/#{merge_request.iid}/discussions" }
+
+    before do
+      project.add_developer(user)
+      new_merge_request.project.add_developer(user)
+    end
+
+    it 'returns only the note that the user should see' do
+      hidden_merge_request = create(:merge_request)
+      new_cross_reference = "test commit #{hidden_merge_request.project.commit}"
+      new_note = create(:system_note, noteable: merge_request, project: project, note: new_cross_reference)
+      create(:system_note_metadata, note: new_note, action: 'cross_reference')
+
+      get api(url, user)
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response.count).to eq(1)
+      expect(json_response.first['notes'].count).to eq(1)
+
+      parsed_note = json_response.first['notes'].first
+      expect(parsed_note['id']).to eq(note.id)
+      expect(parsed_note['body']).to eq(cross_reference)
+      expect(parsed_note['system']).to be true
+    end
+
+    it 'avoids Git calls and N+1 SQL queries' do
+      expect_any_instance_of(Repository).not_to receive(:find_commit).with(commit.id)
+
+      control = ActiveRecord::QueryRecorder.new do
+        get api(url, user)
+      end
+
+      expect(response).to have_gitlab_http_status(200)
+
+      RequestStore.clear!
+
+      new_note = create(:system_note, noteable: merge_request, project: project, note: cross_reference)
+      create(:system_note_metadata, note: new_note, action: 'cross_reference')
+
+      RequestStore.clear!
+
+      expect { get api(url, user) }.not_to exceed_query_limit(control)
+      expect(response).to have_gitlab_http_status(200)
+    end
+  end
+
   context 'when noteable is an Issue' do
     let!(:issue) { create(:issue, project: project, author: user) }
     let!(:issue_note) { create(:discussion_note_on_issue, noteable: issue, project: project, author: user) }