Fix bug where merge request comments created by API would not trigger notifications

Closes https://github.com/gitlabhq/gitlabhq/issues/9715

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.1.0 (unreleased)
   - Make diff file view easier to use on mobile screens (Stan Hu)
+  - Fix bug where merge request comments created by API would not trigger notifications (Stan Hu)
   - Add support for creating directories from Files page (Stan Hu)
   - Allow removing of project without confirmation when JavaScript is disabled (Stan Hu)
   - Support filtering by "Any" milestone or issue and fix "No Milestone" and "No Label" filters (Stan Hu)

lib/api/merge_requests.rb

@@ -249,8 +249,16 @@ module API
         required_attributes! [:note]
         merge_request = user_project.merge_requests.find(params[:merge_request_id])
-        note = merge_request.notes.new(note: params[:note], project_id: user_project.id)
-        note.author = current_user
+
+        authorize! :create_note, merge_request
+
+        opts = {
+          note: params[:note],
+          noteable_type: 'MergeRequest',
+          noteable_id: merge_request.id
+        }
+
+        note = ::Notes::CreateService.new(user_project, current_user, opts).execute
         if note.save
           present note, with: Entities::MRNote

spec/requests/api/merge_requests_spec.rb

@@ -379,9 +379,14 @@ describe API::API, api: true  do
   describe "POST /projects/:id/merge_request/:merge_request_id/comments" do
     it "should return comment" do
+      original_count = merge_request.notes.size
+
       post api("/projects/#{project.id}/merge_request/#{merge_request.id}/comments", user), note: "My comment"
       expect(response.status).to eq(201)
       expect(json_response['note']).to eq('My comment')
+      expect(json_response['author']['name']).to eq(user.name)
+      expect(json_response['author']['username']).to eq(user.username)
+      expect(merge_request.notes.size).to eq(original_count + 1)
     end
     it "should return 400 if note is missing" do