Feature flag for merge requestion push options

https://gitlab.com/gitlab-org/gitlab-ce/issues/43263
https://gitlab.com/gitlab-org/gitlab-ce/issues/53198

lib/api/internal.rb

@@ -264,8 +264,10 @@ module API
         PostReceive.perform_async(params[:gl_repository], params[:identifier],
           params[:changes], push_options.as_json)
 
-        mr_options = push_options.get(:merge_request)
-        output.merge!(process_mr_push_options(mr_options, project, user, params[:changes])) if mr_options.present?
+        if Feature.enabled?(:mr_push_options, default_enabled: true)
+          mr_options = push_options.get(:merge_request)
+          output.merge!(process_mr_push_options(mr_options, project, user, params[:changes])) if mr_options.present?
+        end
 
         broadcast_message = BroadcastMessage.current&.last&.message
         reference_counter_decreased = Gitlab::ReferenceCounter.new(params[:gl_repository]).decrease

spec/requests/api/internal_spec.rb

@@ -995,6 +995,20 @@ describe API::Internal do
 
         expect(json_response['warnings']).to eq('Error encountered with push options \'merge_request.create\': my error')
       end
+
+      context 'when the feature is disabled' do
+        it 'does not invoke MergeRequests::PushOptionsHandlerService' do
+          Feature.disable(:mr_push_options)
+
+          expect(MergeRequests::PushOptionsHandlerService).to receive(:new)
+
+          expect do
+            post api('/internal/post_receive'), params: valid_params
+          end.not_to change { MergeRequest.count }
+
+          Feature.enable(:mr_push_options)
+        end
+      end
     end
 
     context 'broadcast message exists' do