Fix race condition on merge train ref generation

Today, Pipelines for merge train run on `refs/merge`,
however, this causes a race condition that it can be
overwritten by CheckMergeabilityService.

This patch fixes the problem by generating `refs/train`
for those pipelines.

2 files changed, 39 insertions, 0 deletions

diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index fe6d68aff3f..9b0c232f370 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -3220,4 +3220,34 @@ describe MergeRequest do
       it { is_expected.to be_truthy }
     end
   end
+
+  describe '#cleanup_refs' do
+    subject { merge_request.cleanup_refs(only: only) }
+
+    let(:merge_request) { build(:merge_request) }
+
+    context 'when removing all refs' do
+      let(:only) { :all }
+
+      it 'deletes all refs from the target project' do
+        expect(merge_request.target_project.repository)
+          .to receive(:delete_refs)
+          .with(merge_request.ref_path, merge_request.merge_ref_path, merge_request.train_ref_path)
+
+        subject
+      end
+    end
+
+    context 'when removing only train ref' do
+      let(:only) { :train }
+
+      it 'deletes train ref from the target project' do
+        expect(merge_request.target_project.repository)
+          .to receive(:delete_refs)
+          .with(merge_request.train_ref_path)
+
+        subject
+      end
+    end
+  end
 end
diff --git a/spec/services/merge_requests/merge_to_ref_service_spec.rb b/spec/services/merge_requests/merge_to_ref_service_spec.rb
index e2f201677fa..14012b4ea2d 100644
--- a/spec/services/merge_requests/merge_to_ref_service_spec.rb
+++ b/spec/services/merge_requests/merge_to_ref_service_spec.rb
@@ -191,6 +191,15 @@ describe MergeRequests::MergeToRefService do
       it { expect(todo).not_to be_done }
     end
 
+    context 'when target ref is passed as a parameter' do
+      let(:params) { { commit_message: 'merge train', target_ref: target_ref } }
+
+      it_behaves_like 'successfully merges to ref with merge method' do
+        let(:first_parent_ref) { 'refs/heads/master' }
+        let(:target_ref) { 'refs/merge-requests/1/train' }
+      end
+    end
+
     describe 'cascading merge refs' do
       set(:project) { create(:project, :repository) }
       let(:params) { { commit_message: 'Cascading merge', first_parent_ref: first_parent_ref, target_ref: target_ref } }