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.

app/models/merge_request.rb

@@ -1127,6 +1127,19 @@ class MergeRequest < ApplicationRecord
     "refs/#{Repository::REF_MERGE_REQUEST}/#{iid}/merge"
   end
+  def train_ref_path
+    "refs/#{Repository::REF_MERGE_REQUEST}/#{iid}/train"
+  end
+
+  def cleanup_refs(only: :all)
+    target_refs = []
+    target_refs << ref_path       if %i[all head].include?(only)
+    target_refs << merge_ref_path if %i[all merge].include?(only)
+    target_refs << train_ref_path if %i[all train].include?(only)
+
+    project.repository.delete_refs(*target_refs)
+  end
+
   def self.merge_request_ref?(ref)
     ref.start_with?("refs/#{Repository::REF_MERGE_REQUEST}/")
   end

app/models/repository.rb

@@ -845,6 +845,10 @@ class Repository
     raw.merge_to_ref(user, source_sha, branch, target_ref, message, first_parent_ref)
   end
+  def delete_refs(*ref_names)
+    raw.delete_refs(*ref_names)
+  end
+
   def ff_merge(user, source, target_branch, merge_request: nil)
     their_commit_id = commit(source)&.id
     raise 'Invalid merge source' if their_commit_id.nil?

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

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 } }