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.

1 files changed, 13 insertions, 0 deletions

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index e96e26cc773..53977748c30 100644
--- a/app/models/merge_request.rb
+++ b/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