1 files changed, 50 insertions, 0 deletions
diff --git a/app/services/merge_requests/reload_merge_head_diff_service.rb b/app/services/merge_requests/reload_merge_head_diff_service.rb
new file mode 100644
index 00000000000..f02a9bd3139
--- /dev/null
+++ b/app/services/merge_requests/reload_merge_head_diff_service.rb
@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+module MergeRequests
+  class ReloadMergeHeadDiffService
+    include BaseServiceUtility
+
+    def initialize(merge_request)
+      @merge_request = merge_request
+    end
+
+    def execute
+      return error("default_merge_ref_for_diffs feature flag is disabled") unless enabled?
+      return error("Merge request has no merge ref head.") unless merge_request.merge_ref_head.present?
+
+      error_msg = recreate_merge_head_diff
+
+      return error(error_msg) if error_msg
+
+      success
+    end
+
+    private
+
+    attr_reader :merge_request
+
+    def enabled?
+      Feature.enabled?(:default_merge_ref_for_diffs, merge_request.project, default_enabled: :yaml)
+    end
+
+    def recreate_merge_head_diff
+      merge_request.merge_head_diff&.destroy!
+
+      # n+1: https://gitlab.com/gitlab-org/gitlab/-/issues/19377
+      Gitlab::GitalyClient.allow_n_plus_1_calls do
+        merge_request.create_merge_head_diff!
+      end
+
+      # Reset the merge request so it won't load the merge head diff as the
+      # MergeRequest#merge_request_diff.
+      merge_request.reset
+
+      nil
+    rescue StandardError => e
+      message = "Failed to recreate merge head diff: #{e.message}"
+
+      Gitlab::AppLogger.error(message: message, merge_request_id: merge_request.id)
+      message
+    end
+  end
+end