Fix unexpected exception by failed to find an actual head pipeline

Add changelog

app/models/merge_request.rb

@@ -1108,10 +1108,11 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def update_head_pipeline
-    self.head_pipeline = find_actual_head_pipeline
-
+    find_actual_head_pipeline.try do |pipeline|
+      self.head_pipeline = pipeline
       update_column(:head_pipeline_id, head_pipeline.id) if head_pipeline_id_changed?
     end
+  end
 
   def merge_request_pipeline_exists?
     merge_request_pipelines.exists?(sha: diff_head_sha)

changelogs/unreleased/fix-udpate-head-pipeline-method.yml

+---
+title: Fix unexpected exception by failure of finding an actual head pipeline
+merge_request: 24257
+author:
+type: fixed

spec/models/merge_request_spec.rb

@@ -1418,6 +1418,23 @@ describe MergeRequest do
           .to change { merge_request.reload.head_pipeline }
           .from(nil).to(pipeline)
       end
+
+      context 'when merge request has already had head pipeline' do
+        before do
+          merge_request.update!(head_pipeline: pipeline)
+        end
+
+        context 'when failed to find an actual head pipeline' do
+          before do
+            allow(merge_request).to receive(:find_actual_head_pipeline) { }
+          end
+
+          it 'does not update the current head pipeline' do
+            expect { subject }
+              .not_to change { merge_request.reload.head_pipeline }
+          end
+        end
+      end
     end
 
     context 'when there are no pipelines with the diff head sha' do