Cancel running pipelines when merge request is dropped from merge train

This commit improves the canceletion mechanizm of merge train pipelines

changelogs/unreleased/cancel-merge-train-pipeline-when-dropped.yml

+---
+title: Cancel running pipelines when merge request is dropped from merge train
+merge_request: 22146
+author:
+type: fixed

ee/app/models/merge_train.rb

@@ -12,7 +12,10 @@ class MergeTrain < ApplicationRecord
   after_commit :refresh_async, if: -> { saved_change_to_status? && stale? }
 
   after_destroy do |merge_train|
-    run_after_commit { merge_train.cleanup_ref }
+    run_after_commit do
+      merge_train.pipeline&.cancel_running(retries: 1)
+      merge_train.cleanup_ref
+    end
   end
 
   enum status: %i[created merged stale fresh]

ee/spec/models/merge_train_spec.rb

@@ -543,6 +543,20 @@ describe MergeTrain do
     end
   end
 
+  describe '#destroy' do
+    subject { merge_train.destroy }
+
+    context 'when merge train has a pipeline' do
+      let(:merge_train) { create(:merge_train, pipeline: pipeline) }
+      let(:pipeline) { create(:ci_pipeline, :running) }
+      let(:build) { create(:ci_build, :running, pipeline: pipeline) }
+
+      it 'cancels the jobs in the pipeline' do
+        expect { subject }.to change { build.reload.status }.from('running').to('canceled')
+      end
+    end
+  end
+
   describe '#cleanup_ref' do
     subject { merge_train.cleanup_ref }
 

ee/spec/services/auto_merge/merge_train_service_spec.rb

@@ -125,6 +125,19 @@ describe AutoMerge::MergeTrainService do
       subject
     end
 
+    context 'when pipeline exists' do
+      before do
+        merge_request.merge_train.update!(pipeline: pipeline)
+      end
+
+      let(:pipeline) { create(:ci_pipeline) }
+      let(:build) { create(:ci_build, :running, pipeline: pipeline) }
+
+      it 'cancels the jobs in the pipeline' do
+        expect { subject }.to change { build.reload.status }.from('running').to('canceled')
+      end
+    end
+
     context 'when train ref exists' do
       before do
         merge_request.project.repository.create_ref(merge_request.target_branch, merge_request.train_ref_path)