Cancel auto merge when merge request is closed

We should cancel auto merge when merge request is closed.

app/services/merge_requests/close_service.rb

@@ -18,6 +18,7 @@ module MergeRequests
         invalidate_cache_counts(merge_request, users: merge_request.assignees)
         merge_request.update_project_counter_caches
         cleanup_environments(merge_request)
+        cancel_auto_merge(merge_request)
       end
 
       merge_request
@@ -33,5 +34,9 @@ module MergeRequests
         merge_request_metrics_service(merge_request).close(close_event)
       end
     end
+
+    def cancel_auto_merge(merge_request)
+      AutoMergeService.new(project, current_user).cancel(merge_request)
+    end
   end
 end

changelogs/unreleased/cancel-auto-merge-when-merge-request-is-closed.yml

+---
+title: Cancel auto merge when merge request is closed
+merge_request: 28782
+author:
+type: fixed

spec/services/merge_requests/close_service_spec.rb

@@ -52,6 +52,14 @@ describe MergeRequests::CloseService do
       it 'marks todos as done' do
         expect(todo.reload).to be_done
       end
+
+      context 'when auto merge is enabled' do
+        let(:merge_request) { create(:merge_request, :merge_when_pipeline_succeeds) }
+
+        it 'cancels the auto merge' do
+          expect(@merge_request).not_to be_auto_merge_enabled
+        end
+      end
     end
 
     it 'updates metrics' do