During January 2026, Zequn Li focused on improving the reliability of unit tests in the apache/spark repository, specifically targeting the CoarseGrainedExecutorBackend component. By introducing mock implementations for killMarks, Zequn addressed gaps in task-kill tracking within the test suite, reducing flaky failures and guarding against unexpected exceptions. This Scala-based work leveraged advanced mocking and unit testing techniques to stabilize the CoarseGrainedExecutorBackendSuite, directly supporting safer iteration on core execution logic. The patch, delivered as a test-only improvement, enhanced CI reliability and reduced deployment risk, demonstrating a thoughtful approach to test harness design and maintenance within a complex distributed system.