If Secondary NN crashes right after calling dfsadmin -rollingUpgrade prepare, it fails to checkpoint because it fails to download the newest fsimage whose name is fsimage_rollback_txid.
To reproduce this bug:
- Run NN and SNN.
- Run dfsadmin -rollingUpgrade prepare.
- Shutdown SNN, clear all fsimage created by SNN.
- Restart SNN, SNN tries to download the fsimage_txid instead of fsimage_rollback_txid, resulting the failure.