If master's processing its failover and ServerShutdownHandler's processing happen concurrently, it may appear following case.
1.master completed splitLogAfterStartup()
2.RegionserverA restarts, and ServerShutdownHandler is processing.
3.master starts to rebuildUserRegions, and RegionserverA is considered as dead server.
4.master starts to assign regions of RegionserverA because it is a dead server by step3.
However, when doing step4(assigning region), ServerShutdownHandler may be doing split log, Therefore, it may cause data loss.