HADOOP-6965 introduced a getTGT() method and prevents reloginFromKeytab() from happening when TGT is not found. This results in the RPC layer not being able to refresh TGT after TGT expires. The reason is RPC layer only does relogin when the expired TGT is used and an exception is thrown. However, when that happens, the expired TGT will be removed from Subject. Therefore, getTGT() will return null and relogin will not be performed. We observed, for example, JT will not be able to re-connect to NN after TGT expires.