default UserGroupInformation getRealUser(User ugi) {
    return ugi.getUGI();
  }
Returns the "real" user, the user who has the credentials being authenticated by the remote
service, in the form of an {@link UserGroupInformation} object. It is common in the Hadoop
"world" to have distinct notions of a "real" user and a "proxy" user. A "real" user is the user
which actually has the credentials (often, a Kerberos ticket), but some code may be running as
some other user who has no credentials. This method gives the authentication provider a chance
to acknowledge this is happening and ensure that any RPCs are executed with the real user's
credentials, because executing them as the proxy user would result in failure because no
credentials exist to authenticate the RPC. Not all implementations will need to implement this
method. By default, the provided User's UGI is returned directly.
hbase-client/src/main/java/org/apache/hadoop/hbase/security/provider/SaslClientAuthenticationProvider.java