In Impala, we have FLAGS_principal and FLAGS_be_principal flags.
If only FLAGS_principal is set, we use it as both the internal and external principals.
If both FLAGS_principal and FLAGS_be_principal are set, we use FLAGS_be_principal as the internal principal and FLAGS_principal as the external principal.
However, in Kudu, they only source the internal principal from FLAGS_principal and aren't aware of a flag called FLAGS_be_principal.
So as of the time
IMPALA-5129 went in, if FLAGS_be_principal is explicitly set to something different from FLAGS_principal, we would be using the external principal as the internal principal, which is incorrect.