kudu::rpc::Proxy ctor may fail in GetLoggedInUser() for various reason:
This resulted in an empty user name being used in kudu::rpc::ConnectionId. With plaintext SASL (e.g. in an insecure Impala cluster), this may result in the following error:
While one can argue that Kudu should fall back to some default username (e.g. "cpp-client") when GetLoggedInUserName() fails, it may have non-trivial consequence (e.g. generating an authn token with some random username on one machine while using the real user name on another machine). Therefore, it's best for Impala to explicitly set the user credentials (impala/<some-hostname>) after creating the proxy.