presto-main/src/main/java/io/prestosql/operator/scalar/SessionFunctions.java/SessionFunctions/currentUser
class SessionFunctions:    @ScalarFunction(value = "$current_user", hidden = true)
    @Description("current user")
    @SqlType(StandardTypes.VARCHAR)
    public static Slice currentUser(ConnectorSession session)
    {
        return utf8Slice(session.getUser());
    }