sub sysread(...) {
   if ($self->{decoded_remaining}) { return ... }
   else if (sysread($self->{underlying_socket}, $self->{buffer}, ...) > 0) {
      $self->{next_frame_size} //= decode_frame_size($self->{buffer});
      if (length $self->{buffer} >= $self->{next_frame_size}) {
        ... # decode it
        return ...
      }
      $SSL_ERROR= ...;
      return undef;
   }
}