The streaming_hello_freeflow loop reads messages from the upstream by calling AsyncReadExt::read. If the response bytes have only been partially written, then we might send an invalid message to the downstream. This is unlikely to be an issue in practice because hello responses are small, but the code should use Message::parse_bytes for correctness. Note that Message::parse_bytes is not cancellation-safe, so it cannot be called in the select! loop directly.