Function read_packet 

pub async fn read_packet<R: AsyncRead + Unpin>(
    read: &mut R,
    reader: &mut PacketReader<GrowBuffer>,
) -> Result<(FixedHeader, usize), Error>

Reads from read into reader until one whole packet is framed, returning its fixed header and total on-wire length. The packet stays buffered in reader (decode it with PacketReader::decode, then consume it); any bytes read past it remain buffered too, so the same reader can be handed to the steady-state read loop without losing data.