Function copy_bidirectional_with_sizes 

pub async fn copy_bidirectional_with_sizes<A, B>(
    a: &mut A,
    b: &mut B,
    a_to_b: usize,
    b_to_a: usize,
) -> Result<(u64, u64)>
where
    A: AsyncRead + AsyncWrite + Unpin + ?Sized,
    B: AsyncRead + AsyncWrite + Unpin + ?Sized,

copy_bidirectional with a buffer of a_to_b bytes for one direction and of b_to_a bytes for the other.