Support for `recv_into`

[sk1p]

My use case was sending numpy arrays via nng, where it really makes sense to reduce the number of allocations and copies that need to be performed.

Reading the code, I saw that there is already the general idea to support a recv_into operation on sockets. I built a prototype that looks like this, based on the normal read method:

def recv_into(s, buffer):
    """Receive data on the socket s into buffer
    If the request times out the exception
    :class:`pynng.Timeout` is raised.  If the socket cannot perform that
    operation (e.g., a :class:`Pub0`, which can only
    :meth:`~Socket.send`), the exception :class:`pynng.NotSupported`
    is raised.
    """
    from pynng import ffi, lib
    from pynng.nng import check_err
    # we DON'T set lib.NNG_FLAG_ALLOC here, because we want to give
    # our own recv buffer
    flags = 0
    size_t = ffi.new('size_t *')
    size_t[0] = len(buffer)
    ret = lib.nng_recv(s.socket, buffer, size_t, flags)
    check_err(ret)
    assert size_t[0] == len(buffer)

Used like this:

recv_mem = np.zeros(arr_shape, dtype=np.float32)
recv_buf = ffi.from_buffer(
    recv_mem,
    require_writable=True,
)
recv_into(your_socket, recv_buf)
# the received data is now available in `recv_mem`

If there is interest, I could convert this into a PR including test cases etc. and add proper support for returning the part of the buffer that was written to etc. (this prototype was only used for evaluation purposes / benchmarking). Let me know if anyone is interested!

[aqc-carlodri]

Thank you, yes I would definitely be interested in this!