cpython/Misc/NEWS.d/next/Library/2018-10-13-19-15-23.bpo-34521.YPaiTK.rst

Use :func:`socket.CMSG_SPACE` to calculate ancillary data size instead of
:func:`socket.CMSG_LEN` in :func:`multiprocessing.reduction.recvfds` as
:rfc:`3542` requires the use of the former for portable applications.