Now that sendmsg()/recvmsg() are exposed in socketmodule, we could use them to replace the ad-hoc FD-passing routines used by multiprocessing.reduction.
Antoine suggested adding sendfd()/recvfd() methods to socket objects, but I'm not sure about this, since those only make sense for Unix domain sockets.
Two remarks on the patch attached:
- this removes sendfd()/recvfd() from _multiprocessing (but AFAICT those were never documented as part of the public API)
- EOF/invalid data received result in a RuntimeError