[Python-ideas] Add socket utilities for IPv4/6 dual-stack servers

[Giampaolo Rodola']

Some years ago I started working on a patch for the socket module which
added a couple of utility functions for being able to easily create a
server socket, with the addition of being able to accept both IPv4 and IPv6
connections (as a single socket):
https://bugs.python.org/issue17561

Given that not all OSes (Windows, many UNIXes) support this natively, I
later submitted a recipe adding a "multiple socket listener" class.
https://code.activestate.com/recipes/578504-server-supporting-ipv4-and-ipv6/

>From the user perspective, the whole thing can be summarized as follows:

>>> sock = create_server_sock(("", 8000))
>>> if not has_dual_stack(sock):
...     sock.close()
...     sock = MultipleSocketsListener([("0.0.0.0", 8000), ("::", 8000)])
>>>

Part of this work ended up being included into Tulip internals:
https://github.com/python/cpython/blob/70d28a184c42d107cc8c69a95aa52a4469e7929c/Lib/asyncio/base_events.py#L966-L1067
...and after that I basically forgot about the original patch. The other
day I bumped into a case where I needed exactly this (on Windows), so here
I am, trying to revamp the original proposal.

To be clear, the proposal is to add 3 new APIs in order to avoid the
low-level cruft needed when creating a server socket
(SO_REUSEADDR, getaddrinfo() etc.)
and being able to support IPv4/6 dual-stack socket servers in a
cross-platform fashion:

- socket.has_dual_stack()
- socket.create_server_sock()
- socket.MultipleSocketsListener

Whereas the first two functions are relatively straightforward,
MultipleSocketsListener is more debatable because, for instance, it's not
clear what methods like getsockname() should return (because there are 2
sockets involved). One possible solution is to *not* expose such (all
get*?) methods and simply expose the underlying socket objects as in:

>>> socket.MultipleSocketsListener(...).socks[0].getsockname()

On the other hand, all set* / write methods (setblocking(), setsockopt(),
shutdown(), ...) can be exposed and internally they can simply operate
against both sockets.

Thoughts?

-- 
Giampaolo - http://grodola.blogspot.com
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