Add UDP support to `socket.create_server()` for easy dual-stack (IPv4/IPv6) servers

[jaraco]

BPO	25667
Nosy	@gpshead, @jaraco, @pitrou, @bitdancer, @jleedev
PRs	bpo-38907: In http.server script, restore binding to IPv4 on Windows. #17851
Superseder	bpo-17561: Add socket.bind_socket() convenience function

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2015-11-19.13:32:53.803>
labels = ['3.7', 'type-feature', 'library']
title = 'Supply dual-stack (IPv4/IPv6) socket bind routine'
updated_at = <Date 2020-01-06.02:11:41.244>
user = 'https://github.com/jaraco'

bugs.python.org fields:

activity = <Date 2020-01-06.02:11:41.244>
actor = 'jaraco'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2015-11-19.13:32:53.803>
creator = 'jaraco'
dependencies = []
files = []
hgrepos = []
issue_num = 25667
keywords = ['patch']
message_count = 5.0
messages = ['254895', '254900', '261317', '286354', '331942']
nosy_count = 5.0
nosy_names = ['gregory.p.smith', 'jaraco', 'pitrou', 'r.david.murray', 'jleedev']
pr_nums = ['17851']
priority = 'normal'
resolution = 'duplicate'
stage = 'patch review'
status = 'open'
superseder = '17561'
type = 'enhancement'
url = 'https://bugs.python.org/issue25667'
versions = ['Python 3.7']

[jaraco]

When binding to a socket for creating a service, the socket API provides a mechanism for selecting only IPv4 or IPv6, but the simple operation of binding to all interfaces in a dual-stack environment is not easy, as is revealed in bpo-20215 and bpo-24209, but also in third-party packages. I ran into this during the implementation of aiosmtpd (https://gitlab.com/python-smtpd-hackers/aiosmtpd). As a long time maintainer of CherryPy, I've enjoyed the dual stack support there.

I propose the Python stdlib provide a routine to facilitate this need more generally, and then invoke that implementation to address these other use-cases.

As it is a new feature, a backports package should be created to supply the same functionality for older Pythons.

Would this effort require a PEP? Is there a good place for such a routine (maybe socketserver)? Any other thoughts?

[bitdancer]

I don't think it should require a PEP, it looks like a fairly straightforward small enhancement. What is the reason to not put it in the socket module?

[jaraco]

No reason not to put it in the socket module. I was thinking originally that the socket module was C only, but on further reflection, it could easily be one of the "additional facilities implemented in Python". Great suggestion.

[pitrou]

Related though with a different strategy: Tornado's bind_sockets() routine:
https://github.com/tornadoweb/tornado/blob/master/tornado/netutil.py#L114-L138

[jaraco]

I believe this issue is a duplicate of 17561 (#61761), which I stumbled onto today.

[lgommans]

As jaraco mentioned, I think this can be closed in favor of #61761 unless we want another solution for UDP specifically (because the create_server function in which this was implemented creates a TCP socket only).

[gpshead]

the other issue added the likely API, I've retitled this one to be about having that support more than just TCP.