Expose RFC 3542 IPv6 socket options on macOS

[chrysn]

BPO	35569
Nosy	@ronaldoussoren, @ned-deily, @chrysn, @erlend-aasland
PRs	bpo-35569: Expose RFC 3542 IPv6 socket options on macOS #19526 bpo-35569: add Erlend to Misc/ACKS #20146
Files	0001-bpo-35569-Expose-RFC-3542-socket-options-on-macOS.patch 0002-bpo-35569-Add-unit-tests.patch

^{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 = <Date 2020-05-17.06:37:41.536>
created_at = <Date 2018-12-23.12:00:47.132>
labels = ['OS-mac', 'type-feature', '3.9', 'expert-IO']
title = 'Expose RFC 3542 IPv6 socket options on macOS'
updated_at = <Date 2020-05-17.06:57:31.851>
user = 'https://github.com/chrysn'

bugs.python.org fields:

activity = <Date 2020-05-17.06:57:31.851>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-17.06:37:41.536>
closer = 'ned.deily'
components = ['macOS', 'IO']
creation = <Date 2018-12-23.12:00:47.132>
creator = 'chrysn'
dependencies = []
files = ['48937', '49059']
hgrepos = []
issue_num = 35569
keywords = ['patch']
message_count = 9.0
messages = ['332389', '332391', '363032', '363830', '363831', '366323', '369095', '369096', '369098']
nosy_count = 4.0
nosy_names = ['ronaldoussoren', 'ned.deily', 'chrysn', 'erlendaasland']
pr_nums = ['19526', '20146']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue35569'
versions = ['Python 3.9']

[chrysn]

Python builds on MacOS do not expose the IPV6_RECVPKTINFO flag specified in [RFC3842], which is required for UDP protocols that need control over their servers' sending ports like CoAP.

While I don't own Apple hardware and thus can't test it, the nginx code indicates that this API is available on OSX and is just gated behind -D__APPLE_USE_RFC_3542. Searching the web for that define indicates that other interpreted langues and applications use the flag as well (PHP, Ruby; PowerDNS, nmap, libcoap).

Please consider enabling this on future releases of Python on OSX.

[ronaldoussoren]

#include <netinet6/in6.h> in the macOS 10.14 SDK says:

/*
 * RFC 3542 define the following socket options in a manner incompatible
 * with RFC 2292:
 *   IPV6_PKTINFO
 *   IPV6_HOPLIMIT
 *   IPV6_NEXTHOP
 *   IPV6_HOPOPTS
 *   IPV6_DSTOPTS
 *   IPV6_RTHDR
 *
 * To use the new IPv6 Sockets options introduced by RFC 3542
 * the constant __APPLE_USE_RFC_3542 must be defined before
 * including <netinet/in.h>
 *
 * To use the old IPv6 Sockets options from RFC 2292
 * the constant __APPLE_USE_RFC_2292 must be defined before
 * including <netinet/in.h>
 *
 * Note that eventually RFC 3542 is going to be the
 * default and RFC 2292 will be obsolete.
 */

My conclusion from reading this: the name might suggest that this is an internal macro, but should be safe to use.

[erlend-aasland]

Proposed patch attached.

[chrysn]

Testing the application to current git master (on a borrowed machine with Darwin Kernel Version 18.5.0), the provided patch does enable the desired IPV6_RECVPKTINFO flag, and the received pktinfo struct complies to the spec.

[erlend-aasland]

Thanks for testing @chrysn. I guess I should add unit tests if this is to be applied to master.

[erlend-aasland]

Added (Mac OS specific) unit tests.

[ned-deily]

New changeset 9a45bfe by Erlend Egeberg Aasland in branch 'master':
bpo-35569: Expose RFC 3542 IPv6 socket options on macOS (GH-19526)
9a45bfe

[ned-deily]

Thanks for the PR! Merged for 3.9.0b1