socket.TCP_* no longer available with cygwin 3.1.6+

[lazka]

BPO	41374
Nosy	@embray, @lazka, @ZackerySpytz, @iritkatriel, @matzeri
PRs	bpo-41374: Include netinet/tcp.h on Cygwin #21649
Files	3.6.12-socketmodule.patch: Patch used on Cygwin packages

^{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 2021-10-20.17:15:19.285>
created_at = <Date 2020-07-23.07:51:44.046>
labels = ['build', '3.11']
title = 'socket.TCP_* no longer available with cygwin 3.1.6+'
updated_at = <Date 2021-10-20.17:15:24.662>
user = 'https://github.com/lazka'

bugs.python.org fields:

activity = <Date 2021-10-20.17:15:24.662>
actor = 'iritkatriel'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-20.17:15:19.285>
closer = 'iritkatriel'
components = ['Build']
creation = <Date 2020-07-23.07:51:44.046>
creator = 'lazka'
dependencies = []
files = ['49717']
hgrepos = []
issue_num = 41374
keywords = ['patch']
message_count = 4.0
messages = ['374126', '384245', '386918', '404501']
nosy_count = 5.0
nosy_names = ['erik.bray', 'lazka', 'ZackerySpytz', 'iritkatriel', 'matzeri']
pr_nums = ['21649']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue41374'
versions = ['Python 3.11']

[lazka]

The TCP macros are provided by netinet/tcp.h, which for some reason is skipped here:

cpython/Modules/socketmodule.h

Line 11 in 592527f

# if !defined(__CYGWIN__)

Until cygwin 3.1.6 these macros were also provided by sys/socket.h, but this got removed in https://cygwin.com/git/?p=newlib-cygwin.git;a=commit;h=e037192b505b4f233fca9a6deafc9797210f6693

This leads to socket.TCP_NODELAY for example not being available anymore.

git blame leads me to b5daaed introducing this special case, which isn't very helpful. I'd suggest to just remove the cygwin check and always include it (which works fine on my machine)

Downstream bug report for extra context: msys2/MSYS2-packages#2050

[matzeri]

The Analysis is correct.
Removing the test for CYGWIN and always include the
<netinet/tcp.h>

solved the problem building all python (3.6,3.7,3.8) packages

https://sourceware.org/pipermail/cygwin-apps/2020-December/040845.html
https://sourceware.org/pipermail/cygwin-announce/2020-December/009853.html

attached patch used on the build.
Similar one was applied to the rebuild of 2.7.18

[lazka]

ping. The PR looks good to me.

[iritkatriel]

New changeset d8e1819 by Zackery Spytz in branch 'main':
bpo-41374: Remove obsolete exclusion of netinet/tcp.h on Cygwin (GH-21649)
d8e1819