expat: Restore the use of pyexpatns.h to avoid link time conflicts vs other versions

[gpshead]

BPO	35011
Nosy	@gpshead, @benjaminp, @ned-deily, @miss-islington
PRs	bpo-35011: Restore use of pyexpatns.h in libexpat #9939 [3.7] bpo-35011: Restore use of pyexpatns.h in libexpat (GH-9939) #9940 [3.6] bpo-35011: Restore use of pyexpatns.h in libexpat (GH-9939) #9941

^{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 = 'https://github.com/gpshead'
closed_at = <Date 2018-10-18.02:07:24.193>
created_at = <Date 2018-10-18.00:26:15.374>
labels = ['extension-modules', '3.8', 'deferred-blocker', 'build', '3.7']
title = 'expat: Restore the use of pyexpatns.h to avoid link time conflicts vs other versions'
updated_at = <Date 2018-10-18.04:54:49.613>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2018-10-18.04:54:49.613>
actor = 'benjamin.peterson'
assignee = 'gregory.p.smith'
closed = True
closed_date = <Date 2018-10-18.02:07:24.193>
closer = 'gregory.p.smith'
components = ['Build', 'Extension Modules']
creation = <Date 2018-10-18.00:26:15.374>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 35011
keywords = ['patch']
message_count = 7.0
messages = ['327919', '327920', '327921', '327924', '327927', '327928', '327934']
nosy_count = 4.0
nosy_names = ['gregory.p.smith', 'benjamin.peterson', 'ned.deily', 'miss-islington']
pr_nums = ['9939', '9940', '9941']
priority = 'deferred blocker'
resolution = 'fixed'
stage = 'commit review'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue35011'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[gpshead]

These lines used to exist in Modules/expat/expat_external.h:

/* Namespace external symbols to allow multiple libexpat version to
   co-exist. */
#include "pyexpatns.h"

5dc3f23#diff-3afaf7274c90ce1b7405f75ad825f545

removed them during an expat upgrade.

This causes link time conflicts when embedding Python using its own expat in an application that also uses libexpat from the C/C++ side on its own.

[gpshead]

Not a release blocker as most users probably do not run into this problem, but the pyexpatns.h mechanics should be restored.

[gpshead]

This appears to not have been shipped in a release yet. It is new in 3.6.7. cc'ing ned daily to see if he wants to include the fix (the PR is trivial, coming ASAP).

I don't have a good feel for how this impacts the real world or not.

We noticed because we embed CPython in larger applications that use a different copy of libexpat on their own.

[gpshead]

New changeset 9d4712b by Gregory P. Smith in branch 'master':
bpo-35011: Restore use of pyexpatns.h in libexpat (GH-9939)
9d4712b

[miss-islington]

New changeset 4bfecb9 by Miss Islington (bot) in branch '3.6':
[3.6] bpo-35011: Restore use of pyexpatns.h in libexpat (GH-9939) (GH-9941)
4bfecb9

[miss-islington]

New changeset 35ae99d by Miss Islington (bot) in branch '3.7':
[3.7] bpo-35011: Restore use of pyexpatns.h in libexpat (GH-9939) (GH-9940)
35ae99d

[benjaminp]

Sorry for breaking that, and thanks for the fix!

I'm curious, though, why are you still using the embedded expat rather than linking everything against the same expat?