_decimal failed to build with system libmpdec 2.5

[felixonmars]

BPO	41302
Nosy	@doko42, @skrah, @felixonmars, @miss-islington
PRs	bpo-41302: Fix build with system libmpdec #21481 [3.9] bpo-41302: Fix build with system libmpdec (GH-21481) #21486 [3.8] bpo-41302: Support system libmpdec 2.5 for Python 3.8 #21488

^{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/skrah'
closed_at = <Date 2020-07-15.20:30:40.352>
created_at = <Date 2020-07-15.10:54:56.624>
labels = ['extension-modules', 'build', '3.8', '3.9', '3.10']
title = '_decimal failed to build with system libmpdec 2.5'
updated_at = <Date 2020-07-15.20:30:40.351>
user = 'https://github.com/felixonmars'

bugs.python.org fields:

activity = <Date 2020-07-15.20:30:40.351>
actor = 'skrah'
assignee = 'skrah'
closed = True
closed_date = <Date 2020-07-15.20:30:40.352>
closer = 'skrah'
components = ['Extension Modules']
creation = <Date 2020-07-15.10:54:56.624>
creator = 'felixonmars'
dependencies = []
files = []
hgrepos = []
issue_num = 41302
keywords = ['patch']
message_count = 9.0
messages = ['373676', '373677', '373679', '373682', '373691', '373692', '373721', '373722', '373723']
nosy_count = 4.0
nosy_names = ['doko', 'skrah', 'felixonmars', 'miss-islington']
pr_nums = ['21481', '21486', '21488']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'compile error'
url = 'https://bugs.python.org/issue41302'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[felixonmars]

In bpo-40874, mpdecimal.h in the vendored libmpdec has defines of UNUSED while the standalone released version of mpdecimal 2.5.0 doesn't. This breaks _decimal module build with system libmpdec due to UNUSED is undefined.

Errors are like:

cpython/Modules/_decimal/_decimal.c:277:36: error: expected ‘;’, ‘,’ or ‘)’ before ‘UNUSED’
277 | dec_traphandler(mpd_context_t *ctx UNUSED) /* GCOV_NOT_REACHED */
| ^~~~~~

Reproducible in both 3.8 branch and master (didn't test 3.9, but should be affected too).

[skrah]

Thank you for the report, I'll add the define or remove UNUSED in 3.9
and 3.10.

3.8 is still supposed to use libmpdec-2.4.2, though it would be
harmless to use libmpdec-2.5.0.

Are you planning to use libmpdec-2.5.0 with 3.8?

[felixonmars]

Yes, I am currently defining it manually as a workaround for building 3.8.4 in Arch.

Also opened #65680 for this :)

[skrah]

New changeset 015efdb by Felix Yan in branch 'master':
bpo-41302: Fix build with system libmpdec (GH-21481)
015efdb

[miss-islington]

New changeset 8ca63f9 by Miss Islington (bot) in branch '3.9':
bpo-41302: Fix build with system libmpdec (GH-21481)
8ca63f9

[skrah]

Thanks for the patch!

The integrated libmpdec-2.4.2 in Python 3.8 still has a couple of
UNUSED, so the simple approach of moving the define would not work.
It would be easy to replace the few instances of UNUSED with void
casts like in 2.5.0.

Strictly speaking though, this would be a feature for 3.8 and not
a build fix.

On the other hand, if Debian also plans to use 2.5.0 with 3.8,
there would be a stronger case for this build feature.

[skrah]

I'm going to reclassify this as a build fix for 3.8. 3.8 promises:

#if !defined(MPD_VERSION_HEX) || MPD_VERSION_HEX < 0x02040100
  #error "libmpdec version >= 2.4.1 required"
#endif

So it seems reasonable to support at least two or three consecutive
system libmpdec versions, otherwise the >= is pointless.

[skrah]

New changeset 16eea45 by Felix Yan in branch '3.8':
[3.8] bpo-41302: Support system libmpdec 2.5 for Python 3.8 (GH-21488)
16eea45

[skrah]

Closing, thanks for all the patches!