cxx pthread check is not overrideable

[virtuald]

BPO	41916
Nosy	@moreati, @virtuald
PRs	bpo-41916: allow cross-compiled python to have -pthread set for CXX #22525

^{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 2020-10-03.07:20:28.934>
labels = ['build', '3.8']
title = 'cxx pthread check is not overrideable'
updated_at = <Date 2020-10-25.19:38:59.827>
user = 'https://github.com/virtuald'

bugs.python.org fields:

activity = <Date 2020-10-25.19:38:59.827>
actor = 'virtuald'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Cross-Build']
creation = <Date 2020-10-03.07:20:28.934>
creator = 'virtuald'
dependencies = []
files = []
hgrepos = []
issue_num = 41916
keywords = ['patch']
message_count = 2.0
messages = ['377858', '379602']
nosy_count = 2.0
nosy_names = ['Alex.Willmer', 'virtuald']
pr_nums = ['22525']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue41916'
versions = ['Python 3.8']

[virtuald]

I'm cross-compiling python to ARM, following instructions from the crossenv project @ https://crossenv.readthedocs.io/en/latest/quickstart.html#build-or-obtain-host-python

I was getting pthread related errors when using cross-built extension modules. I realized the issue was that distutils wasn't passing -pthread to gcc.

On my native python (Fedora 32 Python 3.8):

>>> import pprint, distutils.sysconfig
>>> pprint.pprint({i: distutils.sysconfig.get_config_vars(i)[0] for i in ('CC', 'CXX', 'LDSHARED')})
{'CC': 'gcc -pthread',
 'CXX': 'g++ -pthread',
 'LDSHARED': 'gcc -pthread -shared -Wl,-z,relro -Wl,--as-needed -Wl,-z,now '
 '-g -Wl,-z,relro -Wl,--as-needed -Wl,-z,now -g'}

In my cross-compiled python environment:

>>> import pprint, distutils.sysconfig
>>> pprint.pprint({i: distutils.sysconfig.get_config_vars(i)[0] for i in ('CC', 'CXX', 'LDSHARED')})
{'CC': 'arm-frc2020-linux-gnueabi-gcc',
 'CXX': 'arm-frc2020-linux-gnueabi-c++',
 'LDSHARED': 'arm-frc2020-linux-gnueabi-gcc -shared'}

As you can see, no -pthread flag. I went digging in the configure script and found this comment in configure.ac:2110:

# Some compilers won't report that they do not support -pthread,
# so we need to run a program to see whether it really made the
# function available.

... well, that doesn't work when cross-compiling. I looked at some other variables set to override things for cross compiling, and tried adding "ac_cv_pthread_is_default=no ac_cv_pthread=yes ac_cv_cxx_thread=yes" to my cross-compile ./configure, and it's closer:

>>> import pprint, distutils.sysconfig
>>> pprint.pprint({i: distutils.sysconfig.get_config_vars(i)[0] for i in ('CC', 'CXX', 'LDSHARED')})
{'CC': 'arm-frc2020-linux-gnueabi-gcc -pthread',
 'CXX': 'arm-frc2020-linux-gnueabi-c++',
 'LDSHARED': 'arm-frc2020-linux-gnueabi-gcc -pthread -shared'}

Closer, but not there yet! It appears that the $ac_cv_cxx_thread is not externally settable (unlike ac_cv_pthread), which I guess is because it's not wrapped in a AC_CACHE_VAL macro?

It seems that the right fix would be for someone to use AC_CACHE_VAL to allow $ac_cv_cxx_thread to be set externally? I'm happy to cargo cult some code from other parts of configure.ac and make a PR for it, but it's not immediately clear (a) what bad side effects this could cause and (b) what the best mechanism to use to regenerate configure et al is?

[virtuald]

FWIW, I've been using the patch in my PR and it seems to work.

[gpshead]

thanks!