CXXFLAGS overrides default flags, CFLAGS doesn't

[mglisse]

distutils/distutils/sysconfig.py

Lines 343 to 345 in 378984e

	cflags = _add_flags(cflags, 'C')
	ldshared = _add_flags(ldshared, 'C')
	cxxflags = os.environ.get('CXXFLAGS', cxxflags)

distutils now uses CXXFLAGS instead of CFLAGS for C++, that's good. However, it was implemented differently: while CFLAGS is appended to the default flags, CXXFLAGS replaces them. Is there a particular reason for this inconsistency? I have an application where I was setting both CFLAGS and CXXFLAGS to the same value, and was quite confused when the behavior changed with the new setuptools.

[cjwatson]

@thesamesam @jaraco I encountered a weird regression which I think is due to this change, and wondered where it ought to be fixed.

The regression is visible in https://bugs.debian.org/1098602, though it's quite confusing. From what I can tell, if Python is built with -DNDEBUG, and one tries to build persistent with CFLAGS set to something that doesn't also contain -DNDEBUG (in particular, Debian's packaging toolchain sets various hardening flags, but the problem can be reproduced by just setting CFLAGS to the empty string in the environment), then its extension fails to compile as follows:

x86_64-linux-gnu-gcc -fPIC -I/usr/include/python3.13 -c src/persistent/cPickleCache.c -o build/temp.linux-x86_64-cpython-313/src/persistent/cPickleCache.o
In file included from /usr/include/python3.13/Python.h:19,
                 from src/persistent/_compat.h:18,
                 from src/persistent/cPersistence.h:18,
                 from src/persistent/cPickleCache.c:96:
src/persistent/cPickleCache.c: In function ‘cc_oid_unreferenced’:
src/persistent/cPickleCache.c:626:25: error: ‘cPersistentObject’ has no member named ‘ob_refcnt’
  626 |     assert(dead_pers_obj->ob_refcnt == 0);
      |                         ^~
src/persistent/cPickleCache.c:626:25: error: ‘cPersistentObject’ has no member named ‘ob_refcnt’
  626 |     assert(dead_pers_obj->ob_refcnt == 0);
      |                         ^~
src/persistent/cPickleCache.c:636:25: error: ‘cPersistentObject’ has no member named ‘ob_refcnt’
  636 |     assert(dead_pers_obj->ob_refcnt == 1);
      |                         ^~
src/persistent/cPickleCache.c:636:25: error: ‘cPersistentObject’ has no member named ‘ob_refcnt’
  636 |     assert(dead_pers_obj->ob_refcnt == 1);
      |                         ^~
src/persistent/cPickleCache.c:665:25: error: ‘cPersistentObject’ has no member named ‘ob_refcnt’
  665 |     assert(dead_pers_obj->ob_refcnt == 1);
      |                         ^~
src/persistent/cPickleCache.c:665:25: error: ‘cPersistentObject’ has no member named ‘ob_refcnt’
  665 |     assert(dead_pers_obj->ob_refcnt == 1);
      |                         ^~
error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1

Previously extensions were built with Python's own CFLAGS even if CFLAGS was set in the environment, so this wasn't a problem.

I don't know Python's C API well enough to determine whether this is a bug in persistent, a bug in this distutils change (via setuptools), or something else. But it seems like something you might want to know about.

[thesamesam]

Thanks @cjwatson. Let me take a look.

[cjwatson]

On second thoughts, based on pypa/setuptools#4836, I think this is just a bug in persistent that's been exposed by this change. I've proposed zopefoundation/persistent#214 to fix it.

[thesamesam]

Many thanks! That looks right to me and that the assertions were broken (and clearly haven't worked for at least some time).