Building against libuuid without pkgconfig fails if OSSP uuid is installed

[tobbez]

Bug report

Bug description:

When building Python on a system where:

• libuuid is installed without pkgconfig
• OSSP uuid is installed (uuid-devel on RHEL/Rocky/Alma)
• OSSP uuid's uuid.h is installed as /usr/include/uuid.h (the case on RHEL/Rocky/Alma)

the build fails with the following error:

./Modules/_uuidmodule.c: In function ‘py_uuid_generate_time_safe’:
./Modules/_uuidmodule.c:31:12: error: storage size of ‘uuid’ isn’t known
   31 |     uuid_t uuid;
      |            ^~~~
./Modules/_uuidmodule.c:35:11: error: implicit declaration of function ‘uuid_generate_time_safe’; did you mean ‘py_uuid_generate_time_safe’? [-Werror=implicit-function-declaration]
   35 |     res = uuid_generate_time_safe(uuid);
      |           ^~~~~~~~~~~~~~~~~~~~~~~
      |           py_uuid_generate_time_safe

This is because on such a system, both HAVE_UUID_H and HAVE_UUID_UUID_H become set:

$ grep 'HAVE_UUID.*_H' pyconfig.h
#define HAVE_UUID_H 1
#define HAVE_UUID_UUID_H 1

-I/usr/include/uuid does not get added to CFLAGS:

$ grep ^MODULE__UUID_CFLAGS Makefile
MODULE__UUID_CFLAGS=

and Modules/_uuidmodule.c currently checks HAVE_UUID_H before HAVE_UUID_UUID_H:

#if defined(HAVE_UUID_H)
  // AIX, FreeBSD, libuuid with pkgconf
  #include <uuid.h>
#elif defined(HAVE_UUID_UUID_H)
  // libuuid without pkgconf
  #include <uuid/uuid.h>
#endif

This causes the wrong uuid.h to be included.

The order in which HAVE_UUID_H and HAVE_UUID_UUID_H are checked was inverted in commit 8af6481, but neither the commit itself or the related issue documents why the order was changed.

Building Python on a system with the properties described in this issue did work as expected in Python 3.10 (3.11 was the first release to include the reordering mentioned above).

Reproducer

docker run --rm -it almalinux:9

dnf install -y 'dnf-command(builddep)'
dnf --enablerepo=crb builddep -y python3
dnf --enablerepo=crb install -y uuid-devel

rm -f /usr/lib64/pkgconfig/uuid.pc

curl -O https://www.python.org/ftp/python/3.14.3/Python-3.14.3.tgz
tar -xf Python-3.14.3.tgz
cd Python-3.14.3

./configure
make -j$(nproc)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[WYSIATI]

This looks like a regression from commit 8af6481f6b7 (bpo-45847, "Port _uuid to PY_STDLIB_MOD") which accidentally inverted the #if check order. The original code (bpo-40501, commit d6b727e2c94) checked HAVE_UUID_UUID_H first, which is correct since <uuid/uuid.h> is more specific than <uuid.h>.

The fix would be to swap the two branches back in Modules/_uuidmodule.c:

#if defined(HAVE_UUID_UUID_H)
  // libuuid without pkgconf
  #include <uuid/uuid.h>
#elif defined(HAVE_UUID_H)
  // AIX, FreeBSD, libuuid with pkgconf
  #include <uuid.h>
#endif

I'd be happy to submit a PR if that works.