test_ctypes: test_generated_data() fails on x86 Debian Installed with X 3.x

[vstinner]

Bug report

x86 Debian Installed with X 3.x build: https://buildbot.python.org/all/#/builders/1244/builds/2499

FAIL: test_generated_data (test.test_ctypes.test_generated_structs.GeneratedTest.test_generated_data) (name='Packed4')
Check that a ctypes struct/union matches its C equivalent.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/buildbot/buildarea/3.x.ware-debian-x86.installed/build/target/lib/python3.14/test/test_ctypes/test_generated_structs.py", line 444, in test_generated_data
    self.assertEqual(sizeof(cls), next(expected))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 12 != 16

The structure:

@register()
class Packed4(Structure):
    _fields_ = [('a', c_int8), ('b', c_int64)]
    _pack_ = 8

configure:

./configure --prefix '$(PWD)/target'

test.pythoninfo:

CC.version: gcc (Debian 12.2.0-14) 12.2.0

sysconfig[CFLAGS]: -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall

sysconfig[PY_CFLAGS]: -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall

sysconfig[PY_CFLAGS_NODIST]: -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden -I./Include/internal -I./Include/internal/mimalloc

sysconfig[PY_STDMODULE_CFLAGS]: -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O3 -Wall -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden -I./Include/internal -I./Include/internal/mimalloc -I. -I./Include

Linked PRs

• gh-121938: ctypes: Skip test of _pack_-ed struct with c_int64 on x86 #125877

[vstinner]

cc @zware

[zware]

According to git bisect, this is due to 18c1a8d

[vstinner]

cc @gpshead @encukou who authored the change introducing the regression.

[hroncok]

We see this on Fedora i686 as well.

[mcepl]

Yes, the same on openSUSE with 3.14.0~a1. Works on x86_64, but breaks on 32bit i586.

Complete build log.

[encukou]

I'll fix it by alpha 2

[encukou]

Took a while, but I'm now convinced that skipping this test is the correct thing to do.

The test code to generate an C struct equivalent to a ctypes struct is far from perfect.