`test_call`'s `test_margin_is_sufficient` (stack protection size check) fails on GCC with -O0

[encukou]

Bug report

On my x86_64 machine after:

./configure -C --prefix '$(PWD)/target'
make 'CFLAGS_NODIST=-O0'

I get:

0:00:00 load avg: 6.18 [1/1] test_call
test test_call failed -- Traceback (most recent call last):
  File "/home/encukou/dev/cpython/Lib/test/test_call.py", line 1054, in test_margin_is_sufficient
    self.assertLess(safe_margin, _testinternalcapi.get_stack_margin())
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 21960.0 not less than 16384

Fedora builds have been getting similar issues on other architectures as well.

Related to #130396

cc @markshannon, @vstinner

Linked PRs

• gh-140222: Increase stack margin on debug build #142452
• [3.14] gh-140222: Increase stack margin on debug build (GH-142452) #142471

[vstinner]

I confirm, I can reproduce the issue on Fedora 42 with gcc (GCC) 15.2.1 20250808 (Red Hat 15.2.1-1).

I can also reproduce the issue with these commands:

set -e -x

make distclean ||:

export BASECFLAGS=
export CFLAGS=
export CFLAGS_NODIST=
export CPPFLAGS=
export CXXFLAGS=
export LDFLAGS=
export LDFLAGS_NODIST=
export OPT=

./configure
make -j12
./python -m test -v test_call -m test_margin_is_sufficient

Output:

FAIL: test_margin_is_sufficient (test.test_call.TestRecursion.test_margin_is_sufficient)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/test/test_call.py", line 1054, in test_margin_is_sufficient
    self.assertLess(safe_margin, _testinternalcapi.get_stack_margin())
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 17568 not less than 16384

[vstinner]

cc @hroncok @befeleme

[hroncok]

This is now also failing (at least on ppc64le) in Fedora with 3.14.1.

[markshannon]

Looks like we need to make the margin even bigger for debug builds. This is just for linux, right?

[hroncok]

Linux is where we see the failure. Not sure if the tests runs elsewhere.