python-mss: Test suite sometimes segfaults due to XQueryExtension() getting NULL Display

General information:

• OS name: Gentoo Linux
• OS version: n/a
• OS architecture: amd64
• Resolutions: n/a
• Python version: 3.11.3 (but also reproduced with 3.9.16)
• MSS version: 8.0.3 and git ef97a27cbeae46cbed8f348679cd9729b4755fc0

For GNU/Linux users:

• Display server protocol and version, if known: Xvfb from xorg-server 21.1.8
• Desktop Environment: n/a
• Composite Window Manager name and version: n/a

Description of the warning/error

When running the test suite under Xvfb, various tests sometimes cause it to segfault seemingly randomly, e.g.:

$ python -m pytest 
========================================================= test session starts =========================================================
platform linux -- Python 3.11.3, pytest-7.3.1, pluggy-1.0.0 -- /tmp/python-mss/.venv/bin/python
cachedir: .pytest_cache
rootdir: /tmp/python-mss
configfile: setup.cfg
plugins: cov-4.0.0
collected 73 items / 1 skipped                                                                                                        

src/tests/test_bgra_to_rgb.py::test_bad_length PASSED                                                                           [  1%]
src/tests/test_bgra_to_rgb.py::test_good_types PASSED                                                                           [  2%]
src/tests/test_cls_image.py::test_custom_cls_image PASSED                                                                       [  4%]
src/tests/test_find_monitors.py::test_get_monitors PASSED                                                                       [  5%]
src/tests/test_find_monitors.py::test_keys_aio PASSED                                                                           [  6%]
src/tests/test_find_monitors.py::test_keys_monitor_1 PASSED                                                                     [  8%]
src/tests/test_find_monitors.py::test_dimensions PASSED                                                                         [  9%]
src/tests/test_get_pixels.py::test_grab_monitor PASSED                                                                          [ 10%]
src/tests/test_get_pixels.py::test_grab_part_of_screen Fatal Python error: Segmentation fault

Current thread 0x00007f4df6e3c740 (most recent call first):
  File "/tmp/python-mss/src/mss/linux.py", line 359 in _is_extension_enabled
  File "/tmp/python-mss/src/mss/linux.py", line 321 in __init__
  File "/tmp/python-mss/src/mss/factory.py", line 34 in mss
  File "/tmp/python-mss/src/tests/test_get_pixels.py", line 25 in test_grab_part_of_screen
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/python.py", line 194 in pytest_pyfunc_call
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/python.py", line 1799 in runtest
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/runner.py", line 169 in pytest_runtest_call
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/runner.py", line 262 in <lambda>
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/runner.py", line 341 in from_call
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/runner.py", line 261 in call_runtest_hook
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/runner.py", line 222 in call_and_report
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/runner.py", line 133 in runtestprotocol
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/runner.py", line 114 in pytest_runtest_protocol
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/main.py", line 348 in pytest_runtestloop
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/main.py", line 323 in _main
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/main.py", line 269 in wrap_session
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/main.py", line 316 in pytest_cmdline_main
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_callers.py", line 39 in _multicall
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_manager.py", line 80 in _hookexec
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pluggy/_hooks.py", line 265 in __call__
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/config/__init__.py", line 166 in main
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/_pytest/config/__init__.py", line 189 in console_main
  File "/tmp/python-mss/.venv/lib/python3.11/site-packages/pytest/__main__.py", line 5 in <module>
  File "<frozen runpy>", line 88 in _run_code
  File "<frozen runpy>", line 198 in _run_module_as_main

Extension modules: numpy.core._multiarray_umath, numpy.core._multiarray_tests, numpy.linalg._umath_linalg, numpy.fft._pocketfft_internal, numpy.random._common, numpy.random.bit_generator, numpy.random._bounded_integers, numpy.random._mt19937, numpy.random.mtrand, numpy.random._philox, numpy.random._pcg64, numpy.random._sfc64, numpy.random._generator, PIL._imaging (total: 14)
Segmentation fault (core dumped)

gdb suggests that XQueryExtension() is receiving a NULL pointer as display:

#0  __pthread_kill_implementation (threadid=<optimized out>, signo=signo@entry=11, no_tid=no_tid@entry=0) at pthread_kill.c:44
#1  0x00007f4df66a38df in __pthread_kill_internal (signo=11, threadid=<optimized out>) at pthread_kill.c:78
#2  0x00007f4df6653af2 in __GI_raise (sig=11) at ../sysdeps/posix/raise.c:26
#3  <signal handler called>
#4  XQueryExtension (dpy=0x0, name=0x7f4dd24e2870 "RANDR", major_opcode=0x7f4dd247e920, first_event=0x7f4dd247ea40, 
    first_error=0x7f4dd247e800) at /usr/src/debug/x11-libs/libX11-1.8.4-r1/libX11-1.8.4/src/QuExt.c:48
#5  0x00007f4df521228a in ?? () from /usr/lib64/libffi.so.8
#6  0x00007f4df52116a4 in ?? () from /usr/lib64/libffi.so.8
#7  0x00007f4df5211dfd in ffi_call () from /usr/lib64/libffi.so.8
#8  0x00007f4df5255060 in ?? () from /usr/lib/python3.11/lib-dynload/_ctypes.cpython-311-x86_64-linux-gnu.so
#9  0x00007f4df524e388 in ?? () from /usr/lib/python3.11/lib-dynload/_ctypes.cpython-311-x86_64-linux-gnu.so
#10 0x00007f4df694ec0b in _PyObject_MakeTpCall () from /usr/lib64/libpython3.11.so.1.0
#11 0x00007f4df68fdeb0 in _PyEval_EvalFrameDefault () from /usr/lib64/libpython3.11.so.1.0
[…]

This seems to be some ugly race condition, I’m trying to investigate further.

Other details

I’m testing via:

python -m venv .venv
. .venv/bin/activate
pip install -r tests-requirements.txt
export DISPLAY=:1
Xvfb :1 &
python -m pytest