Segfault on imshow when setting a matplotlib backend beforehand

[epbuennig]

The python repository issue-template mentioned that issues that have to be fixed in C++ as well should be reported here.
Although the example is purely python. The example relies on matplotlib and may not be caused by opencv directly, but the segfault happens when calling a opencv function, so I'm reporting it here.

System information (version)

• OpenCV => 4.6.0-4 (this is the package I'm using, it looks like it's using gcc, but I honestly can't tell)
• Python-OpenCV => 4.6.0.66
• Operating System / Platform => Arch Linux 5.19.7-arch1-1 #1 SMP PREEMPT_DYNAMIC x86_64 GNU/Linux
• Compiler => gcc (GCC) 12.2.0 (see note above)

Detailed description

When importing matplotlib.pyplot and then setting a backend for matplotlib before using opencv the first call to imshow causes a segmentation fault.

Steps to reproduce

1. Set up a python environment using Python 3.9.13 with the following packages:

cycler          0.11.0
fonttools       4.37.1
gobject         0.1.0
kiwisolver      1.4.4
matplotlib      3.5.3
numpy           1.23.2
opencv-python   4.6.0.66
packaging       21.3
Pillow          9.2.0
pip             22.2.2
pycairo         1.21.0
PyGObject       3.42.2
pyparsing       3.0.9
PyQt5-Qt5       5.15.2
PyQt5-sip       12.11.0
pyrealsense2    2.51.1.4348
python-dateutil 2.8.2
scipy           1.9.1
setuptools      60.2.0
six             1.16.0
wheel           0.37.1

2. Prepare a python script like this:

import matplotlib as mpl
import cv2

# swapping the order of thsese also resolves the issue
from matplotlib import pyplot as plt # removing this import resoves the issue
mpl.use('GTK3Agg') # remvoing this also resovle the issue

def test():
    webcam = cv2.VideoCapture(0)

    try:
        while True:
            ret, frame = webcam.read()

            # segfault happens on the first call to `imshow`
            cv2.imshow('frame', frame)
            if cv2.waitKey(1) & 0xFF == ord('q'):
                break
    finally:
        webcam.release()
        cv2.destroyAllWindows()


test()

3. Running the script.

Issue submission checklist

• I report the issue, it's not a question
• I checked the problem with documentation, FAQ, open issues, forum.opencv.org, Stack Overflow, etc and have not found any solution
• I updated to the latest OpenCV version and the issue is still there
• There is reproducer code and related data files: videos, images, onnx, etc

[tacaswell]

I strongly suspect that this will reproduce using only gtk.

[tacaswell]

matplotlib/matplotlib#24364 (comment) it does reproduce with only gi / gtk.