test_transparent_markers[pdf] is broken on v3.3.x Travis macOS

[QuLogic]

Bug report

Bug summary

Starting with #18545 and the subsequent merge, test_transparent_markers[pdf] has been failing on v3.3.x for Travis macOS. It does not appear to be failing on other systems or other branches. There were no other builds on that branch for 5 days before that, so it may or may not be related to that PR specifically.

I could not reproduce on the Mac Mini either, so I'm not sure how to fix it.

Actual outcome

________________________ test_transparent_markers[pdf] _________________________
[gw0] darwin -- Python 3.8.5 /usr/local/opt/python@3.8/bin/python3.8
expected = '/Users/travis/build/matplotlib/matplotlib/result_images/test_axes/transparent_markers-expected.pdf'
actual = PosixPath('/Users/travis/build/matplotlib/matplotlib/result_images/test_axes/transparent_markers.pdf')
tol = 0
    def _raise_on_image_difference(expected, actual, tol):
        __tracebackhide__ = True
    
        err = compare_images(expected, actual, tol, in_decorator=True)
        if err:
            for key in ["actual", "expected", "diff"]:
                err[key] = os.path.relpath(err[key])
>           raise ImageComparisonFailure(
                ('images not close (RMS %(rms).3f):'
                    '\n\t%(actual)s\n\t%(expected)s\n\t%(diff)s') % err)
E           matplotlib.testing.exceptions.ImageComparisonFailure: images not close (RMS 4.572):
E           	result_images/test_axes/transparent_markers_pdf.png
E           	result_images/test_axes/transparent_markers-expected_pdf.png
E           	result_images/test_axes/transparent_markers_pdf-failed-diff.png
lib/matplotlib/testing/decorators.py:139: ImageComparisonFailure

------------------------------ Captured log call -------------------------------
DEBUG    matplotlib.axes._base:_base.py:2613 title position was updated manually, not adjusting
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IHDR' 16 13
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'iCCP' 41 2354
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:351 iCCP profile name b'default_rgb.icc'
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:352 Compression method 0
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'pHYs' 2407 9
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'tEXt' 2428 31
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IDAT' 2471 8192
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IHDR' 16 13
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'iCCP' 41 2354
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:351 iCCP profile name b'default_rgb.icc'
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:352 Compression method 0
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'pHYs' 2407 9
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'tEXt' 2428 31
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IDAT' 2471 8192
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IHDR' 16 13
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'iCCP' 41 2354
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:351 iCCP profile name b'default_rgb.icc'
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:352 Compression method 0
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'pHYs' 2407 9
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'tEXt' 2428 31
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IDAT' 2471 8192
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IHDR' 16 13
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'iCCP' 41 2354
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:351 iCCP profile name b'default_rgb.icc'
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:352 Compression method 0
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'pHYs' 2407 9
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'tEXt' 2428 31
DEBUG    PIL.PngImagePlugin:PngImagePlugin.py:153 STREAM b'IDAT' 2471 8192

Matplotlib version

• Operating system: macOS
• Matplotlib version: v3.3.x
• Matplotlib backend (print(matplotlib.get_backend())): macosx, I guess, though maybe Agg for the test
• Python version: 3.8

[timhoffm]

1. Do the diff images tell anything.
2. Does that test run the changed code at all?
3. If we don't know further, we can always roll back and see if that fixes CI again. If so, the problem in somehow in the PR, if not it comes from elsewhere.

[QuLogic]

This started failing on Fedora Rawhide as well, which just updated to Ghoscript 9.53. This also changed versions between the working and failing macOS builds. The strange thing is that there is no substantial difference in the test and even the baseline image between v3.3.x and master, unless it has to do with the test framework itself.

[QuLogic]

Ah, it's because ghostscript is installed as a side effect on v3.3.x, but not on master.