Detach PyQt6 QPixmap instance before returning

[radarhere]

Alternative to #8507

The user in that PR reported seeing corrupt output from ImageQt.toqpixmap() on Windows.

Pillow/src/PIL/ImageQt.py

Lines 214 to 216 in 9a4b3e0

	def toqpixmap(im: Image.Image | str | QByteArray) -> QPixmap:
	qimage = toqimage(im)
	return getattr(QPixmap, "fromImage")(qimage)

Testing, I found I was able to use GitHub Actions to reproduce the matter.

Testing further, I found that it was only happening when qimage was inside the function - if I unwrapped the function so that qimage wouldn't be garbage collected, the problem disappeared. I created a reproduction just using PyQt6 and asked about this at https://stackoverflow.com/questions/79133259/corrupted-display-from-qpixmap-fromimage. The response I received was

This is caused by Qt's implicit data sharing. The QImage will be garbage-collected when wrap returns, which may mean some of the shared data is no longer accesible. To avoid these problems, call pixmap.detach() to enforce a copy-on-write.

This fix allows us to test PyQt6 in GitHub Actions with CPython, which would otherwise fail.

[codev8services]

This resolved the issue #8507. RGB and RGBA work correctly.

[hugovk]

This "6" means PyQt, and "side6" would mean PySide6.

Do we know if this also affects PySide6?

[radarhere]

I'm unable to replicate the problem on Windows on GitHub Actions with PySide6.

If I run the test suite with PySide6 without this fix, it passes.

Furthermore, detach() doesn't exist in PySide6 - https://doc.qt.io/qtforpython-6/PySide6/QtGui/QPixmap.html / https://github.com/python-pillow/Pillow/actions/runs/11554805812/job/32158808074#step:6:5698. It is possible they don't offer it precisely because it isn't needed?

[hugovk]

Thanks for testing, let's merge. We'll have to see what happens when/if PyQt7 comes out in the future.