Ensure image is mutable before saving #3724
Merged
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Resolves #3496
In that issue, a bus error occurs when opening an image and saving it back to the same location. It was described as relating to mmap. Investigating, I find that the bus error occurs at
Pillow/src/libImaging/Pack.c
Line 459 in d1f865e
So presumably that is conflicting with mmap.
The same block of code that sets up mmap also marks the image as readonly.
Pillow/src/PIL/ImageFile.py
Lines 185 to 191 in d1f865e
https://pillow.readthedocs.io/en/5.4.1/releasenotes/3.3.2.html
Given the general testing requirement of operating on temp files, and since the test involves opening and saving back the same file, the test can't be run on Windows, as temp files can't be opened twice on that operating system - https://docs.python.org/2/library/tempfile.html