App icns modification breaks the icon display on macOS

[asoluter]

What did you do?

Modified *.icns file inside the app on MacOS

What did you expect to happen?

Updated icon to be visible using Finder/Get Info.

What actually happened?

After upgrading Pillow to 8.3.0 version, updating *.icns file inside the application on MacOS, makes the app icon not visible/invalid in Finder, though all the icons are visible if you open the icns file directly.

What are your OS, Python and Pillow versions?

• OS: MacOS 10.15 Catalina
• Python: 3.7.7, 3.9.7
• Pillow: 8.3.0 or 8.3.1, 8.4.0

with Image.open(image_path) as image:
    draw=ImageDraw.Draw(image)
    draw.rectangle([(0,0), (image.width,image.height)], fill=(255,0,0))
    image.save(image.filename)

[radarhere]

Using an application on my computer, I'm not able to replicate this. Could you upload the original ICNS file? What version of macOS do you have?

[asoluter]

@radarhere Sorry about the invalid example, I've rechecked it and it requires drawing prior to saving in order to break the icon (Finder may cache the old icon, so it's easier to see the change using Get Info option on the application). Not really sure if it's related to #4526 change, but the issue appeared after upgrading to Pillow 8.3.0

[asoluter]

@radarhere I've updated the example section of description. It was tested with both MacOS 10.15 Catalina and 11.6 Big Sur. The issue is present in 8.4.0 as well.

[radarhere]

Testing, I find that the problem is the file length in the header. I've created PR #5845 to resolve this.