Backward seek on closed GifImageFile breaks expected behaviour

[AnonymouX47]

What did you do?

• Opened a GIF image from a file
• Seeked forward
• Closed the image
• Tried to seek backward
• Tried to load the image

What did you expect to happen?

After the image was closed, I expected all operations that involve loading or mutation to raise ValueError: Operation on closed image.
I tested WebPImageFile and it behaved as expected.

SInce PIL.Image.Image provides no public interface to determine if an image is closed or not, the next point of call would be trying an operation on the image to see if a specific exception is raised. The inconsistency shown above breaks that behaviour.

What actually happened?

Operations on the closed image don't raise the expected exception after seeking backward.

What are your OS, Python and Pillow versions?

• OS: Pop OS 21.10 (Linux)
• Python: 3.9.10
• Pillow: 9.1.0

from PIL import Image

img = Image.open("tests/images/lion.gif")
img.seek(4)  # Ok
img.close()  # Ok
img.load()  # Ok; ValueError: Operation on closed image
img.seek(5)  # Ok; ValueError: Operation on closed image
img.seek(0)  # Not Ok; AttributeError: 'NoneType' object has no attribute 'seek'
img.tell()  # Not Ok; -1
img.seek(5)  # Not Ok; AttributeError: 'NoneType' object has no attribute 'seek'
img.load()  # Not Ok; AttributeError: 'NoneType' object has no attribute 'read'

[AnonymouX47]

I don't think this is an issue that could come up often but there's also the possibility of it coming up.

[radarhere]

I've created PR #6213 to resolve this.

[AnonymouX47]

Thanks 😃