GIF files with comments must be GIF89a

[raygard]

What did you do?

Ran the GIF tests in Pillow-9.1.0, looked at output from test_comment_over_255(tmp_path) test.

What did you expect to happen?

The output GIF file should be GIF89a because it contains a comment.

What actually happened?

The file temp.gif has a signature of GIF87a.

What are your OS, Python and Pillow versions?

• OS: Windows 10
• Python: 3.10.4
• Pillow: 9.1.0

For code, see def test_comment_over_255(tmp_path): in Tests\test_file_gif.py.

The generated image file is:

Discussion:

This is due to an obsolete line in _get_global_header(im, info) in GifImagePlugin.py:

                extensionKey == "comment" and not (1 <= len(info[extensionKey]) <= 255)

The len(info[extensionKey]) should be simply tested for 0 to indicate no comment is wanted.
Suggested change is

                extensionKey == "comment" and len(info[extensionKey]) == 0

I have created a pull request #6292 for this.

As a side note, I think the entire feature test for GIF 87a vs. 89a can be changed from:

    version = b"87a"
    for extensionKey in ["transparency", "duration", "loop", "comment"]:
        if info and extensionKey in info:
            if (extensionKey == "duration" and info[extensionKey] == 0) or (
                extensionKey == "comment" and len(info[extensionKey]) == 0
            ):
                continue
            version = b"89a"
            break
    else:
        if im.info.get("version") == b"89a":
            version = b"89a"

to:

    version = b"87a"
    if (im.info.get("version") == b"89a") or info and (
            "transparency" in info or
            "loop" in info or
            ("duration" in info and info["duration"] != 0) or
            ("comment" in info and len(info["comment"]) != 0)):
        version = b"89a"

I think that's correct but would be happy for someone to look it over.

[radarhere]

https://www.w3.org/Graphics/GIF/spec-gif89a.txt

24. Comment Extension.
    ...
    b. Required Version. 89a.

You're right, thanks for catching that.