Dangling IFD pointer tags persisting after deleting their content

[serghov]

Describe the bug

When all tags within a SubIFD (such as GPSInfo or Exif) are deleted from ExifData, the corresponding pointer tag (e.g., Exif.Image.GPSTag, Exif.Image.ExifTag) remains in the metadata. This leads to the creation of empty or invalid IFDs when writing the image, causing inconsistency across platforms and failures in tests like geotag_test which fails quite frequently especially on macos.

To Reproduce

This is a non-deterministic bug, so it does not fail every time, however it fails quite frequently on geotag_test on macos on ci.

[kevinbackhouse]

@kmilos: was this fixed by #3470?

[kmilos]

Unrelated (i.e. still saw the GPS tag failure once or twice on macOS after that was merged), so looks like this is an actual issue in the code somewhere?

[kmilos]

And the currently repeatable failure on macOS Intel only is I think unrelated as well, always fails on the network test...

[kmilos]

@kevinbackhouse Yep, still happening intermittently:

https://github.com/Exiv2/exiv2/actions/runs/22096664410/job/63855411317#step:6:2040

and different to the known failure:

https://github.com/Exiv2/exiv2/actions/runs/22096660679/job/63860861082#step:5:152

[kevinbackhouse]

@kevinbackhouse Yep, still happening intermittently:

https://github.com/Exiv2/exiv2/actions/runs/22096664410/job/63855411317#step:6:2040

I've been trying reproduce that test failure, but haven't been able to. I tried to copy the build instructions exactly, using clang-21 and meson. Then I ran the tests repeatedly but they never failed. @serghov: do you know how to reproduce the bug?

[serghov]

@kevinbackhouse unfortunately I don't know how to reproduce it. And the fix isn't trivial either, I took a shot at it here, but don't like it