Avoid self-copy data loss in avifImageCopy()

[jmestwa-coder]

Summary

avifImageCopy() does not handle self-copy correctly.

When the same image is passed as both source and destination:

avifImageCopy(image, image, AVIF_PLANES_ALL);

the function frees the image planes before copying, causing owned image data to be discarded while still returning AVIF_RESULT_OK.

Fix

Add an early return for self-copy operations:

if (dstImage == srcImage) {
    return AVIF_RESULT_OK;
}

This preserves existing image data and avoids unnecessary work when the source and destination are the same object.

[y-guyon]

Thank you for your interest in libavif.

I would consider this as incorrect use of the API and in my opinion there is no need to handle that case nor explain it in the API comment.

Why would a user call avifImageCopy(image, image, planes)?

[jmestwa-coder]

@y-guyon Thank you for the clarification.

I viewed self-copy as a valid operation because the API does not currently document that the source and destination must be different.

My concern was that the function currently returns AVIF_RESULT_OK while modifying the image when dstImage == srcImage.

Would you prefer this to remain undocumented caller responsibility, or would documenting dstImage != srcImage as a precondition be worthwhile?

[wantehchang]

In API design it seems more elegant to allow self copy and handle it properly, similar to the assignment operator = in C++.

But I agree with Yannis that calling avifImageCopy(image, image, planes) is extremely unlikely and would almost always indicate a programming error. We probably should strive to detect such bugs rather than ignore them. I suggest we change your check for self copy to return an error.

There are two other libavif public functions that take two avifImage parameters:

• avifImageSetViewRect: This should require dstImage != srcImage. See PR Disallow dstImage == srcImage #3236.
• avifImageComputeGainMap: I think it is fine if baseImage == altImage.

[wantehchang]

This PR has been replaced by PR #3236.