MP4: Unexpected upgrade of `gnre` to `©gen`

[uklotzde]

Reproducer

parse_ilst()

Summary

This opinionated behavior not only prevents applications from detecting and fixing/resolving those issues themselves. It causes ambiguities and mixes up metadata which is much more concerning.

Afterwards the imported Ilst contains both regular ©gen atoms side by side with implicitly migrated gnre atoms. They are indistinguishable and alter the metadata unexpectedly when written back to the file.

The only situation when such an implicit migration would be acceptable is if the original Ilst does only contain gnre but no ©gen atoms. But conditional, data-dependent behavior is harder to reason about, so better not try to be clever.

Expected behavior

This feature should either be made optional or provided as a manual, post-processing step on the imported Ilst. I would prefer the latter, because import and migration should be independent.

Assets

No response