bug#78623: cp --preserve=xattr copies attr as well as xattr which breaks cross-FS copy

On 30/05/2025 18:16, Paul Eggert wrote:
> On 2025-05-30 02:37, Pádraig Brady wrote:
>> we only have this issue with --preserve=xattr which diagnoses any
>> issues.
>> Perhaps we would benefit from a --preserve=supported-xattr option?
> 
> If we go that route, it might be a bit better if the new option-arg
> began with 'xattr' rather than ended with 'xattr' so that it's easier to
> find in the doc. Perhaps something like --preserve='xattr-try'?
> 
> I'm not quite seeing the motivation, though. Why are scripts using
> --preserve=xattr rather than the much-simpler '-a'? That is, why
> preserve xattr but not other metadata?

Thinking a bit more about this I think I'll add it, because:

1. ENOTSUP is common with xattrs

2. `--preserve=all --no-preserve=mode,ownership,timestamps,links,context`
is too verbose, and not future proof if we ever add another --preserve option.

3. It's more descriptive to explicitly say that -a implies --preserve=xattr-supported

I would also like to get feedback though on why -a doesn't suffice.
Though I suppose by that argument you could ask why we need the
various --preserve options at all.

cheers,
Pádraig