Sparse file support for `lseek`?

[sharifhsn]

On Linux and some other POSIX systems, lseek supports two additional values for whence relating to sparse files, SEEK_HOLE and SEEK_DATA. I believe these should be supported by rustix.

For reference, some filesystems have files with large amounts of zero data that can be optionally marked "sparse", and these sections won't be allocated on disk. This feature can be exposed to userspace applications to improve the performance of some operations, like cp.

Although this is technically not a part of the POSIX standard, it has been proposed for issue 8.

As an example of prior art, nix already exposes this ability.

[sunfishcode]

Yes, rustix should add this. It will require Rustix to switch to its own SeekFrom type, rather than just using std's, but I think that makes sense to do.

[sharifhsn]

Great! I'll submit a PR for this today if you want.

[sunfishcode]

Sure, sounds great!