Support for fetching disk quota information

[Heliozoa]

Hi, thanks for your work on heim.
Would it be possible to add functionality relating to disk quotas? In particular, I was looking for a cross-platform way to find the remaining disk space with the quota being taken into account. As far as I can tell there's no crate that does it at the moment, so it would be nice to see in heim. Thanks!

[svartalf]

Hi, @Heliozoa! That is a great idea, but I dont have enough knowledge if it's possible to implement it properly (and preferably cross-platform), so it needs to be researched first.
If you have any insights on that matter, that would be really helpful.

[Heliozoa]

I'm not too knowledgeable either, unfortunately. My understanding is that disk quotas are supported at least on all the tier 1 platforms.

On Windows: https://docs.microsoft.com/en-gb/windows/win32/fileio/managing-disk-quotas

nix has a module for disk quotas: https://docs.rs/nix/0.18.0/nix/sys/quota/index.html
But I get an ENOSYS error trying to use it. quotactl's manpage says the error means my "kernel has not been compiled with the CONFIG_QUOTA option", but I don't think that's the case. I might just be using the command wrong, I'm not too experienced with APIs this low-level. I'll try and investigate this later and hopefully fix it so I can play around with the commands a bit.

It looks like Usage::free() is already quota-aware on Windows: https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getdiskfreespaceexa

If per-user quotas are being used, this value may be less than the total number of free bytes on a disk.