Support custom signals for sigaction

[AzariasB]

sigaction currently only accepts the Signal enum and directly passes it to libc::sigaction as a c_int.

I'm working on a linux system that has custom signals superior to 32. And thus not declared in the Signal enum.

The idea would be to have another sigaction function that accepts directly a c_int.

Something like

pub unsafe fn sigaction_raw_signal(signal_raw: libc::c_int sigaction: &SigAction) -> Result<SigAction> {
    let mut oldact = mem::MaybeUninit::<libc::sigaction>::uninit();

    let res = libc::sigaction(signal_raw,
                              &sigaction.sigaction as *const libc::sigaction,
                              oldact.as_mut_ptr());

    Errno::result(res).map(|_| SigAction { sigaction: oldact.assume_init() })
}

It might be a bit out of scope for this library ... let me know!

I looked into implementing the function myself, but since Sigaction's fields are not public, I cannot have this function inside my own library. So another way would be to make sigaction public, but I feel like it's not ideal.

[SteveLauC]

but since Sigaction's fields are not public, I cannot have this function inside my own library. So another way would be to make sigaction public, but I feel like it's not ideal.

This option isn't that bad, and I think this is something we should do, without this, Nix cannot be used under scenarios where interaction with raw libc types is necessary.

Actually, this has been done to a lot of Nix wrapper types, though the way they did this are not consistent, some types directly expose it, some types have functions like into_raw/from_raw, and some types did it through the From/Into traits...

---

cc @asomers, do you think we should make the way how our wrapper type interact with the libc types unified? In my mind, I think we should:

1. Make the wrapper type #[repr(transparent)]
2. Implement a standard way to convert it to/from the inner raw type

[asomers]

Yes, definitely @SteveLauC . That's a good idea.

[AzariasB]

Thank you very much for your answers!
I'm not very familiar with nix's codebase, but if I can do anything to help, let me know!