Creating proper type for sighandler_t using anonymous union on Unix

[landhb]

Resolves #1359

This PR allows you to use the sigaction struct on Unix properly. For instance you'll be able to create a new sigaction struct passing all three types of handlers, below demonstrates both the creation of a handler of type void (*sa_handler)(int) and for SIG_IGN which is a const size_t:

extern "C" fn signal_handler(signal: i32) {todo!()}

let mut act = libc::sigaction {
    sa_sigaction: libc::sighandler_t {
        sa_handler: Some(signal_handler),
    },
    sa_mask: newmask,
    sa_flags: 0,
    sa_restorer: None,
};

// Demonstrating use of SIG_IGN constant 
let _ignore_action = libc::sigaction {
    sa_sigaction: libc::SIG_IGN,
    sa_mask: newmask,
    sa_flags: 0,
    sa_restorer: None,
};

unsafe {
    libc::sigaction(libc::SIGUSR1, &mut act, core::ptr::null_mut());
}

[rust-highfive]

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[landhb]

Working on fixing the test suite errors.

[landhb]

All checks are passing now.

[JohnTitor]

Thanks for the PR!
But this should be a breaking change and I'd hold off unless it's a serious issue.

[JohnTitor]

Could you move this before the consts declaration?

[landhb]

Yep! Moved in 6913b31. Let me know if that's where you meant.

[JohnTitor]

Is the current type wrong on all the UNIX platforms?

[landhb]

I believe so. Here's what I can dig up so far:

BSD/XNU uses one of the function pointer variants void (*sig_t)(int), and calls it sig_t:

https://github.com/apple/darwin-xnu/blob/main/bsd/sys/signal.h#L500
https://github.com/NetBSD/src/blob/trunk/sys/sys/signal.h#L172

Haiku has both function pointer variants:

https://github.com/haiku/haiku/blob/d0e006f5569f24e73a4d83f5c9766f1132160d7a/headers/posix/signal.h#L61-L62

Hermit has the void (*signal_handler_t)(int):

https://github.com/hermitcore/libhermit/blob/63e67e1af1e56fbb51f14c89997bbaac11a37cfa/include/hermit/signal.h#L46

Redox as well:

https://github.com/redox-os/relibc/blob/29e2f2923170a7a52c5645ab002564bfa4b0c43b/src/header/signal/mod.rs#L35

[JohnTitor]

Also cc @Amanieu as I'm unfamiliar with this.

[Amanieu]

The new version is more correct but I'm afraid it will be a significant breaking change so we should hold off on it until the next major release.

[bors]

☔ The latest upstream changes (presumably #3038) made this pull request unmergeable. Please resolve the merge conflicts.

[safinaskar]

@landhb , breaking release of libc will be released soon. So, consider to continue this work