SigSet: A new unsafe helper method to create a SigSet from a sigset_t

Currently,  the only way to create a `SigSet` from a `sigset_t` object
is by using pointer casts, like:

```
unsafe {
    let sigset = *(&sigset as *const libc::sigset_t as *const SigSet)
};
```

This is un-ergonomic for library creators with interfaces to C.
So, let's add a new unsafe method that creates a SigSet from a sigset_t
object.

We can't implement `From` since converting from sigset_t to
SigSet is unsafe because objects of type sigset_t must be initialized
by calling either sigemptyset(3) or sigfillset(3) before being used.
In other case, the results are undefined.
Ee cannot implement `TryFrom` either, because there is no way to check
if an object of type sigset_t is initialized.

Signed-off-by: German Maglione <gmaglione@redhat.com>

CHANGELOG.md

@@ -19,6 +19,7 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1703](https://github.com/nix-rust/nix/pull/1703))
 - Added `ptrace::read_user` and `ptrace::write_user` for Linux.
   (#[1697](https://github.com/nix-rust/nix/pull/1697))
+- Added `from_sigset_t_unchecked()` to `signal::SigSet`.
 
 ### Changed
 

src/sys/signal.rs

@@ -619,6 +619,13 @@ impl SigSet {
             Signal::try_from(signum.assume_init()).unwrap()
         })
     }
+
+    /// Converts a sigset_t object to a SigSet without checking  whether the sigset_t is already
+    /// initialized. Objects of type sigset_t must be initialized by calling either sigemptyset(3)
+    /// or sigfillset(3) before being used. In other case, the results are undefined.
+    pub unsafe fn from_sigset_t_unchecked(sigset: libc::sigset_t) -> SigSet {
+        SigSet { sigset }
+    }
 }
 
 impl AsRef<libc::sigset_t> for SigSet {
@@ -1344,4 +1351,22 @@ mod tests {
             assert_eq!(mask.wait().unwrap(), SIGUSR1);
         }).join().unwrap();
     }
+
+    #[test]
+    fn test_from_sigset_t_unchecked() {
+        let src_set = SigSet::empty();
+        let set = unsafe { SigSet::from_sigset_t_unchecked(src_set.sigset) };
+
+        for signal in Signal::iterator() {
+            assert!(!set.contains(signal));
+        }
+
+        let src_set = SigSet::all();
+        let set = unsafe { SigSet::from_sigset_t_unchecked(src_set.sigset) };
+
+        for signal in Signal::iterator() {
+            assert!(set.contains(signal));
+        }
+    }
+
 }