Merge #1741

1741: SigSet: A new unsafe helper method to create a SigSet from a sigset_t r=rtzoeller a=germag

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 
`libc::sigset_t` object.

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

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

Co-authored-by: German Maglione <gmaglione@redhat.com>

CHANGELOG.md

@@ -21,6 +21,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1697](https://github.com/nix-rust/nix/pull/1697))
 - Added `getrusage` and helper types `UsageWho` and `Usage`
   (#[1747](https://github.com/nix-rust/nix/pull/1747))
+- Added `from_sigset_t_unchecked()` to `signal::SigSet`.
+  (#[1741](https://github.com/nix-rust/nix/pull/1741))
 
 ### Changed
 
@@ -32,6 +34,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1713](https://github.com/nix-rust/nix/pull/1713))
 - `nix::poll::ppoll`: `sigmask` parameter is now optional.
   (#[1739](https://github.com/nix-rust/nix/pull/1739))
+- `signal:SigSet` it's now marked as `repr(trasparent)`.
+  (#[1741](https://github.com/nix-rust/nix/pull/1741))
 
 ### Fixed
 

src/sys/signal.rs

@@ -465,6 +465,9 @@ use std::iter::FromIterator;
 use std::iter::IntoIterator;
 
 /// Specifies a set of [`Signal`]s that may be blocked, waited for, etc.
+// We are using `transparent` here to be super sure that `SigSet`
+// is represented exactly like the `sigset_t` struct from C.
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct SigSet {
     sigset: libc::sigset_t
@@ -567,6 +570,19 @@ impl SigSet {
             Signal::try_from(signum.assume_init()).unwrap()
         })
     }
+
+    /// Converts a `libc::sigset_t` object to a [`SigSet`] without checking  whether the
+    /// `libc::sigset_t` is already initialized.
+    ///
+    /// # Safety
+    ///
+    /// The `sigset` passed in must be a valid an initialized `libc::sigset_t` by calling either
+    /// [`sigemptyset(3)`](https://man7.org/linux/man-pages/man3/sigemptyset.3p.html) or
+    /// [`sigfillset(3)`](https://man7.org/linux/man-pages/man3/sigfillset.3p.html).
+    /// Otherwise, 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 {
@@ -1311,4 +1327,22 @@ mod tests {
         .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));
+        }
+    }
+
 }