Replace custom ucred struct with newtype wrapper around libc::ucred

Author: Bryant Mairs

CHANGELOG.md

@@ -22,7 +22,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#739](https://github.com/nix-rust/nix/pull/739))
 - Expose `signalfd` module on Android as well.
   ([#739](https://github.com/nix-rust/nix/pull/739))
-- Added nix::sys::ptrace::detach. 
+- Added nix::sys::ptrace::detach.
   ([#749](https://github.com/nix-rust/nix/pull/749))
 - Added timestamp socket control message variant:
   `nix::sys::socket::ControlMessage::ScmTimestamp`
@@ -75,11 +75,14 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#731](https://github.com/nix-rust/nix/pull/731))
 - Marked `pty::ptsname` function as `unsafe`
   ([#744](https://github.com/nix-rust/nix/pull/744))
-- Moved constants ptrace request, event and options to enums and updated ptrace functions and argument types accordingly. 
+- Moved constants ptrace request, event and options to enums and updated ptrace functions and argument types accordingly.
   ([#749](https://github.com/nix-rust/nix/pull/749))
 - `AioCb::Drop` will now panic if the `AioCb` is still in-progress ([#715](https://github.com/nix-rust/nix/pull/715))
 - Restricted `nix::sys::socket::UnixAddr::new_abstract` to Linux and Android only.
   ([#785](https://github.com/nix-rust/nix/pull/785))
+- The `ucred` struct has been removed in favor of a `UserCredentials` struct that
+  contains only getters for its fields.
+  ([#814](https://github.com/nix-rust/nix/pull/814))
 
 
 ### Fixed

src/sys/socket/mod.rs

@@ -4,7 +4,7 @@
 use {Error, Result};
 use errno::Errno;
 use features;
-use libc::{self, c_void, c_int, socklen_t, size_t, pid_t, uid_t, gid_t};
+use libc::{self, c_void, c_int, socklen_t, size_t};
 use std::{mem, ptr, slice};
 use std::os::unix::io::RawFd;
 use sys::time::TimeVal;
@@ -174,6 +174,53 @@ libc_bitflags!{
     }
 }
 
+cfg_if! {
+    if #[cfg(all(target_os = "linux", not(target_arch = "arm")))] {
+        use std::fmt;
+
+        /// Unix credentials of the sending process.
+        ///
+        /// This struct is used with the `SO_PEERCRED` ancillary message for UNIX sockets.
+        #[repr(C)]
+        #[derive(Clone, Copy)]
+        pub struct UnixCredentials(libc::ucred);
+
+        impl UnixCredentials {
+            /// Returns the process identifier
+            pub fn pid(&self) -> libc::pid_t {
+                self.0.pid
+            }
+
+            /// Returns the user identifier
+            pub fn uid(&self) -> libc::uid_t {
+                self.0.uid
+            }
+
+            /// Returns the group identifier
+            pub fn gid(&self) -> libc::gid_t {
+                self.0.gid
+            }
+        }
+
+        impl PartialEq for UnixCredentials {
+            fn eq(&self, other: &Self) -> bool {
+                self.0.pid == other.0.pid && self.0.uid == other.0.uid && self.0.gid == other.0.gid
+            }
+        }
+        impl Eq for UnixCredentials {}
+
+        impl fmt::Debug for UnixCredentials {
+            fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                f.debug_struct("UnixCredentials")
+                    .field("pid", &self.0.pid)
+                    .field("uid", &self.0.uid)
+                    .field("gid", &self.0.gid)
+                    .finish()
+            }
+        }
+    }
+}
+
 /// Copy the in-memory representation of src into the byte slice dst,
 /// updating the slice to point to the remainder of dst only. Unsafe
 /// because it exposes all bytes in src, which may be UB if some of them
@@ -799,14 +846,6 @@ pub fn send(fd: RawFd, buf: &[u8], flags: MsgFlags) -> Result<usize> {
     Errno::result(ret).map(|r| r as usize)
 }
 
-#[repr(C)]
-#[derive(Clone, Copy, PartialEq, Eq, Debug)]
-pub struct ucred {
-    pid: pid_t,
-    uid: uid_t,
-    gid: gid_t,
-}
-
 /*
  *
  * ===== Socket Options =====

src/sys/socket/sockopt.rs

@@ -152,7 +152,7 @@ sockopt_impl!(Both, OobInline, libc::SOL_SOCKET, libc::SO_OOBINLINE, bool);
 sockopt_impl!(GetOnly, SocketError, libc::SOL_SOCKET, libc::SO_ERROR, i32);
 sockopt_impl!(Both, KeepAlive, libc::SOL_SOCKET, libc::SO_KEEPALIVE, bool);
 #[cfg(all(target_os = "linux", not(target_arch="arm")))]
-sockopt_impl!(GetOnly, PeerCredentials, libc::SOL_SOCKET, libc::SO_PEERCRED, super::ucred);
+sockopt_impl!(GetOnly, PeerCredentials, libc::SOL_SOCKET, libc::SO_PEERCRED, super::UnixCredentials);
 #[cfg(any(target_os = "macos",
           target_os = "ios"))]
 sockopt_impl!(Both, TcpKeepAlive, libc::IPPROTO_TCP, libc::TCP_KEEPALIVE, u32);
@@ -384,7 +384,7 @@ mod test {
         let a_cred = getsockopt(a, super::PeerCredentials).unwrap();
         let b_cred = getsockopt(b, super::PeerCredentials).unwrap();
         assert_eq!(a_cred, b_cred);
-        assert!(a_cred.pid != 0);
+        assert!(a_cred.pid() != 0);
     }
 
     #[test]