unistd: Add getpeereid(3)

Closes #1339.

Author: woodruffw

CHANGELOG.md

@@ -7,6 +7,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ### Added
 - Added `mremap` (#[1306](https://github.com/nix-rust/nix/pull/1306))
 
+- Added `getpeereid` (#[1342](https://github.com/nix-rust/nix/pull/1342))
+### Changed
 ### Fixed
 ### Changed
 ### Removed

src/unistd.rs

@@ -2785,3 +2785,23 @@ pub fn ttyname(fd: RawFd) -> Result<PathBuf> {
     buf.truncate(nul);
     Ok(OsString::from_vec(buf).into())
 }
+
+/// Get the effective user ID and group ID associated with a Unix domain socket.
+///
+/// See also [getpeereid(3)](https://www.freebsd.org/cgi/man.cgi?query=getpeereid)
+#[cfg(any(
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "freebsd",
+    target_os = "openbsd",
+    target_os = "netbsd",
+    target_os = "dragonfly",
+))]
+pub fn getpeereid(fd: RawFd) -> Result<(Uid, Gid)> {
+    let mut uid = 1;
+    let mut gid = 1;
+
+    let ret = unsafe { libc::getpeereid(fd, &mut uid, &mut gid) };
+
+    Errno::result(ret).map(|_| (Uid(uid), Gid(gid)))
+}

test/test_unistd.rs

@@ -1044,3 +1044,42 @@ fn test_ttyname_invalid_fd() {
 fn test_ttyname_invalid_fd() {
     assert_eq!(ttyname(-1), Err(Error::Sys(Errno::ENOTTY)));
 }
+
+#[test]
+#[cfg(any(
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "freebsd",
+    target_os = "openbsd",
+    target_os = "netbsd",
+    target_os = "dragonfly",
+))]
+fn test_getpeereid() {
+    use std::os::unix::net::UnixStream;
+    let (sock_a, sock_b) = UnixStream::pair().unwrap();
+
+    let (uid_a, gid_a) = getpeereid(sock_a.as_raw_fd()).unwrap();
+    let (uid_b, gid_b) = getpeereid(sock_b.as_raw_fd()).unwrap();
+
+    let uid = geteuid();
+    let gid = getegid();
+
+    assert_eq!(uid, uid_a);
+    assert_eq!(gid, gid_a);
+    assert_eq!(uid_a, uid_b);
+    assert_eq!(gid_a, gid_b);
+}
+
+#[test]
+#[cfg(any(
+    target_os = "macos",
+    target_os = "ios",
+    target_os = "freebsd",
+    target_os = "openbsd",
+    target_os = "netbsd",
+    target_os = "dragonfly",
+))]
+fn test_getpeereid_invalid_fd() {
+    // getpeereid is not POSIX, so error codes are inconsistent between different Unices.
+    assert!(getpeereid(-1).is_err());
+}