Merge #1342

1342: feat(unistd): Add getpeereid(3) r=asomers a=woodruffw

`getpeereid(3)` is not POSIX, but it's present on many BSD-derived Unices. It's also the standard mechanism on those OSes for retrieving socket peer credentials (compare `getsockopt` + `SO_PEERCRED` on Linux, which `nix` already supports).

Closes #1339.

Co-authored-by: William Woodruff <william@yossarian.net>

3 files changed, 60 insertions, 3 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cdd06714..64e33f33 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,11 +6,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased] - ReleaseDate
 ### Added
 - Added `mremap` (#[1306](https://github.com/nix-rust/nix/pull/1306))
-
 - Added `personality` (#[1331](https://github.com/nix-rust/nix/pull/1331))
-
 - Added limited Fuchsia support (#[1285](https://github.com/nix-rust/nix/pull/1285))
-
+- Added `getpeereid` (#[1342](https://github.com/nix-rust/nix/pull/1342))
 ### Fixed
 ### Changed
 
diff --git a/src/unistd.rs b/src/unistd.rs
index 20d7c79a..dd335596 100644
--- a/src/unistd.rs
+++ b/src/unistd.rs
@@ -2804,3 +2804,23 @@ 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)))
+}
diff --git a/test/test_unistd.rs b/test/test_unistd.rs
index 0f6cf7d3..16a8a05d 100644
--- a/test/test_unistd.rs
+++ b/test/test_unistd.rs
@@ -1069,3 +1069,42 @@ fn test_ttyname_not_pty() {
 fn test_ttyname_invalid_fd() {
     assert_eq!(ttyname(-1), Err(Error::Sys(Errno::EBADF)));
 }
+
+#[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());
+}