Add fchown(2) wrapper

CHANGELOG.md

@@ -5,6 +5,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased] - ReleaseDate
 ### Added
+- Added `fchown(2)` wrapper.
+  (#[1257](https://github.com/nix-rust/nix/pull/1257))
 - Added support on linux systems for `MAP_HUGE_`_`SIZE`_ family of flags.
   (#[1211](https://github.com/nix-rust/nix/pull/1211))
 - Added support for `F_OFD_*` `fcntl` commands on Linux and Android.

src/unistd.rs

@@ -651,6 +651,20 @@ pub fn chown<P: ?Sized + NixPath>(path: &P, owner: Option<Uid>, group: Option<Gi
     Errno::result(res).map(drop)
 }
 
+/// Change the ownership of the file referred to by the open file descriptor `fd` to be owned by
+/// the specified `owner` (user) and `group` (see
+/// [fchown(2)](https://pubs.opengroup.org/onlinepubs/9699919799/functions/fchown.html)).
+///
+/// The owner/group for the provided file will not be modified if `None` is
+/// provided for that argument.  Ownership change will be attempted for the path
+/// only if `Some` owner/group is provided.
+#[inline]
+pub fn fchown(fd: RawFd, owner: Option<Uid>, group: Option<Gid>) -> Result<()> {
+    let (uid, gid) = chown_raw_ids(owner, group);
+    let res = unsafe { libc::fchown(fd, uid, gid) };
+    Errno::result(res).map(drop)
+}
+
 /// Flags for `fchownat` function.
 #[derive(Clone, Copy, Debug)]
 pub enum FchownatFlags {

test/test_unistd.rs

@@ -17,6 +17,7 @@ use std::ffi::CString;
 use std::fs::DirBuilder;
 use std::fs::{self, File};
 use std::io::Write;
+use std::mem;
 use std::os::unix::prelude::*;
 use tempfile::{tempdir, tempfile};
 use libc::{_exit, off_t};
@@ -409,6 +410,23 @@ fn test_chown() {
     chown(&path, uid, gid).unwrap_err();
 }
 
+#[test]
+fn test_fchown() {
+    // Testing for anything other than our own UID/GID is hard.
+    let uid = Some(getuid());
+    let gid = Some(getgid());
+
+    let path = tempfile().unwrap();
+    let fd = path.as_raw_fd();
+
+    fchown(fd, uid, gid).unwrap();
+    fchown(fd, uid, None).unwrap();
+    fchown(fd, None, gid).unwrap();
+
+    mem::drop(path);
+    fchown(fd, uid, gid).unwrap_err();
+}
+
 #[test]
 #[cfg(not(target_os = "redox"))]
 fn test_fchownat() {