unistd: add fchdir(2)

This introduces a wrapper for fchdir(2), allowing a process to change
directory based on an open file descriptor.

The underlying function is available in libc crate since 0.2.20.

src/unistd.rs

@@ -249,6 +249,19 @@ pub fn chdir<P: ?Sized + NixPath>(path: &P) -> Result<()> {
     Errno::result(res).map(drop)
 }
 
+/// Change the current working directory of the process to the one
+/// given as an open file descriptor (see
+/// [fchdir(2)](http://man7.org/linux/man-pages/man2/fchdir.2.html)).
+///
+/// This function may fail in a number of different scenarios.  See the man
+/// pages for additional details on possible failure cases.
+#[inline]
+pub fn fchdir(dirfd: RawFd) -> Result<()> {
+    let res = unsafe { libc::fchdir(dirfd) };
+
+    Errno::result(res).map(drop)
+}
+
 /// Creates new directory `path` with access rights `mode`.
 ///
 /// # Errors

test/test_unistd.rs

@@ -6,6 +6,7 @@ use nix::sys::wait::*;
 use nix::sys::stat;
 use std::iter;
 use std::ffi::CString;
+use std::fs::File;
 use std::io::{Write, Read};
 use std::os::unix::prelude::*;
 use std::env::current_dir;
@@ -141,6 +142,24 @@ macro_rules! execve_test_factory(
     )
 );
 
+#[test]
+fn test_fchdir() {
+    let tmpdir = TempDir::new("test_fchdir").unwrap();
+    let tmpdir_path = tmpdir.path().canonicalize().unwrap();
+    let tmpdir_fd = File::open(&tmpdir_path).unwrap().into_raw_fd();
+    let olddir_path = getcwd().unwrap();
+    let olddir_fd = File::open(&olddir_path).unwrap().into_raw_fd();
+
+    assert!(fchdir(tmpdir_fd).is_ok());
+    assert_eq!(getcwd().unwrap(), tmpdir_path);
+
+    assert!(fchdir(olddir_fd).is_ok());
+    assert_eq!(getcwd().unwrap(), olddir_path);
+
+    assert!(close(olddir_fd).is_ok());
+    assert!(close(tmpdir_fd).is_ok());
+}
+
 #[test]
 fn test_getcwd() {
     let tmp_dir = TempDir::new("test_getcwd").unwrap();