Add support for sendmsg(2), recvmsg(2), and cmsg(3)

The best specification for control message layout appears to be
[RFC 2292, section 4](https://tools.ietf.org/html/rfc2292#section-4),
despite this not being a wire protocol. These definitions have also been
checked against glibc 2.19 <bits/socket.h> and Linux 4.0
<linux/socket.h>, and tested on Debian 8.1 and FreeBSD 10.2 x86_64.

The API differs a bit from the cmsg(3) API for type-safety reasons (and
also because the cmsg(3) API is terrible). See test/sys/test_socket.rs
for an example.

Only supports SCM_RIGHTS at the moment.

Fixes #88.

1 files changed, 54 insertions, 1 deletions

diff --git a/test/sys/test_socket.rs b/test/sys/test_socket.rs
index e8cb4bed..7b95767e 100644
--- a/test/sys/test_socket.rs
+++ b/test/sys/test_socket.rs
@@ -2,7 +2,7 @@ use nix::sys::socket::{InetAddr, UnixAddr, getsockname};
 use std::{mem, net};
 use std::path::Path;
 use std::str::FromStr;
-use std::os::unix::io::AsRawFd;
+use std::os::unix::io::{AsRawFd, RawFd};
 use ports::localhost;
 
 #[test]
@@ -63,3 +63,56 @@ pub fn test_socketpair() {
 
     assert_eq!(&buf[..], b"hello");
 }
+
+#[test]
+pub fn test_scm_rights() {
+    use nix::sys::uio::IoVec;
+    use nix::unistd::{pipe, read, write, close};
+    use nix::sys::socket::{socketpair, sendmsg, recvmsg,
+                           AddressFamily, SockType, SockFlag,
+                           ControlMessage, CmsgSpace,
+                           MSG_TRUNC, MSG_CTRUNC};
+
+    let (fd1, fd2) = socketpair(AddressFamily::Unix, SockType::Stream, 0,
+                                SockFlag::empty())
+                     .unwrap();
+    let (r, w) = pipe().unwrap();
+    let mut received_r: Option<RawFd> = None;
+
+    {
+        let iov = [IoVec::from_slice(b"hello")];
+        let fds = [r];
+        let cmsg = ControlMessage::ScmRights(&fds);
+        assert_eq!(sendmsg(fd1, &iov, &[cmsg], 0, None).unwrap(), 5);
+        close(r).unwrap();
+        close(fd1).unwrap();
+    }
+
+    {
+        let mut buf = [0u8; 5];
+        let iov = [IoVec::from_mut_slice(&mut buf[..])];
+        let mut cmsgspace: CmsgSpace<[RawFd; 1]> = CmsgSpace::new();
+        let msg = recvmsg(fd2, &iov, Some(&mut cmsgspace), 0).unwrap();
+
+        for cmsg in msg.cmsgs() {
+            if let ControlMessage::ScmRights(fd) = cmsg {
+                assert_eq!(received_r, None);
+                assert_eq!(fd.len(), 1);
+                received_r = Some(fd[0]);
+            } else {
+                panic!("unexpected cmsg");
+            }
+        }
+        assert_eq!(msg.flags & (MSG_TRUNC | MSG_CTRUNC), 0);
+        close(fd2).unwrap();
+    }
+
+    let received_r = received_r.expect("Did not receive passed fd");
+    // Ensure that the received file descriptor works
+    write(w, b"world").unwrap();
+    let mut buf = [0u8; 5];
+    read(received_r, &mut buf).unwrap();
+    assert_eq!(&buf[..], b"world");
+    close(received_r).unwrap();
+    close(w).unwrap();
+}