Fix recvmmsg(2) implementation

There were two problems discovered with the `recvmmsg(2)` implementation
that this changeset attempts to fix:

1. As mentioned in nix-rust/issues/1325, `recvmmsg(2)` can return fewer
   messages than requested, and
2. Passing the return value of `recvmmsg(2)` as the number of bytes in
   the messages received is incorrect.

This changeset incorporates the proposed fix from nix-rust/issues/1325,
as well as passing the correct value (`mmsghdr.msg_len`) for the number
of bytes in a given message.

src/sys/socket/mod.rs

@@ -1217,17 +1217,18 @@ pub fn recvmmsg<'a, I>(
 
     let ret = unsafe { libc::recvmmsg(fd, output.as_mut_ptr(), output.len() as _, flags.bits() as _, timeout) };
 
-    let r = Errno::result(ret)?;
+    let _ = Errno::result(ret)?;
 
     Ok(output
         .into_iter()
+        .take(ret as usize)
         .zip(addresses.iter().map(|addr| unsafe{addr.assume_init()}))
         .zip(results.into_iter())
         .map(|((mmsghdr, address), (msg_controllen, cmsg_buffer))| {
             unsafe {
                 read_mhdr(
                     mmsghdr.msg_hdr,
-                    r as isize,
+                    mmsghdr.msg_len as isize,
                     msg_controllen,
                     address,
                     cmsg_buffer

test/sys/test_socket.rs

@@ -437,6 +437,76 @@ mod recvfrom {
 
         send_thread.join().unwrap();
     }
+
+    #[cfg(any(
+        target_os = "linux",
+        target_os = "android",
+        target_os = "freebsd",
+        target_os = "netbsd",
+    ))]
+    #[test]
+    pub fn udp_recvmmsg_dontwait_short_read() {
+        use nix::sys::uio::IoVec;
+        use nix::sys::socket::{MsgFlags, recvmmsg};
+
+        const NUM_MESSAGES_SENT: usize = 2;
+        const DATA: [u8; 4] = [1,2,3,4];
+
+        let std_sa = SocketAddr::from_str("127.0.0.1:6799").unwrap();
+        let inet_addr = InetAddr::from_std(&std_sa);
+        let sock_addr = SockAddr::new_inet(inet_addr);
+
+        let rsock = socket(AddressFamily::Inet,
+            SockType::Datagram,
+            SockFlag::empty(),
+            None
+        ).unwrap();
+        bind(rsock, &sock_addr).unwrap();
+        let ssock = socket(
+            AddressFamily::Inet,
+            SockType::Datagram,
+            SockFlag::empty(),
+            None,
+        ).expect("send socket failed");
+
+        let send_thread = thread::spawn(move || {
+            for _ in 0..NUM_MESSAGES_SENT {
+                sendto(ssock, &DATA[..], &sock_addr, MsgFlags::empty()).unwrap();
+            }
+        });
+        // Ensure we've sent all the messages before continuing so `recvmmsg`
+        // will return right away
+        send_thread.join().unwrap();
+
+        let mut msgs = std::collections::LinkedList::new();
+
+        // Buffers to receive >`NUM_MESSAGES_SENT` messages to ensure `recvmmsg`
+        // will return when there are fewer than requested messages in the
+        // kernel buffers when using `MSG_DONTWAIT`.
+        let mut receive_buffers = [[0u8; 32]; NUM_MESSAGES_SENT + 2];
+        let iovs: Vec<_> = receive_buffers.iter_mut().map(|buf| {
+            [IoVec::from_mut_slice(&mut buf[..])]
+        }).collect();
+
+        for iov in &iovs {
+            msgs.push_back(RecvMmsgData {
+                iov: iov,
+                cmsg_buffer: None,
+            })
+        };
+
+        let res = recvmmsg(rsock, &mut msgs, MsgFlags::MSG_DONTWAIT, None).expect("recvmmsg");
+        assert_eq!(res.len(), NUM_MESSAGES_SENT);
+
+        for RecvMsg { address, bytes, .. } in res.into_iter() {
+            assert_eq!(AddressFamily::Inet, address.unwrap().family());
+            assert_eq!(DATA.len(), bytes);
+        }
+
+        for buf in &receive_buffers[..NUM_MESSAGES_SENT] {
+            assert_eq!(&buf[..DATA.len()], DATA);
+        }
+    }
 }
 
 // Test error handling of our recvmsg wrapper