Turn on SO_LINGER for client communication sockets.

This reduces the probability that the receiver receives errors when we
close our end of the socket with data remaining.

There is deadlock potential with this patch, because turning on `SO_LINGER`
causes `close()` to block until the receiver has received all the data. If
deadlocks happen, a workaround will be to close sockets in a separate thread.
This is ugly and slow, so I don't want to do that unless we need to.

Might fix servo#29.

platform/linux/mod.rs

@@ -480,6 +480,7 @@ impl UnixOneShotServer {
             if client_fd < 0 {
                 return Err(UnixError::last())
             }
+            make_socket_lingering(client_fd);
 
             let receiver = UnixReceiver {
                 fd: client_fd,
@@ -490,6 +491,26 @@ impl UnixOneShotServer {
     }
 }
 
+// Make sure that the kernel doesn't return errors to readers if there's still data left after we
+// close our end.
+//
+// See, for example, https://github.com/servo/ipc-channel/issues/29
+fn make_socket_lingering(sockfd: c_int) -> Result<(),UnixError> {
+    let linger = linger {
+        l_onoff: 1,
+        l_linger: 30,
+    };
+    let err = libc::setsockopt(sockfd,
+                               SOL_SOCKET,
+                               SO_LINGER,
+                               &linger as *const linger as *const c_void,
+                               mem::size_of::<linger>() as socklen_t);
+    if err < 0 {
+        return Err(UnixError::last())
+    }
+    Ok(())
+}
+
 pub struct UnixSharedMemory {
     ptr: *mut u8,
     length: usize,
@@ -853,10 +874,11 @@ fn is_socket(fd: c_int) -> bool {
 // FFI stuff follows:
 
 const POLLIN: c_short = 0x01;
-const S_IFMT: mode_t = 0o00170000;
-const S_IFSOCK: mode_t = 0o0140000;
 const SCM_RIGHTS: c_int = 0x01;
 const SOCK_SEQPACKET: c_int = 0x05;
+const SO_LINGER: c_int = 13;
+const S_IFMT: mode_t = 0o00170000;
+const S_IFSOCK: mode_t = 0o0140000;
 
 #[allow(non_camel_case_types)]
 type nfds_t = c_ulong;
@@ -893,6 +915,12 @@ extern {
     fn poll(fds: *mut pollfd, nfds: nfds_t, timeout: c_int) -> c_int;
     fn recvmsg(socket: c_int, message: *mut msghdr, flags: c_int) -> ssize_t;
     fn sendmsg(socket: c_int, message: *const msghdr, flags: c_int) -> ssize_t;
+    fn setsockopt(socket: c_int,
+                  level: c_int,
+                  option_name: c_int,
+                  option_value: *const c_void,
+                  option_len: socklen_t)
+                  -> c_int;
     fn socketpair(domain: c_int, socket_type: c_int, protocol: c_int, sv: *mut c_int) -> c_int;
     fn strdup(string: *const c_char) -> *mut c_char;
 }