Bring back UdpSocket::only_v6 (fixes #1424)

Author: djc

src/net/udp.rs

@@ -511,6 +511,12 @@ impl UdpSocket {
         self.inner.leave_multicast_v6(multiaddr, interface)
     }
 
+    /// Get the value of the `IPV6_V6ONLY` option on this socket.
+    #[allow(clippy::trivially_copy_pass_by_ref)]
+    pub fn only_v6(&self) -> io::Result<bool> {
+        sys::udp::only_v6(&self.inner)
+    }
+
     /// Get the value of the `SO_ERROR` option on this socket.
     ///
     /// This will retrieve the stored error in the underlying socket, clearing

src/sys/shell/udp.rs

@@ -4,3 +4,7 @@ use std::net::{self, SocketAddr};
 pub fn bind(_: SocketAddr) -> io::Result<net::UdpSocket> {
     os_required!()
 }
+
+pub(crate) fn only_v6(_: &net::UdpSocket) -> io::Result<bool> {
+    os_required!()
+}

src/sys/unix/udp.rs

@@ -1,8 +1,9 @@
 use crate::sys::unix::net::{new_ip_socket, socket_addr};
 
 use std::io;
+use std::mem;
 use std::net::{self, SocketAddr};
-use std::os::unix::io::FromRawFd;
+use std::os::unix::io::{AsRawFd, FromRawFd};
 
 pub fn bind(addr: SocketAddr) -> io::Result<net::UdpSocket> {
     // Gives a warning for non Apple platforms.
@@ -21,3 +22,18 @@ pub fn bind(addr: SocketAddr) -> io::Result<net::UdpSocket> {
             .map(|_| unsafe { net::UdpSocket::from_raw_fd(socket) })
     })
 }
+
+pub(crate) fn only_v6(socket: &net::UdpSocket) -> io::Result<bool> {
+    let mut optval: libc::c_int = 0;
+    let mut optlen = mem::size_of::<libc::c_int>() as libc::socklen_t;
+
+    syscall!(getsockopt(
+        socket.as_raw_fd(),
+        libc::IPPROTO_IPV6,
+        libc::IPV6_V6ONLY,
+        &mut optval as *mut _ as *mut _,
+        &mut optlen,
+    ))?;
+
+    Ok(optval != 0)
+}

src/sys/windows/udp.rs

@@ -1,9 +1,13 @@
 use std::io;
+use std::mem::{self, MaybeUninit};
 use std::net::{self, SocketAddr};
-use std::os::windows::io::FromRawSocket;
+use std::os::windows::io::{AsRawSocket, FromRawSocket};
 use std::os::windows::raw::SOCKET as StdSocket; // winapi uses usize, stdlib uses u32/u64.
 
-use winapi::um::winsock2::{bind as win_bind, closesocket, SOCKET_ERROR, SOCK_DGRAM};
+use winapi::ctypes::c_int;
+use winapi::shared::ws2def::IPPROTO_IPV6;
+use winapi::shared::ws2ipdef::IPV6_V6ONLY;
+use winapi::um::winsock2::{bind as win_bind, closesocket, getsockopt, SOCKET_ERROR, SOCK_DGRAM};
 
 use crate::sys::windows::net::{init, new_ip_socket, socket_addr};
 
@@ -25,3 +29,25 @@ pub fn bind(addr: SocketAddr) -> io::Result<net::UdpSocket> {
         .map(|_| unsafe { net::UdpSocket::from_raw_socket(socket as StdSocket) })
     })
 }
+
+pub(crate) fn only_v6(socket: &net::UdpSocket) -> io::Result<bool> {
+    let mut optval: MaybeUninit<c_int> = MaybeUninit::uninit();
+    let mut optlen = mem::size_of::<c_int>() as c_int;
+
+    syscall!(
+        getsockopt(
+            socket.as_raw_socket() as usize,
+            IPPROTO_IPV6 as c_int,
+            IPV6_V6ONLY as c_int,
+            optval.as_mut_ptr().cast(),
+            &mut optlen,
+        ),
+        PartialEq::eq,
+        SOCKET_ERROR
+    )?;
+
+    debug_assert_eq!(optlen as usize, mem::size_of::<c_int>());
+    // Safety: `getsockopt` initialised `optval` for us.
+    let optval = unsafe { optval.assume_init() };
+    Ok(optval != 0)
+}