fix only_v6() panic on windows

Author: zh-jq

src/socket.rs

@@ -1932,10 +1932,13 @@ impl Socket {
     ///
     /// [`set_only_v6`]: Socket::set_only_v6
     pub fn only_v6(&self) -> io::Result<bool> {
+        #[cfg(not(windows))]
         unsafe {
-            getsockopt::<c_int>(self.as_raw(), sys::IPPROTO_IPV6, sys::IPV6_V6ONLY)
+            getsockopt::<Bool>(self.as_raw(), sys::IPPROTO_IPV6, sys::IPV6_V6ONLY)
                 .map(|only_v6| only_v6 != 0)
         }
+        #[cfg(windows)]
+        sys::only_v6(self.as_raw())
     }
 
     /// Set the value for the `IPV6_V6ONLY` option on this socket.

src/sys/windows.rs

@@ -892,6 +892,26 @@ pub(crate) fn to_mreqn(
     }
 }
 
+pub(crate) fn only_v6(socket: RawSocket) -> io::Result<bool> {
+    let mut optval: c_int = 0; // input must be 4 bytes (DWORD)
+    let mut optlen = mem::size_of_val(&optval) as c_int;
+    syscall!(
+        getsockopt(
+            socket,
+            IPPROTO_IPV6 as i32,
+            IPV6_V6ONLY,
+            ptr::from(&mut optval).cast(),
+            &mut optlen,
+        ),
+        PartialEq::eq,
+        SOCKET_ERROR
+    )
+    .map(|_| {
+        // optlen may be 1, which won't match the input size of optval
+        optval != 0
+    })
+}
+
 #[cfg(feature = "all")]
 pub(crate) fn original_dst_v4(socket: RawSocket) -> io::Result<SockAddr> {
     unsafe {