windows: Fix NLMTU for ipv6 (#1871)

udp/udp_rio_windows.go

@@ -31,7 +31,8 @@ func procyield(cycles uint32)
 
 const (
 	packetsPerRing = 1024
-	bytesPerPacket = 2048 - 32
+	// Caps tun.mtu at MTU-32 direct, MTU-64 relayed, unenforced anywhere else. 17.6MB page locked per socket.
+	bytesPerPacket = MTU
 	receiveSpins   = 15
 )
 
@@ -69,12 +70,14 @@ func NewRIOListener(l *slog.Logger, addr netip.Addr, port int) (*RIOConn, error)
 
 	err := u.bind(l, &windows.SockaddrInet6{Addr: addr.As16(), Port: port})
 	if err != nil {
+		u.close()
 		return nil, fmt.Errorf("bind: %w", err)
 	}
 
 	for i := 0; i < packetsPerRing; i++ {
 		err = u.insertReceiveRequest()
 		if err != nil {
+			u.close()
 			return nil, fmt.Errorf("init rx ring: %w", err)
 		}
 	}
@@ -356,15 +359,25 @@ func (u *RIOConn) Close() error {
 		return nil
 	}
 
+	u.close()
+	return nil
+}
+
+// Also unwinds a partial build from NewRIOListener, where isOpen is false and Close would no-op.
+// Socket first, unlike wireguard-go: receive() re-arms every slot, so freeing the rings under a live socket
+// hands the kernel freed pages for all packetsPerRing outstanding receives.
+func (u *RIOConn) close() {
+	// WSASocket reports failure as InvalidHandle, not zero.
+	if u.sock != 0 && u.sock != windows.InvalidHandle {
+		windows.CloseHandle(u.sock)
+	}
+	u.sock = 0
+
 	windows.PostQueuedCompletionStatus(u.rx.iocp, 0, 0, nil)
 	windows.PostQueuedCompletionStatus(u.tx.iocp, 0, 0, nil)
 
 	u.rx.CloseAndZero()
 	u.tx.CloseAndZero()
-	if u.sock != 0 {
-		windows.CloseHandle(u.sock)
-	}
-	return nil
 }
 
 func (ring *ringBuffer) Push() *ringPacket {