net: allow WriteMsgUDPAddrPort write with the default addrport

src/net/ipsock_posix.go

@@ -238,7 +238,7 @@ func addrPortToSockaddrInet4(ap netip.AddrPort) (syscall.SockaddrInet4, error) {
 	// ipToSockaddrInet4 has special handling here for zero length slices.
 	// We do not, because netip has no concept of a generic zero IP address.
 	addr := ap.Addr()
-	if !addr.Is4() {
+	if addr.IsValid() && !addr.Is4() {
 		return syscall.SockaddrInet4{}, &AddrError{Err: "non-IPv4 address", Addr: addr.String()}
 	}
 	sa := syscall.SockaddrInet4{
@@ -256,7 +256,7 @@ func addrPortToSockaddrInet6(ap netip.AddrPort) (syscall.SockaddrInet6, error) {
 	// to an IPv4-mapped IPv6 address.
 	// The error message is kept consistent with ipToSockaddrInet6.
 	addr := ap.Addr()
-	if !addr.IsValid() {
+	if addr.IsValid() && !addr.Is6() {
 		return syscall.SockaddrInet6{}, &AddrError{Err: "non-IPv6 address", Addr: addr.String()}
 	}
 	sa := syscall.SockaddrInet6{

src/net/udpsock_posix.go

@@ -186,17 +186,29 @@ func (c *UDPConn) writeMsgAddrPort(b, oob []byte, addr netip.AddrPort) (n, oobn
 
 	switch c.fd.family {
 	case syscall.AF_INET:
-		sa, err := addrPortToSockaddrInet4(addr)
-		if err != nil {
-			return 0, 0, err
+		saPtr := &syscall.SockaddrInet4{}
+		if addr.IsValid() {
+			sa, err := addrPortToSockaddrInet4(addr)
+			if err != nil {
+				return 0, 0, err
+			}
+			saPtr = &sa
+		} else {
+			saPtr = nil
 		}
-		return c.fd.writeMsgInet4(b, oob, &sa)
+		return c.fd.writeMsgInet4(b, oob, saPtr)
 	case syscall.AF_INET6:
-		sa, err := addrPortToSockaddrInet6(addr)
-		if err != nil {
-			return 0, 0, err
+		saPtr := &syscall.SockaddrInet6{}
+		if addr.IsValid() {
+			sa, err := addrPortToSockaddrInet6(addr)
+			if err != nil {
+				return 0, 0, err
+			}
+			saPtr = &sa
+		} else {
+			saPtr = nil
 		}
-		return c.fd.writeMsgInet6(b, oob, &sa)
+		return c.fd.writeMsgInet6(b, oob, saPtr)
 	default:
 		return 0, 0, &AddrError{Err: "invalid address family", Addr: addr.Addr().String()}
 	}

src/net/udpsock_test.go

@@ -705,3 +705,31 @@ func TestIPv6WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestWriteToUDPAddrPort(t *testing.T) {
+	dialer := Dialer{}
+	c, err := dialer.Dial("udp", "[::1]:12345")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	ra, err := ResolveUDPAddr("udp", "[::1]:12345")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	b := []byte("CONNECTED-MODE SOCKET")
+
+	_, _, err = c.(*UDPConn).WriteMsgUDPAddrPort(b, nil, ra.AddrPort())
+	if err == nil {
+		t.Fatal("should fail")
+	}
+	if err != nil && err.(*OpError).Err != ErrWriteToConnected {
+		t.Fatalf("should fail as ErrWriteToConnected: %v", err)
+	}
+	_, _, err = c.(*UDPConn).WriteMsgUDPAddrPort(b, nil, netip.AddrPort{})
+	if err != nil {
+		t.Fatal(err)
+	}
+}

src/syscall/syscall_unix.go

@@ -410,17 +410,25 @@ func SendmsgN(fd int, p, oob []byte, to Sockaddr, flags int) (n int, err error)
 }
 
 func sendmsgNInet4(fd int, p, oob []byte, to *SockaddrInet4, flags int) (n int, err error) {
-	ptr, salen, err := to.sockaddr()
-	if err != nil {
-		return 0, err
+	var ptr unsafe.Pointer
+	var salen _Socklen
+	if to != nil {
+		ptr, salen, err = to.sockaddr()
+		if err != nil {
+			return 0, err
+		}
 	}
 	return sendmsgN(fd, p, oob, ptr, salen, flags)
 }
 
 func sendmsgNInet6(fd int, p, oob []byte, to *SockaddrInet6, flags int) (n int, err error) {
-	ptr, salen, err := to.sockaddr()
-	if err != nil {
-		return 0, err
+	var ptr unsafe.Pointer
+	var salen _Socklen
+	if to != nil {
+		ptr, salen, err = to.sockaddr()
+		if err != nil {
+			return 0, err
+		}
 	}
 	return sendmsgN(fd, p, oob, ptr, salen, flags)
 }