net: fix WriteMsgUDPAddrPort addr handling

WriteMsgUDPAddrPort should accept IPv4 target addresses on IPv6 UDP sockets.
An IPv4 target address will be converted to an IPv4-mapped IPv6 address.

Fixes golang#52264.

src/net/ipsock_posix.go

@@ -215,8 +215,12 @@ func addrPortToSockaddrInet4(ap netip.AddrPort) (syscall.SockaddrInet4, error) {
 func addrPortToSockaddrInet6(ap netip.AddrPort) (syscall.SockaddrInet6, error) {
 	// ipToSockaddrInet6 has special handling here for zero length slices.
 	// We do not, because netip has no concept of a generic zero IP address.
+	//
+	// addr is allowed to be an IPv4 address, because As16 will convert it
+	// to an IPv4-mapped IPv6 address.
+	// The error message is kept consistent with ipToSockaddrInet6.
 	addr := ap.Addr()
-	if !addr.Is6() {
+	if !addr.IsValid() {
 		return syscall.SockaddrInet6{}, &AddrError{Err: "non-IPv6 address", Addr: addr.String()}
 	}
 	sa := syscall.SockaddrInet6{

src/net/udpsock_test.go

@@ -9,6 +9,7 @@ package net
 import (
 	"errors"
 	"internal/testenv"
+	"net/netip"
 	"os"
 	"reflect"
 	"runtime"
@@ -622,3 +623,38 @@ func TestUDPIPVersionReadMsg(t *testing.T) {
 		t.Error("returned UDPAddr is not IPv4")
 	}
 }
+
+// TestIPv6WriteMsgUDPAddrPortTargetAddrIPVersion verifies that
+// WriteMsgUDPAddrPort accepts IPv4, IPv4-mapped IPv6, and IPv6 target addresses
+// on a UDPConn listening on "::".
+func TestIPv6WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
+	if !supportsIPv6() {
+		t.Skip("IPv6 is not supported")
+	}
+
+	conn, err := ListenUDP("udp", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	daddr4 := netip.AddrPortFrom(netip.MustParseAddr("127.0.0.1"), 12345)
+	daddr4in6 := netip.AddrPortFrom(netip.MustParseAddr("::ffff:127.0.0.1"), 12345)
+	daddr6 := netip.AddrPortFrom(netip.MustParseAddr("::1"), 12345)
+	buf := make([]byte, 8)
+
+	_, _, err = conn.WriteMsgUDPAddrPort(buf, nil, daddr4)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, _, err = conn.WriteMsgUDPAddrPort(buf, nil, daddr4in6)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, _, err = conn.WriteMsgUDPAddrPort(buf, nil, daddr6)
+	if err != nil {
+		t.Fatal(err)
+	}
+}