net: allow zero value destination address in WriteMsgUDPAddrPort

The existing address validity checks already cover both connected and
non-connected sockets. Pass a nil sockaddr just like WriteMsgUDP, when
the address is zero value.

TestWriteToUDP is extended to cover the netip APIs.

Fixes #74841

Change-Id: I2708e7747e224958198fe7abb3fcd8d59bc5a88a
Reviewed-on: https://go-review.googlesource.com/c/go/+/692437
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Damien Neil <dneil@google.com>

4 files changed, 47 insertions, 33 deletions

diff --git a/src/net/ipsock_posix.go b/src/net/ipsock_posix.go
index 52712f932f..1ed37d7e6e 100644
--- a/src/net/ipsock_posix.go
+++ b/src/net/ipsock_posix.go
@@ -260,9 +260,6 @@ 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() {
-		return syscall.SockaddrInet6{}, &AddrError{Err: "non-IPv6 address", Addr: addr.String()}
-	}
 	sa := syscall.SockaddrInet6{
 		Addr:   addr.As16(),
 		Port:   int(ap.Port()),
diff --git a/src/net/net_fake.go b/src/net/net_fake.go
index f7eb28e01a..cc35d90a26 100644
--- a/src/net/net_fake.go
+++ b/src/net/net_fake.go
@@ -1082,11 +1082,19 @@ func (ffd *fakeNetFD) writeMsg(p []byte, oob []byte, sa syscall.Sockaddr) (n int
 	return n, 0, err
 }
 
-func (ffd *fakeNetFD) writeMsgInet4(p []byte, oob []byte, sa *syscall.SockaddrInet4) (n int, oobn int, err error) {
+func (ffd *fakeNetFD) writeMsgInet4(p []byte, oob []byte, sa4 *syscall.SockaddrInet4) (n int, oobn int, err error) {
+	var sa syscall.Sockaddr
+	if sa4 != nil {
+		sa = sa4
+	}
 	return ffd.writeMsg(p, oob, sa)
 }
 
-func (ffd *fakeNetFD) writeMsgInet6(p []byte, oob []byte, sa *syscall.SockaddrInet6) (n int, oobn int, err error) {
+func (ffd *fakeNetFD) writeMsgInet6(p []byte, oob []byte, sa6 *syscall.SockaddrInet6) (n int, oobn int, err error) {
+	var sa syscall.Sockaddr
+	if sa6 != nil {
+		sa = sa6
+	}
 	return ffd.writeMsg(p, oob, sa)
 }
 
diff --git a/src/net/udpsock_posix.go b/src/net/udpsock_posix.go
index 3cd1d0a762..99e4359e71 100644
--- a/src/net/udpsock_posix.go
+++ b/src/net/udpsock_posix.go
@@ -186,17 +186,25 @@ 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
+		var sap *syscall.SockaddrInet4
+		if addr.IsValid() {
+			sa, err := addrPortToSockaddrInet4(addr)
+			if err != nil {
+				return 0, 0, err
+			}
+			sap = &sa
 		}
-		return c.fd.writeMsgInet4(b, oob, &sa)
+		return c.fd.writeMsgInet4(b, oob, sap)
 	case syscall.AF_INET6:
-		sa, err := addrPortToSockaddrInet6(addr)
-		if err != nil {
-			return 0, 0, err
+		var sap *syscall.SockaddrInet6
+		if addr.IsValid() {
+			sa, err := addrPortToSockaddrInet6(addr)
+			if err != nil {
+				return 0, 0, err
+			}
+			sap = &sa
 		}
-		return c.fd.writeMsgInet6(b, oob, &sa)
+		return c.fd.writeMsgInet6(b, oob, sap)
 	default:
 		return 0, 0, &AddrError{Err: "invalid address family", Addr: addr.Addr().String()}
 	}
diff --git a/src/net/udpsock_test.go b/src/net/udpsock_test.go
index 3c8da43bdd..7b4bf328e2 100644
--- a/src/net/udpsock_test.go
+++ b/src/net/udpsock_test.go
@@ -141,36 +141,37 @@ func testWriteToConn(t *testing.T, raddr string) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	rap := ra.AddrPort()
+
+	assertErrWriteToConnected := func(t *testing.T, err error) {
+		t.Helper()
+		if e, ok := err.(*OpError); !ok || e.Err != ErrWriteToConnected {
+			t.Errorf("got %v; want ErrWriteToConnected", err)
+		}
+	}
 
 	b := []byte("CONNECTED-MODE SOCKET")
+	_, err = c.(*UDPConn).WriteToUDPAddrPort(b, rap)
+	assertErrWriteToConnected(t, err)
 	_, err = c.(*UDPConn).WriteToUDP(b, ra)
-	if err == nil {
-		t.Fatal("should fail")
-	}
-	if err != nil && err.(*OpError).Err != ErrWriteToConnected {
-		t.Fatalf("should fail as ErrWriteToConnected: %v", err)
-	}
+	assertErrWriteToConnected(t, err)
 	_, err = c.(*UDPConn).WriteTo(b, ra)
-	if err == nil {
-		t.Fatal("should fail")
-	}
-	if err != nil && err.(*OpError).Err != ErrWriteToConnected {
-		t.Fatalf("should fail as ErrWriteToConnected: %v", err)
-	}
+	assertErrWriteToConnected(t, err)
 	_, err = c.Write(b)
 	if err != nil {
-		t.Fatal(err)
+		t.Errorf("c.Write(b) = %v; want nil", err)
 	}
 	_, _, err = c.(*UDPConn).WriteMsgUDP(b, nil, ra)
-	if err == nil {
-		t.Fatal("should fail")
-	}
-	if err != nil && err.(*OpError).Err != ErrWriteToConnected {
-		t.Fatalf("should fail as ErrWriteToConnected: %v", err)
-	}
+	assertErrWriteToConnected(t, err)
 	_, _, err = c.(*UDPConn).WriteMsgUDP(b, nil, nil)
 	if err != nil {
-		t.Fatal(err)
+		t.Errorf("c.WriteMsgUDP(b, nil, nil) = %v; want nil", err)
+	}
+	_, _, err = c.(*UDPConn).WriteMsgUDPAddrPort(b, nil, rap)
+	assertErrWriteToConnected(t, err)
+	_, _, err = c.(*UDPConn).WriteMsgUDPAddrPort(b, nil, netip.AddrPort{})
+	if err != nil {
+		t.Errorf("c.WriteMsgUDPAddrPort(b, nil, netip.AddrPort{}) = %v; want nil", err)
 	}
 }