windows: add GetBestInterfaceEx

Change-Id: I56f9212420f96a0f275e79b4a9766b09b175254d
Reviewed-on: https://go-review.googlesource.com/c/sys/+/413495
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Dan Kortschak <dan@kortschak.io>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

windows/syscall_windows.go

@@ -861,6 +861,7 @@ const socket_error = uintptr(^uint32(0))
 //sys	GetAdaptersAddresses(family uint32, flags uint32, reserved uintptr, adapterAddresses *IpAdapterAddresses, sizePointer *uint32) (errcode error) = iphlpapi.GetAdaptersAddresses
 //sys	GetACP() (acp uint32) = kernel32.GetACP
 //sys	MultiByteToWideChar(codePage uint32, dwFlags uint32, str *byte, nstr int32, wchar *uint16, nwchar int32) (nwrite int32, err error) = kernel32.MultiByteToWideChar
+//sys	getBestInterfaceEx(sockaddr unsafe.Pointer, pdwBestIfIndex *uint32) (errcode error) = iphlpapi.GetBestInterfaceEx
 
 // For testing: clients can set this flag to force
 // creation of IPv6 sockets to return EAFNOSUPPORT.
@@ -1045,6 +1046,14 @@ func Connect(fd Handle, sa Sockaddr) (err error) {
 	return connect(fd, ptr, n)
 }
 
+func GetBestInterfaceEx(sa Sockaddr, pdwBestIfIndex *uint32) (err error) {
+	ptr, _, err := sa.sockaddr()
+	if err != nil {
+		return err
+	}
+	return getBestInterfaceEx(ptr, pdwBestIfIndex)
+}
+
 func Getsockname(fd Handle) (sa Sockaddr, err error) {
 	var rsa RawSockaddrAny
 	l := int32(unsafe.Sizeof(rsa))

windows/zsyscall_windows.go

@@ -177,6 +177,7 @@ var (
 	procDnsRecordListFree                                    = moddnsapi.NewProc("DnsRecordListFree")
 	procGetAdaptersAddresses                                 = modiphlpapi.NewProc("GetAdaptersAddresses")
 	procGetAdaptersInfo                                      = modiphlpapi.NewProc("GetAdaptersInfo")
+	procGetBestInterfaceEx                                   = modiphlpapi.NewProc("GetBestInterfaceEx")
 	procGetIfEntry                                           = modiphlpapi.NewProc("GetIfEntry")
 	procAssignProcessToJobObject                             = modkernel32.NewProc("AssignProcessToJobObject")
 	procCancelIo                                             = modkernel32.NewProc("CancelIo")
@@ -1539,6 +1540,14 @@ func GetAdaptersInfo(ai *IpAdapterInfo, ol *uint32) (errcode error) {
 	return
 }
 
+func getBestInterfaceEx(sockaddr unsafe.Pointer, pdwBestIfIndex *uint32) (errcode error) {
+	r0, _, _ := syscall.Syscall(procGetBestInterfaceEx.Addr(), 2, uintptr(sockaddr), uintptr(unsafe.Pointer(pdwBestIfIndex)), 0)
+	if r0 != 0 {
+		errcode = syscall.Errno(r0)
+	}
+	return
+}
+
 func GetIfEntry(pIfRow *MibIfRow) (errcode error) {
 	r0, _, _ := syscall.Syscall(procGetIfEntry.Addr(), 1, uintptr(unsafe.Pointer(pIfRow)), 0, 0)
 	if r0 != 0 {