Add durable HostID generation for Windows.

https://github.com/shirou/gopsutil/pull/312

vendor/github.com/shirou/gopsutil/host/host_windows.go

@@ -7,7 +7,9 @@ import (
 	"os"
 	"runtime"
 	"strings"
+	"syscall"
 	"time"
+	"unsafe"
 
 	"github.com/StackExchange/wmi"
 
@@ -33,11 +35,14 @@ func Info() (*InfoStat, error) {
 		OS: runtime.GOOS,
 	}
 
+	{
 		hostname, err := os.Hostname()
 		if err == nil {
 			ret.Hostname = hostname
 		}
+	}
 
+	{
 		platform, family, version, err := PlatformInformation()
 		if err == nil {
 			ret.Platform = platform
@@ -46,21 +51,61 @@ func Info() (*InfoStat, error) {
 		} else {
 			return ret, err
 		}
+	}
 
+	{
 		boot, err := BootTime()
 		if err == nil {
 			ret.BootTime = boot
 			ret.Uptime, _ = Uptime()
 		}
+	}
 
+	{
+		hostID, err := getMachineGuid()
+		if err == nil {
+			ret.HostID = hostID
+		}
+	}
+
+	{
 		procs, err := process.Pids()
 		if err == nil {
 			ret.Procs = uint64(len(procs))
 		}
+	}
 
 	return ret, nil
 }
 
+func getMachineGuid() (string, error) {
+	var h syscall.Handle
+	err := syscall.RegOpenKeyEx(syscall.HKEY_LOCAL_MACHINE, syscall.StringToUTF16Ptr(`SOFTWARE\Microsoft\Cryptography`), 0, syscall.KEY_READ, &h)
+	if err != nil {
+		return "", err
+	}
+	defer syscall.RegCloseKey(h)
+
+	const windowsRegBufLen = 74 // len(`{`) + len(`abcdefgh-1234-456789012-123345456671` * 2) + len(`}`) // 2 == bytes/UTF16
+	const uuidLen = 36
+
+	var regBuf [windowsRegBufLen]uint16
+	bufLen := uint32(windowsRegBufLen)
+	var valType uint32
+	err = syscall.RegQueryValueEx(h, syscall.StringToUTF16Ptr(`MachineGuid`), nil, &valType, (*byte)(unsafe.Pointer(&regBuf[0])), &bufLen)
+	if err != nil {
+		return "", err
+	}
+
+	hostID := syscall.UTF16ToString(regBuf[:])
+	hostIDLen := len(hostID)
+	if hostIDLen != uuidLen {
+		return "", fmt.Errorf("HostID incorrect: %q\n", hostID)
+	}
+
+	return hostID, nil
+}
+
 func GetOSInfo() (Win32_OperatingSystem, error) {
 	var dst []Win32_OperatingSystem
 	q := wmi.CreateQuery(&dst, "")
@@ -130,7 +175,6 @@ func PlatformInformation() (platform string, family string, version string, err
 }
 
 func Users() ([]UserStat, error) {
-
 	var ret []UserStat
 
 	return ret, nil