internal/windows: cache interface rather than instance ID

Apparently instance ID is not stable between S3 sleep on recent Windows
builds.

Reported-by: J. Michael McAtee <[email protected]>
Reported-by: Menno Finlay-Smits <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>

Author: zx2c4

internal/wgwindows/client_windows.go

@@ -17,8 +17,8 @@ var _ wginternal.Client = &Client{}
 
 // A Client provides access to WireGuardNT ioctl information.
 type Client struct {
-	cachedAdapters map[string]string
-	lastLenGuess   uint32
+	cachedInterfaces map[string]*uint16
+	lastLenGuess     uint32
 }
 
 var (
@@ -38,8 +38,8 @@ func init() {
 	}
 }
 
-func (c *Client) refreshInstanceIdCache() error {
-	cachedAdapters := make(map[string]string, 5)
+func (c *Client) refreshInterfaceCache() error {
+	cachedInterfaces := make(map[string]*uint16, 5)
 	devInfo, err := windows.SetupDiGetClassDevsEx(&deviceClassNetGUID, enumerator, 0, windows.DIGCF_PRESENT, 0, "")
 	if err != nil {
 		return err
@@ -70,43 +70,51 @@ func (c *Client) refreshInstanceIdCache() error {
 		if err != nil {
 			continue
 		}
-		cachedAdapters[adapterName] = instanceId
+		interfaces, err := windows.CM_Get_Device_Interface_List(instanceId, &deviceInterfaceNetGUID, windows.CM_GET_DEVICE_INTERFACE_LIST_PRESENT)
+		if err != nil {
+			continue
+		}
+		interface16, err := windows.UTF16PtrFromString(interfaces[0])
+		if err != nil {
+			continue
+		}
+		cachedInterfaces[adapterName] = interface16
 	}
-	c.cachedAdapters = cachedAdapters
+	c.cachedInterfaces = cachedInterfaces
 	return nil
 }
 
-func (c *Client) interfaceHandle(name string) (windows.Handle, error) {
-	instanceId, ok := c.cachedAdapters[name]
-	if !ok {
-		err := c.refreshInstanceIdCache()
-		if err != nil {
-			return 0, err
-		}
-		instanceId, ok = c.cachedAdapters[name]
+func (c *Client) interfaceHandle(name string) (handle windows.Handle, err error) {
+	hasRefreshed := false
+	for !hasRefreshed {
+		fileName, ok := c.cachedInterfaces[name]
 		if !ok {
-			return 0, os.ErrNotExist
+			err := c.refreshInterfaceCache()
+			if err != nil {
+				return 0, err
+			}
+			hasRefreshed = true
+			fileName, ok = c.cachedInterfaces[name]
+			if !ok {
+				return 0, os.ErrNotExist
+			}
+		}
+		handle, err = windows.CreateFile(fileName, windows.GENERIC_READ|windows.GENERIC_WRITE, windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE, nil, windows.OPEN_EXISTING, 0, 0)
+		if err == nil {
+			break
 		}
 	}
-	interfaces, err := windows.CM_Get_Device_Interface_List(instanceId, &deviceInterfaceNetGUID, windows.CM_GET_DEVICE_INTERFACE_LIST_PRESENT)
-	if err != nil {
-		return 0, err
-	}
-	interface16, err := windows.UTF16PtrFromString(interfaces[0])
-	if err != nil {
-		return 0, err
-	}
-	return windows.CreateFile(interface16, windows.GENERIC_READ|windows.GENERIC_WRITE, windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE, nil, windows.OPEN_EXISTING, 0, 0)
+	return
 }
 
 // Devices implements wginternal.Client.
 func (c *Client) Devices() ([]*wgtypes.Device, error) {
-	err := c.refreshInstanceIdCache()
+	err := c.refreshInterfaceCache()
 	if err != nil {
 		return nil, err
 	}
-	ds := make([]*wgtypes.Device, 0, len(c.cachedAdapters))
-	for name := range c.cachedAdapters {
+	ds := make([]*wgtypes.Device, 0, len(c.cachedInterfaces))
+	for name := range c.cachedInterfaces {
 		d, err := c.Device(name)
 		if err != nil {
 			return nil, err