Add API to trigger data disk move only (home-assistant#129)

* Add API to trigger data disk move only

Add a new D-Bus API which allows to trigger the data disk move only.
This allows to handle the selection and partitioning from Supervisor
directly (via UDisks2 D-Bus API). Handling partitioning directly in
Supervisor should improve device detection/selection and allows for
better error handling.

* Drop return value from MarkDataMove

* Fix linter error

datadisk/datadisk.go

@@ -40,6 +40,21 @@ type datadisk struct {
 	props *prop.Properties
 }
 
+func (d datadisk) MarkDataMove() *dbus.Error {
+	/* Move request marker for hassos-data.service */
+	fileName := "/mnt/overlay/move-data"
+	_, err := os.Stat(fileName)
+	if os.IsNotExist(err) {
+		file, err := os.Create(fileName)
+		if err != nil {
+			return dbus.MakeFailedError(err)
+		}
+		defer file.Close()
+	}
+
+	return nil
+}
+
 func (d datadisk) ChangeDevice(newDevice string) (bool, *dbus.Error) {
 	logging.Info.Printf("Request to change data disk to %s.", newDevice)
 
@@ -59,15 +74,9 @@ func (d datadisk) ChangeDevice(newDevice string) (bool, *dbus.Error) {
 		return false, dbus.MakeFailedError(err)
 	}
 
-	/* Move request marker for hassos-data.service */
-	fileName := "/mnt/overlay/move-data"
-	_, err = os.Stat(fileName)
-	if os.IsNotExist(err) {
-		file, err := os.Create(fileName)
-		if err != nil {
-			return false, dbus.MakeFailedError(err)
-		}
-		defer file.Close()
+	dbuserr := d.MarkDataMove()
+	if dbuserr != nil {
+		return false, dbuserr
 	}
 
 	return true, nil