Merge pull request #240 from runcom/fix-kata-devmapper

layers: do not try to unmount if not mounted at all on delete

layers.go

@@ -841,8 +841,12 @@ func (r *layerStore) Delete(id string) error {
 		return ErrLayerUnknown
 	}
 	id = layer.ID
-	if _, err := r.Unmount(id, true); err != nil {
-		return err
+	// This check is needed for idempotency of delete where the layer could have been
+	// already unmounted (since c/storage gives you that API directly)
+	for layer.MountCount > 0 {
+		if _, err := r.Unmount(id, false); err != nil {
+			return err
+		}
 	}
 	err := r.driver.Remove(id)
 	if err == nil {