Better sparse image for podman

This library uses the fact that uploaded tar may miss some layers when
pushed to docker provided the missing layers are already in daemon.
It works also with podman but the layers have to be present in the tar
at least as empty entries.

Signed-off-by: Matej Vasek <[email protected]>

local/local.go

@@ -511,23 +511,30 @@ func (i *Image) doSave() (types.ImageInspect, error) {
 		return types.ImageInspect{}, err
 	}
 
+	var blankIdx int
 	var layerPaths []string
 	for _, path := range i.layerPaths {
 		if path == "" {
-			layerPaths = append(layerPaths, "")
-			continue
-		}
-		layerName := fmt.Sprintf("/%x.tar", sha256.Sum256([]byte(path)))
-		f, err := os.Open(filepath.Clean(path))
-		if err != nil {
-			return types.ImageInspect{}, err
-		}
-		defer f.Close()
-		if err := addFileToTar(tw, layerName, f); err != nil {
-			return types.ImageInspect{}, err
+			layerName := fmt.Sprintf("blank_%d", blankIdx)
+			blankIdx++
+			hdr := &tar.Header{Name: layerName, Mode: 0644, Size: 0}
+			if err := tw.WriteHeader(hdr); err != nil {
+				return types.ImageInspect{}, err
+			}
+			layerPaths = append(layerPaths, layerName)
+		} else {
+			layerName := fmt.Sprintf("/%x.tar", sha256.Sum256([]byte(path)))
+			f, err := os.Open(filepath.Clean(path))
+			if err != nil {
+				return types.ImageInspect{}, err
+			}
+			defer f.Close()
+			if err := addFileToTar(tw, layerName, f); err != nil {
+				return types.ImageInspect{}, err
+			}
+			f.Close()
+			layerPaths = append(layerPaths, layerName)
 		}
-		f.Close()
-		layerPaths = append(layerPaths, layerName)
 	}
 
 	manifest, err := json.Marshal([]map[string]interface{}{