Add interface method, AddOrReuseLayerWithHistory

cnb_image.go

@@ -357,6 +357,32 @@ func (i *CNBImageCore) AddLayerWithHistory(layer v1.Layer, history v1.History) e
 	return err
 }
 
+func (i *CNBImageCore) AddOrReuseLayerWithHistory(path string, diffID string, history v1.History) error {
+	prevLayerExists, err := i.PreviousImageHasLayer(diffID)
+	if err != nil {
+		return err
+	}
+	if !prevLayerExists {
+		return i.AddLayerWithDiffIDAndHistory(path, diffID, history)
+	}
+	return i.ReuseLayerWithHistory(diffID, history)
+}
+
+func (i *CNBImageCore) PreviousImageHasLayer(diffID string) (bool, error) {
+	if i.previousImage == nil {
+		return false, nil
+	}
+	layerHash, err := v1.NewHash(diffID)
+	if err != nil {
+		return false, fmt.Errorf("failed to get layer hash: %w", err)
+	}
+	prevConfigFile, err := getConfigFile(i.previousImage)
+	if err != nil {
+		return false, fmt.Errorf("failed to get previous image config: %w", err)
+	}
+	return contains(prevConfigFile.RootFS.DiffIDs, layerHash), nil
+}
+
 func (i *CNBImageCore) Rebase(baseTopLayerDiffID string, withNewBase Image) error {
 	newBase := withNewBase.UnderlyingImage() // FIXME: when all imgutil.Images are v1.Images, we can remove this part
 	var err error
@@ -457,6 +483,14 @@ func getHistory(forIndex int, fromImage v1.Image) (v1.History, error) {
 }
 
 func (i *CNBImageCore) ReuseLayerWithHistory(diffID string, history v1.History) error {
+	var err error
+	// ensure existing history
+	if err = i.MutateConfigFile(func(c *v1.ConfigFile) {
+		c.History = NormalizedHistory(c.History, len(c.RootFS.DiffIDs))
+	}); err != nil {
+		return err
+	}
+
 	layerHash, err := v1.NewHash(diffID)
 	if err != nil {
 		return fmt.Errorf("failed to get layer hash: %w", err)

fakes/image.go

@@ -18,6 +18,8 @@ import (
 	"github.com/buildpacks/imgutil"
 )
 
+var _ imgutil.Image = &Image{}
+
 func NewImage(name, topLayerSha string, identifier imgutil.Identifier) *Image {
 	return &Image{
 		labels:           nil,
@@ -238,6 +240,10 @@ func shaForFile(path string) (string, error) {
 	return hex.EncodeToString(hasher.Sum(make([]byte, 0, hasher.Size()))), nil
 }
 
+func (i *Image) AddOrReuseLayerWithHistory(_, _ string, _ v1.History) error {
+	panic("implement me")
+}
+
 func (i *Image) GetLayer(sha string) (io.ReadCloser, error) {
 	path, ok := i.layersMap[sha]
 	if !ok {

image.go

@@ -62,6 +62,7 @@ type Image interface {
 	AddLayer(path string) error
 	AddLayerWithDiffID(path, diffID string) error
 	AddLayerWithDiffIDAndHistory(path, diffID string, history v1.History) error
+	AddOrReuseLayerWithHistory(path, diffID string, history v1.History) error
 	Delete() error
 	Rebase(string, Image) error
 	RemoveLabel(string) error

local/local.go

@@ -166,6 +166,17 @@ func (i *Image) addLayerToStore(fromPath, withDiffID string) (v1.Layer, error) {
 	return layer, nil
 }
 
+func (i *Image) AddOrReuseLayerWithHistory(path string, diffID string, history v1.History) error {
+	prevLayerExists, err := i.PreviousImageHasLayer(diffID)
+	if err != nil {
+		return err
+	}
+	if !prevLayerExists {
+		return i.AddLayerWithDiffIDAndHistory(path, diffID, history)
+	}
+	return i.ReuseLayerWithHistory(diffID, history)
+}
+
 func (i *Image) Rebase(baseTopLayerDiffID string, withNewBase imgutil.Image) error {
 	if err := i.ensureLayers(); err != nil {
 		return err