1 files changed, 144 insertions, 0 deletions
diff --git a/pkg/v1/mutate/rebase.go b/pkg/v1/mutate/rebase.go
new file mode 100644
index 0000000..c606e0b
--- /dev/null
+++ b/pkg/v1/mutate/rebase.go
@@ -0,0 +1,144 @@
+// Copyright 2018 Google LLC All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package mutate
+
+import (
+	"fmt"
+
+	v1 "github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/empty"
+)
+
+// Rebase returns a new v1.Image where the oldBase in orig is replaced by newBase.
+func Rebase(orig, oldBase, newBase v1.Image) (v1.Image, error) {
+	// Verify that oldBase's layers are present in orig, otherwise orig is
+	// not based on oldBase at all.
+	origLayers, err := orig.Layers()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get layers for original: %w", err)
+	}
+	oldBaseLayers, err := oldBase.Layers()
+	if err != nil {
+		return nil, err
+	}
+	if len(oldBaseLayers) > len(origLayers) {
+		return nil, fmt.Errorf("image %q is not based on %q (too few layers)", orig, oldBase)
+	}
+	for i, l := range oldBaseLayers {
+		oldLayerDigest, err := l.Digest()
+		if err != nil {
+			return nil, fmt.Errorf("failed to get digest of layer %d of %q: %w", i, oldBase, err)
+		}
+		origLayerDigest, err := origLayers[i].Digest()
+		if err != nil {
+			return nil, fmt.Errorf("failed to get digest of layer %d of %q: %w", i, orig, err)
+		}
+		if oldLayerDigest != origLayerDigest {
+			return nil, fmt.Errorf("image %q is not based on %q (layer %d mismatch)", orig, oldBase, i)
+		}
+	}
+
+	oldConfig, err := oldBase.ConfigFile()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get config for old base: %w", err)
+	}
+
+	origConfig, err := orig.ConfigFile()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get config for original: %w", err)
+	}
+
+	newConfig, err := newBase.ConfigFile()
+	if err != nil {
+		return nil, fmt.Errorf("could not get config for new base: %w", err)
+	}
+
+	// Stitch together an image that contains:
+	// - original image's config
+	// - new base image's os/arch properties
+	// - new base image's layers + top of original image's layers
+	// - new base image's history + top of original image's history
+	rebasedImage, err := Config(empty.Image, *origConfig.Config.DeepCopy())
+	if err != nil {
+		return nil, fmt.Errorf("failed to create empty image with original config: %w", err)
+	}
+
+	// Add new config properties from existing images.
+	rebasedConfig, err := rebasedImage.ConfigFile()
+	if err != nil {
+		return nil, fmt.Errorf("could not get config for rebased image: %w", err)
+	}
+	// OS/Arch properties from new base
+	rebasedConfig.Architecture = newConfig.Architecture
+	rebasedConfig.OS = newConfig.OS
+	rebasedConfig.OSVersion = newConfig.OSVersion
+
+	// Apply config properties to rebased.
+	rebasedImage, err = ConfigFile(rebasedImage, rebasedConfig)
+	if err != nil {
+		return nil, fmt.Errorf("failed to replace config for rebased image: %w", err)
+	}
+
+	// Get new base layers and config for history.
+	newBaseLayers, err := newBase.Layers()
+	if err != nil {
+		return nil, fmt.Errorf("could not get new base layers for new base: %w", err)
+	}
+	// Add new base layers.
+	rebasedImage, err = Append(rebasedImage, createAddendums(0, 0, newConfig.History, newBaseLayers)...)
+	if err != nil {
+		return nil, fmt.Errorf("failed to append new base image: %w", err)
+	}
+
+	// Add original layers above the old base.
+	rebasedImage, err = Append(rebasedImage, createAddendums(len(oldConfig.History), len(oldBaseLayers)+1, origConfig.History, origLayers)...)
+	if err != nil {
+		return nil, fmt.Errorf("failed to append original image: %w", err)
+	}
+
+	return rebasedImage, nil
+}
+
+// createAddendums makes a list of addendums from a history and layers starting from a specific history and layer
+// indexes.
+func createAddendums(startHistory, startLayer int, history []v1.History, layers []v1.Layer) []Addendum {
+	var adds []Addendum
+	// History should be a superset of layers; empty layers (e.g. ENV statements) only exist in history.
+	// They cannot be iterated identically but must be walked independently, only advancing the iterator for layers
+	// when a history entry for a non-empty layer is seen.
+	layerIndex := 0
+	for historyIndex := range history {
+		var layer v1.Layer
+		emptyLayer := history[historyIndex].EmptyLayer
+		if !emptyLayer {
+			layer = layers[layerIndex]
+			layerIndex++
+		}
+		if historyIndex >= startHistory || layerIndex >= startLayer {
+			adds = append(adds, Addendum{
+				Layer:   layer,
+				History: history[historyIndex],
+			})
+		}
+	}
+	// In the event history was malformed or non-existent, append the remaining layers.
+	for i := layerIndex; i < len(layers); i++ {
+		if i >= startLayer {
+			adds = append(adds, Addendum{Layer: layers[layerIndex]})
+		}
+	}
+
+	return adds
+}