add v1tarreader for images and use it for docker load

Signed-off-by: Avi Deitcher <avi@deitcher.net>

src/cmd/linuxkit/cache/source.go

@@ -6,8 +6,10 @@ import (
 	"io"
 
 	"github.com/containerd/containerd/reference"
+	"github.com/google/go-containerregistry/pkg/name"
 	"github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/mutate"
+	"github.com/google/go-containerregistry/pkg/v1/tarball"
 	lktspec "github.com/linuxkit/linuxkit/src/cmd/linuxkit/spec"
 	imagespec "github.com/opencontainers/image-spec/specs-go/v1"
 )
@@ -70,6 +72,28 @@ func (c ImageSource) TarReader() (io.ReadCloser, error) {
 	return mutate.Extract(image), nil
 }
 
+// V1TarReader return an io.ReadCloser to read the image as a v1 tarball
+func (c ImageSource) V1TarReader() (io.ReadCloser, error) {
+	imageName := c.ref.String()
+
+	refName, err := name.ParseReference(imageName)
+	if err != nil {
+		return nil, fmt.Errorf("error parsing image name: %v", err)
+	}
+	// get a reference to the image
+	image, err := c.provider.findImage(imageName, c.architecture)
+	if err != nil {
+		return nil, err
+	}
+	// convert the writer to a reader
+	r, w := io.Pipe()
+	go func() {
+		defer w.Close()
+		tarball.Write(refName, image, w)
+	}()
+	return r, nil
+}
+
 // Descriptor return the descriptor of the image.
 func (c ImageSource) Descriptor() *v1.Descriptor {
 	return c.descriptor

src/cmd/linuxkit/docker/cmd.go

@@ -88,6 +88,16 @@ func Export(container string) (io.ReadCloser, error) {
 	return responseBody, err
 }
 
+// Save save the provided image ref.
+func Save(image string) (io.ReadCloser, error) {
+	log.Debugf("docker save: %s", image)
+	cli, err := Client()
+	if err != nil {
+		return nil, errors.New("could not initialize Docker API client")
+	}
+	return cli.ImageSave(context.Background(), []string{image})
+}
+
 // Rm remove the given container from docker.
 func Rm(container string) error {
 	log.Debugf("docker rm: %s", container)

src/cmd/linuxkit/docker/source.go

@@ -77,6 +77,11 @@ func (d ImageSource) TarReader() (io.ReadCloser, error) {
 	}, nil
 }
 
+// V1TarReader return an io.ReadCloser to read the save of the image
+func (d ImageSource) V1TarReader() (io.ReadCloser, error) {
+	return Save(d.ref.String())
+}
+
 // Descriptor return the descriptor of the image.
 func (d ImageSource) Descriptor() *v1.Descriptor {
 	return nil

src/cmd/linuxkit/pkglib/build.go

@@ -289,7 +289,7 @@ func (p Pkg) Build(bos ...BuildOpt) error {
 	// if requested docker, load the image up
 	if bo.targetDocker {
 		cacheSource := c.NewSource(&ref, arch, desc)
-		reader, err := cacheSource.TarReader()
+		reader, err := cacheSource.V1TarReader()
 		if err != nil {
 			return fmt.Errorf("unable to get reader from cache: %v", err)
 		}

src/cmd/linuxkit/pkglib/build_test.go

@@ -270,6 +270,9 @@ func (c cacheMockerSource) Config() (imagespec.ImageConfig, error) {
 func (c cacheMockerSource) TarReader() (io.ReadCloser, error) {
 	return nil, errors.New("unsupported")
 }
+func (c cacheMockerSource) V1TarReader() (io.ReadCloser, error) {
+	return nil, errors.New("unsupported")
+}
 func (c cacheMockerSource) Descriptor() *registry.Descriptor {
 	return c.descriptor
 }

src/cmd/linuxkit/spec/image.go

@@ -10,7 +10,12 @@ import (
 // ImageSource interface to an image. It can have its config read, and a its containers
 // can be read via an io.ReadCloser tar stream.
 type ImageSource interface {
+	// Config get the config for the image
 	Config() (imagespec.ImageConfig, error)
+	// TarReader get the flattened filesystem of the image as a tar stream/
 	TarReader() (io.ReadCloser, error)
+	// Descriptor get the v1.Descriptor of the image
 	Descriptor() *v1.Descriptor
+	// V1TarReader get the image as v1 tarball, also compatibel with `docker load`
+	V1TarReader() (io.ReadCloser, error)
 }