@@ -26,6 +26,7 @@ type mockLayer struct {

 	diffID    layer.DiffID

 	chainID   layer.ChainID

 	parent    layer.Layer

+	platform  layer.Platform

 }

 func (ml *mockLayer) TarStream() (io.ReadCloser, error) {

@@ -56,6 +57,10 @@ func (ml *mockLayer) DiffSize() (size int64, err error) {

 	return 0, nil

 }

+func (ml *mockLayer) Platform() layer.Platform {

+	return ml.platform

+}

+

 func (ml *mockLayer) Metadata() (map[string]string, error) {

 	return make(map[string]string), nil

 }

@@ -55,6 +55,10 @@ func (el *emptyLayer) Metadata() (map[string]string, error) {

 	return make(map[string]string), nil

 }

+func (el *emptyLayer) Platform() Platform {

+	return ""

+}

+

 // IsEmpty returns true if the layer is an EmptyLayer

 func IsEmpty(diffID DiffID) bool {

 	return diffID == DigestSHA256EmptyTar

new file mode 100644

@@ -0,0 +1,13 @@

+// +build !windows

+

+package layer

+

+// SetPlatform writes the "platform" file to the layer filestore

+func (fm *fileMetadataTransaction) SetPlatform(platform Platform) error {

+	return nil

+}

+

+// GetPlatform reads the "platform" file from the layer filestore

+func (fms *fileMetadataStore) GetPlatform(layer ChainID) (Platform, error) {

+	return "", nil

+}

new file mode 100644

@@ -0,0 +1,35 @@

+package layer

+

+import (

+	"fmt"

+	"io/ioutil"

+	"os"

+	"strings"

+)

+

+// SetPlatform writes the "platform" file to the layer filestore

+func (fm *fileMetadataTransaction) SetPlatform(platform Platform) error {

+	if platform == "" {

+		return nil

+	}

+	return fm.ws.WriteFile("platform", []byte(platform), 0644)

+}

+

+// GetPlatform reads the "platform" file from the layer filestore

+func (fms *fileMetadataStore) GetPlatform(layer ChainID) (Platform, error) {

+	contentBytes, err := ioutil.ReadFile(fms.getLayerFilename(layer, "platform"))

+	if err != nil {

+		// For backwards compatibility, the platform file may not exist. Default to "windows" if missing.

+		if os.IsNotExist(err) {

+			return "windows", nil

+		}

+		return "", err

+	}

+	content := strings.TrimSpace(string(contentBytes))

+

+	if content != "windows" && content != "linux" {

+		return "", fmt.Errorf("invalid platform value: %s", content)

+	}

+

+	return Platform(content), nil

+}

@@ -64,6 +64,14 @@ func (id ChainID) String() string {

 	return string(id)

 }

+// Platform is the platform of a layer

+type Platform string

+

+// String returns a string rendition of layers target platform

+func (id Platform) String() string {

+	return string(id)

+}

+

 // DiffID is the hash of an individual layer tar.

 type DiffID digest.Digest

@@ -99,6 +107,9 @@ type Layer interface {

 	// Parent returns the next layer in the layer chain.

 	Parent() Layer

+	// Platform returns the platform of the layer

+	Platform() Platform

+

 	// Size returns the size of the entire layer chain. The size

 	// is calculated from the total size of all files in the layers.

 	Size() (int64, error)

@@ -208,6 +219,7 @@ type MetadataTransaction interface {

 	SetDiffID(DiffID) error

 	SetCacheID(string) error

 	SetDescriptor(distribution.Descriptor) error

+	SetPlatform(Platform) error

 	TarSplitWriter(compressInput bool) (io.WriteCloser, error)

 	Commit(ChainID) error

@@ -228,6 +240,7 @@ type MetadataStore interface {

 	GetDiffID(ChainID) (DiffID, error)

 	GetCacheID(ChainID) (string, error)

 	GetDescriptor(ChainID) (distribution.Descriptor, error)

+	GetPlatform(ChainID) (Platform, error)

 	TarSplitReader(ChainID) (io.ReadCloser, error)

 	SetMountID(string, string) error

@@ -144,6 +144,11 @@ func (ls *layerStore) loadLayer(layer ChainID) (*roLayer, error) {

 		return nil, fmt.Errorf("failed to get descriptor for %s: %s", layer, err)

 	}

+	platform, err := ls.store.GetPlatform(layer)

+	if err != nil {

+		return nil, fmt.Errorf("failed to get platform for %s: %s", layer, err)

+	}

+

 	cl = &roLayer{

 		chainID:    layer,

 		diffID:     diff,

@@ -152,6 +157,7 @@ func (ls *layerStore) loadLayer(layer ChainID) (*roLayer, error) {

 		layerStore: ls,

 		references: map[Layer]struct{}{},

 		descriptor: descriptor,

+		platform:   platform,

 	}

 	if parent != "" {

@@ -16,6 +16,7 @@ type roLayer struct {

 	size       int64

 	layerStore *layerStore

 	descriptor distribution.Descriptor

+	platform   Platform

 	referenceCount int

 	references     map[Layer]struct{}

@@ -142,6 +143,9 @@ func storeLayer(tx MetadataTransaction, layer *roLayer) error {

 			return err

 		}

 	}

+	if err := tx.SetPlatform(layer.platform); err != nil {

+		return err

+	}

 	return nil

 }

new file mode 100644

@@ -0,0 +1,7 @@

+// +build !windows

+

+package layer

+

+func (rl *roLayer) Platform() Platform {

+	return ""

+}

@@ -7,3 +7,10 @@ var _ distribution.Describable = &roLayer{}

 func (rl *roLayer) Descriptor() distribution.Descriptor {

 	return rl.descriptor

 }

+

+func (rl *roLayer) Platform() Platform {

+	if rl.platform == "" {

+		return "windows"

+	}

+	return rl.platform

+}

@@ -433,6 +433,10 @@ func (l *mockLayer) DiffSize() (int64, error) {

 	return 0, nil

 }

+func (l *mockLayer) Platform() layer.Platform {

+	return ""

+}

+

 func (l *mockLayer) Metadata() (map[string]string, error) {

 	return nil, nil

 }