image: Add ImageLayoutVersion and check oci-layout in tar engines

The new engine code isn't very DRY, but I haven't decided where to
collect this sort of shared stuff yet.  And it's not very long, so
it's not too terrible to leave it WET.

Signed-off-by: W. Trevor King <wking@tremily.us>

image/cas/layout/tar.go

@@ -16,13 +16,15 @@ package layout
 
 import (
 	"archive/tar"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
 	"os"
 	"strings"
 
 	"github.com/opencontainers/image-spec/image/cas"
+	"github.com/opencontainers/image-spec/image/structure"
 )
 
 // TarEngine is a cas.Engine backed by a tar file.
@@ -31,11 +33,40 @@ type TarEngine struct {
 }
 
 // GetTarEngine returns a TarEngine.
-func GetTarEngine(file ReadSeekCloser) (engine cas.Engine, err error) {
-	engine = &TarEngine{
+func GetTarEngine(file ReadSeekCloser) (eng cas.Engine, err error) {
+	engine := &TarEngine{
 		reader: file,
 	}
-	return engine, nil
+
+	_, err = engine.reader.Seek(0, os.SEEK_SET)
+	if err != nil {
+		return nil, err
+	}
+
+	tarReader := tar.NewReader(engine.reader)
+	for {
+		header, err := tarReader.Next()
+		if err == io.EOF {
+			return nil, errors.New("oci-layout not found")
+		}
+		if err != nil {
+			return nil, err
+		}
+
+		if header.Name == "./oci-layout" {
+			decoder := json.NewDecoder(tarReader)
+			var version structure.ImageLayoutVersion
+			err = decoder.Decode(&version)
+			if err != nil {
+				return nil, err
+			}
+			if version.Version != "1.0.0" {
+				return nil, fmt.Errorf("unrecognized imageLayoutVersion: %q", version.Version)
+			}
+
+			return engine, nil
+		}
+	}
 }
 
 // Put adds a new blob to the store.

image/refs/layout/tar.go

@@ -34,12 +34,40 @@ type TarEngine struct {
 }
 
 // GetTarEngine returns a TarEngine.
-func GetTarEngine(file casLayout.ReadSeekCloser) (engine refs.Engine, err error) {
-	engine = &TarEngine{
+func GetTarEngine(file casLayout.ReadSeekCloser) (eng refs.Engine, err error) {
+	engine := &TarEngine{
 		reader: file,
 	}
 
-	return engine, nil
+	_, err = engine.reader.Seek(0, os.SEEK_SET)
+	if err != nil {
+		return nil, err
+	}
+
+	tarReader := tar.NewReader(engine.reader)
+	for {
+		header, err := tarReader.Next()
+		if err == io.EOF {
+			return nil, errors.New("oci-layout not found")
+		}
+		if err != nil {
+			return nil, err
+		}
+
+		if header.Name == "./oci-layout" {
+			decoder := json.NewDecoder(tarReader)
+			var version structure.ImageLayoutVersion
+			err = decoder.Decode(&version)
+			if err != nil {
+				return nil, err
+			}
+			if version.Version != "1.0.0" {
+				return nil, fmt.Errorf("unrecognized imageLayoutVersion: %q", version.Version)
+			}
+
+			return engine, nil
+		}
+	}
 }
 
 // Put adds a new reference to the store.

image/structure/image_layout.go

@@ -0,0 +1,21 @@
+// Copyright 2016 The Linux Foundation
+//
+// 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 structure
+
+// ImageLayoutVersion represents the oci-version content for the image
+// layout format.
+type ImageLayoutVersion struct {
+	Version string `json:"imageLayoutVersion"`
+}