image/cas: Add a generic CAS interface

cmd/oci-image-tool/cas-get.go

@@ -0,0 +1,80 @@
+// 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 main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	"github.com/opencontainers/go-digest"
+	"github.com/opencontainers/image-tools/image/cas/layout"
+	"github.com/urfave/cli"
+	"golang.org/x/net/context"
+)
+
+type casGetCmd struct {
+	path   string
+	digest digest.Digest
+}
+
+var casGetCommand = cli.Command{
+	Name:      "get",
+	Usage:     "Retrieve a blob from the store and write it to stdout.",
+	ArgsUsage: "PATH DIGEST",
+	Action:    casGetHandle,
+}
+
+func casGetHandle(ctx *cli.Context) (err error) {
+	if ctx.NArg() != 2 {
+		return fmt.Errorf("both PATH and DIGEST must be provided")
+	}
+
+	state := &casGetCmd{}
+	state.path = ctx.Args().Get(0)
+	state.digest, err = digest.Parse(ctx.Args().Get(1))
+	if err != nil {
+		return err
+	}
+
+	engineContext := context.Background()
+
+	engine, err := layout.NewEngine(engineContext, state.path)
+	if err != nil {
+		return err
+	}
+	defer engine.Close()
+
+	reader, err := engine.Get(engineContext, state.digest)
+	if err != nil {
+		return err
+	}
+	defer reader.Close()
+
+	bytes, err := ioutil.ReadAll(reader)
+	if err != nil {
+		return err
+	}
+
+	n, err := os.Stdout.Write(bytes)
+	if err != nil {
+		return err
+	}
+	if n < len(bytes) {
+		return fmt.Errorf("wrote %d of %d bytes", n, len(bytes))
+	}
+
+	return nil
+}

cmd/oci-image-tool/cas.go

@@ -0,0 +1,32 @@
+// 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 main
+
+import (
+	"github.com/urfave/cli"
+)
+
+func casHandle(context *cli.Context) error {
+	return cli.ShowCommandHelp(context, context.Command.Name)
+}
+
+var casCommand = cli.Command{
+	Name:   "cas",
+	Usage:  "Content-addressable storage manipulation",
+	Action: casHandle,
+	Subcommands: []cli.Command{
+		casGetCommand,
+	},
+}

cmd/oci-image-tool/main.go

@@ -52,6 +52,7 @@ func main() {
 		validateCommand,
 		unpackCommand,
 		createCommand,
+		casCommand,
 	}
 
 	if err := app.Run(os.Args); err != nil {

image/cas/interface.go

@@ -0,0 +1,34 @@
+// 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 cas implements generic content-addressable storage.
+package cas
+
+import (
+	"io"
+
+	"github.com/opencontainers/go-digest"
+	"golang.org/x/net/context"
+)
+
+// Engine represents a content-addressable storage engine.
+type Engine interface {
+	// Get returns a reader for retrieving a blob from the store.
+	// Returns os.ErrNotExist if the digest is not found.
+	Get(ctx context.Context, digest digest.Digest) (reader io.ReadCloser, err error)
+
+	// Close releases resources held by the engine.  Subsequent engine
+	// method calls will fail.
+	Close() (err error)
+}

image/cas/layout/interface.go

@@ -0,0 +1,25 @@
+// 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 layout
+
+import (
+	"io"
+)
+
+// ReadWriteSeekCloser wraps the Read, Write, Seek, and Close methods.
+type ReadWriteSeekCloser interface {
+	io.ReadWriteSeeker
+	io.Closer
+}

image/cas/layout/main.go

@@ -0,0 +1,51 @@
+// 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 layout implements the cas interface using the image-spec's
+// image-layout [1].
+//
+// [1]: https://github.com/opencontainers/image-spec/blob/master/image-layout.md
+package layout
+
+import (
+	"os"
+	"strings"
+
+	"github.com/opencontainers/go-digest"
+	"github.com/opencontainers/image-tools/image/cas"
+	"golang.org/x/net/context"
+)
+
+// NewEngine instantiates an engine with the appropriate backend (tar,
+// HTTP, ...).
+func NewEngine(ctx context.Context, path string) (engine cas.Engine, err error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+
+	return NewTarEngine(ctx, file)
+}
+
+// blobPath returns the PATH to the DIGEST blob.  SEPARATOR selects
+// the path separator used between components.
+func blobPath(digest digest.Digest, separator string) (path string, err error) {
+	err = digest.Validate()
+	if err != nil {
+		return "", err
+	}
+	algorithm := digest.Algorithm().String()
+	components := []string{".", "blobs", algorithm, digest.Hex()}
+	return strings.Join(components, separator), nil
+}

image/cas/layout/tar.go

@@ -0,0 +1,78 @@
+// 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 layout
+
+import (
+	"archive/tar"
+	"io"
+	"io/ioutil"
+	"os"
+
+	"github.com/opencontainers/go-digest"
+	"github.com/opencontainers/image-tools/image/cas"
+	"golang.org/x/net/context"
+)
+
+// TarEngine is a cas.Engine backed by a tar file.
+type TarEngine struct {
+	file ReadWriteSeekCloser
+}
+
+// NewTarEngine returns a new TarEngine.
+func NewTarEngine(ctx context.Context, file ReadWriteSeekCloser) (engine cas.Engine, err error) {
+	engine = &TarEngine{
+		file: file,
+	}
+
+	return engine, nil
+}
+
+// Get returns a reader for retrieving a blob from the store.
+func (engine *TarEngine) Get(ctx context.Context, digest digest.Digest) (reader io.ReadCloser, err error) {
+	targetName, err := blobPath(digest, "/")
+	if err != nil {
+		return nil, err
+	}
+
+	_, err = engine.file.Seek(0, io.SeekStart)
+	if err != nil {
+		return nil, err
+	}
+
+	tarReader := tar.NewReader(engine.file)
+	for {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+
+		header, err := tarReader.Next()
+		if err == io.EOF {
+			return nil, os.ErrNotExist
+		} else if err != nil {
+			return nil, err
+		}
+
+		if header.Name == targetName {
+			return ioutil.NopCloser(tarReader), nil
+		}
+	}
+}
+
+// Close releases resources held by the engine.
+func (engine *TarEngine) Close() (err error) {
+	return engine.file.Close()
+}