image/layout/tar: Add a CreateTarFile helper

The NewEngine commands for the tar-backed image-layout engines (both
the CAS and refs engines) open files O_RDWR and expect image-layout
compatible content in the tarball.  That makes sense, but for folks
who *don't* have such a tarball, a helper like CreateTarFile makes it
easy to explicitly create an empty one.

The 0666 file modes and 0777 directory modes rely on the caller's
umask to appropriately limit user/group/other permissions for the
tarball itself and any content extracted to the filesystem from the
tarball.

The trailing slashes are based on part of libarchive's description of
old-style archives [1]:

  name
    Pathname, stored as a null-terminated string.  Early tar
    implementations only stored regular files (including hardlinks to
    those files).  One common early convention used a trailing "/"
    character to indicate a directory name, allowing directory
    permissions and owner information to be archived and restored.

and POSIX ustar archives [2]:

  name, prefix
    ... The standard does not require a trailing / character on
    directory names, though most implementations still include this
    for compatibility reasons.

Expose this new functionality on the command line as:

  $ oci-image-init image-layout PATH

where 'image-layout' is a separate level in case we support
initializing additional types of repositories in the future.

[1]: https://github.com/libarchive/libarchive/wiki/ManPageTar5#old-style-archive-format
[2]: https://github.com/libarchive/libarchive/wiki/ManPageTar5#posix-ustar-archives

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

.gitignore

@@ -1,5 +1,6 @@
 /oci-cas
 /oci-create-runtime-bundle
+/oci-image-init
 /oci-image-validate
 /oci-refs
 /oci-unpack

Makefile

@@ -5,6 +5,7 @@ EPOCH_TEST_COMMIT ?= v0.2.0
 TOOLS := \
 	oci-cas \
 	oci-create-runtime-bundle \
+	oci-image-init \
 	oci-image-validate \
 	oci-refs \
 	oci-unpack

cmd/oci-image-init/image_layout.go

@@ -0,0 +1,57 @@
+// 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"
+	"os"
+
+	"github.com/opencontainers/image-tools/image/layout"
+	"github.com/spf13/cobra"
+	"golang.org/x/net/context"
+)
+
+type imageLayout struct{}
+
+func newImageLayoutCmd() *cobra.Command {
+	state := &imageLayout{}
+
+	return &cobra.Command{
+		Use:   "image-layout PATH",
+		Short: "Initialize an OCI image-layout repository",
+		Run:   state.Run,
+	}
+}
+
+func (state *imageLayout) Run(cmd *cobra.Command, args []string) {
+	if len(args) != 1 {
+		if err := cmd.Usage(); err != nil {
+			fmt.Fprintln(os.Stderr, err)
+		}
+		os.Exit(1)
+	}
+
+	path := args[0]
+
+	ctx := context.Background()
+
+	err := layout.CreateTarFile(ctx, path)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+
+	os.Exit(0)
+}

cmd/oci-image-init/main.go

@@ -0,0 +1,37 @@
+// 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"
+	"os"
+
+	"github.com/spf13/cobra"
+)
+
+func main() {
+	cmd := &cobra.Command{
+		Use:   "oci-image-init",
+		Short: "Initialize an OCI image",
+	}
+
+	cmd.AddCommand(newImageLayoutCmd())
+
+	err := cmd.Execute()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}

image/layout/tar.go

@@ -218,3 +218,50 @@ func CheckTarVersion(ctx context.Context, reader io.ReadSeeker) (err error) {
 
 	return nil
 }
+
+// CreateTarFile creates a new image-layout tar file at the given path.
+func CreateTarFile(ctx context.Context, path string) (err error) {
+	file, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0666)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	tarWriter := tar.NewWriter(file)
+	defer tarWriter.Close()
+
+	now := time.Now()
+	for _, name := range []string{"./blobs/", "./refs/"} {
+		header := &tar.Header{
+			Name:     name,
+			Mode:     0777,
+			ModTime:  now,
+			Typeflag: tar.TypeDir,
+		}
+		err = tarWriter.WriteHeader(header)
+		if err != nil {
+			return err
+		}
+	}
+
+	imageLayoutVersion := ImageLayoutVersion{
+		Version: "1.0.0",
+	}
+	imageLayoutVersionBytes, err := json.Marshal(imageLayoutVersion)
+	if err != nil {
+		return err
+	}
+	header := &tar.Header{
+		Name:     "./oci-layout",
+		Mode:     0666,
+		Size:     int64(len(imageLayoutVersionBytes)),
+		ModTime:  now,
+		Typeflag: tar.TypeReg,
+	}
+	err = tarWriter.WriteHeader(header)
+	if err != nil {
+		return err
+	}
+	_, err = tarWriter.Write(imageLayoutVersionBytes)
+	return err
+}