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image/cas: Add a generic CAS interface
And implement that interface for tarballs based on the specs image-layout. I plan on adding other backends later, but this is enough for a proof of concept. Also add a new 'cas get' subcommand to oci-image-tool so folks can access the new read functionality from the command line. In a subsequent commit, I'll replace oci-image-tools walker functionality with this new API. Signed-off-by: W. Trevor King <[email protected]>
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‎cmd/oci-image-tool/cas.go‎

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// Copyright 2016 The Linux Foundation
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package main
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import (
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"io"
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"log"
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"github.com/spf13/cobra"
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)
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func newCASCmd(stdout io.Writer, stderr *log.Logger) *cobra.Command {
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cmd := &cobra.Command{
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Use: "cas",
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Short: "Content-addressable storage manipulation",
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}
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cmd.AddCommand(newCASGetCmd(stdout, stderr))
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return cmd
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}

‎cmd/oci-image-tool/cas_get.go‎

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// Copyright 2016 The Linux Foundation
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package main
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import (
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"fmt"
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"io"
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"io/ioutil"
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"log"
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"os"
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"github.com/opencontainers/image-spec/image/cas/layout"
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"github.com/spf13/cobra"
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)
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type casGetCmd struct {
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stdout io.Writer
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stderr *log.Logger
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path string
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digest string
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}
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func newCASGetCmd(stdout io.Writer, stderr *log.Logger) *cobra.Command {
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state := &casGetCmd{
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stdout: stdout,
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stderr: stderr,
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}
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return &cobra.Command{
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Use: "get PATH DIGEST",
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Short: "Retrieve a blob from the store",
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Run: state.Run,
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}
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}
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func (state *casGetCmd) Run(cmd *cobra.Command, args []string) {
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if len(args) != 2 {
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state.stderr.Print("both PATH and DIGEST must be provided")
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if err := cmd.Usage(); err != nil {
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state.stderr.Println(err)
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}
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os.Exit(1)
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}
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state.path = args[0]
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state.digest = args[1]
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err := state.run()
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if err != nil {
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state.stderr.Println(err)
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os.Exit(1)
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}
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os.Exit(1)
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}
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func (state *casGetCmd) run() (err error) {
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engine, err := layout.GetEngine(state.path)
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if err != nil {
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return err
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}
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reader, err := engine.Get(state.digest)
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if err != nil {
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return err
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}
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bytes, err := ioutil.ReadAll(reader)
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if err != nil {
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return err
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}
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n, err := state.stdout.Write(bytes)
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if err != nil {
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return err
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}
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if n < len(bytes) {
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return fmt.Errorf("wrote %d of %d bytes", n, len(bytes))
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}
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return engine.Close()
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}

‎cmd/oci-image-tool/main.go‎

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cmd.AddCommand(newValidateCmd(stdout, stderr))
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cmd.AddCommand(newUnpackCmd(stdout, stderr))
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cmd.AddCommand(newBundleCmd(stdout, stderr))
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cmd.AddCommand(newCASCmd(os.Stdout, stderr))
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if err := cmd.Execute(); err != nil {
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stderr.Println(err)

‎image/cas/interface.go‎

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// Copyright 2016 The Linux Foundation
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package cas implements generic content-addressable storage.
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package cas
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import (
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"io"
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)
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// Engine represents a content-addressable storage engine.
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type Engine interface {
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// Put adds a new blob to the store.
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Put(writer io.Writer) (digest string, err error)
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// Get returns a reader for retrieving a blob from the store.
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Get(digest string) (reader io.Reader, err error)
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// Delete removes a blob from the store.
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Delete(digest string) (err error)
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// Close releases resources held by the engine. Subsequent engine
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// method calls will fail.
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Close() (err error)
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}

‎image/cas/layout/interface.go‎

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// Copyright 2016 The Linux Foundation
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package layout
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import (
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"io"
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)
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// ReadSeekCloser wraps the Read, Seek, and Close methods.
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type ReadSeekCloser interface {
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io.Reader
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io.Seeker
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io.Closer
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}

‎image/cas/layout/main.go‎

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// Copyright 2016 The Linux Foundation
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Package layout implements the cas interface using the image-spec's
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// image-layout [1].
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//
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// [1]: https://github.com/opencontainers/image-spec/blob/master/image-layout.md
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package layout
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import (
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"os"
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"github.com/opencontainers/image-spec/image/cas"
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)
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// GetEngine guesses the appropriate backend (tar, HTTP, ...) and
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// returns that engine.
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func GetEngine(path string) (engine cas.Engine, err error) {
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file, err := os.Open(path)
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if err != nil {
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return nil, err
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}
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return GetTarEngine(file)
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}

‎image/cas/layout/tar.go‎

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// Copyright 2016 The Linux Foundation
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package layout
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import (
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"archive/tar"
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"errors"
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"fmt"
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"io"
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"os"
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"strings"
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"github.com/opencontainers/image-spec/image/cas"
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)
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// TarEngine is a cas.Engine backed by a tar file.
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type TarEngine struct {
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reader ReadSeekCloser
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}
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// GetTarEngine returns a TarEngine.
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func GetTarEngine(file ReadSeekCloser) (engine cas.Engine, err error) {
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engine = &TarEngine{
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reader: file,
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}
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return engine, nil
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}
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// Put adds a new blob to the store.
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func (engine *TarEngine) Put(writer io.Writer) (digest string, err error) {
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// FIXME
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return "", errors.New("TarEngine.Put is not supported yet")
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}
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// Get returns a reader for retrieving a blob from the store.
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func (engine *TarEngine) Get(digest string) (reader io.Reader, err error) {
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fields := strings.SplitN(digest, ":", 2)
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if len(fields) != 2 {
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return nil, fmt.Errorf("invalid digest: %q, %v", digest, fields)
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}
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algorithm := fields[0]
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hash := fields[1]
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targetName := fmt.Sprintf("./blobs/%s-%s", algorithm, hash)
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_, err = engine.reader.Seek(0, os.SEEK_SET)
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if err != nil {
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return nil, err
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}
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tarReader := tar.NewReader(engine.reader)
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for {
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header, err := tarReader.Next()
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if err != nil {
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return nil, err
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}
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if header.Name == targetName {
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return tarReader, nil
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}
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}
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}
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// Delete removes a blob from the store.
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func (engine *TarEngine) Delete(digest string) (err error) {
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// FIXME
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return errors.New("TarEngine.Delete is not supported yet")
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}
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// Close releases resources held by the engine.
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func (engine *TarEngine) Close() (err error) {
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return engine.reader.Close()
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}

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