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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 oci-cas command so folks can access the new read functionality from the command line. In a subsequent commit, I'll replace the image/walker.go functionality with this new API. The Context interface follows the pattern recommended in [1], allowing callers to cancel long running actions (e.g. push/pull over the network for engine implementations that communicate with a remote store). [1]: https://blog.golang.org/context Signed-off-by: W. Trevor King <[email protected]>
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‎.gitignore‎

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/oci-cas
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/oci-create-runtime-bundle
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/oci-unpack
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/oci-image-validate
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/oci-unpack

‎Makefile‎

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@@ -3,6 +3,7 @@ export GO15VENDOREXPERIMENT
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EPOCH_TEST_COMMIT ?= v0.2.0
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TOOLS := \
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oci-cas \
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oci-create-runtime-bundle \
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oci-image-validate \
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oci-unpack

‎cmd/oci-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/ioutil"
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"os"
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"github.com/opencontainers/image-tools/image/cas/layout"
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"github.com/spf13/cobra"
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"golang.org/x/net/context"
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)
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type getCmd struct {
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path string
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digest string
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}
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func newGetCmd() *cobra.Command {
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state := &getCmd{}
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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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Long: "Retrieve a blob from the store and write it to stdout.",
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Run: state.Run,
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}
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}
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func (state *getCmd) Run(cmd *cobra.Command, args []string) {
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if len(args) != 2 {
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fmt.Fprintln(os.Stderr, "both PATH and DIGEST must be provided")
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if err := cmd.Usage(); err != nil {
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fmt.Fprintln(os.Stderr, 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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fmt.Fprintln(os.Stderr, err)
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os.Exit(1)
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}
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os.Exit(0)
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}
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func (state *getCmd) run() (err error) {
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ctx := context.Background()
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engine, err := layout.NewEngine(state.path)
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if err != nil {
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return err
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}
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defer engine.Close()
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reader, err := engine.Get(ctx, state.digest)
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if err != nil {
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return err
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}
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defer reader.Close()
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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 := os.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 nil
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}

‎cmd/oci-cas/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 main
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import (
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"fmt"
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"os"
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"github.com/spf13/cobra"
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)
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func main() {
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cmd := &cobra.Command{
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Use: "oci-cas",
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Short: "Content-addressable storage manipulation",
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}
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cmd.AddCommand(newGetCmd())
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err := cmd.Execute()
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if err != nil {
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fmt.Fprintln(os.Stderr, err)
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os.Exit(1)
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}
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}

‎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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"golang.org/x/net/context"
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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. The action is idempotent; a
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// nil return means "that content is stored at DIGEST" without
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// implying "because of your Put()".
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Put(ctx context.Context, reader io.Reader) (digest string, err error)
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// Get returns a reader for retrieving a blob from the store.
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// Returns os.ErrNotExist if the digest is not found.
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Get(ctx context.Context, digest string) (reader io.ReadCloser, err error)
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// Delete removes a blob from the store. Returns os.ErrNotExist if
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// the digest is not found.
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Delete(ctx context.Context, 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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// ReadWriteSeekCloser wraps the Read, Write, Seek, and Close methods.
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type ReadWriteSeekCloser interface {
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io.ReadWriteSeeker
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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-tools/image/cas"
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)
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// NewEngine instantiates an engine with the appropriate backend (tar,
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// HTTP, ...).
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func NewEngine(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 NewTarEngine(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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"io/ioutil"
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"os"
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"strings"
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"github.com/opencontainers/image-tools/image/cas"
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"golang.org/x/net/context"
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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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file ReadWriteSeekCloser
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}
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// NewTarEngine returns a new TarEngine.
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func NewTarEngine(file ReadWriteSeekCloser) (engine cas.Engine, err error) {
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engine = &TarEngine{
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file: 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(ctx context.Context, reader io.Reader) (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(ctx context.Context, digest string) (reader io.ReadCloser, 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.file.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.file)
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for {
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select {
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case <-ctx.Done():
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return nil, ctx.Err()
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default:
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}
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header, err := tarReader.Next()
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if err == io.EOF {
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return nil, os.ErrNotExist
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} else 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 ioutil.NopCloser(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(ctx context.Context, 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.file.Close()
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}

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