distribution/storage/paths.go

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package storage
import (
"fmt"
"path"
"strings"
"github.com/docker/distribution/digest"
"github.com/docker/distribution/storagedriver"
)
const storagePathVersion = "v2"
// pathMapper maps paths based on "object names" and their ids. The "object
// names" mapped by pathMapper are internal to the storage system.
//
// The path layout in the storage backend will be roughly as follows:
//
// <root>/v2
// -> repositories/
// -><name>/
// -> manifests/
// <manifests by tag name>
// -> layers/
// <layer links to blob store>
// -> blob/<algorithm>
// <split directory content addressable storage>
//
// There are few important components to this path layout. First, we have the
// repository store identified by name. This contains the image manifests and
// a layer store with links to CAS blob ids. Outside of the named repo area,
// we have the the blob store. It contains the actual layer data and any other
// data that can be referenced by a CAS id.
//
// We cover the path formats implemented by this path mapper below.
//
// manifestPathSpec: <root>/v2/repositories/<name>/manifests/<tag>
// layerLinkPathSpec: <root>/v2/repositories/<name>/layers/tarsum/<tarsum version>/<tarsum hash alg>/<tarsum hash>
// blobPathSpec: <root>/v2/blob/<algorithm>/<first two hex bytes of digest>/<hex digest>
//
// For more information on the semantic meaning of each path and their
// contents, please see the path spec documentation.
type pathMapper struct {
root string
version string // should be a constant?
}
var defaultPathMapper = &pathMapper{
root: "/docker/registry/",
version: storagePathVersion,
}
// path returns the path identified by spec.
func (pm *pathMapper) path(spec pathSpec) (string, error) {
// Switch on the path object type and return the appropriate path. At
// first glance, one may wonder why we don't use an interface to
// accomplish this. By keep the formatting separate from the pathSpec, we
// keep separate the path generation componentized. These specs could be
// passed to a completely different mapper implementation and generate a
// different set of paths.
//
// For example, imagine migrating from one backend to the other: one could
// build a filesystem walker that converts a string path in one version,
// to an intermediate path object, than can be consumed and mapped by the
// other version.
rootPrefix := []string{pm.root, pm.version}
repoPrefix := append(rootPrefix, "repositories")
switch v := spec.(type) {
case manifestTagsPath:
return path.Join(append(repoPrefix, v.name, "manifests")...), nil
case manifestPathSpec:
// TODO(sday): May need to store manifest by architecture.
return path.Join(append(repoPrefix, v.name, "manifests", v.tag)...), nil
case layerLinkPathSpec:
components, err := digestPathComoponents(v.digest)
if err != nil {
return "", err
}
// For now, only map tarsum paths.
if components[0] != "tarsum" {
// Only tarsum is supported, for now
return "", fmt.Errorf("unsupported content digest: %v", v.digest)
}
layerLinkPathComponents := append(repoPrefix, v.name, "layers")
return path.Join(append(layerLinkPathComponents, components...)...), nil
case blobPathSpec:
components, err := digestPathComoponents(v.digest)
if err != nil {
return "", err
}
// For now, only map tarsum paths.
if components[0] != "tarsum" {
// Only tarsum is supported, for now
return "", fmt.Errorf("unsupported content digest: %v", v.digest)
}
blobPathPrefix := append(rootPrefix, "blob")
return path.Join(append(blobPathPrefix, components...)...), nil
default:
// TODO(sday): This is an internal error. Ensure it doesn't escape (panic?).
return "", fmt.Errorf("unknown path spec: %#v", v)
}
}
// pathSpec is a type to mark structs as path specs. There is no
// implementation because we'd like to keep the specs and the mappers
// decoupled.
type pathSpec interface {
pathSpec()
}
// manifestTagsPath describes the path elements required to point to the
// directory with all manifest tags under the repository.
type manifestTagsPath struct {
name string
}
func (manifestTagsPath) pathSpec() {}
// manifestPathSpec describes the path elements used to build a manifest path.
// The contents should be a signed manifest json file.
type manifestPathSpec struct {
name string
tag string
}
func (manifestPathSpec) pathSpec() {}
// layerLink specifies a path for a layer link, which is a file with a blob
// id. The layer link will contain a content addressable blob id reference
// into the blob store. The format of the contents is as follows:
//
// <algorithm>:<hex digest of layer data>
//
// The following example of the file contents is more illustrative:
//
// sha256:96443a84ce518ac22acb2e985eda402b58ac19ce6f91980bde63726a79d80b36
//
// This says indicates that there is a blob with the id/digest, calculated via
// sha256 that can be fetched from the blob store.
type layerLinkPathSpec struct {
name string
digest digest.Digest
}
func (layerLinkPathSpec) pathSpec() {}
// blobAlgorithmReplacer does some very simple path sanitization for user
// input. Mostly, this is to provide some heirachry for tarsum digests. Paths
// should be "safe" before getting this far due to strict digest requirements
// but we can add further path conversion here, if needed.
var blobAlgorithmReplacer = strings.NewReplacer(
"+", "/",
".", "/",
";", "/",
)
// blobPath contains the path for the registry global blob store. For now,
// this contains layer data, exclusively.
type blobPathSpec struct {
digest digest.Digest
}
func (blobPathSpec) pathSpec() {}
// digestPathComoponents provides a consistent path breakdown for a given
// digest. For a generic digest, it will be as follows:
//
// <algorithm>/<first two bytes of digest>/<full digest>
//
// Most importantly, for tarsum, the layout looks like this:
//
// tarsum/<version>/<digest algorithm>/<first two bytes of digest>/<full digest>
//
// This is slightly specialized to store an extra version path for version 0
// tarsums.
func digestPathComoponents(dgst digest.Digest) ([]string, error) {
if err := dgst.Validate(); err != nil {
return nil, err
}
algorithm := blobAlgorithmReplacer.Replace(dgst.Algorithm())
hex := dgst.Hex()
prefix := []string{algorithm}
suffix := []string{
hex[:2], // Breaks heirarchy up.
hex,
}
if tsi, err := digest.ParseTarSum(dgst.String()); err == nil {
// We have a tarsum!
version := tsi.Version
if version == "" {
version = "v0"
}
prefix = []string{
"tarsum",
version,
tsi.Algorithm,
}
}
return append(prefix, suffix...), nil
}
// resolveBlobPath looks up the blob location in the repositories from a
// layer/blob link file, returning blob path or an error on failure.
func resolveBlobPath(driver storagedriver.StorageDriver, pm *pathMapper, name string, dgst digest.Digest) (string, error) {
pathSpec := layerLinkPathSpec{name: name, digest: dgst}
layerLinkPath, err := pm.path(pathSpec)
if err != nil {
return "", err
}
layerLinkContent, err := driver.GetContent(layerLinkPath)
if err != nil {
return "", err
}
// NOTE(stevvooe): The content of the layer link should match the digest.
// This layer of indirection is for name-based content protection.
linked, err := digest.ParseDigest(string(layerLinkContent))
if err != nil {
return "", err
}
bp := blobPathSpec{digest: linked}
return pm.path(bp)
}