vendor/github.com/containerd/continuity/resource.go - third_party/github.com/moby/moby - Git at Google

 /*
    Copyright The containerd Authors.

    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 continuity

 import (
 	"errors"
 	"fmt"
 	"os"
 	"reflect"
 	"sort"

 	pb "github.com/containerd/continuity/proto"
 	"github.com/opencontainers/go-digest"
 )

 // TODO(stevvooe): A record based model, somewhat sketched out at the bottom
 // of this file, will be more flexible. Another possibly is to tie the package
 // interface directly to the protobuf type. This will have efficiency
 // advantages at the cost coupling the nasty codegen types to the exported
 // interface.

 type Resource interface {
 	// Path provides the primary resource path relative to the bundle root. In
 	// cases where resources have more than one path, such as with hard links,
 	// this will return the primary path, which is often just the first entry.
 	Path() string

 	// Mode returns the
 	Mode() os.FileMode

 	UID() int64
 	GID() int64
 }

 // ByPath provides the canonical sort order for a set of resources. Use with
 // sort.Stable for deterministic sorting.
 type ByPath []Resource

 func (bp ByPath) Len() int           { return len(bp) }
 func (bp ByPath) Swap(i, j int)      { bp[i], bp[j] = bp[j], bp[i] }
 func (bp ByPath) Less(i, j int) bool { return bp[i].Path() < bp[j].Path() }

 type XAttrer interface {
 	XAttrs() map[string][]byte
 }

 // Hardlinkable is an interface that a resource type satisfies if it can be a
 // hardlink target.
 type Hardlinkable interface {
 	// Paths returns all paths of the resource, including the primary path
 	// returned by Resource.Path. If len(Paths()) > 1, the resource is a hard
 	// link.
 	Paths() []string
 }

 type RegularFile interface {
 	Resource
 	XAttrer
 	Hardlinkable

 	Size() int64
 	Digests() []digest.Digest
 }

 // Merge two or more Resources into new file. Typically, this should be
 // used to merge regular files as hardlinks. If the files are not identical,
 // other than Paths and Digests, the merge will fail and an error will be
 // returned.
 func Merge(fs ...Resource) (Resource, error) {
 	if len(fs) < 1 {
 		return nil, fmt.Errorf("please provide a resource to merge")
 	}

 	if len(fs) == 1 {
 		return fs[0], nil
 	}

 	var paths []string
 	var digests []digest.Digest
 	bypath := map[string][]Resource{}

 	// The attributes are all compared against the first to make sure they
 	// agree before adding to the above collections. If any of these don't
 	// correctly validate, the merge fails.
 	prototype := fs[0]
 	xattrs := make(map[string][]byte)

 	// initialize xattrs for use below. All files must have same xattrs.
 	if prototypeXAttrer, ok := prototype.(XAttrer); ok {
 		for attr, value := range prototypeXAttrer.XAttrs() {
 			xattrs[attr] = value
 		}
 	}

 	for _, f := range fs {
 		h, isHardlinkable := f.(Hardlinkable)
 		if !isHardlinkable {
 			return nil, errNotAHardLink
 		}

 		if f.Mode() != prototype.Mode() {
 			return nil, fmt.Errorf("modes do not match: %v != %v", f.Mode(), prototype.Mode())
 		}

 		if f.UID() != prototype.UID() {
 			return nil, fmt.Errorf("uid does not match: %v != %v", f.UID(), prototype.UID())
 		}

 		if f.GID() != prototype.GID() {
 			return nil, fmt.Errorf("gid does not match: %v != %v", f.GID(), prototype.GID())
 		}

 		if xattrer, ok := f.(XAttrer); ok {
 			fxattrs := xattrer.XAttrs()
 			if !reflect.DeepEqual(fxattrs, xattrs) {
 				return nil, fmt.Errorf("resource %q xattrs do not match: %v != %v", f, fxattrs, xattrs)
 			}
 		}

 		for _, p := range h.Paths() {
 			pfs, ok := bypath[p]
 			if !ok {
 				// ensure paths are unique by only appending on a new path.
 				paths = append(paths, p)
 			}

 			bypath[p] = append(pfs, f)
 		}

 		if regFile, isRegFile := f.(RegularFile); isRegFile {
 			prototypeRegFile, prototypeIsRegFile := prototype.(RegularFile)
 			if !prototypeIsRegFile {
 				return nil, errors.New("prototype is not a regular file")
 			}

 			if regFile.Size() != prototypeRegFile.Size() {
 				return nil, fmt.Errorf("size does not match: %v != %v", regFile.Size(), prototypeRegFile.Size())
 			}

 			digests = append(digests, regFile.Digests()...)
 		} else if device, isDevice := f.(Device); isDevice {
 			prototypeDevice, prototypeIsDevice := prototype.(Device)
 			if !prototypeIsDevice {
 				return nil, errors.New("prototype is not a device")
 			}

 			if device.Major() != prototypeDevice.Major() {
 				return nil, fmt.Errorf("major number does not match: %v != %v", device.Major(), prototypeDevice.Major())
 			}
 			if device.Minor() != prototypeDevice.Minor() {
 				return nil, fmt.Errorf("minor number does not match: %v != %v", device.Minor(), prototypeDevice.Minor())
 			}
 		} else if _, isNamedPipe := f.(NamedPipe); isNamedPipe {
 			_, prototypeIsNamedPipe := prototype.(NamedPipe)
 			if !prototypeIsNamedPipe {
 				return nil, errors.New("prototype is not a named pipe")
 			}
 		} else {
 			return nil, errNotAHardLink
 		}
 	}

 	sort.Stable(sort.StringSlice(paths))

 	// Choose a "canonical" file. Really, it is just the first file to sort
 	// against. We also effectively select the very first digest as the
 	// "canonical" one for this file.
 	first := bypath[paths[0]][0]

 	resource := resource{
 		paths:  paths,
 		mode:   first.Mode(),
 		uid:    first.UID(),
 		gid:    first.GID(),
 		xattrs: xattrs,
 	}

 	switch typedF := first.(type) {
 	case RegularFile:
 		var err error
 		digests, err = uniqifyDigests(digests...)
 		if err != nil {
 			return nil, err
 		}

 		return &regularFile{
 			resource: resource,
 			size:     typedF.Size(),
 			digests:  digests,
 		}, nil
 	case Device:
 		return &device{
 			resource: resource,
 			major:    typedF.Major(),
 			minor:    typedF.Minor(),
 		}, nil

 	case NamedPipe:
 		return &namedPipe{
 			resource: resource,
 		}, nil

 	default:
 		return nil, errNotAHardLink
 	}
 }

 type Directory interface {
 	Resource
 	XAttrer

 	// Directory is a no-op method to identify directory objects by interface.
 	Directory()
 }

 type SymLink interface {
 	Resource

 	// Target returns the target of the symlink contained in the .
 	Target() string
 }

 type NamedPipe interface {
 	Resource
 	Hardlinkable
 	XAttrer

 	// Pipe is a no-op method to allow consistent resolution of NamedPipe
 	// interface.
 	Pipe()
 }

 type Device interface {
 	Resource
 	Hardlinkable
 	XAttrer

 	Major() uint64
 	Minor() uint64
 }

 type resource struct {
 	paths    []string
 	mode     os.FileMode
 	uid, gid int64
 	xattrs   map[string][]byte
 }

 var _ Resource = &resource{}

 func (r *resource) Path() string {
 	if len(r.paths) < 1 {
 		return ""
 	}

 	return r.paths[0]
 }

 func (r *resource) Mode() os.FileMode {
 	return r.mode
 }

 func (r *resource) UID() int64 {
 	return r.uid
 }

 func (r *resource) GID() int64 {
 	return r.gid
 }

 type regularFile struct {
 	resource
 	size    int64
 	digests []digest.Digest
 }

 var _ RegularFile = &regularFile{}

 // newRegularFile returns the RegularFile, using the populated base resource
 // and one or more digests of the content.
 func newRegularFile(base resource, paths []string, size int64, dgsts ...digest.Digest) (RegularFile, error) {
 	if !base.Mode().IsRegular() {
 		return nil, fmt.Errorf("not a regular file")
 	}

 	base.paths = make([]string, len(paths))
 	copy(base.paths, paths)

 	// make our own copy of digests
 	ds := make([]digest.Digest, len(dgsts))
 	copy(ds, dgsts)

 	return &regularFile{
 		resource: base,
 		size:     size,
 		digests:  ds,
 	}, nil
 }

 func (rf *regularFile) Paths() []string {
 	paths := make([]string, len(rf.paths))
 	copy(paths, rf.paths)
 	return paths
 }

 func (rf *regularFile) Size() int64 {
 	return rf.size
 }

 func (rf *regularFile) Digests() []digest.Digest {
 	digests := make([]digest.Digest, len(rf.digests))
 	copy(digests, rf.digests)
 	return digests
 }

 func (rf *regularFile) XAttrs() map[string][]byte {
 	xattrs := make(map[string][]byte, len(rf.xattrs))

 	for attr, value := range rf.xattrs {
 		xattrs[attr] = append(xattrs[attr], value...)
 	}

 	return xattrs
 }

 type directory struct {
 	resource
 }

 var _ Directory = &directory{}

 func newDirectory(base resource) (Directory, error) {
 	if !base.Mode().IsDir() {
 		return nil, fmt.Errorf("not a directory")
 	}

 	return &directory{
 		resource: base,
 	}, nil
 }

 func (d *directory) Directory() {}

 func (d *directory) XAttrs() map[string][]byte {
 	xattrs := make(map[string][]byte, len(d.xattrs))

 	for attr, value := range d.xattrs {
 		xattrs[attr] = append(xattrs[attr], value...)
 	}

 	return xattrs
 }

 type symLink struct {
 	resource
 	target string
 }

 var _ SymLink = &symLink{}

 func newSymLink(base resource, target string) (SymLink, error) {
 	if base.Mode()&os.ModeSymlink == 0 {
 		return nil, fmt.Errorf("not a symlink")
 	}

 	return &symLink{
 		resource: base,
 		target:   target,
 	}, nil
 }

 func (l *symLink) Target() string {
 	return l.target
 }

 type namedPipe struct {
 	resource
 }

 var _ NamedPipe = &namedPipe{}

 func newNamedPipe(base resource, paths []string) (NamedPipe, error) {
 	if base.Mode()&os.ModeNamedPipe == 0 {
 		return nil, fmt.Errorf("not a namedpipe")
 	}

 	base.paths = make([]string, len(paths))
 	copy(base.paths, paths)

 	return &namedPipe{
 		resource: base,
 	}, nil
 }

 func (np *namedPipe) Pipe() {}

 func (np *namedPipe) Paths() []string {
 	paths := make([]string, len(np.paths))
 	copy(paths, np.paths)
 	return paths
 }

 func (np *namedPipe) XAttrs() map[string][]byte {
 	xattrs := make(map[string][]byte, len(np.xattrs))

 	for attr, value := range np.xattrs {
 		xattrs[attr] = append(xattrs[attr], value...)
 	}

 	return xattrs
 }

 type device struct {
 	resource
 	major, minor uint64
 }

 var _ Device = &device{}

 func newDevice(base resource, paths []string, major, minor uint64) (Device, error) {
 	if base.Mode()&os.ModeDevice == 0 {
 		return nil, fmt.Errorf("not a device")
 	}

 	base.paths = make([]string, len(paths))
 	copy(base.paths, paths)

 	return &device{
 		resource: base,
 		major:    major,
 		minor:    minor,
 	}, nil
 }

 func (d *device) Paths() []string {
 	paths := make([]string, len(d.paths))
 	copy(paths, d.paths)
 	return paths
 }

 func (d *device) XAttrs() map[string][]byte {
 	xattrs := make(map[string][]byte, len(d.xattrs))

 	for attr, value := range d.xattrs {
 		xattrs[attr] = append(xattrs[attr], value...)
 	}

 	return xattrs
 }

 func (d device) Major() uint64 {
 	return d.major
 }

 func (d device) Minor() uint64 {
 	return d.minor
 }

 // toProto converts a resource to a protobuf record. We'd like to push this
 // the individual types but we want to keep this all together during
 // prototyping.
 func toProto(resource Resource) *pb.Resource {
 	b := &pb.Resource{
 		Path: []string{resource.Path()},
 		Mode: uint32(resource.Mode()),
 		Uid:  resource.UID(),
 		Gid:  resource.GID(),
 	}

 	if xattrer, ok := resource.(XAttrer); ok {
 		// Sorts the XAttrs by name for consistent ordering.
 		keys := []string{}
 		xattrs := xattrer.XAttrs()
 		for k := range xattrs {
 			keys = append(keys, k)
 		}
 		sort.Strings(keys)

 		for _, k := range keys {
 			b.Xattr = append(b.Xattr, &pb.XAttr{Name: k, Data: xattrs[k]})
 		}
 	}

 	switch r := resource.(type) {
 	case RegularFile:
 		b.Path = r.Paths()
 		b.Size = uint64(r.Size())

 		for _, dgst := range r.Digests() {
 			b.Digest = append(b.Digest, dgst.String())
 		}
 	case SymLink:
 		b.Target = r.Target()
 	case Device:
 		b.Major, b.Minor = r.Major(), r.Minor()
 		b.Path = r.Paths()
 	case NamedPipe:
 		b.Path = r.Paths()
 	}

 	// enforce a few stability guarantees that may not be provided by the
 	// resource implementation.
 	sort.Strings(b.Path)

 	return b
 }

 // fromProto converts from a protobuf Resource to a Resource interface.
 func fromProto(b *pb.Resource) (Resource, error) {
 	base := &resource{
 		paths: b.Path,
 		mode:  os.FileMode(b.Mode),
 		uid:   b.Uid,
 		gid:   b.Gid,
 	}

 	base.xattrs = make(map[string][]byte, len(b.Xattr))

 	for _, attr := range b.Xattr {
 		base.xattrs[attr.Name] = attr.Data
 	}

 	switch {
 	case base.Mode().IsRegular():
 		dgsts := make([]digest.Digest, len(b.Digest))
 		for i, dgst := range b.Digest {
 			// TODO(stevvooe): Should we be validating at this point?
 			dgsts[i] = digest.Digest(dgst)
 		}

 		return newRegularFile(*base, b.Path, int64(b.Size), dgsts...)
 	case base.Mode().IsDir():
 		return newDirectory(*base)
 	case base.Mode()&os.ModeSymlink != 0:
 		return newSymLink(*base, b.Target)
 	case base.Mode()&os.ModeNamedPipe != 0:
 		return newNamedPipe(*base, b.Path)
 	case base.Mode()&os.ModeDevice != 0:
 		return newDevice(*base, b.Path, b.Major, b.Minor)
 	}

 	return nil, fmt.Errorf("unknown resource record (%#v): %s", b, base.Mode())
 }

 // NOTE(stevvooe): An alternative model that supports inline declaration.
 // Convenient for unit testing where inline declarations may be desirable but
 // creates an awkward API for the standard use case.

 // type ResourceKind int

 // const (
 // 	ResourceRegularFile = iota + 1
 // 	ResourceDirectory
 // 	ResourceSymLink
 // 	Resource
 // )

 // type Resource struct {
 // 	Kind         ResourceKind
 // 	Paths        []string
 // 	Mode         os.FileMode
 // 	UID          string
 // 	GID          string
 // 	Size         int64
 // 	Digests      []digest.Digest
 // 	Target       string
 // 	Major, Minor int
 // 	XAttrs       map[string][]byte
 // }

 // type RegularFile struct {
 // 	Paths   []string
 //  Size 	int64
 // 	Digests []digest.Digest
 // 	Perm    os.FileMode // os.ModePerm + sticky, setuid, setgid
 // }
	/*
	Copyright The containerd Authors.

	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 continuity

	import (
	"errors"
	"fmt"
	"os"
	"reflect"
	"sort"

	pb "github.com/containerd/continuity/proto"
	"github.com/opencontainers/go-digest"
	)

	// TODO(stevvooe): A record based model, somewhat sketched out at the bottom
	// of this file, will be more flexible. Another possibly is to tie the package
	// interface directly to the protobuf type. This will have efficiency
	// advantages at the cost coupling the nasty codegen types to the exported
	// interface.

	type Resource interface {
	// Path provides the primary resource path relative to the bundle root. In
	// cases where resources have more than one path, such as with hard links,
	// this will return the primary path, which is often just the first entry.
	Path() string

	// Mode returns the
	Mode() os.FileMode

	UID() int64
	GID() int64
	}

	// ByPath provides the canonical sort order for a set of resources. Use with
	// sort.Stable for deterministic sorting.
	type ByPath []Resource

	func (bp ByPath) Len() int { return len(bp) }
	func (bp ByPath) Swap(i, j int) { bp[i], bp[j] = bp[j], bp[i] }
	func (bp ByPath) Less(i, j int) bool { return bp[i].Path() < bp[j].Path() }

	type XAttrer interface {
	XAttrs() map[string][]byte
	}

	// Hardlinkable is an interface that a resource type satisfies if it can be a
	// hardlink target.
	type Hardlinkable interface {
	// Paths returns all paths of the resource, including the primary path
	// returned by Resource.Path. If len(Paths()) > 1, the resource is a hard
	// link.
	Paths() []string
	}

	type RegularFile interface {
	Resource
	XAttrer
	Hardlinkable

	Size() int64
	Digests() []digest.Digest
	}

	// Merge two or more Resources into new file. Typically, this should be
	// used to merge regular files as hardlinks. If the files are not identical,
	// other than Paths and Digests, the merge will fail and an error will be
	// returned.
	func Merge(fs ...Resource) (Resource, error) {
	if len(fs) < 1 {
	return nil, fmt.Errorf("please provide a resource to merge")
	}

	if len(fs) == 1 {
	return fs[0], nil
	}

	var paths []string
	var digests []digest.Digest
	bypath := map[string][]Resource{}

	// The attributes are all compared against the first to make sure they
	// agree before adding to the above collections. If any of these don't
	// correctly validate, the merge fails.
	prototype := fs[0]
	xattrs := make(map[string][]byte)

	// initialize xattrs for use below. All files must have same xattrs.
	if prototypeXAttrer, ok := prototype.(XAttrer); ok {
	for attr, value := range prototypeXAttrer.XAttrs() {
	xattrs[attr] = value
	}
	}

	for _, f := range fs {
	h, isHardlinkable := f.(Hardlinkable)
	if !isHardlinkable {
	return nil, errNotAHardLink
	}

	if f.Mode() != prototype.Mode() {
	return nil, fmt.Errorf("modes do not match: %v != %v", f.Mode(), prototype.Mode())
	}

	if f.UID() != prototype.UID() {
	return nil, fmt.Errorf("uid does not match: %v != %v", f.UID(), prototype.UID())
	}

	if f.GID() != prototype.GID() {
	return nil, fmt.Errorf("gid does not match: %v != %v", f.GID(), prototype.GID())
	}

	if xattrer, ok := f.(XAttrer); ok {
	fxattrs := xattrer.XAttrs()
	if !reflect.DeepEqual(fxattrs, xattrs) {
	return nil, fmt.Errorf("resource %q xattrs do not match: %v != %v", f, fxattrs, xattrs)
	}
	}

	for _, p := range h.Paths() {
	pfs, ok := bypath[p]
	if !ok {
	// ensure paths are unique by only appending on a new path.
	paths = append(paths, p)
	}

	bypath[p] = append(pfs, f)
	}

	if regFile, isRegFile := f.(RegularFile); isRegFile {
	prototypeRegFile, prototypeIsRegFile := prototype.(RegularFile)
	if !prototypeIsRegFile {
	return nil, errors.New("prototype is not a regular file")
	}

	if regFile.Size() != prototypeRegFile.Size() {
	return nil, fmt.Errorf("size does not match: %v != %v", regFile.Size(), prototypeRegFile.Size())
	}

	digests = append(digests, regFile.Digests()...)
	} else if device, isDevice := f.(Device); isDevice {
	prototypeDevice, prototypeIsDevice := prototype.(Device)
	if !prototypeIsDevice {
	return nil, errors.New("prototype is not a device")
	}

	if device.Major() != prototypeDevice.Major() {
	return nil, fmt.Errorf("major number does not match: %v != %v", device.Major(), prototypeDevice.Major())
	}
	if device.Minor() != prototypeDevice.Minor() {
	return nil, fmt.Errorf("minor number does not match: %v != %v", device.Minor(), prototypeDevice.Minor())
	}
	} else if _, isNamedPipe := f.(NamedPipe); isNamedPipe {
	_, prototypeIsNamedPipe := prototype.(NamedPipe)
	if !prototypeIsNamedPipe {
	return nil, errors.New("prototype is not a named pipe")
	}
	} else {
	return nil, errNotAHardLink
	}
	}

	sort.Stable(sort.StringSlice(paths))

	// Choose a "canonical" file. Really, it is just the first file to sort
	// against. We also effectively select the very first digest as the
	// "canonical" one for this file.
	first := bypath[paths[0]][0]

	resource := resource{
	paths: paths,
	mode: first.Mode(),
	uid: first.UID(),
	gid: first.GID(),
	xattrs: xattrs,
	}

	switch typedF := first.(type) {
	case RegularFile:
	var err error
	digests, err = uniqifyDigests(digests...)
	if err != nil {
	return nil, err
	}

	return &regularFile{
	resource: resource,
	size: typedF.Size(),
	digests: digests,
	}, nil
	case Device:
	return &device{
	resource: resource,
	major: typedF.Major(),
	minor: typedF.Minor(),
	}, nil

	case NamedPipe:
	return &namedPipe{
	resource: resource,
	}, nil

	default:
	return nil, errNotAHardLink
	}
	}

	type Directory interface {
	Resource
	XAttrer

	// Directory is a no-op method to identify directory objects by interface.
	Directory()
	}

	type SymLink interface {
	Resource

	// Target returns the target of the symlink contained in the .
	Target() string
	}

	type NamedPipe interface {
	Resource
	Hardlinkable
	XAttrer

	// Pipe is a no-op method to allow consistent resolution of NamedPipe
	// interface.
	Pipe()
	}

	type Device interface {
	Resource
	Hardlinkable
	XAttrer

	Major() uint64
	Minor() uint64
	}

	type resource struct {
	paths []string
	mode os.FileMode
	uid, gid int64
	xattrs map[string][]byte
	}

	var _ Resource = &resource{}

	func (r *resource) Path() string {
	if len(r.paths) < 1 {
	return ""
	}

	return r.paths[0]
	}

	func (r *resource) Mode() os.FileMode {
	return r.mode
	}

	func (r *resource) UID() int64 {
	return r.uid
	}

	func (r *resource) GID() int64 {
	return r.gid
	}

	type regularFile struct {
	resource
	size int64
	digests []digest.Digest
	}

	var _ RegularFile = &regularFile{}

	// newRegularFile returns the RegularFile, using the populated base resource
	// and one or more digests of the content.
	func newRegularFile(base resource, paths []string, size int64, dgsts ...digest.Digest) (RegularFile, error) {
	if !base.Mode().IsRegular() {
	return nil, fmt.Errorf("not a regular file")
	}

	base.paths = make([]string, len(paths))
	copy(base.paths, paths)

	// make our own copy of digests
	ds := make([]digest.Digest, len(dgsts))
	copy(ds, dgsts)

	return &regularFile{
	resource: base,
	size: size,
	digests: ds,
	}, nil
	}

	func (rf *regularFile) Paths() []string {
	paths := make([]string, len(rf.paths))
	copy(paths, rf.paths)
	return paths
	}

	func (rf *regularFile) Size() int64 {
	return rf.size
	}

	func (rf *regularFile) Digests() []digest.Digest {
	digests := make([]digest.Digest, len(rf.digests))
	copy(digests, rf.digests)
	return digests
	}

	func (rf *regularFile) XAttrs() map[string][]byte {
	xattrs := make(map[string][]byte, len(rf.xattrs))

	for attr, value := range rf.xattrs {
	xattrs[attr] = append(xattrs[attr], value...)
	}

	return xattrs
	}

	type directory struct {
	resource
	}

	var _ Directory = &directory{}

	func newDirectory(base resource) (Directory, error) {
	if !base.Mode().IsDir() {
	return nil, fmt.Errorf("not a directory")
	}

	return &directory{
	resource: base,
	}, nil
	}

	func (d *directory) Directory() {}

	func (d *directory) XAttrs() map[string][]byte {
	xattrs := make(map[string][]byte, len(d.xattrs))

	for attr, value := range d.xattrs {
	xattrs[attr] = append(xattrs[attr], value...)
	}

	return xattrs
	}

	type symLink struct {
	resource
	target string
	}

	var _ SymLink = &symLink{}

	func newSymLink(base resource, target string) (SymLink, error) {
	if base.Mode()&os.ModeSymlink == 0 {
	return nil, fmt.Errorf("not a symlink")
	}

	return &symLink{
	resource: base,
	target: target,
	}, nil
	}

	func (l *symLink) Target() string {
	return l.target
	}

	type namedPipe struct {
	resource
	}

	var _ NamedPipe = &namedPipe{}

	func newNamedPipe(base resource, paths []string) (NamedPipe, error) {
	if base.Mode()&os.ModeNamedPipe == 0 {
	return nil, fmt.Errorf("not a namedpipe")
	}

	base.paths = make([]string, len(paths))
	copy(base.paths, paths)

	return &namedPipe{
	resource: base,
	}, nil
	}

	func (np *namedPipe) Pipe() {}

	func (np *namedPipe) Paths() []string {
	paths := make([]string, len(np.paths))
	copy(paths, np.paths)
	return paths
	}

	func (np *namedPipe) XAttrs() map[string][]byte {
	xattrs := make(map[string][]byte, len(np.xattrs))

	for attr, value := range np.xattrs {
	xattrs[attr] = append(xattrs[attr], value...)
	}

	return xattrs
	}

	type device struct {
	resource
	major, minor uint64
	}

	var _ Device = &device{}

	func newDevice(base resource, paths []string, major, minor uint64) (Device, error) {
	if base.Mode()&os.ModeDevice == 0 {
	return nil, fmt.Errorf("not a device")
	}

	base.paths = make([]string, len(paths))
	copy(base.paths, paths)

	return &device{
	resource: base,
	major: major,
	minor: minor,
	}, nil
	}

	func (d *device) Paths() []string {
	paths := make([]string, len(d.paths))
	copy(paths, d.paths)
	return paths
	}

	func (d *device) XAttrs() map[string][]byte {
	xattrs := make(map[string][]byte, len(d.xattrs))

	for attr, value := range d.xattrs {
	xattrs[attr] = append(xattrs[attr], value...)
	}

	return xattrs
	}

	func (d device) Major() uint64 {
	return d.major
	}

	func (d device) Minor() uint64 {
	return d.minor
	}

	// toProto converts a resource to a protobuf record. We'd like to push this
	// the individual types but we want to keep this all together during
	// prototyping.
	func toProto(resource Resource) *pb.Resource {
	b := &pb.Resource{
	Path: []string{resource.Path()},
	Mode: uint32(resource.Mode()),
	Uid: resource.UID(),
	Gid: resource.GID(),
	}

	if xattrer, ok := resource.(XAttrer); ok {
	// Sorts the XAttrs by name for consistent ordering.
	keys := []string{}
	xattrs := xattrer.XAttrs()
	for k := range xattrs {
	keys = append(keys, k)
	}
	sort.Strings(keys)

	for _, k := range keys {
	b.Xattr = append(b.Xattr, &pb.XAttr{Name: k, Data: xattrs[k]})
	}
	}

	switch r := resource.(type) {
	case RegularFile:
	b.Path = r.Paths()
	b.Size = uint64(r.Size())

	for _, dgst := range r.Digests() {
	b.Digest = append(b.Digest, dgst.String())
	}
	case SymLink:
	b.Target = r.Target()
	case Device:
	b.Major, b.Minor = r.Major(), r.Minor()
	b.Path = r.Paths()
	case NamedPipe:
	b.Path = r.Paths()
	}

	// enforce a few stability guarantees that may not be provided by the
	// resource implementation.
	sort.Strings(b.Path)

	return b
	}

	// fromProto converts from a protobuf Resource to a Resource interface.
	func fromProto(b *pb.Resource) (Resource, error) {
	base := &resource{
	paths: b.Path,
	mode: os.FileMode(b.Mode),
	uid: b.Uid,
	gid: b.Gid,
	}

	base.xattrs = make(map[string][]byte, len(b.Xattr))

	for _, attr := range b.Xattr {
	base.xattrs[attr.Name] = attr.Data
	}

	switch {
	case base.Mode().IsRegular():
	dgsts := make([]digest.Digest, len(b.Digest))
	for i, dgst := range b.Digest {
	// TODO(stevvooe): Should we be validating at this point?
	dgsts[i] = digest.Digest(dgst)
	}

	return newRegularFile(*base, b.Path, int64(b.Size), dgsts...)
	case base.Mode().IsDir():
	return newDirectory(*base)
	case base.Mode()&os.ModeSymlink != 0:
	return newSymLink(*base, b.Target)
	case base.Mode()&os.ModeNamedPipe != 0:
	return newNamedPipe(*base, b.Path)
	case base.Mode()&os.ModeDevice != 0:
	return newDevice(*base, b.Path, b.Major, b.Minor)
	}

	return nil, fmt.Errorf("unknown resource record (%#v): %s", b, base.Mode())
	}

	// NOTE(stevvooe): An alternative model that supports inline declaration.
	// Convenient for unit testing where inline declarations may be desirable but
	// creates an awkward API for the standard use case.

	// type ResourceKind int

	// const (
	// ResourceRegularFile = iota + 1
	// ResourceDirectory
	// ResourceSymLink
	// Resource
	// )

	// type Resource struct {
	// Kind ResourceKind
	// Paths []string
	// Mode os.FileMode
	// UID string
	// GID string
	// Size int64
	// Digests []digest.Digest
	// Target string
	// Major, Minor int
	// XAttrs map[string][]byte
	// }

	// type RegularFile struct {
	// Paths []string
	// Size int64
	// Digests []digest.Digest
	// Perm os.FileMode // os.ModePerm + sticky, setuid, setgid
	// }