blob: d0c7fa86083721c415a8c6ac7209c9ae2119898d [file] [log] [blame]
// Package layer is package for managing read-only
// and read-write mounts on the union file system
// driver. Read-only mounts are referenced using a
// content hash and are protected from mutation in
// the exposed interface. The tar format is used
// to create read-only layers and export both
// read-only and writable layers. The exported
// tar data for a read-only layer should match
// the tar used to create the layer.
package layer // import ""
import (
var (
// ErrLayerDoesNotExist is used when an operation is
// attempted on a layer which does not exist.
ErrLayerDoesNotExist = errors.New("layer does not exist")
// ErrLayerNotRetained is used when a release is
// attempted on a layer which is not retained.
ErrLayerNotRetained = errors.New("layer not retained")
// ErrMountDoesNotExist is used when an operation is
// attempted on a mount layer which does not exist.
ErrMountDoesNotExist = errors.New("mount does not exist")
// ErrMountNameConflict is used when a mount is attempted
// to be created but there is already a mount with the name
// used for creation.
ErrMountNameConflict = errors.New("mount already exists with name")
// ErrActiveMount is used when an operation on a
// mount is attempted but the layer is still
// mounted and the operation cannot be performed.
ErrActiveMount = errors.New("mount still active")
// ErrNotMounted is used when requesting an active
// mount but the layer is not mounted.
ErrNotMounted = errors.New("not mounted")
// ErrMaxDepthExceeded is used when a layer is attempted
// to be created which would result in a layer depth
// greater than the 125 max.
ErrMaxDepthExceeded = errors.New("max depth exceeded")
// ErrNotSupported is used when the action is not supported
// on the current host operating system.
ErrNotSupported = errors.New("not support on this host operating system")
// ChainID is the content-addressable ID of a layer.
type ChainID digest.Digest
// String returns a string rendition of a layer ID
func (id ChainID) String() string {
return string(id)
// DiffID is the hash of an individual layer tar.
type DiffID digest.Digest
// String returns a string rendition of a layer DiffID
func (diffID DiffID) String() string {
return string(diffID)
// TarStreamer represents an object which may
// have its contents exported as a tar stream.
type TarStreamer interface {
// TarStream returns a tar archive stream
// for the contents of a layer.
TarStream() (io.ReadCloser, error)
// Layer represents a read-only layer
type Layer interface {
// TarStreamFrom returns a tar archive stream for all the layer chain with
// arbitrary depth.
TarStreamFrom(ChainID) (io.ReadCloser, error)
// ChainID returns the content hash of the entire layer chain. The hash
// chain is made up of DiffID of top layer and all of its parents.
ChainID() ChainID
// DiffID returns the content hash of the layer
// tar stream used to create this layer.
DiffID() DiffID
// Parent returns the next layer in the layer chain.
Parent() Layer
// Size returns the size of the entire layer chain. The size
// is calculated from the total size of all files in the layers.
Size() (int64, error)
// DiffSize returns the size difference of the top layer
// from parent layer.
DiffSize() (int64, error)
// Metadata returns the low level storage metadata associated
// with layer.
Metadata() (map[string]string, error)
// RWLayer represents a layer which is
// read and writable
type RWLayer interface {
// Name of mounted layer
Name() string
// Parent returns the layer which the writable
// layer was created from.
Parent() Layer
// Mount mounts the RWLayer and returns the filesystem path
// the to the writable layer.
Mount(mountLabel string) (containerfs.ContainerFS, error)
// Unmount unmounts the RWLayer. This should be called
// for every mount. If there are multiple mount calls
// this operation will only decrement the internal mount counter.
Unmount() error
// Size represents the size of the writable layer
// as calculated by the total size of the files
// changed in the mutable layer.
Size() (int64, error)
// Changes returns the set of changes for the mutable layer
// from the base layer.
Changes() ([]archive.Change, error)
// Metadata returns the low level metadata for the mutable layer
Metadata() (map[string]string, error)
// Metadata holds information about a
// read-only layer
type Metadata struct {
// ChainID is the content hash of the layer
ChainID ChainID
// DiffID is the hash of the tar data used to
// create the layer
DiffID DiffID
// Size is the size of the layer and all parents
Size int64
// DiffSize is the size of the top layer
DiffSize int64
// MountInit is a function to initialize a
// writable mount. Changes made here will
// not be included in the Tar stream of the
// RWLayer.
type MountInit func(root containerfs.ContainerFS) error
// CreateRWLayerOpts contains optional arguments to be passed to CreateRWLayer
type CreateRWLayerOpts struct {
MountLabel string
InitFunc MountInit
StorageOpt map[string]string
// Store represents a backend for managing both
// read-only and read-write layers.
type Store interface {
Register(io.Reader, ChainID) (Layer, error)
Get(ChainID) (Layer, error)
Map() map[ChainID]Layer
Release(Layer) ([]Metadata, error)
CreateRWLayer(id string, parent ChainID, opts *CreateRWLayerOpts) (RWLayer, error)
GetRWLayer(id string) (RWLayer, error)
GetMountID(id string) (string, error)
ReleaseRWLayer(RWLayer) ([]Metadata, error)
Cleanup() error
DriverStatus() [][2]string
DriverName() string
// DescribableStore represents a layer store capable of storing
// descriptors for layers.
type DescribableStore interface {
RegisterWithDescriptor(io.Reader, ChainID, distribution.Descriptor) (Layer, error)
// CreateChainID returns ID for a layerDigest slice
func CreateChainID(dgsts []DiffID) ChainID {
return createChainIDFromParent("", dgsts...)
func createChainIDFromParent(parent ChainID, dgsts ...DiffID) ChainID {
if len(dgsts) == 0 {
return parent
if parent == "" {
return createChainIDFromParent(ChainID(dgsts[0]), dgsts[1:]...)
// H = "H(n-1) SHA256(n)"
dgst := digest.FromBytes([]byte(string(parent) + " " + string(dgsts[0])))
return createChainIDFromParent(ChainID(dgst), dgsts[1:]...)
// ReleaseAndLog releases the provided layer from the given layer
// store, logging any error and release metadata
func ReleaseAndLog(ls Store, l Layer) {
metadata, err := ls.Release(l)
if err != nil {
logrus.Errorf("Error releasing layer %s: %v", l.ChainID(), err)
// LogReleaseMetadata logs a metadata array, uses this to
// ensure consistent logging for release metadata
func LogReleaseMetadata(metadatas []Metadata) {
for _, metadata := range metadatas {
logrus.Infof("Layer %s cleaned up", metadata.ChainID)