docs/layer/layer.go

// 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 (
	"errors"
	"io"

	"github.com/Sirupsen/logrus"
	"github.com/docker/distribution"
	"github.com/docker/distribution/digest"
	"github.com/docker/docker/pkg/archive"
)

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 platform
	ErrNotSupported = errors.New("not support on this platform")
)

// 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 {
	TarStreamer

	// 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 {
	TarStreamer

	// 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) (string, 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 string) error

// 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)
	Release(Layer) ([]Metadata, error)

	CreateRWLayer(id string, parent ChainID, mountLabel string, initFunc MountInit, storageOpt map[string]string) (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)
}

// MetadataTransaction represents functions for setting layer metadata
// with a single transaction.
type MetadataTransaction interface {
	SetSize(int64) error
	SetParent(parent ChainID) error
	SetDiffID(DiffID) error
	SetCacheID(string) error
	SetDescriptor(distribution.Descriptor) error
	TarSplitWriter(compressInput bool) (io.WriteCloser, error)

	Commit(ChainID) error
	Cancel() error
	String() string
}

// MetadataStore represents a backend for persisting
// metadata about layers and providing the metadata
// for restoring a Store.
type MetadataStore interface {
	// StartTransaction starts an update for new metadata
	// which will be used to represent an ID on commit.
	StartTransaction() (MetadataTransaction, error)

	GetSize(ChainID) (int64, error)
	GetParent(ChainID) (ChainID, error)
	GetDiffID(ChainID) (DiffID, error)
	GetCacheID(ChainID) (string, error)
	GetDescriptor(ChainID) (distribution.Descriptor, error)
	TarSplitReader(ChainID) (io.ReadCloser, error)

	SetMountID(string, string) error
	SetInitID(string, string) error
	SetMountParent(string, ChainID) error

	GetMountID(string) (string, error)
	GetInitID(string) (string, error)
	GetMountParent(string) (ChainID, error)

	// List returns the full list of referenced
	// read-only and read-write layers
	List() ([]ChainID, []string, error)

	Remove(ChainID) error
	RemoveMount(string) 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(metadata)
}

// 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)
	}
}