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.



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    const DigestSHA256EmptyTar = DiffID("sha256:5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef")

      DigestSHA256EmptyTar is the canonical sha256 digest of empty tar file - (1024 NULL bytes)


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      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")
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      var EmptyLayer = &emptyLayer{}

        EmptyLayer is a layer that corresponds to empty tar.


        func IsEmpty

        func IsEmpty(diffID DiffID) bool

          IsEmpty returns true if the layer is an EmptyLayer

          func LogReleaseMetadata

          func LogReleaseMetadata(metadatas []Metadata)

            LogReleaseMetadata logs a metadata array, uses this to ensure consistent logging for release metadata

            func ReleaseAndLog

            func ReleaseAndLog(ls Store, l Layer)

              ReleaseAndLog releases the provided layer from the given layer store, logging any error and release metadata


              type ChainID

              type ChainID digest.Digest

                ChainID is the content-addressable ID of a layer.

                func CreateChainID

                func CreateChainID(dgsts []DiffID) ChainID

                  CreateChainID returns ID for a layerDigest slice

                  func (ChainID) String

                  func (id ChainID) String() string

                    String returns a string rendition of a layer ID

                    type CreateRWLayerOpts

                    type CreateRWLayerOpts struct {
                    	MountLabel string
                    	InitFunc   MountInit
                    	StorageOpt map[string]string

                      CreateRWLayerOpts contains optional arguments to be passed to CreateRWLayer

                      type DescribableStore

                      type DescribableStore interface {
                      	RegisterWithDescriptor(io.Reader, ChainID, distribution.Descriptor) (Layer, error)

                        DescribableStore represents a layer store capable of storing descriptors for layers.

                        type DiffID

                        type DiffID digest.Digest

                          DiffID is the hash of an individual layer tar.

                          func (DiffID) String

                          func (diffID DiffID) String() string

                            String returns a string rendition of a layer DiffID

                            type 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)

                              Layer represents a read-only layer

                              type Metadata

                              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

                                Metadata holds information about a read-only layer

                                type MountInit

                                type MountInit func(root containerfs.ContainerFS) error

                                  MountInit is a function to initialize a writable mount. Changes made here will not be included in the Tar stream of the RWLayer.

                                  type RWLayer

                                  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)
                                  	// ApplyDiff applies the diff to the RW layer
                                  	ApplyDiff(diff io.Reader) (int64, error)

                                    RWLayer represents a layer which is read and writable

                                    type Store

                                    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

                                      Store represents a backend for managing both read-only and read-write layers.

                                      func NewStoreFromOptions

                                      func NewStoreFromOptions(options StoreOptions) (Store, error)

                                        NewStoreFromOptions creates a new Store instance

                                        type StoreOptions

                                        type StoreOptions struct {
                                        	Root                      string
                                        	MetadataStorePathTemplate string
                                        	GraphDriver               string
                                        	GraphDriverOptions        []string
                                        	IDMapping                 *idtools.IdentityMapping
                                        	PluginGetter              plugingetter.PluginGetter
                                        	ExperimentalEnabled       bool
                                        	OS                        string

                                          StoreOptions are the options used to create a new Store instance

                                          type TarStreamer

                                          type TarStreamer interface {
                                          	// TarStream returns a tar archive stream
                                          	// for the contents of a layer.
                                          	TarStream() (io.ReadCloser, error)

                                            TarStreamer represents an object which may have its contents exported as a tar stream.