c8d: align "Size" and "VirtualSize" for images
- relates to https://github.com/moby/moby/pull/45346
- fixes https://github.com/moby/moby/issues/43862


In versions of Docker before v1.10, this field was calculated from the image itself and all of its parent images. Images are now stored self-contained, and no longer use a parent-chain, making this field an equivalent of the Size field.

For the containerd integration, the Size should be the sum of the image's compressed / packaged and unpacked (snapshots) layers.

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daemon/containerd/service.go
package containerd

import (
	"context"
	"encoding/json"
	"sync/atomic"

	"github.com/containerd/containerd"
	"github.com/containerd/containerd/content"
	"github.com/containerd/containerd/plugin"
	"github.com/containerd/containerd/remotes/docker"
	"github.com/containerd/containerd/snapshots"
	"github.com/docker/docker/container"
	daemonevents "github.com/docker/docker/daemon/events"
	"github.com/docker/docker/daemon/images"
	"github.com/docker/docker/errdefs"
	"github.com/docker/docker/image"
	"github.com/docker/docker/layer"
	"github.com/opencontainers/go-digest"
	"github.com/opencontainers/image-spec/identity"
	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
	"github.com/pkg/errors"
)

// ImageService implements daemon.ImageService
type ImageService struct {
	client          *containerd.Client
	containers      container.Store
	snapshotter     string
	registryHosts   RegistryHostsProvider
	registryService RegistryConfigProvider
	eventsService   *daemonevents.Events
	pruneRunning    atomic.Bool
}

type RegistryHostsProvider interface {
	RegistryHosts() docker.RegistryHosts
}

type RegistryConfigProvider interface {
	IsInsecureRegistry(host string) bool
}

type ImageServiceConfig struct {
	Client        *containerd.Client
	Containers    container.Store
	Snapshotter   string
	HostsProvider RegistryHostsProvider
	Registry      RegistryConfigProvider
	EventsService *daemonevents.Events
}

// NewService creates a new ImageService.
func NewService(config ImageServiceConfig) *ImageService {
	return &ImageService{
		client:          config.Client,
		containers:      config.Containers,
		snapshotter:     config.Snapshotter,
		registryHosts:   config.HostsProvider,
		registryService: config.Registry,
		eventsService:   config.EventsService,
	}
}

// DistributionServices return services controlling daemon image storage.
func (i *ImageService) DistributionServices() images.DistributionServices {
	return images.DistributionServices{}
}

// CountImages returns the number of images stored by ImageService
// called from info.go
func (i *ImageService) CountImages() int {
	imgs, err := i.client.ListImages(context.TODO())
	if err != nil {
		return 0
	}

	return len(imgs)
}

// CreateLayer creates a filesystem layer for a container.
// called from create.go
// TODO: accept an opt struct instead of container?
func (i *ImageService) CreateLayer(container *container.Container, initFunc layer.MountInit) (layer.RWLayer, error) {
	return nil, errdefs.NotImplemented(errdefs.NotImplemented(errors.New("not implemented")))
}

// LayerStoreStatus returns the status for each layer store
// called from info.go
func (i *ImageService) LayerStoreStatus() [][2]string {
	// TODO(thaJeztah) do we want to add more details about the driver here?
	return [][2]string{
		{"driver-type", string(plugin.SnapshotPlugin)},
	}
}

// GetLayerMountID returns the mount ID for a layer
// called from daemon.go Daemon.Shutdown(), and Daemon.Cleanup() (cleanup is actually continerCleanup)
// TODO: needs to be refactored to Unmount (see callers), or removed and replaced with GetLayerByID
func (i *ImageService) GetLayerMountID(cid string) (string, error) {
	return "", errdefs.NotImplemented(errors.New("not implemented"))
}

// Cleanup resources before the process is shutdown.
// called from daemon.go Daemon.Shutdown()
func (i *ImageService) Cleanup() error {
	return nil
}

// StorageDriver returns the name of the default storage-driver (snapshotter)
// used by the ImageService.
func (i *ImageService) StorageDriver() string {
	return i.snapshotter
}

// ReleaseLayer releases a layer allowing it to be removed
// called from delete.go Daemon.cleanupContainer(), and Daemon.containerExport()
func (i *ImageService) ReleaseLayer(rwlayer layer.RWLayer) error {
	return errdefs.NotImplemented(errors.New("not implemented"))
}

// LayerDiskUsage returns the number of bytes used by layer stores
// called from disk_usage.go
func (i *ImageService) LayerDiskUsage(ctx context.Context) (int64, error) {
	var allLayersSize int64
	// TODO(thaJeztah): do we need to take multiple snapshotters into account? See https://github.com/moby/moby/issues/45273
	snapshotter := i.client.SnapshotService(i.snapshotter)
	snapshotter.Walk(ctx, func(ctx context.Context, info snapshots.Info) error {
		usage, err := snapshotter.Usage(ctx, info.Name)
		if err != nil {
			return err
		}
		allLayersSize += usage.Size
		return nil
	})
	return allLayersSize, nil
}

// UpdateConfig values
//
// called from reload.go
func (i *ImageService) UpdateConfig(maxDownloads, maxUploads int) {
	panic("not implemented")
}

// GetLayerFolders returns the layer folders from an image RootFS.
func (i *ImageService) GetLayerFolders(img *image.Image, rwLayer layer.RWLayer) ([]string, error) {
	return nil, errdefs.NotImplemented(errors.New("not implemented"))
}

// GetContainerLayerSize returns the real size & virtual size of the container.
func (i *ImageService) GetContainerLayerSize(ctx context.Context, containerID string) (int64, int64, error) {
	ctr := i.containers.Get(containerID)
	if ctr == nil {
		return 0, 0, nil
	}
	cs := i.client.ContentStore()

	imageManifestBytes, err := content.ReadBlob(ctx, cs, *ctr.ImageManifest)
	if err != nil {
		return 0, 0, err
	}

	var manifest ocispec.Manifest
	if err := json.Unmarshal(imageManifestBytes, &manifest); err != nil {
		return 0, 0, err
	}

	imageConfigBytes, err := content.ReadBlob(ctx, cs, manifest.Config)
	if err != nil {
		return 0, 0, err
	}
	var img ocispec.Image
	if err := json.Unmarshal(imageConfigBytes, &img); err != nil {
		return 0, 0, err
	}

	snapshotter := i.client.SnapshotService(ctr.Driver)
	usage, err := snapshotter.Usage(ctx, containerID)
	if err != nil {
		return 0, 0, err
	}

	sizeCache := make(map[digest.Digest]int64)
	snapshotSizeFn := func(d digest.Digest) (int64, error) {
		if s, ok := sizeCache[d]; ok {
			return s, nil
		}
		u, err := snapshotter.Usage(ctx, d.String())
		if err != nil {
			return 0, err
		}
		sizeCache[d] = u.Size
		return u.Size, nil
	}

	chainIDs := identity.ChainIDs(img.RootFS.DiffIDs)
	virtualSize, err := computeVirtualSize(chainIDs, snapshotSizeFn)
	if err != nil {
		return 0, 0, err
	}

	return usage.Size, usage.Size + virtualSize, nil
}

package containerd

import (
	"context"
	"encoding/json"
	"sync/atomic"

	"github.com/containerd/containerd"
	"github.com/containerd/containerd/content"
	"github.com/containerd/containerd/plugin"
	"github.com/containerd/containerd/remotes/docker"
	"github.com/containerd/containerd/snapshots"
	"github.com/docker/docker/container"
	daemonevents "github.com/docker/docker/daemon/events"
	"github.com/docker/docker/daemon/images"
	"github.com/docker/docker/errdefs"
	"github.com/docker/docker/image"
	"github.com/docker/docker/layer"
	"github.com/opencontainers/go-digest"
	"github.com/opencontainers/image-spec/identity"
	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
	"github.com/pkg/errors"
)

// ImageService implements daemon.ImageService
type ImageService struct {
	client          *containerd.Client
	containers      container.Store
	snapshotter     string
	registryHosts   RegistryHostsProvider
	registryService RegistryConfigProvider
	eventsService   *daemonevents.Events
	pruneRunning    atomic.Bool
}

type RegistryHostsProvider interface {
	RegistryHosts() docker.RegistryHosts
}

type RegistryConfigProvider interface {
	IsInsecureRegistry(host string) bool
}

type ImageServiceConfig struct {
	Client        *containerd.Client
	Containers    container.Store
	Snapshotter   string
	HostsProvider RegistryHostsProvider
	Registry      RegistryConfigProvider
	EventsService *daemonevents.Events
}

// NewService creates a new ImageService.
func NewService(config ImageServiceConfig) *ImageService {
	return &ImageService{
		client:          config.Client,
		containers:      config.Containers,
		snapshotter:     config.Snapshotter,
		registryHosts:   config.HostsProvider,
		registryService: config.Registry,
		eventsService:   config.EventsService,
	}
}

// DistributionServices return services controlling daemon image storage.
func (i *ImageService) DistributionServices() images.DistributionServices {
	return images.DistributionServices{}
}

// CountImages returns the number of images stored by ImageService
// called from info.go
func (i *ImageService) CountImages() int {
	imgs, err := i.client.ListImages(context.TODO())
	if err != nil {
		return 0
	}

	return len(imgs)
}

// CreateLayer creates a filesystem layer for a container.
// called from create.go
// TODO: accept an opt struct instead of container?
func (i *ImageService) CreateLayer(container *container.Container, initFunc layer.MountInit) (layer.RWLayer, error) {
	return nil, errdefs.NotImplemented(errdefs.NotImplemented(errors.New("not implemented")))
}

// LayerStoreStatus returns the status for each layer store
// called from info.go
func (i *ImageService) LayerStoreStatus() [][2]string {
	// TODO(thaJeztah) do we want to add more details about the driver here?
	return [][2]string{
		{"driver-type", string(plugin.SnapshotPlugin)},
	}
}

// GetLayerMountID returns the mount ID for a layer
// called from daemon.go Daemon.Shutdown(), and Daemon.Cleanup() (cleanup is actually continerCleanup)
// TODO: needs to be refactored to Unmount (see callers), or removed and replaced with GetLayerByID
func (i *ImageService) GetLayerMountID(cid string) (string, error) {
	return "", errdefs.NotImplemented(errors.New("not implemented"))
}

// Cleanup resources before the process is shutdown.
// called from daemon.go Daemon.Shutdown()
func (i *ImageService) Cleanup() error {
	return nil
}

// StorageDriver returns the name of the default storage-driver (snapshotter)
// used by the ImageService.
func (i *ImageService) StorageDriver() string {
	return i.snapshotter
}

// ReleaseLayer releases a layer allowing it to be removed
// called from delete.go Daemon.cleanupContainer(), and Daemon.containerExport()
func (i *ImageService) ReleaseLayer(rwlayer layer.RWLayer) error {
	return errdefs.NotImplemented(errors.New("not implemented"))
}

// LayerDiskUsage returns the number of bytes used by layer stores
// called from disk_usage.go
func (i *ImageService) LayerDiskUsage(ctx context.Context) (int64, error) {
	var allLayersSize int64
	// TODO(thaJeztah): do we need to take multiple snapshotters into account? See https://github.com/moby/moby/issues/45273
	snapshotter := i.client.SnapshotService(i.snapshotter)
	snapshotter.Walk(ctx, func(ctx context.Context, info snapshots.Info) error {
		usage, err := snapshotter.Usage(ctx, info.Name)
		if err != nil {
			return err
		}
		allLayersSize += usage.Size
		return nil
	})
	return allLayersSize, nil
}

// UpdateConfig values
//
// called from reload.go
func (i *ImageService) UpdateConfig(maxDownloads, maxUploads int) {
	panic("not implemented")
}

// GetLayerFolders returns the layer folders from an image RootFS.
func (i *ImageService) GetLayerFolders(img *image.Image, rwLayer layer.RWLayer) ([]string, error) {
	return nil, errdefs.NotImplemented(errors.New("not implemented"))
}

// GetContainerLayerSize returns the real size & virtual size of the container.
func (i *ImageService) GetContainerLayerSize(ctx context.Context, containerID string) (int64, int64, error) {
	ctr := i.containers.Get(containerID)
	if ctr == nil {
		return 0, 0, nil
	}
	cs := i.client.ContentStore()

	imageManifestBytes, err := content.ReadBlob(ctx, cs, *ctr.ImageManifest)
	if err != nil {
		return 0, 0, err
	}

	var manifest ocispec.Manifest
	if err := json.Unmarshal(imageManifestBytes, &manifest); err != nil {
		return 0, 0, err
	}

	imageConfigBytes, err := content.ReadBlob(ctx, cs, manifest.Config)
	if err != nil {
		return 0, 0, err
	}
	var img ocispec.Image
	if err := json.Unmarshal(imageConfigBytes, &img); err != nil {
		return 0, 0, err
	}

	snapshotter := i.client.SnapshotService(ctr.Driver)
	usage, err := snapshotter.Usage(ctx, containerID)
	if err != nil {
		return 0, 0, err
	}

	sizeCache := make(map[digest.Digest]int64)
	snapshotSizeFn := func(d digest.Digest) (int64, error) {
		if s, ok := sizeCache[d]; ok {
			return s, nil
		}
		u, err := snapshotter.Usage(ctx, d.String())
		if err != nil {
			return 0, err
		}
		sizeCache[d] = u.Size
		return u.Size, nil
	}

	chainIDs := identity.ChainIDs(img.RootFS.DiffIDs)
	snapShotSize, err := computeSnapshotSize(chainIDs, snapshotSizeFn)
	if err != nil {
		return 0, 0, err
	}

	// TODO(thaJeztah): include content-store size for the image (similar to "GET /images/json")
	return usage.Size, usage.Size + snapShotSize, nil
}

As I noted in the meeting today, I'd personally be surprised to find that the `docker image ls` listed size doesn't include the layers still in the content store as well as the extracted content in the snapshotter.  Just because we _can_ remove the image from the content store when it's extracted doesn't mean we would or even should, and IMO the default top-level size field should absolutely reflect all parts of "this image taking up space" including any content and all snapshotters (ideally).

It'd be great if we could dial into that somehow (maybe a separate struct in the API response that includes the individual details of the content store and each snapshotter/snapshot size?), but I don't think that's critical (especially given we can mostly query that from containerd itself outside of Docker if absolutely necessary).