implement docker system df
**- What I did**
upstream https://github.com/rumpl/moby/pull/29
daemon/containerd/service.go
package containerd

import (
	"context"
	"errors"

	"github.com/containerd/containerd"
	"github.com/containerd/containerd/plugin"
	"github.com/docker/docker/api/types"
	"github.com/docker/docker/container"
	"github.com/docker/docker/daemon/images"
	"github.com/docker/docker/errdefs"
	"github.com/docker/docker/image"
	"github.com/docker/docker/layer"
)

// ImageService implements daemon.ImageService
type ImageService struct {
	client      *containerd.Client
	snapshotter string
}

// NewService creates a new ImageService.
func NewService(c *containerd.Client, snapshotter string) *ImageService {
	return &ImageService{
		client:      c,
		snapshotter: snapshotter,
	}
}

// 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)
}

// Children returns the children image.IDs for a parent image.
// called from list.go to filter containers
// TODO: refactor to expose an ancestry for image.ID?
func (i *ImageService) Children(id image.ID) []image.ID {
	panic("not implemented")
}

// 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")))
}

// GetLayerByID returns a layer by ID
// called from daemon.go Daemon.restore(), and Daemon.containerExport().
func (i *ImageService) GetLayerByID(cid string) (layer.RWLayer, error) {
	return nil, 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) {
	return 0, errdefs.NotImplemented(errors.New("not implemented"))
}

// ImageDiskUsage returns information about image data disk usage.
func (i *ImageService) ImageDiskUsage(ctx context.Context) ([]*types.ImageSummary, error) {
	return nil, errdefs.NotImplemented(errors.New("not implemented"))
}

// 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(containerID string) (int64, int64) {
	panic("not implemented")
}

package containerd

import (
	"context"

	"github.com/containerd/containerd"
	"github.com/containerd/containerd/plugin"
	"github.com/containerd/containerd/snapshots"
	"github.com/docker/docker/api/types"
	"github.com/docker/docker/api/types/filters"
	"github.com/docker/docker/container"
	"github.com/docker/docker/daemon/images"
	"github.com/docker/docker/errdefs"
	"github.com/docker/docker/image"
	"github.com/docker/docker/layer"
	"github.com/pkg/errors"
	"golang.org/x/sync/singleflight"
)

// ImageService implements daemon.ImageService
type ImageService struct {
	client      *containerd.Client
	snapshotter string
	usage       singleflight.Group
}

// NewService creates a new ImageService.
func NewService(c *containerd.Client, snapshotter string) *ImageService {
	return &ImageService{
		client:      c,
		snapshotter: snapshotter,
	}
}

// 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)
}

// Children returns the children image.IDs for a parent image.
// called from list.go to filter containers
// TODO: refactor to expose an ancestry for image.ID?
func (i *ImageService) Children(id image.ID) []image.ID {
	panic("not implemented")
}

// 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")))
}

// GetLayerByID returns a layer by ID
// called from daemon.go Daemon.restore(), and Daemon.containerExport().
func (i *ImageService) GetLayerByID(cid string) (layer.RWLayer, error) {
	return nil, 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) {
	ch := i.usage.DoChan("LayerDiskUsage", func() (interface{}, error) {
		var allLayersSize int64
		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
	})
	select {
	case <-ctx.Done():
		return 0, ctx.Err()
	case res := <-ch:
		if res.Err != nil {
			return 0, res.Err
		}
		return res.Val.(int64), nil
	}
}

// ImageDiskUsage returns information about image data disk usage.
func (i *ImageService) ImageDiskUsage(ctx context.Context) ([]*types.ImageSummary, error) {
	ch := i.usage.DoChan("ImageDiskUsage", func() (interface{}, error) {
		// Get all top images with extra attributes
		imgs, err := i.Images(ctx, types.ImageListOptions{
			Filters:        filters.NewArgs(),
			SharedSize:     true,
			ContainerCount: true,
		})
		if err != nil {
			return nil, errors.Wrap(err, "failed to retrieve image list")
		}
		return imgs, nil
	})
	select {
	case <-ctx.Done():
		return nil, ctx.Err()
	case res := <-ch:
		if res.Err != nil {
			return nil, res.Err
		}
		return res.Val.([]*types.ImageSummary), 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(containerID string) (int64, int64) {
	panic("not implemented")
}

Looks like the `images` variable colliding is not an _actual_ issue in the graphdriver implementation, but perhaps it's good to make the same changes in that code as well (as a separate commit) to keep the code the same.