@@ -118,6 +118,7 @@ func (b *Backend) PruneCache(ctx context.Context) (*types.BuildCachePruneReport,

 	return &types.BuildCachePruneReport{SpaceReclaimed: fsCacheSize + uint64(buildCacheSize)}, nil

 }

+// Cancel cancels the build by ID

 func (b *Backend) Cancel(ctx context.Context, id string) error {

 	return b.buildkit.Cancel(ctx, id)

 }

@@ -76,10 +76,7 @@ func (s *systemRouter) getDiskUsage(ctx context.Context, w http.ResponseWriter,

 	eg.Go(func() error {

 		var err error

 		du, err = s.backend.SystemDiskUsage(ctx)

-		if err != nil {

-			return err

-		}

-		return nil

+		return err

 	})

 	var builderSize int64 // legacy

@@ -189,8 +189,10 @@ type ImageBuildOptions struct {

 type BuilderVersion string

 const (

-	BuilderV1       BuilderVersion = "1"

-	BuilderBuildKit                = "2"

+	// BuilderV1 is the first generation builder in docker daemon

+	BuilderV1 BuilderVersion = "1"

+	// BuilderBuildKit is builder based on moby/buildkit project

+	BuilderBuildKit = "2"

 )

 // ImageBuildResponse holds information

@@ -43,6 +43,7 @@ import (

 const preferLocal = true // FIXME: make this optional from the op

+// SourceOpt is options for creating the image source

 type SourceOpt struct {

 	SessionManager  *session.Manager

 	ContentStore    content.Store

@@ -58,6 +59,7 @@ type imageSource struct {

 	g flightcontrol.Group

 }

+// NewSource creates a new image source

 func NewSource(opt SourceOpt) (source.Source, error) {

 	is := &imageSource{

 		SourceOpt: opt,

@@ -69,10 +69,7 @@ func (s *snapshotter) EnsureLayer(ctx context.Context, key string) ([]layer.Diff

 			}

 		}

 		diffID, size, err = s.reg.ChecksumForGraphID(id, parent, "", tarSplitPath)

-		if err != nil {

-			return err

-		}

-		return nil

+		return err

 	})

 	if err := eg.Wait(); err != nil {

@@ -23,6 +23,7 @@ var keyCommitted = []byte("committed")

 var keyChainID = []byte("chainid")

 var keySize = []byte("size")

+// Opt defines options for creating the snapshotter

 type Opt struct {

 	GraphDriver graphdriver.Driver

 	LayerStore  layer.Store

@@ -50,6 +51,7 @@ type snapshotter struct {

 var _ snapshot.SnapshotterBase = &snapshotter{}

+// NewSnapshotter creates a new snapshotter

 func NewSnapshotter(opt Opt) (snapshot.SnapshotterBase, error) {

 	dbPath := filepath.Join(opt.Root, "snapshots.db")

 	db, err := bolt.Open(dbPath, 0600, nil)

@@ -196,7 +198,7 @@ func (s *snapshotter) Stat(ctx context.Context, key string) (snapshots.Info, err

 			inf.Parent = p.ChainID().String()

 		}

 		inf.Kind = snapshots.KindCommitted

-		inf.Name = string(key)

+		inf.Name = key

 		return inf, nil

 	}

@@ -215,7 +217,7 @@ func (s *snapshotter) Stat(ctx context.Context, key string) (snapshots.Info, err

 		if b == nil && l == nil {

 			return errors.Errorf("snapshot %s not found", id) // TODO: typed

 		}

-		inf.Name = string(key)

+		inf.Name = key

 		if b != nil {

 			v := b.Get(keyParent)

 			if v != nil {

@@ -322,7 +324,7 @@ func (s *snapshotter) Remove(ctx context.Context, key string) error {

 }

 func (s *snapshotter) Commit(ctx context.Context, name, key string, opts ...snapshots.Opt) error {

-	if err := s.db.Update(func(tx *bolt.Tx) error {

+	return s.db.Update(func(tx *bolt.Tx) error {

 		b, err := tx.CreateBucketIfNotExists([]byte(name))

 		if err != nil {

 			return err

@@ -331,10 +333,7 @@ func (s *snapshotter) Commit(ctx context.Context, name, key string, opts ...snap

 			return err

 		}

 		return nil

-	}); err != nil {

-		return err

-	}

-	return nil

+	})

 }

 func (s *snapshotter) View(ctx context.Context, key, parent string, opts ...snapshots.Opt) (snapshot.Mountable, error) {

@@ -421,7 +420,7 @@ func (s *snapshotter) Usage(ctx context.Context, key string) (us snapshots.Usage

 	}); err != nil {

 		return usage, err

 	}

-	usage.Size = int64(diffSize)

+	usage.Size = diffSize

 	return usage, nil

 }

@@ -24,12 +24,14 @@ import (

 	grpcmetadata "google.golang.org/grpc/metadata"

 )

+// Opt is option struct required for creating the builder

 type Opt struct {

 	SessionManager *session.Manager

 	Root           string

 	Dist           images.DistributionServices

 }

+// Builder can build using BuildKit backend

 type Builder struct {

 	controller     *control.Controller

 	reqBodyHandler *reqBodyHandler

@@ -38,6 +40,7 @@ type Builder struct {

 	jobs map[string]*buildJob

 }

+// New creates a new builder

 func New(opt Opt) (*Builder, error) {

 	reqHandler := newReqBodyHandler(tracing.DefaultTransport)

@@ -53,6 +56,7 @@ func New(opt Opt) (*Builder, error) {

 	return b, nil

 }

+// Cancel cancels a build using ID

 func (b *Builder) Cancel(ctx context.Context, id string) error {

 	b.mu.Lock()

 	if j, ok := b.jobs[id]; ok && j.cancel != nil {

@@ -62,6 +66,7 @@ func (b *Builder) Cancel(ctx context.Context, id string) error {

 	return nil

 }

+// DiskUsage returns a report about space used by build cache

 func (b *Builder) DiskUsage(ctx context.Context) ([]*types.BuildCache, error) {

 	duResp, err := b.controller.DiskUsage(ctx, &controlapi.DiskUsageRequest{})

 	if err != nil {

@@ -86,6 +91,7 @@ func (b *Builder) DiskUsage(ctx context.Context) ([]*types.BuildCache, error) {

 	return items, nil

 }

+// Prune clears all reclaimable build cache

 func (b *Builder) Prune(ctx context.Context) (int64, error) {

 	ch := make(chan *controlapi.UsageRecord)

@@ -114,6 +120,7 @@ func (b *Builder) Prune(ctx context.Context) (int64, error) {

 	return size, nil

 }

+// Build executes a build request

 func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.Result, error) {

 	var rc = opt.Source

@@ -181,10 +188,8 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.

 		frontendAttrs["context"] = url

 	}

-	var cacheFrom []string

-	for _, v := range opt.Options.CacheFrom {

-		cacheFrom = append(cacheFrom, v)

-	}

+	cacheFrom := append([]string{}, opt.Options.CacheFrom...)

+

 	frontendAttrs["cache-from"] = strings.Join(cacheFrom, ",")

 	for k, v := range opt.Options.BuildArgs {

@@ -117,7 +117,7 @@ func newController(rt http.RoundTripper, opt Opt) (*control.Controller, error) {

 	frontends["dockerfile.v0"] = dockerfile.NewDockerfileFrontend()

 	frontends["gateway.v0"] = gateway.NewGatewayFrontend()

-	wopt := mobyworker.WorkerOpt{

+	wopt := mobyworker.Opt{

 		ID:                "moby",

 		SessionManager:    opt.SessionManager,

 		MetadataStore:     md,

@@ -19,10 +19,12 @@ const (

 	exporterImageConfig = "containerimage.config"

 )

+// Differ can make a moby layer from a snapshot

 type Differ interface {

 	EnsureLayer(ctx context.Context, key string) ([]layer.DiffID, error)

 }

+// Opt defines a struct for creating new exporter

 type Opt struct {

 	ImageStore     image.Store

 	ReferenceStore reference.Store

@@ -33,6 +35,7 @@ type imageExporter struct {

 	opt Opt

 }

+// New creates a new moby imagestore exporter

 func New(opt Opt) (exporter.Exporter, error) {

 	im := &imageExporter{opt: opt}

 	return im, nil

@@ -49,9 +49,8 @@ func patchImageConfig(dt []byte, dps []digest.Digest, history []ocispec.History)

 	var rootFS ocispec.RootFS

 	rootFS.Type = "layers"

-	for _, dp := range dps {

-		rootFS.DiffIDs = append(rootFS.DiffIDs, dp)

-	}

+	rootFS.DiffIDs = append(rootFS.DiffIDs, dps...)

+

 	dt, err := json.Marshal(rootFS)

 	if err != nil {

 		return nil, errors.Wrap(err, "failed to marshal rootfs")

@@ -87,7 +86,7 @@ func normalizeLayersAndHistory(diffs []digest.Digest, history []ocispec.History,

 	var historyLayers int

 	for _, h := range history {

 		if !h.EmptyLayer {

-			historyLayers += 1

+			historyLayers++

 		}

 	}

 	if historyLayers > len(diffs) {

@@ -65,8 +65,3 @@ func (h *reqBodyHandler) RoundTrip(req *http.Request) (*http.Response, error) {

 	}

 	return h.rt.RoundTrip(req)

 }

-

-type readCloser struct {

-	io.Reader

-	io.Closer

-}

@@ -39,9 +39,8 @@ import (

 	"github.com/pkg/errors"

 )

-// WorkerOpt is specific to a worker.

-// See also CommonOpt.

-type WorkerOpt struct {

+// Opt defines a structure for creating a worker.

+type Opt struct {

 	ID                string

 	Labels            map[string]string

 	SessionManager    *session.Manager

@@ -60,12 +59,12 @@ type WorkerOpt struct {

 // Worker is a local worker instance with dedicated snapshotter, cache, and so on.

 // TODO: s/Worker/OpWorker/g ?

 type Worker struct {

-	WorkerOpt

+	Opt

 	SourceManager *source.Manager

 }

 // NewWorker instantiates a local worker

-func NewWorker(opt WorkerOpt) (*Worker, error) {

+func NewWorker(opt Opt) (*Worker, error) {

 	sm, err := source.NewManager()

 	if err != nil {

 		return nil, err

@@ -106,23 +105,27 @@ func NewWorker(opt WorkerOpt) (*Worker, error) {

 	sm.Register(ss)

 	return &Worker{

-		WorkerOpt:     opt,

+		Opt:           opt,

 		SourceManager: sm,

 	}, nil

 }

+// ID returns worker ID

 func (w *Worker) ID() string {

-	return w.WorkerOpt.ID

+	return w.Opt.ID

 }

+// Labels returns map of all worker labels

 func (w *Worker) Labels() map[string]string {

-	return w.WorkerOpt.Labels

+	return w.Opt.Labels

 }

+// LoadRef loads a reference by ID

 func (w *Worker) LoadRef(id string) (cache.ImmutableRef, error) {

 	return w.CacheManager.Get(context.TODO(), id)

 }

+// ResolveOp converts a LLB vertex into a LLB operation

 func (w *Worker) ResolveOp(v solver.Vertex, s frontend.FrontendLLBBridge) (solver.Op, error) {

 	switch op := v.Sys().(type) {

 	case *pb.Op_Source:

@@ -136,6 +139,7 @@ func (w *Worker) ResolveOp(v solver.Vertex, s frontend.FrontendLLBBridge) (solve

 	}

 }

+// ResolveImageConfig returns image config for an image

 func (w *Worker) ResolveImageConfig(ctx context.Context, ref string) (digest.Digest, []byte, error) {

 	// ImageSource is typically source/containerimage

 	resolveImageConfig, ok := w.ImageSource.(resolveImageConfig)

@@ -145,10 +149,7 @@ func (w *Worker) ResolveImageConfig(ctx context.Context, ref string) (digest.Dig

 	return resolveImageConfig.ResolveImageConfig(ctx, ref)

 }

-type resolveImageConfig interface {

-	ResolveImageConfig(ctx context.Context, ref string) (digest.Digest, []byte, error)

-}

-

+// Exec executes a process directly on a worker

 func (w *Worker) Exec(ctx context.Context, meta executor.Meta, rootFS cache.ImmutableRef, stdin io.ReadCloser, stdout, stderr io.WriteCloser) error {

 	active, err := w.CacheManager.New(ctx, rootFS)

 	if err != nil {

@@ -158,14 +159,17 @@ func (w *Worker) Exec(ctx context.Context, meta executor.Meta, rootFS cache.Immu

 	return w.Executor.Exec(ctx, meta, active, nil, stdin, stdout, stderr)

 }

+// DiskUsage returns disk usage report

 func (w *Worker) DiskUsage(ctx context.Context, opt client.DiskUsageInfo) ([]*client.UsageInfo, error) {

 	return w.CacheManager.DiskUsage(ctx, opt)

 }

+// Prune deletes reclaimable build cache

 func (w *Worker) Prune(ctx context.Context, ch chan client.UsageInfo) error {

 	return w.CacheManager.Prune(ctx, ch)

 }

+// Exporter returns exporter by name

 func (w *Worker) Exporter(name string) (exporter.Exporter, error) {

 	exp, ok := w.Exporters[name]

 	if !ok {

@@ -174,10 +178,12 @@ func (w *Worker) Exporter(name string) (exporter.Exporter, error) {

 	return exp, nil

 }

+// GetRemote returns a remote snapshot reference for a local one

 func (w *Worker) GetRemote(ctx context.Context, ref cache.ImmutableRef, createIfNeeded bool) (*solver.Remote, error) {

 	return nil, errors.Errorf("getremote not implemented")

 }

+// FromRemote converts a remote snapshot reference to a local one

 func (w *Worker) FromRemote(ctx context.Context, remote *solver.Remote) (cache.ImmutableRef, error) {

 	rootfs, err := getLayers(ctx, remote.Descriptors)

 	if err != nil {

@@ -219,7 +225,7 @@ func (w *Worker) FromRemote(ctx context.Context, remote *solver.Remote) (cache.I

 type discardProgress struct{}

-func (_ *discardProgress) WriteProgress(_ pkgprogress.Progress) error {

+func (*discardProgress) WriteProgress(_ pkgprogress.Progress) error {

 	return nil

 }

@@ -309,3 +315,7 @@ func oneOffProgress(ctx context.Context, id string) func(err error) error {

 		return err

 	}

 }

+

+type resolveImageConfig interface {

+	ResolveImageConfig(ctx context.Context, ref string) (digest.Digest, []byte, error)

+}

@@ -922,6 +922,7 @@ func NewDaemon(config *config.Config, registryService registry.Service, containe

 	return d, nil

 }

+// DistributionServices returns services controlling daemon storage

 func (daemon *Daemon) DistributionServices() images.DistributionServices {

 	return daemon.imageService.DistributionServices()

 }

@@ -76,6 +76,7 @@ type ImageService struct {

 	uploadManager             *xfer.LayerUploadManager

 }

+// DistributionServices provides daemon image storage services

 type DistributionServices struct {

 	DownloadManager   distribution.RootFSDownloadManager

 	V2MetadataService metadata.V2MetadataService

@@ -84,6 +85,7 @@ type DistributionServices struct {

 	ReferenceStore    dockerreference.Store

 }

+// DistributionServices return services controlling daemon image storage

 func (i *ImageService) DistributionServices() DistributionServices {

 	return DistributionServices{

 		DownloadManager:   i.downloadManager,