@@ -63,6 +63,10 @@ func (b *Backend) Build(ctx context.Context, config backend.BuildConfig) (string

 		}

 	}

+	if build == nil {

+		return "", nil

+	}

+

 	var imageID = build.ImageID

 	if options.Squash {

 		if imageID, err = squashBuild(build, b.imageComponent); err != nil {

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

 	"io"

 	"strings"

 	"sync"

+	"time"

 	"github.com/containerd/containerd/content"

 	"github.com/docker/docker/api/types"

@@ -34,7 +35,7 @@ type Builder struct {

 	reqBodyHandler *reqBodyHandler

 	mu   sync.Mutex

-	jobs map[string]func()

+	jobs map[string]*buildJob

 }

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

@@ -47,15 +48,15 @@ func New(opt Opt) (*Builder, error) {

 	b := &Builder{

 		controller:     c,

 		reqBodyHandler: reqHandler,

-		jobs:           map[string]func(){},

+		jobs:           map[string]*buildJob{},

 	}

 	return b, nil

 }

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

 	b.mu.Lock()

-	if cancel, ok := b.jobs[id]; ok {

-		cancel()

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

+		j.cancel()

 	}

 	b.mu.Unlock()

 	return nil

@@ -114,10 +115,43 @@ func (b *Builder) Prune(ctx context.Context) (int64, error) {

 }

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

+	var rc = opt.Source

+

 	if buildID := opt.Options.BuildID; buildID != "" {

 		b.mu.Lock()

-		ctx, b.jobs[buildID] = context.WithCancel(ctx)

+

+		upload := false

+		if strings.HasPrefix(buildID, "upload-request:") {

+			upload = true

+			buildID = strings.TrimPrefix(buildID, "upload-request:")

+		}

+

+		if _, ok := b.jobs[buildID]; !ok {

+			b.jobs[buildID] = newBuildJob()

+		}

+		j := b.jobs[buildID]

+		ctx, cancel := context.WithCancel(ctx)

+		j.cancel = cancel

 		b.mu.Unlock()

+

+		if upload {

+			ctx2, cancel := context.WithTimeout(ctx, 5*time.Second)

+			defer cancel()

+			err := j.SetUpload(ctx2, rc)

+			return nil, err

+		}

+

+		if remoteContext := opt.Options.RemoteContext; remoteContext == "upload-request" {

+			ctx2, cancel := context.WithTimeout(ctx, 5*time.Second)

+			defer cancel()

+			var err error

+			rc, err = j.WaitUpload(ctx2)

+			if err != nil {

+				return nil, err

+			}

+			opt.Options.RemoteContext = ""

+		}

+

 		defer func() {

 			delete(b.jobs, buildID)

 		}()

@@ -142,7 +176,7 @@ func (b *Builder) Build(ctx context.Context, opt backend.BuildConfig) (*builder.

 			frontendAttrs["context"] = opt.Options.RemoteContext

 		}

 	} else {

-		url, cancel := b.reqBodyHandler.newRequest(opt.Source)

+		url, cancel := b.reqBodyHandler.newRequest(rc)

 		defer cancel()

 		frontendAttrs["context"] = url

 	}

@@ -295,3 +329,78 @@ type contentStoreNoLabels struct {

 func (c *contentStoreNoLabels) Update(ctx context.Context, info content.Info, fieldpaths ...string) (content.Info, error) {

 	return content.Info{}, nil

 }

+

+type wrapRC struct {

+	io.ReadCloser

+	once   sync.Once

+	err    error

+	waitCh chan struct{}

+}

+

+func (w *wrapRC) Read(b []byte) (int, error) {

+	n, err := w.ReadCloser.Read(b)

+	if err != nil {

+		e := err

+		if e == io.EOF {

+			e = nil

+		}

+		w.close(e)

+	}

+	return n, err

+}

+

+func (w *wrapRC) Close() error {

+	err := w.ReadCloser.Close()

+	w.close(err)

+	return err

+}

+

+func (w *wrapRC) close(err error) {

+	w.once.Do(func() {

+		w.err = err

+		close(w.waitCh)

+	})

+}

+

+func (w *wrapRC) wait() error {

+	<-w.waitCh

+	return w.err

+}

+

+type buildJob struct {

+	cancel func()

+	waitCh chan func(io.ReadCloser) error

+}

+

+func newBuildJob() *buildJob {

+	return &buildJob{waitCh: make(chan func(io.ReadCloser) error)}

+}

+

+func (j *buildJob) WaitUpload(ctx context.Context) (io.ReadCloser, error) {

+	done := make(chan struct{})

+

+	var upload io.ReadCloser

+	fn := func(rc io.ReadCloser) error {

+		w := &wrapRC{ReadCloser: rc, waitCh: make(chan struct{})}

+		upload = w

+		close(done)

+		return w.wait()

+	}

+

+	select {

+	case <-ctx.Done():

+		return nil, ctx.Err()

+	case j.waitCh <- fn:

+		<-done

+		return upload, nil

+	}

+}

+

+func (j *buildJob) SetUpload(ctx context.Context, rc io.ReadCloser) error {

+	select {

+	case <-ctx.Done():

+		return ctx.Err()

+	case fn := <-j.waitCh:

+		return fn(rc)

+	}

+}