@@ -1,6 +1,7 @@

 package build // import "github.com/docker/docker/api/server/router/build"

 import (

+	"bufio"

 	"bytes"

 	"context"

 	"encoding/base64"

@@ -192,8 +193,19 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

 	w.Header().Set("Content-Type", "application/json")

-	output := ioutils.NewWriteFlusher(w)

+	var output writeCloseFlusher = ioutils.NewWriteFlusher(w)

 	defer output.Close()

+

+	body := r.Body

+	if body != nil {

+		// there is a possibility that output is written before request body

+		// has been fully read so we need to protect against it.

+		// this can be removed when

+		// https://github.com/golang/go/issues/15527

+		// https://github.com/golang/go/issues/22209

+		// has been fixed

+		body, output = wrapOutputBufferedUntilRequestRead(body, output)

+	}

 	errf := func(err error) error {

 		if httputils.BoolValue(r, "q") && notVerboseBuffer.Len() > 0 {

 			output.Write(notVerboseBuffer.Bytes())

@@ -235,7 +247,7 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

 	wantAux := versions.GreaterThanOrEqualTo(version, "1.30")

 	imgID, err := br.backend.Build(ctx, backend.BuildConfig{

-		Source:         r.Body,

+		Source:         body,

 		Options:        buildOptions,

 		ProgressWriter: buildProgressWriter(out, wantAux, createProgressReader),

 	})

@@ -294,3 +306,92 @@ func buildProgressWriter(out io.Writer, wantAux bool, createProgressReader func(

 		ProgressReaderFunc: createProgressReader,

 	}

 }

+

+type writeCloseFlusher interface {

+	Flush()

+	Flushed() bool

+	io.WriteCloser

+}

+

+func wrapOutputBufferedUntilRequestRead(rc io.ReadCloser, out writeCloseFlusher) (io.ReadCloser, writeCloseFlusher) {

+	w := &wcf{

+		buf:               bytes.NewBuffer(nil),

+		writeCloseFlusher: out,

+	}

+	r := bufio.NewReader(rc)

+	_, err := r.Peek(1)

+	if err != nil {

+		return rc, out

+	}

+	rc = &rcNotifier{

+		Reader: r,

+		Closer: rc,

+		notify: w.notify,

+	}

+	return rc, w

+}

+

+type rcNotifier struct {

+	io.Reader

+	io.Closer

+	notify func()

+}

+

+func (r *rcNotifier) Read(b []byte) (int, error) {

+	n, err := r.Reader.Read(b)

+	if err != nil {

+		r.notify()

+	}

+	return n, err

+}

+

+type wcf struct {

+	writeCloseFlusher

+	mu      sync.Mutex

+	ready   bool

+	buf     *bytes.Buffer

+	flushed bool

+}

+

+func (w *wcf) Flush() {

+	w.mu.Lock()

+	w.flushed = true

+	if !w.ready {

+		w.mu.Unlock()

+		return

+	}

+	w.mu.Unlock()

+	w.writeCloseFlusher.Flush()

+}

+

+func (w *wcf) Flushed() bool {

+	w.mu.Lock()

+	b := w.flushed

+	w.mu.Unlock()

+	return b

+}

+

+func (w *wcf) Write(b []byte) (int, error) {

+	w.mu.Lock()

+	if !w.ready {

+		n, err := w.buf.Write(b)

+		w.mu.Unlock()

+		return n, err

+	}

+	w.mu.Unlock()

+	return w.writeCloseFlusher.Write(b)

+}

+

+func (w *wcf) notify() {

+	w.mu.Lock()

+	if !w.ready {

+		if w.buf.Len() > 0 {

+			io.Copy(w.writeCloseFlusher, w.buf)

+		}

+		if w.flushed {

+			w.writeCloseFlusher.Flush()

+		}

+		w.ready = true

+	}

+	w.mu.Unlock()

+}

@@ -1,7 +1,6 @@

 package buildkit

 import (

-	"bufio"

 	"io"

 	"net/http"

 	"strings"

@@ -28,12 +27,9 @@ func newReqBodyHandler(rt http.RoundTripper) *reqBodyHandler {

 }

 func (h *reqBodyHandler) newRequest(rc io.ReadCloser) (string, func()) {

-	// handle expect-continue vs chunked output

-	r := bufio.NewReader(rc)

-	r.Peek(1)

 	id := identity.NewID()

 	h.mu.Lock()

-	h.requests[id] = &readCloser{Reader: r, Closer: rc}

+	h.requests[id] = rc

 	h.mu.Unlock()

 	return "http://" + urlPrefix + id, func() {

 		h.mu.Lock()

@@ -58,12 +54,14 @@ func (h *reqBodyHandler) RoundTrip(req *http.Request) (*http.Response, error) {

 			return nil, errors.Errorf("context not found")

 		}

-		return &http.Response{

+		resp := &http.Response{

 			Status:        "200 OK",

 			StatusCode:    200,

 			Body:          rc,

 			ContentLength: -1,

-		}, nil

+		}

+

+		return resp, nil

 	}

 	return h.rt.RoundTrip(req)

 }