@@ -193,10 +193,8 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

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

-	var output writeCloseFlusher = ioutils.NewWriteFlusher(w)

-	defer output.Close()

-

 	body := r.Body

+	var ww io.Writer = w

 	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.

@@ -204,18 +202,25 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

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

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

 		// has been fixed

-		body, output = wrapOutputBufferedUntilRequestRead(body, output)

+		body, ww = wrapOutputBufferedUntilRequestRead(body, ww)

 	}

+

+	output := ioutils.NewWriteFlusher(ww)

+	defer output.Close()

+

 	errf := func(err error) error {

+

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

 			output.Write(notVerboseBuffer.Bytes())

 		}

+

+		logrus.Debugf("isflushed %v", output.Flushed())

 		// Do not write the error in the http output if it's still empty.

 		// This prevents from writing a 200(OK) when there is an internal error.

 		if !output.Flushed() {

 			return err

 		}

-		_, err = w.Write(streamformatter.FormatError(err))

+		_, err = output.Write(streamformatter.FormatError(err))

 		if err != nil {

 			logrus.Warnf("could not write error response: %v", err)

 		}

@@ -311,16 +316,20 @@ func buildProgressWriter(out io.Writer, wantAux bool, createProgressReader func(

 	}

 }

-type writeCloseFlusher interface {

+type flusher interface {

 	Flush()

-	Flushed() bool

-	io.WriteCloser

 }

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

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

+	var fl flusher = &ioutils.NopFlusher{}

+	if f, ok := out.(flusher); ok {

+		fl = f

+	}

+

 	w := &wcf{

-		buf:               bytes.NewBuffer(nil),

-		writeCloseFlusher: out,

+		buf:     bytes.NewBuffer(nil),

+		Writer:  out,

+		flusher: fl,

 	}

 	r := bufio.NewReader(rc)

 	_, err := r.Peek(1)

@@ -355,7 +364,8 @@ func (r *rcNotifier) Close() error {

 }

 type wcf struct {

-	writeCloseFlusher

+	io.Writer

+	flusher

 	mu      sync.Mutex

 	ready   bool

 	buf     *bytes.Buffer

@@ -370,7 +380,7 @@ func (w *wcf) Flush() {

 		return

 	}

 	w.mu.Unlock()

-	w.writeCloseFlusher.Flush()

+	w.flusher.Flush()

 }

 func (w *wcf) Flushed() bool {

@@ -388,17 +398,17 @@ func (w *wcf) Write(b []byte) (int, error) {

 		return n, err

 	}

 	w.mu.Unlock()

-	return w.writeCloseFlusher.Write(b)

+	return w.Writer.Write(b)

 }

 func (w *wcf) notify() {

 	w.mu.Lock()

 	if !w.ready {

 		if w.buf.Len() > 0 {

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

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

 		}

 		if w.flushed {

-			w.writeCloseFlusher.Flush()

+			w.flusher.Flush()

 		}

 		w.ready = true

 	}