@@ -169,9 +169,7 @@ func getContainersExport(srv *Server, version float64, w http.ResponseWriter, r

 		return fmt.Errorf("Missing parameter")

 	}

 	job := srv.Eng.Job("export", vars["name"])

-	if err := job.Stdout.Add(w); err != nil {

-		return err

-	}

+	job.Stdout.Add(w)

 	if err := job.Run(); err != nil {

 		return err

 	}

@@ -573,9 +571,7 @@ func getImagesGet(srv *Server, version float64, w http.ResponseWriter, r *http.R

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

 	}

 	job := srv.Eng.Job("image_export", vars["name"])

-	if err := job.Stdout.Add(w); err != nil {

-		return err

-	}

+	job.Stdout.Add(w)

 	return job.Run()

 }

@@ -806,7 +802,7 @@ func postContainersAttach(srv *Server, version float64, w http.ResponseWriter, r

 	job.Setenv("stderr", r.Form.Get("stderr"))

 	job.Stdin.Add(inStream)

 	job.Stdout.Add(outStream)

-	job.Stderr.Add(errStream)

+	job.Stderr.Set(errStream)

 	if err := job.Run(); err != nil {

 		fmt.Fprintf(outStream, "Error: %s\n", err)

@@ -836,7 +832,7 @@ func wsContainersAttach(srv *Server, version float64, w http.ResponseWriter, r *

 		job.Setenv("stderr", r.Form.Get("stderr"))

 		job.Stdin.Add(ws)

 		job.Stdout.Add(ws)

-		job.Stderr.Add(ws)

+		job.Stderr.Set(ws)

 		if err := job.Run(); err != nil {

 			utils.Errorf("Error: %s", err)

 		}

@@ -31,12 +31,20 @@ func (o *Output) Used() bool {

 // Add attaches a new destination to the Output. Any data subsequently written

 // to the output will be written to the new destination in addition to all the others.

 // This method is thread-safe.

-// FIXME: Add cannot fail

-func (o *Output) Add(dst io.Writer) error {

+func (o *Output) Add(dst io.Writer) {

 	o.Mutex.Lock()

 	defer o.Mutex.Unlock()

 	o.dests = append(o.dests, dst)

-	return nil

+}

+

+// Set closes and remove existing destination and then attaches a new destination to

+// the Output. Any data subsequently written to the output will be written to the new

+// destination in addition to all the others. This method is thread-safe.

+func (o *Output) Set(dst io.Writer) {

+	o.Close()

+	o.Mutex.Lock()

+	defer o.Mutex.Unlock()

+	o.dests = []io.Writer{dst}

 }

 // AddPipe creates an in-memory pipe with io.Pipe(), adds its writing end as a destination,

@@ -95,9 +95,7 @@ func TestOutputAddEnv(t *testing.T) {

 func TestOutputAddClose(t *testing.T) {

 	o := NewOutput()

 	var s sentinelWriteCloser

-	if err := o.Add(&s); err != nil {

-		t.Fatal(err)

-	}

+	o.Add(&s)

 	if err := o.Close(); err != nil {

 		t.Fatal(err)

 	}