packer: Preserve original stdout/stderr on StartWithUi

/cc @sit

packer/communicator.go

@@ -67,8 +67,17 @@ func (r *RemoteCmd) StartWithUi(c Communicator, ui Ui) error {
 	stderr_r, stderr_w := io.Pipe()
 
 	// Set the writers for the output so that we get it streamed to us
-	r.Stdout = stdout_w
-	r.Stderr = stderr_w
+	if r.Stdout == nil {
+		r.Stdout = stdout_w
+	} else {
+		r.Stdout = io.MultiWriter(r.Stdout, stdout_w)
+	}
+
+	if r.Stderr == nil {
+		r.Stderr = stderr_w
+	} else {
+		r.Stderr = io.MultiWriter(r.Stderr, stderr_w)
+	}
 
 	// Start the command
 	if err := c.Start(r); err != nil {

packer/communicator_test.go

@@ -38,6 +38,7 @@ func (c *TestCommunicator) Download(string, io.Writer) error {
 func TestRemoteCmd_StartWithUi(t *testing.T) {
 	data := "hello\nworld\nthere"
 
+	originalOutput := new(bytes.Buffer)
 	rcOutput := new(bytes.Buffer)
 	uiOutput := new(bytes.Buffer)
 	rcOutput.WriteString(data)
@@ -53,6 +54,7 @@ func TestRemoteCmd_StartWithUi(t *testing.T) {
 
 	rc := &RemoteCmd{
 		Command: "test",
+		Stdout:  originalOutput,
 	}
 
 	go func() {
@@ -68,6 +70,10 @@ func TestRemoteCmd_StartWithUi(t *testing.T) {
 	if uiOutput.String() != strings.TrimSpace(data)+"\n" {
 		t.Fatalf("bad output: '%s'", uiOutput.String())
 	}
+
+	if originalOutput.String() != data {
+		t.Fatalf("original is bad: '%s'", originalOutput.String())
+	}
 }
 
 func TestRemoteCmd_Wait(t *testing.T) {