packer: RemoteCmd.StartWithUi thanks to @sit

I ended up working on this on my own outside your PR, sorry, but it is
basically the same as yours.

packer/communicator.go

 package packer
 
 import (
+	"github.com/mitchellh/iochan"
 	"io"
+	"strings"
 	"time"
 )
 
@@ -57,6 +59,61 @@ type Communicator interface {
 	Download(string, io.Writer) error
 }
 
+// StartWithUi runs the remote command and streams the output to any
+// configured Writers for stdout/stderr, while also writing each line
+// as it comes to a Ui.
+func (r *RemoteCmd) StartWithUi(c Communicator, ui Ui) error {
+	stdout_r, stdout_w := io.Pipe()
+	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
+
+	// Start the command
+	if err := c.Start(r); err != nil {
+		return err
+	}
+
+	// Create the channels we'll use for data
+	exitCh := make(chan int, 1)
+	stdoutCh := iochan.DelimReader(stdout_r, '\n')
+	stderrCh := iochan.DelimReader(stderr_r, '\n')
+
+	// Start the goroutine to watch for the exit
+	go func() {
+		defer stdout_w.Close()
+		defer stderr_w.Close()
+		r.Wait()
+		exitCh <- r.ExitStatus
+	}()
+
+	// Loop and get all our output
+OutputLoop:
+	for {
+		select {
+		case output := <-stderrCh:
+			ui.Message(strings.TrimSpace(output))
+		case output := <-stdoutCh:
+			ui.Message(strings.TrimSpace(output))
+		case <-exitCh:
+			break OutputLoop
+		}
+	}
+
+	// Make sure we finish off stdout/stderr because we may have gotten
+	// a message from the exit channel before finishing these first.
+	for output := range stdoutCh {
+		ui.Message(strings.TrimSpace(output))
+	}
+
+	for output := range stderrCh {
+		ui.Message(strings.TrimSpace(output))
+	}
+
+	return nil
+}
+
 // Wait waits for the remote command to complete.
 func (r *RemoteCmd) Wait() {
 	for !r.Exited {

packer/communicator_test.go

 package packer
 
 import (
+	"bytes"
+	"io"
+	"strings"
 	"testing"
 	"time"
 )
 
+type TestCommunicator struct {
+	Stderr io.Reader
+	Stdout io.Reader
+}
+
+func (c *TestCommunicator) Start(rc *RemoteCmd) error {
+	go func() {
+		if rc.Stdout != nil && c.Stdout != nil {
+			io.Copy(rc.Stdout, c.Stdout)
+		}
+
+		if rc.Stderr != nil && c.Stderr != nil {
+			io.Copy(rc.Stderr, c.Stderr)
+		}
+	}()
+
+	return nil
+}
+
+func (c *TestCommunicator) Upload(string, io.Reader) error {
+	return nil
+}
+
+func (c *TestCommunicator) Download(string, io.Writer) error {
+	return nil
+}
+
+func TestRemoteCmd_StartWithUi(t *testing.T) {
+	data := "hello\nworld\nthere"
+
+	rcOutput := new(bytes.Buffer)
+	uiOutput := new(bytes.Buffer)
+	rcOutput.WriteString(data)
+
+	testComm := &TestCommunicator{
+		Stdout: rcOutput,
+	}
+
+	testUi := &ReaderWriterUi{
+		Reader: new(bytes.Buffer),
+		Writer: uiOutput,
+	}
+
+	rc := &RemoteCmd{
+		Command: "test",
+	}
+
+	go func() {
+		time.Sleep(100 * time.Millisecond)
+		rc.Exited = true
+	}()
+
+	err := rc.StartWithUi(testComm, testUi)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if uiOutput.String() != strings.TrimSpace(data)+"\n" {
+		t.Fatalf("bad output: '%s'", uiOutput.String())
+	}
+}
+
 func TestRemoteCmd_Wait(t *testing.T) {
 	var cmd RemoteCmd