packer: errors go to stderr [GH-868]

packer/environment.go

@@ -83,6 +83,7 @@ func DefaultEnvironmentConfig() *EnvironmentConfig {
 	config.Ui = &BasicUi{
 		Reader: os.Stdin,
 		Writer: os.Stdout,
+		ErrorWriter: os.Stderr,
 	}
 
 	return config

packer/environment_test.go

@@ -32,6 +32,7 @@ func testEnvironment() Environment {
 	config.Ui = &BasicUi{
 		Reader: new(bytes.Buffer),
 		Writer: new(bytes.Buffer),
+		ErrorWriter: new(bytes.Buffer),
 	}
 
 	env, err := NewEnvironment(config)

packer/ui.go

@@ -61,6 +61,7 @@ type TargettedUi struct {
 type BasicUi struct {
 	Reader      io.Reader
 	Writer      io.Writer
+	ErrorWriter io.Writer
 	l           sync.Mutex
 	interrupted bool
 }
@@ -235,8 +236,13 @@ func (rw *BasicUi) Error(message string) {
 	rw.l.Lock()
 	defer rw.l.Unlock()
 
+	writer := rw.ErrorWriter
+	if writer == nil {
+		writer = rw.Writer
+	}
+
 	log.Printf("ui error: %s", message)
-	_, err := fmt.Fprint(rw.Writer, message+"\n")
+	_, err := fmt.Fprint(writer, message+"\n")
 	if err != nil {
 		log.Printf("[ERR] Failed to write to UI: %s", err)
 	}

packer/ui_test.go

@@ -7,10 +7,27 @@ import (
 	"testing"
 )
 
+// This reads the output from the bytes.Buffer in our test object
+// and then resets the buffer.
+func readWriter(ui *BasicUi) (result string) {
+	buffer := ui.Writer.(*bytes.Buffer)
+	result = buffer.String()
+	buffer.Reset()
+	return
+}
+
+func readErrorWriter(ui *BasicUi) (result string) {
+	buffer := ui.ErrorWriter.(*bytes.Buffer)
+	result = buffer.String()
+	buffer.Reset()
+	return
+}
+
 func testUi() *BasicUi {
 	return &BasicUi{
 		Reader: new(bytes.Buffer),
 		Writer: new(bytes.Buffer),
+		ErrorWriter: new(bytes.Buffer),
 	}
 }
 
@@ -32,6 +49,11 @@ func TestColoredUi(t *testing.T) {
 
 	ui.Error("foo")
 	result = readWriter(bufferUi)
+	if result != "" {
+		t.Fatalf("invalid output: %s", result)
+	}
+
+	result = readErrorWriter(bufferUi)
 	if result != "\033[1;31mfoo\033[0m\n" {
 		t.Fatalf("invalid output: %s", result)
 	}
@@ -59,7 +81,7 @@ func TestColoredUi_noColorEnv(t *testing.T) {
 	}
 
 	ui.Error("foo")
-	result = readWriter(bufferUi)
+	result = readErrorWriter(bufferUi)
 	if result != "foo\n" {
 		t.Fatalf("invalid output: %s", result)
 	}
@@ -88,7 +110,7 @@ func TestTargettedUi(t *testing.T) {
 	}
 
 	targettedUi.Error("bar")
-	actual = readWriter(bufferUi)
+	actual = readErrorWriter(bufferUi)
 	expected = "==> foo: bar\n"
 	if actual != expected {
 		t.Fatalf("bad: %#v", actual)
@@ -131,12 +153,13 @@ func TestBasicUi_Error(t *testing.T) {
 
 	var actual, expected string
 	bufferUi.Error("foo")
-	actual = readWriter(bufferUi)
+	actual = readErrorWriter(bufferUi)
 	expected = "foo\n"
 	if actual != expected {
 		t.Fatalf("bad: %#v", actual)
 	}
 
+	bufferUi.ErrorWriter = nil
 	bufferUi.Error("5")
 	actual = readWriter(bufferUi)
 	expected = "5\n"
@@ -214,12 +237,3 @@ func TestMachineReadableUi(t *testing.T) {
 		t.Fatalf("bad: %#v", data)
 	}
 }
-
-// This reads the output from the bytes.Buffer in our test object
-// and then resets the buffer.
-func readWriter(ui *BasicUi) (result string) {
-	buffer := ui.Writer.(*bytes.Buffer)
-	result = buffer.String()
-	buffer.Reset()
-	return
-}