packer: Ui has Error method

command/build/command.go

@@ -9,7 +9,7 @@ type Command byte
 
 func (Command) Run(env packer.Environment, args []string) int {
 	if len(args) != 1 {
-		// TODO: Error message
+		env.Ui().Error("A single template argument is required.\n")
 		return 1
 	}
 

packer/rpc/ui.go

@@ -22,11 +22,23 @@ type UiSayArgs struct {
 	Vars []interface{}
 }
 
+func (u *Ui) Error(format string, a ...interface{}) {
+	args := &UiSayArgs{format, a}
+	u.client.Call("Ui.Error", args, new(interface{}))
+}
+
 func (u *Ui) Say(format string, a ...interface{}) {
 	args := &UiSayArgs{format, a}
 	u.client.Call("Ui.Say", args, new(interface{}))
 }
 
+func (u *UiServer) Error(args *UiSayArgs, reply *interface{}) error {
+	u.ui.Error(args.Format, args.Vars...)
+
+	*reply = nil
+	return nil
+}
+
 func (u *UiServer) Say(args *UiSayArgs, reply *interface{}) error {
 	u.ui.Say(args.Format, args.Vars...)
 

packer/rpc/ui_test.go

@@ -7,11 +7,20 @@ import (
 )
 
 type testUi struct {
+	errorCalled bool
+	errorFormat string
+	errorVars []interface{}
 	sayCalled bool
 	sayFormat string
 	sayVars []interface{}
 }
 
+func (u *testUi) Error(format string, a ...interface{}) {
+	u.errorCalled = true
+	u.errorFormat = format
+	u.errorVars = a
+}
+
 func (u *testUi) Say(format string, a ...interface{}) {
 	u.sayCalled = true
 	u.sayFormat = format
@@ -36,7 +45,10 @@ func TestUiRPC(t *testing.T) {
 	}
 
 	uiClient := &Ui{client}
-	uiClient.Say("format", "arg0", 42)
 
+	uiClient.Error("format", "arg0", 42)
+	assert.Equal(ui.errorFormat, "format", "format should be correct")
+
+	uiClient.Say("format", "arg0", 42)
 	assert.Equal(ui.sayFormat, "format", "format should be correct")
 }

packer/ui.go

@@ -8,6 +8,7 @@ import "io"
 // is formatted and various levels of output.
 type Ui interface {
 	Say(format string, a ...interface{})
+	Error(format string, a...interface{})
 }
 
 // The ReaderWriterUi is a UI that writes and reads from standard Go
@@ -20,3 +21,7 @@ type ReaderWriterUi struct {
 func (rw *ReaderWriterUi) Say(format string, a ...interface{}) {
 	fmt.Fprintf(rw.Writer, format, a...)
 }
+
+func (rw *ReaderWriterUi) Error(format string, a ...interface{}) {
+	fmt.Fprintf(rw.Writer, format, a...)
+}

packer/ui_test.go

@@ -13,6 +13,18 @@ func testUi() *ReaderWriterUi {
 	}
 }
 
+func TestReaderWriterUi_Error(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	bufferUi := testUi()
+
+	bufferUi.Error("foo")
+	assert.Equal(readWriter(bufferUi), "foo", "basic output")
+
+	bufferUi.Error("%d", 5)
+	assert.Equal(readWriter(bufferUi), "5", "formatting")
+}
+
 func TestReaderWriterUi_Say(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)