Fail on unknown values of -on-error

command/build.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"sync"
 
+	"github.com/mitchellh/packer/helper/enumflag"
 	"github.com/mitchellh/packer/packer"
 	"github.com/mitchellh/packer/template"
 )
@@ -26,7 +27,8 @@ func (c BuildCommand) Run(args []string) int {
 	flags.BoolVar(&cfgColor, "color", true, "")
 	flags.BoolVar(&cfgDebug, "debug", false, "")
 	flags.BoolVar(&cfgForce, "force", false, "")
-	flags.StringVar(&cfgOnError, "on-error", "cleanup", "")
+	flagOnError := enumflag.New(&cfgOnError, "cleanup", "abort", "ask")
+	flags.Var(flagOnError, "on-error", "")
 	flags.BoolVar(&cfgParallel, "parallel", true, "")
 	if err := flags.Parse(args); err != nil {
 		return 1

common/multistep_runner.go

@@ -14,7 +14,7 @@ import (
 
 func newRunner(steps []multistep.Step, config PackerConfig, ui packer.Ui) (multistep.Runner, multistep.DebugPauseFn) {
 	switch config.PackerOnError {
-	case "cleanup":
+	case "", "cleanup":
 	case "abort":
 		for i, step := range steps {
 			steps[i] = abortStep{step, ui}
@@ -23,8 +23,6 @@ func newRunner(steps []multistep.Step, config PackerConfig, ui packer.Ui) (multi
 		for i, step := range steps {
 			steps[i] = askStep{step, ui}
 		}
-	default:
-		ui.Error(fmt.Sprintf("Ignoring -on-error=%q argument: unknown on-error value", config.PackerOnError))
 	}
 
 	if config.PackerDebug {

helper/enumflag/flag.go

@@ -0,0 +1,28 @@
+package enumflag
+
+import "fmt"
+
+type enumFlag struct {
+	target  *string
+	options []string
+}
+
+// New returns a flag.Value implementation for parsing flags with a one-of-a-set value
+func New(target *string, options ...string) *enumFlag {
+	return &enumFlag{target: target, options: options}
+}
+
+func (f *enumFlag) String() string {
+	return *f.target
+}
+
+func (f *enumFlag) Set(value string) error {
+	for _, v := range f.options {
+		if v == value {
+			*f.target = value
+			return nil
+		}
+	}
+
+	return fmt.Errorf("expected one of %q", f.options)
+}