builder/common: CheckUnusedConfig

builder/common/config.go

 package common
 
 import (
+	"fmt"
 	"github.com/mitchellh/mapstructure"
+	"github.com/mitchellh/packer/packer"
+	"sort"
+	"strings"
 )
 
+// CheckUnusedConfig is a helper that makes sure that the there are no
+// unused configuration keys, properly ignoring keys that don't matter.
+func CheckUnusedConfig(md *mapstructure.Metadata) error {
+	errs := make([]error, 0)
+
+	if md.Unused != nil && len(md.Unused) > 0 {
+		sort.Strings(md.Unused)
+		for _, unused := range md.Unused {
+			if unused != "type" && !strings.HasPrefix(unused, "packer_") {
+				errs = append(
+					errs, fmt.Errorf("Unknown configuration key: %s", unused))
+			}
+		}
+	}
+
+	if len(errs) > 0 {
+		return &packer.MultiError{errs}
+	}
+
+	return nil
+}
+
 // DecodeConfig is a helper that handles decoding raw configuration using
 // mapstructure. It returns the metadata and any errors that may happen.
 // If you need extra configuration for mapstructure, you should configure

builder/common/config_test.go

 package common
 
 import (
+	"github.com/mitchellh/mapstructure"
 	"reflect"
 	"testing"
 )
 
+func TestCheckUnusedConfig(t *testing.T) {
+	md := &mapstructure.Metadata{
+		Unused: make([]string, 0),
+	}
+
+	err := CheckUnusedConfig(md)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	md.Unused = []string{"foo", "bar"}
+	err = CheckUnusedConfig(md)
+	if err == nil {
+		t.Fatal("should have error")
+	}
+}
+
 func TestDecodeConfig(t *testing.T) {
 	type Local struct {
 		Foo string