builder/common: extract out decode config

builder/amazon/ebs/builder.go

@@ -10,7 +10,6 @@ import (
 	"fmt"
 	"github.com/mitchellh/goamz/aws"
 	"github.com/mitchellh/goamz/ec2"
-	"github.com/mitchellh/mapstructure"
 	"github.com/mitchellh/multistep"
 	"github.com/mitchellh/packer/builder/common"
 	"github.com/mitchellh/packer/packer"
@@ -53,24 +52,11 @@ type Builder struct {
 }
 
 func (b *Builder) Prepare(raws ...interface{}) error {
-	var md mapstructure.Metadata
-	decoderConfig := &mapstructure.DecoderConfig{
-		Metadata: &md,
-		Result:   &b.config,
-	}
-
-	decoder, err := mapstructure.NewDecoder(decoderConfig)
+	md, err := common.DecodeConfig(&b.config, raws...)
 	if err != nil {
 		return err
 	}
 
-	for _, raw := range raws {
-		err := decoder.Decode(raw)
-		if err != nil {
-			return err
-		}
-	}
-
 	// Accumulate any errors
 	errs := make([]error, 0)
 

builder/common/config.go

+package common
+
+import (
+	"github.com/mitchellh/mapstructure"
+)
+
+// 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
+// it manually and not use this helper function.
+func DecodeConfig(target interface{}, raws ...interface{}) (*mapstructure.Metadata, error) {
+	var md mapstructure.Metadata
+	decoderConfig := &mapstructure.DecoderConfig{
+		Metadata: &md,
+		Result:   target,
+	}
+
+	decoder, err := mapstructure.NewDecoder(decoderConfig)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, raw := range raws {
+		err := decoder.Decode(raw)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return &md, nil
+}

builder/common/config_test.go

+package common
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestDecodeConfig(t *testing.T) {
+	type Local struct {
+		Foo string
+		Bar string
+	}
+
+	raws := []interface{}{
+		map[string]interface{}{
+			"foo": "bar",
+		},
+		map[string]interface{}{
+			"bar": "baz",
+			"baz": "what",
+		},
+	}
+
+	var result Local
+	md, err := DecodeConfig(&result, raws...)
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	if result.Foo != "bar" {
+		t.Fatalf("invalid: %#v", result.Foo)
+	}
+
+	if result.Bar != "baz" {
+		t.Fatalf("invalid: %#v", result.Bar)
+	}
+
+	if md == nil {
+		t.Fatal("metadata should not be nil")
+	}
+
+	if !reflect.DeepEqual(md.Unused, []string{"baz"}) {
+		t.Fatalf("unused: %#v", md.Unused)
+	}
+}