Make Build an interface

packer/build.go

 package packer
 
+// A Build represents a single job within Packer that is responsible for
+// building some machine image artifact. Builds are meant to be parallelized.
+type Build interface {
+	Prepare()
+	Run(ui Ui)
+}
+
 // A build struct represents a single build job, the result of which should
 // be a single machine image artifact. This artifact may be comprised of
 // multiple files, of course, but it should be for only a single provider
 // (such as VirtualBox, EC2, etc.).
-type Build struct {
+type coreBuild struct {
 	name    string
 	builder Builder
 	rawConfig interface{}
@@ -23,7 +30,7 @@ type Build struct {
 // Run is where the actual build should take place. It takes a Build and a Ui.
 type Builder interface {
 	Prepare(config interface{})
-	Run(build *Build, ui Ui)
+	Run(build Build, ui Ui)
 }
 
 // This factory is responsible for returning Builders for the given name.
@@ -44,13 +51,13 @@ func (NilBuilderFactory) CreateBuilder(name string) Builder {
 
 // Prepare prepares the build by doing some initialization for the builder
 // and any hooks. This _must_ be called prior to Run.
-func (b *Build) Prepare() {
+func (b *coreBuild) Prepare() {
 	b.prepareCalled = true
 	b.builder.Prepare(b.rawConfig)
 }
 
 // Runs the actual build. Prepare must be called prior to running this.
-func (b *Build) Run(ui Ui) {
+func (b *coreBuild) Run(ui Ui) {
 	if !b.prepareCalled {
 		panic("Prepare must be called first")
 	}

packer/build_test.go

@@ -17,7 +17,7 @@ type TestBuilder struct {
 	prepareCalled bool
 	prepareConfig interface{}
 	runCalled bool
-	runBuild *Build
+	runBuild Build
 	runUi Ui
 }
 
@@ -26,14 +26,14 @@ func (tb *TestBuilder) Prepare(config interface{}) {
 	tb.prepareConfig = config
 }
 
-func (tb *TestBuilder) Run(b *Build, ui Ui) {
+func (tb *TestBuilder) Run(b Build, ui Ui) {
 	tb.runCalled = true
 	tb.runBuild = b
 	tb.runUi = ui
 }
 
-func testBuild() *Build {
-	return &Build{
+func testBuild() Build {
+	return &coreBuild{
 		name: "test",
 		builder: &TestBuilder{},
 		rawConfig: 42,
@@ -52,7 +52,7 @@ func TestBuild_Prepare(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)
 
 	build := testBuild()
-	builder := build.builder.(*TestBuilder)
+	builder := build.(*coreBuild).builder.(*TestBuilder)
 
 	build.Prepare()
 	assert.True(builder.prepareCalled, "prepare should be called")
@@ -68,7 +68,7 @@ func TestBuild_Run(t *testing.T) {
 	build.Prepare()
 	build.Run(ui)
 
-	builder := build.builder.(*TestBuilder)
+	builder := build.(*coreBuild).builder.(*TestBuilder)
 
 	assert.True(builder.runCalled, "run should be called")
 	assert.Equal(builder.runBuild, build, "run should be called with build")

packer/template.go

@@ -95,7 +95,7 @@ func (t *Template) BuildNames() []string {
 //
 // If the build does not exist as part of this template, an error is
 // returned.
-func (t *Template) Build(name string, bf BuilderFactory) (b *Build, err error) {
+func (t *Template) Build(name string, bf BuilderFactory) (b Build, err error) {
 	builderConfig, ok := t.Builders[name]
 	if !ok {
 		err = fmt.Errorf("No such build found in template: %s", name)
@@ -108,7 +108,7 @@ func (t *Template) Build(name string, bf BuilderFactory) (b *Build, err error) {
 		return
 	}
 
-	b = &Build{
+	b = &coreBuild{
 		name: name,
 		builder: builder,
 		rawConfig: builderConfig.rawConfig,