Require Prepare to be called on Build

packer/build.go

@@ -7,6 +7,8 @@ package packer
 type Build struct {
 	name    string
 	builder Builder
+
+	prepareCalled bool
 }
 
 // Implementers of Builder are responsible for actually building images
@@ -42,10 +44,15 @@ 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(config interface{}) {
+	b.prepareCalled = true
 	b.builder.Prepare(config)
 }
 
 // Runs the actual build. Prepare must be called prior to running this.
 func (b *Build) Run(ui Ui) {
+	if !b.prepareCalled {
+		panic("Prepare must be called first")
+	}
+
 	b.builder.Run(b, ui)
 }

packer/build_test.go

@@ -49,6 +49,7 @@ func TestBuild_Run(t *testing.T) {
 	ui := testUi()
 
 	build := testBuild()
+	build.Prepare(nil)
 	build.Run(ui)
 
 	builder := build.builder.(*TestBuilder)
@@ -57,3 +58,15 @@ func TestBuild_Run(t *testing.T) {
 	assert.Equal(builder.runBuild, build, "run should be called with build")
 	assert.Equal(builder.runUi, ui, "run should be called with ui")
 }
+
+func TestBuild_RunBeforePrepare(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	defer func() {
+		p := recover()
+		assert.NotNil(p, "should panic")
+		assert.Equal(p.(string), "Prepare must be called first", "right panic")
+	}()
+
+	testBuild().Run(testUi())
+}