packer: panic if Prepare called twice on build, lock

packer/build.go

 package packer
 
-import "log"
+import (
+	"log"
+	"sync"
+)
 
 // This is the key in configurations that is set to "true" when Packer
 // debugging is enabled.
@@ -48,6 +51,7 @@ type coreBuild struct {
 	provisioners  []coreBuildProvisioner
 
 	debug         bool
+	l             sync.Mutex
 	prepareCalled bool
 }
 
@@ -66,7 +70,13 @@ func (b *coreBuild) Name() string {
 // Prepare prepares the build by doing some initialization for the builder
 // and any hooks. This _must_ be called prior to Run.
 func (b *coreBuild) Prepare() (err error) {
-	// TODO: lock
+	b.l.Lock()
+	defer b.l.Unlock()
+
+	if b.prepareCalled {
+		panic("prepare already called")
+	}
+
 	b.prepareCalled = true
 
 	debugConfig := map[string]interface{}{

packer/build_test.go

@@ -49,6 +49,26 @@ func TestBuild_Prepare(t *testing.T) {
 	assert.Equal(prov.prepConfigs, []interface{}{42, debugFalseConfig}, "prepare should be called with proper config")
 }
 
+func TestBuild_Prepare_Twice(t *testing.T) {
+	build := testBuild()
+	if err := build.Prepare(); err != nil {
+		t.Fatalf("bad error: %s", err)
+	}
+
+	defer func() {
+		p := recover()
+		if p == nil {
+			t.Fatalf("should've paniced")
+		}
+
+		if p.(string) != "prepare already called" {
+			t.Fatalf("Invalid panic: %s", p)
+		}
+	}()
+
+	build.Prepare()
+}
+
 func TestBuild_Prepare_Debug(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)