builder/virtualbox: Error if output directory already exists

builder/virtualbox/builder.go

@@ -164,6 +164,10 @@ func (b *Builder) Prepare(raws ...interface{}) error {
 		}
 	}
 
+	if _, err := os.Stat(b.config.OutputDir); err == nil {
+		errs = append(errs, errors.New("Output directory already exists. It must not exist."))
+	}
+
 	if b.config.RawBootWait != "" {
 		b.config.BootWait, err = time.ParseDuration(b.config.RawBootWait)
 		if err != nil {

builder/virtualbox/builder_test.go

@@ -224,6 +224,31 @@ func TestBuilderPrepare_ISOUrl(t *testing.T) {
 	}
 }
 
+func TestBuilderPrepare_OutputDir(t *testing.T) {
+	var b Builder
+	config := testConfig()
+
+	// Test with existing dir
+	dir, err := ioutil.TempDir("", "packer")
+	if err != nil {
+		t.Fatalf("err: %s", err)
+	}
+	defer os.RemoveAll(dir)
+
+	config["output_directory"] = dir
+	err = b.Prepare(config)
+	if err == nil {
+		t.Fatal("should have error")
+	}
+
+	// Test with a good one
+	config["output_directory"] = "i-hope-i-dont-exist"
+	err = b.Prepare(config)
+	if err != nil {
+		t.Fatalf("should not have error: %s", err)
+	}
+}
+
 func TestBuilderPrepare_ShutdownTimeout(t *testing.T) {
 	var b Builder
 	config := testConfig()