packer: Panic on template if no builder function

packer/template.go

@@ -156,6 +156,12 @@ func (t *Template) Build(name string, components *ComponentFinder) (b Build, err
 		return
 	}
 
+	// We panic if there is no builder function because this is really
+	// an internal bug that always needs to be fixed, not an error.
+	if components.Builder == nil {
+		panic("no builder function")
+	}
+
 	builder, err := components.Builder(builderConfig.builderType)
 	if err != nil {
 		return

packer/template_test.go

@@ -291,6 +291,42 @@ func TestTemplate_BuildUnknownBuilder(t *testing.T) {
 	assert.NotNil(err, "should have error")
 }
 
+func TestTemplate_Build_NilBuilderFunc(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	data := `
+	{
+		"name": "my-image",
+		"builders": [
+			{
+				"name": "test1",
+				"type": "test-builder"
+			}
+		],
+
+		"provisioners": [
+			{
+				"type": "test-prov"
+			}
+		]
+	}
+	`
+
+	template, err := ParseTemplate([]byte(data))
+	assert.Nil(err, "should not error")
+
+	defer func() {
+		p := recover()
+		assert.NotNil(p, "should panic")
+
+		if p != nil {
+			assert.Equal(p.(string), "no builder function", "right panic")
+		}
+	}()
+
+	template.Build("test1", &ComponentFinder{})
+}
+
 func TestTemplate_Build(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)