packer: Template parsing error if name/type is not string

packer/template.go

@@ -52,7 +52,7 @@ func ParseTemplate(data []byte) (t *Template, err error) {
 	for i, v := range rawTpl.Builders {
 		rawType, ok := v["type"]
 		if !ok {
-			errors = append(errors, fmt.Errorf("missing 'type' for builder %d", i + 1))
+			errors = append(errors, fmt.Errorf("builder %d: missing 'type'", i + 1))
 			continue
 		}
 
@@ -64,9 +64,18 @@ func ParseTemplate(data []byte) (t *Template, err error) {
 			rawName = v["type"]
 		}
 
-		// TODO: Error checking if we can't convert
-		name := rawName.(string)
-		typeName := rawType.(string)
+		// Attempt to convert the name/type to strings, but error if we can't
+		name, ok := rawName.(string)
+		if !ok {
+			errors = append(errors, fmt.Errorf("builder %d: name must be a string", i + 1))
+			continue
+		}
+
+		typeName, ok := rawType.(string)
+		if !ok {
+			errors = append(errors, fmt.Errorf("builder %d: type must be a string", i + 1))
+			continue
+		}
 
 		// Check if we already have a builder with this name and error if so
 		if _, ok := t.Builders[name]; ok {

packer/template_test.go

@@ -54,6 +54,22 @@ func TestParseTemplate_BuilderWithoutType(t *testing.T) {
 	assert.NotNil(err, "should have error")
 }
 
+func TestParseTemplate_BuilderWithNonStringType(t *testing.T) {
+	assert := asserts.NewTestingAsserts(t, true)
+
+	data := `
+	{
+		"name": "my-image",
+		"builders": [{
+			"type": 42
+		}]
+	}
+	`
+
+	_, err := ParseTemplate([]byte(data))
+	assert.NotNil(err, "should have error")
+}
+
 func TestParseTemplate_BuilderWithoutName(t *testing.T) {
 	assert := asserts.NewTestingAsserts(t, true)