fix: escape non-scalar literals in variables

fixes #559

Signed-off-by: Max Brauer <mbrauer@vmware.com>

Author: mamachanko

internal/builder/interpolator.go

@@ -13,7 +13,7 @@ import (
 	"github.com/pivotal-cf/kiln/pkg/proofing"
 
 	yamlConverter "github.com/ghodss/yaml"
-	"gopkg.in/yaml.v2"
+	"gopkg.in/yaml.v3"
 )
 
 const (
@@ -274,8 +274,8 @@ func (i Interpolator) interpolateValueIntoYAML(input InterpolateInput, name stri
 		return "", err // should never happen
 	}
 
-	if stringScalar, ok := val.(string); ok {
-		return stringScalar, nil
+	if stringVal, ok := val.(string); ok {
+		return i.yamlMarshalString(stringVal)
 	}
 
 	interpolatedYAML, err := i.interpolate(input, name, initialYAML)
@@ -291,6 +291,39 @@ func (i Interpolator) interpolateValueIntoYAML(input InterpolateInput, name stri
 	return string(inlinedYAML), nil
 }
 
+// yamlMarshalString escapes a string so it can be embedded into a YAML document literally
+//
+// For example:
+//   - "hello" → "hello"
+//   - "true" → "true"
+//   - "[one, 2]" → "\"[one, 2]\""
+func (i Interpolator) yamlMarshalString(val string) (string, error) {
+	var node yaml.Node
+	err := yaml.Unmarshal([]byte(val), &node)
+	if err != nil {
+		return "", err
+	}
+
+	// when it's a document node jump to the content node
+	contentNode := &node
+	if len(node.Content) > 0 {
+		contentNode = node.Content[0]
+	}
+
+	// when the node is a scalar, e.g. string, int, double, etc., no escaping is needed
+	if contentNode.Kind == yaml.ScalarNode {
+		return val, nil
+	}
+
+	// escape objects and arrays
+	var escaped []byte
+	escaped, err = yaml.Marshal(val)
+	if err != nil {
+		return "", err
+	}
+	return string(escaped), nil
+}
+
 // Workaround to avoid YAML indentation being incorrect when value is interpolated into the metadata
 func (i Interpolator) yamlMarshalOneLine(yamlContents []byte) ([]byte, error) {
 	return yamlConverter.YAMLToJSON(yamlContents)

internal/builder/interpolator_test.go

@@ -272,6 +272,29 @@ some_form_types:
 `))
 	})
 
+	It("interpolates literal variables", func() {
+		templateYAML := `
+---
+literal_object: $(variable "variable-literal-object")
+literal_array: $(variable "variable-literal-array")
+`
+
+		input = builder.InterpolateInput{
+			SkipKilnMetadata: true,
+			Variables: map[string]any{
+				"variable-literal-object": `{"some": "value"}`,
+				"variable-literal-array": `[some, "things"]`,
+			},
+		}
+
+		interpolatedYAML, err := interpolator.Interpolate(input, "", []byte(templateYAML))
+		Expect(err).NotTo(HaveOccurred())
+		Expect(interpolatedYAML).To(HelpfullyMatchYAML(`
+literal_object: "{\"some\": \"value\"}"
+literal_array: "[some, \"things\"]"
+`))
+	})
+
 	Context("when multiple stemcells are specified", func() {
 		var templateYAML string