parser: Fix YAML maps key type

Recurse through result of yaml package parsing and change all
maps from map[interface{}]interface{} to map[string]interface{}
making them jsonable and sortable.

Fixes #2441, #4083

parser/frontmatter.go

@@ -201,9 +201,46 @@ func removeTOMLIdentifier(datum []byte) []byte {
 func HandleYAMLMetaData(datum []byte) (interface{}, error) {
 	m := map[string]interface{}{}
 	err := yaml.Unmarshal(datum, &m)
+
+	// To support boolean keys, the `yaml` package unmarshals maps to
+	// map[interface{}]interface{}. Here we recurse through the result
+	// and change all maps to map[string]interface{} like we would've
+	// gotten from `json`.
+	if err == nil {
+		for k, v := range m {
+			m[k] = stringifyYAMLMapKeys(v)
+		}
+	}
+
 	return m, err
 }
 
+// stringifyKeysMapValue recurses into in and changes all instances of
+// map[interface{}]interface{} to map[string]interface{}. This is useful to
+// work around the impedence mismatch between JSON and YAML unmarshaling that's
+// described here: https://github.com/go-yaml/yaml/issues/139
+//
+// Inspired by https://github.com/stripe/stripe-mock
+func stringifyYAMLMapKeys(in interface{}) interface{} {
+	switch in := in.(type) {
+	case []interface{}:
+		res := make([]interface{}, len(in))
+		for i, v := range in {
+			res[i] = stringifyYAMLMapKeys(v)
+		}
+		return res
+	case map[interface{}]interface{}:
+		res := make(map[string]interface{})
+		for k, v := range in {
+			kStr, _ := k.(string)
+			res[kStr] = stringifyYAMLMapKeys(v)
+		}
+		return res
+	default:
+		return in
+	}
+}
+
 // HandleJSONMetaData unmarshals JSON-encoded datum and returns a Go interface
 // representing the encoded data structure.
 func HandleJSONMetaData(datum []byte) (interface{}, error) {

parser/frontmatter_test.go

@@ -169,7 +169,7 @@ func TestHandleYAMLMetaData(t *testing.T) {
 	}{
 		{nil, map[string]interface{}{}, false},
 		{[]byte("title: test 1"), map[string]interface{}{"title": "test 1"}, false},
-		{[]byte("a: Easy!\nb:\n  c: 2\n  d: [3, 4]"), map[string]interface{}{"a": "Easy!", "b": map[interface{}]interface{}{"c": 2, "d": []interface{}{3, 4}}}, false},
+		{[]byte("a: Easy!\nb:\n  c: 2\n  d: [3, 4]"), map[string]interface{}{"a": "Easy!", "b": map[string]interface{}{"c": 2, "d": []interface{}{3, 4}}}, false},
 		// errors
 		{[]byte("z = not toml"), nil, true},
 	}