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 gohugoio#2441, gohugoio#4083

parser/frontmatter.go

@@ -19,6 +19,7 @@ import (
 	"bytes"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"strings"
 
@@ -201,9 +202,45 @@ 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, MIT licensed
+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 {
+			res[fmt.Sprintf("%v", k)] = 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 +169,8 @@ 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},
+		{[]byte("a:\n  true: 1\n  false: 2"), map[string]interface{}{"a": map[string]interface{}{"true": 1, "false": 2}}, false},
 		// errors
 		{[]byte("z = not toml"), nil, true},
 	}
@@ -320,6 +321,41 @@ func TestRemoveTOMLIdentifier(t *testing.T) {
 	}
 }
 
+func TestStringifyYAMLMapKeys(t *testing.T) {
+	cases := []struct {
+		input map[interface{}]interface{}
+		want  map[string]interface{}
+	}{
+		{
+			map[interface{}]interface{}{"a": 1, "b": 2},
+			map[string]interface{}{"a": 1, "b": 2},
+		},
+		{
+			map[interface{}]interface{}{"a": []interface{}{1, map[interface{}]interface{}{"b": 2}}},
+			map[string]interface{}{"a": []interface{}{1, map[string]interface{}{"b": 2}}},
+		},
+		{
+			map[interface{}]interface{}{true: 1, "b": false},
+			map[string]interface{}{"true": 1, "b": false},
+		},
+		{
+			map[interface{}]interface{}{1: "a", 2: "b"},
+			map[string]interface{}{"1": "a", "2": "b"},
+		},
+		{
+			map[interface{}]interface{}{"a": map[interface{}]interface{}{"b": 1}},
+			map[string]interface{}{"a": map[string]interface{}{"b": 1}},
+		},
+	}
+
+	for i, c := range cases {
+		res := stringifyYAMLMapKeys(c.input)
+		if !reflect.DeepEqual(res, c.want) {
+			t.Errorf("[%d] given %q\nwant: %q\n got: %q", i, c.input, c.want, res)
+		}
+	}
+}
+
 func BenchmarkFrontmatterTags(b *testing.B) {
 
 	for _, frontmatter := range []string{"JSON", "YAML", "YAML2", "TOML"} {