Encode strings with YAML 1.1 boolean values with quotes

To allow better compatibility with YAML 1.1 parsers, when outputting a
string that is equal to one of the YAML 1.1 boolean values ("yes", "no",
"on", "off", etc), enclose the string in quotes. The string will then
parse correctly by either YAML 1.1 or YAML 1.2 parsers.

This change does not affect decoding: Bare `yes` and `no` will still be
decoded as strings.

See: go-yaml#214

encode.go

@@ -298,6 +298,23 @@ func isBase60Float(s string) (result bool) {
 // is bogus. In practice parsers do not enforce the "\.[0-9_]*" suffix.
 var base60float = regexp.MustCompile(`^[-+]?[0-9][0-9_]*(?::[0-5]?[0-9])+(?:\.[0-9_]*)?$`)
 
+// isYaml11Bool returns whether s is a boolean value as defined in YAML 1.1.
+//
+// The extra boolean values in YAML 1.1 are a terrible idea and is unsupported
+// in YAML 1.2 and by this package, but these should be marshalled quoted for
+// the time being for compatibility with other parsers.
+func isYaml11Bool(s string) (result bool) {
+	// Fast path.
+	if s == "" {
+		return false
+	}
+	return yaml11Bool.MatchString(s)
+}
+
+// From https://yaml.org/type/bool.html, except "true" and "false" are removed
+// since those are supported in YAML 1.2
+var yaml11Bool = regexp.MustCompile(`^(y|Y|yes|Yes|YES|n|N|no|No|NO|True|TRUE|False|FALSE|on|On|ON|off|Off|OFF)$`)
+
 func (e *encoder) stringv(tag string, in reflect.Value) {
 	var style yaml_scalar_style_t
 	s := in.String()
@@ -319,7 +336,7 @@ func (e *encoder) stringv(tag string, in reflect.Value) {
 		// tag when encoded unquoted. If it doesn't,
 		// there's no need to quote it.
 		rtag, _ := resolve("", s)
-		canUsePlain = rtag == strTag && !isBase60Float(s)
+		canUsePlain = rtag == strTag && !isBase60Float(s) && !isYaml11Bool(s)
 	}
 	// Note: it's possible for user code to emit invalid YAML
 	// if they explicitly specify a tag and a string containing
@@ -425,6 +442,9 @@ func (e *encoder) node(node *Node, tail string) {
 					tag = ""
 					forceQuoting = true
 				}
+				if stag == strTag && isYaml11Bool(node.Value) {
+					forceQuoting = true
+				}
 			}
 		} else {
 			switch node.Kind {

encode_test.go

@@ -19,13 +19,12 @@ import (
 	"bytes"
 	"fmt"
 	"math"
+	"net"
+	"os"
 	"strconv"
 	"strings"
 	"time"
 
-	"net"
-	"os"
-
 	. "gopkg.in/check.v1"
 	"gopkg.in/yaml.v3"
 )
@@ -422,6 +421,40 @@ var marshalTests = []struct {
 		map[string]string{"a": "\tB\n\tC\n"},
 		"a: |\n    \tB\n    \tC\n",
 	},
+
+	// Strings that are equal to the YAML 1.1 boolean values should be quoted
+	// so that YAML 1.1 parsers will parse them as booleans.
+	{
+		map[string]string{"v": "yes"},
+		"v: \"yes\"\n",
+	}, {
+		map[string]interface{}{"v": "no"},
+		"v: \"no\"\n",
+	}, {
+		&yaml.Node{Value: `yes`, Tag: "!!str", Kind: yaml.ScalarNode},
+		"\"yes\"\n",
+	}, {
+		&yaml.Node{Value: `no`, Tag: "!!str", Kind: yaml.ScalarNode},
+		"\"no\"\n",
+	}, {
+		&yaml.Node{Value: `true`, Tag: "!!str", Kind: yaml.ScalarNode},
+		"\"true\"\n",
+	}, {
+		&yaml.Node{Value: `false`, Tag: "!!str", Kind: yaml.ScalarNode},
+		"\"false\"\n",
+	}, {
+		&yaml.Node{Value: `yes`, Tag: "", Kind: yaml.ScalarNode},
+		"yes\n",
+	}, {
+		&yaml.Node{Value: `no`, Tag: "", Kind: yaml.ScalarNode},
+		"no\n",
+	}, {
+		&yaml.Node{Value: `true`, Tag: "", Kind: yaml.ScalarNode},
+		"true\n",
+	}, {
+		&yaml.Node{Value: `false`, Tag: "", Kind: yaml.ScalarNode},
+		"false\n",
+	},
 }
 
 func (s *S) TestMarshal(c *C) {