exp/template: add an html escaping function.

R=golang-dev, dsymonds, adg
CC=golang-dev
https://golang.org/cl/4626092

src/pkg/exp/template/exec_test.go

@@ -150,6 +150,10 @@ var execTests = []execTest{
 	{"printf field", `{{printf "%s" .U.V}}`, "v", tVal, true},
 	{"printf method", `{{printf "%s" .Method0}}`, "resultOfMethod0", tVal, true},
 	{"printf lots", `{{printf "%d %s %g %s" 127 "hello" 7-3i .Method0}}`, "127 hello (7-3i) resultOfMethod0", tVal, true},
+	{"html", `{{html "<script>alert(\"XSS\");</script>"}}`,
+		"&lt;script&gt;alert(&#34;XSS&#34;);&lt;/script&gt;", tVal, true},
+	{"html pipeline", `{{printf "<script>alert(\"XSS\");</script>" | html}}`,
+		"&lt;script&gt;alert(&#34;XSS&#34;);&lt;/script&gt;", tVal, true},
 	// With.
 	{"with true", "{{with true}}{{.}}{{end}}", "true", tVal, true},
 	{"with false", "{{with false}}{{.}}{{else}}FALSE{{end}}", "FALSE", tVal, true},

src/pkg/exp/template/funcs.go

@@ -5,8 +5,11 @@
 package template
 
 import (
+	"bytes"
+	"io"
 	"fmt"
 	"reflect"
+	"strings"
 )
 
 // FuncMap is the type of the map defining the mapping from names to functions.
@@ -16,6 +19,7 @@ type FuncMap map[string]interface{}
 
 var funcs = map[string]reflect.Value{
 	"printf": reflect.ValueOf(fmt.Sprintf),
+	"html":   reflect.ValueOf(HTMLEscaper),
 }
 
 // addFuncs adds to values the functions in funcs, converting them to reflect.Values.
@@ -61,3 +65,64 @@ func findFunction(name string, tmpl *Template, set *Set) (reflect.Value, bool) {
 	}
 	return reflect.Value{}, false
 }
+
+// HTML escaping
+
+var (
+	escQuot = []byte(""") // shorter than """
+	escApos = []byte("'") // shorter than "'"
+	escAmp  = []byte("&")
+	escLt   = []byte("<")
+	escGt   = []byte(">")
+)
+
+// HTMLEscape writes to w the escaped HTML equivalent of the plain text data b.
+func HTMLEscape(w io.Writer, b []byte) {
+	last := 0
+	for i, c := range b {
+		var esc []byte
+		switch c {
+		case '"':
+			esc = escQuot
+		case '\'':
+			esc = escApos
+		case '&':
+			esc = escAmp
+		case '<':
+			esc = escLt
+		case '>':
+			esc = escGt
+		default:
+			continue
+		}
+		w.Write(b[last:i])
+		w.Write(esc)
+		last = i + 1
+	}
+	w.Write(b[last:])
+}
+
+// HTMLEscapeString returns the escaped HTML equivalent of the plain text data s.
+func HTMLEscapeString(s string) string {
+	// Avoid allocation if we can.
+	if strings.IndexAny(s, `'"&<>`) < 0 {
+		return s
+	}
+	var b bytes.Buffer
+	HTMLEscape(&b, []byte(s))
+	return b.String()
+}
+
+// HTMLEscaper returns the escaped HTML equivalent of the textual
+// representation of its arguments.
+func HTMLEscaper(args ...interface{}) string {
+	ok := false
+	var s string
+	if len(args) == 1 {
+		s, ok = args[0].(string)
+	}
+	if !ok {
+		s = fmt.Sprint(args...)
+	}
+	return HTMLEscapeString(s)
+}