exp/template/html: fix bug, '<' normalization for text nodes that change context

R=nigeltao
CC=golang-dev
https://golang.org/cl/5080042

src/pkg/exp/template/html/escape.go

@@ -551,8 +551,17 @@ func (e *escaper) escapeText(c context, n *parse.TextNode) context {
 	for i != len(s) {
 		c1, nread := contextAfterText(c, s[i:])
 		i1 := i + nread
-		if c.state == c1.state && (c.state == stateText || c.state == stateRCDATA) {
-			for j := i; j < i1; j++ {
+		if c.state == stateText || c.state == stateRCDATA {
+			end := i1
+			if c1.state != c.state {
+				for j := end - 1; j >= i; j-- {
+					if s[j] == '<' {
+						end = j
+						break
+					}
+				}
+			}
+			for j := i; j < end; j++ {
 				if s[j] == '<' {
 					b.Write(s[written:j])
 					b.WriteString("&lt;")

src/pkg/exp/template/html/escape_test.go

@@ -366,6 +366,26 @@ func TestEscape(t *testing.T) {
 			// TODO: Elide comment.
 			"<b>Hello, <!-- name of world -->&lt;Cincinatti&gt;</b>",
 		},
+		{
+			"HTML comment not first < in text node.",
+			"<<!-- -->!--",
+			"&lt;<!-- -->!--",
+		},
+		{
+			"HTML normalization 1",
+			"a < b",
+			"a &lt; b",
+		},
+		{
+			"HTML normalization 2",
+			"a << b",
+			"a &lt;&lt; b",
+		},
+		{
+			"HTML normalization 3",
+			"a<<!-- --><!-- -->b",
+			"a&lt;<!-- --><!-- -->b",
+		},
 		{
 			"Split HTML comment",
 			"<b>Hello, <!-- name of {{if .T}}city -->{{.C}}{{else}}world -->{{.W}}{{end}}</b>",