exp/template/html: allow commenting out of actions

Instead of erroring on actions inside comments, use existing escaping
pipeline to quash the output of actions inside comments.

If a template maintainer uses a comment to disable template code:

  {{if .}}Hello, {{.}}!{{end}}

->

  <!--{{if true}}Hello, {{.}}!{{end}}-->

will result in

  <!--Hello, !-->

regardless of the value of {{.}}.

In a later CL, comment elision will result in the entire commented-out
section being dropped from the template output.

Any side-effects in pipelines, such as panics, will still be realized.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5078041

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

@@ -100,19 +100,6 @@ const (
 	// produce a valid JavaScript Program.
 	ErrEndContext
 
-	// ErrInsideComment: "... appears inside a comment"
-	// Example:
-	//  <!-- {{.X}} -->
-	//  <script>/* {{.X}} */</script>
-	//  <style>/* {{.X}} */</style>
-	//
-	// Discussion:
-	//  {{.X}} appears inside a comment. There is no escaping convention for
-	//  comments. To use IE conditional comments, inject the  whole comment
-	//  as an HTML, JS, or CSS value (see content.go).
-	//  To comment out code, break the {{...}}.
-	ErrInsideComment
-
 	// ErrNoNames: "must specify names of top level templates"
 	// 
 	//   EscapeSet does not assume that all templates in a set produce HTML.

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

@@ -64,6 +64,7 @@ func EscapeSet(s *template.Set, names ...string) (*template.Set, os.Error) {
 // funcMap maps command names to functions that render their inputs safe.
 var funcMap = template.FuncMap{
 	"exp_template_html_attrescaper":     attrEscaper,
+	"exp_template_html_commentescaper":  commentEscaper,
 	"exp_template_html_cssescaper":      cssEscaper,
 	"exp_template_html_cssvaluefilter":  cssValueFilter,
 	"exp_template_html_htmlnamefilter":  htmlNameFilter,
@@ -200,12 +201,10 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		s = append(s, "exp_template_html_htmlnamefilter")
 	default:
 		if isComment(c.state) {
-			return context{
-				state: stateError,
-				err:   errorf(ErrInsideComment, n.Line, "%s appears inside a comment", n),
-			}
+			s = append(s, "exp_template_html_commentescaper")
+		} else {
+			panic("unexpected state " + c.state.String())
 		}
-		panic("unexpected state " + c.state.String())
 	}
 	switch c.delim {
 	case delimNone:

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

@@ -361,11 +361,94 @@ func TestEscape(t *testing.T) {
 			`<a style="border-image: url(/**/%27%22;://%20%5c), url(&quot;/**/%27%22;://%20%5c&quot;), url('/**/%27%22;://%20%5c'), 'http://www.example.com/?q=%2f%2a%2a%2f%27%22%3b%3a%2f%2f%20%5c''">`,
 		},
 		{
-			"comment",
+			"HTML comment",
 			"<b>Hello, <!-- name of world -->{{.C}}</b>",
 			// TODO: Elide comment.
 			"<b>Hello, <!-- name of world -->&lt;Cincinatti&gt;</b>",
 		},
+		{
+			"Split HTML comment",
+			"<b>Hello, <!-- name of {{if .T}}city -->{{.C}}{{else}}world -->{{.W}}{{end}}</b>",
+			"<b>Hello, <!-- name of city -->&lt;Cincinatti&gt;</b>",
+		},
+		{
+			"JS line comment",
+			"<script>for (;;) { if (c()) break// foo not a label\n" +
+				"foo({{.T}});}</script>",
+			"<script>for (;;) { if (c()) break// foo not a label\n" +
+				"foo( true );}</script>",
+		},
+		{
+			"JS multiline block comment",
+			"<script>for (;;) { if (c()) break/* foo not a label\n" +
+				" */foo({{.T}});}</script>",
+			// Newline separates break from call. If newline
+			// removed, then break will consume label leaving
+			// code invalid.
+			"<script>for (;;) { if (c()) break/* foo not a label\n" +
+				" */foo( true );}</script>",
+		},
+		{
+			"JS single-line block comment",
+			"<script>for (;;) {\n" +
+				"if (c()) break/* foo a label */foo;" +
+				"x({{.T}});}</script>",
+			// Newline separates break from call. If newline
+			// removed, then break will consume label leaving
+			// code invalid.
+			"<script>for (;;) {\n" +
+				"if (c()) break/* foo a label */foo;" +
+				"x( true );}</script>",
+		},
+		{
+			"JS block comment flush with mathematical division",
+			"<script>var a/*b*//c\nd</script>",
+			"<script>var a/*b*//c\nd</script>",
+		},
+		{
+			"JS mixed comments",
+			"<script>var a/*b*///c\nd</script>",
+			"<script>var a/*b*///c\nd</script>",
+		},
+		{
+			"CSS comments",
+			"<style>p// paragraph\n" +
+				`{border: 1px/* color */{{"#00f"}}}</style>`,
+			"<style>p// paragraph\n" +
+				"{border: 1px/* color */#00f}</style>",
+		},
+		{
+			"JS attr block comment",
+			`<a onclick="f(&quot;&quot;); /* alert({{.H}}) */">`,
+			// Attribute comment tests should pass if the comments
+			// are successfully elided.
+			`<a onclick="f(&quot;&quot;); /* alert() */">`,
+		},
+		{
+			"JS attr line comment",
+			`<a onclick="// alert({{.G}})">`,
+			`<a onclick="// alert()">`,
+		},
+		{
+			"CSS attr block comment",
+			`<a style="/* color: {{.H}} */">`,
+			`<a style="/* color:  */">`,
+		},
+		{
+			"CSS attr line comment",
+			`<a style="// color: {{.G}}">`,
+			`<a style="// color: ">`,
+		},
+		{
+			"HTML substitution commented out",
+			"<p><!-- {{.H}} --></p>",
+			"<p><!--  --></p>",
+		},
+		{
+			"Comment ends flush with start",
+			"<!--{{.}}--><script>/*{{.}}*///{{.}}\n</script><style>/*{{.}}*///{{.}}\n</style><a onclick='/*{{.}}*///{{.}}' style='/*{{.}}*///{{.}}'>",
+			"<!----><script>/**///\n</script><style>/**///\n</style><a onclick='/**///' style='/**///'>",
+		},
 		{
 			"typed HTML in text",
 			`{{.W}}`,
@@ -717,26 +800,6 @@ func TestErrors(t *testing.T) {
 			`<a onclick="/foo[\]/`,
 			`unfinished JS regexp charset: "foo[\\]/"`,
 		},
-		{
-			`<a onclick="/* alert({{.X}}) */">`,
-			`z:1: (action: [(command: [F=[X]])]) appears inside a comment`,
-		},
-		{
-			`<a onclick="// alert({{.X}})">`,
-			`z:1: (action: [(command: [F=[X]])]) appears inside a comment`,
-		},
-		{
-			`<a style="/* color: {{.X}} */">`,
-			`z:1: (action: [(command: [F=[X]])]) appears inside a comment`,
-		},
-		{
-			`<a style="// color: {{.X}}">`,
-			`z:1: (action: [(command: [F=[X]])]) appears inside a comment`,
-		},
-		{
-			"<!-- {{.H}} -->",
-			"z:1: (action: [(command: [F=[H]])]) appears inside a comment",
-		},
 		{
 			// It is ambiguous whether 1.5 should be 1\.5 or 1.5.
 			// Either `var x = 1/- 1.5 /i.test(x)`

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

@@ -224,3 +224,13 @@ func htmlNameFilter(args ...interface{}) string {
 	}
 	return s
 }
+
+// commentEscaper returns the empty string regardless of input.
+// Comment content does not correspond to any parsed structure or
+// human-readable content, so the simplest and most secure policy is to drop
+// content interpolated into comments.
+// This approach is equally valid whether or not static comment content is
+// removed from the template.
+func commentEscaper(args ...interface{}) string {
+	return ""
+}