exp/template/html: avoid redundant escaping directives.

This is a possible optimization.  I'm not sure the complexity is worth it.
The new benchmark in escape_test is 46us without and 35us with the optimization.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5168041

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

@@ -262,19 +262,54 @@ func ensurePipelineContains(p *parse.PipeNode, s []string) {
 		i := indexOfStr((id.Args[0].(*parse.IdentifierNode)).Ident, s, escFnsEq)
 		if i != -1 {
 			for _, name := range s[:i] {
-				newCmds = append(newCmds, newIdentCmd(name))
+				newCmds = appendCmd(newCmds, newIdentCmd(name))
 			}
 			s = s[i+1:]
 		}
-		newCmds = append(newCmds, id)
+		newCmds = appendCmd(newCmds, id)
 	}
 	// Create any remaining sanitizers.
 	for _, name := range s {
-		newCmds = append(newCmds, newIdentCmd(name))
+		newCmds = appendCmd(newCmds, newIdentCmd(name))
 	}
 	p.Cmds = newCmds
 }
 
+// redundantFuncs[a][b] implies that funcMap[b](funcMap[a](x)) == funcMap[a](x)
+// for all x.
+var redundantFuncs = map[string]map[string]bool{
+	"exp_template_html_commentescaper": {
+		"exp_template_html_attrescaper":    true,
+		"exp_template_html_nospaceescaper": true,
+		"exp_template_html_htmlescaper":    true,
+	},
+	"exp_template_html_cssescaper": {
+		"exp_template_html_attrescaper": true,
+	},
+	"exp_template_html_jsregexpescaper": {
+		"exp_template_html_attrescaper": true,
+	},
+	"exp_template_html_jsstrescaper": {
+		"exp_template_html_attrescaper": true,
+	},
+	"exp_template_html_urlescaper": {
+		"exp_template_html_urlnormalizer": true,
+	},
+}
+
+// appendCmd appends the given command to the end of the command pipeline
+// unless it is redundant with the last command.
+func appendCmd(cmds []*parse.CommandNode, cmd *parse.CommandNode) []*parse.CommandNode {
+	if n := len(cmds); n != 0 {
+		last, ok := cmds[n-1].Args[0].(*parse.IdentifierNode)
+		next, _ := cmd.Args[0].(*parse.IdentifierNode)
+		if ok && redundantFuncs[last.Ident][next.Ident] {
+			return cmds
+		}
+	}
+	return append(cmds, cmd)
+}
+
 // indexOfStr is the first i such that eq(s, strs[i]) or -1 if s was not found.
 func indexOfStr(s string, strs []string, eq func(a, b string) bool) int {
 	for i, t := range strs {

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

@@ -1577,3 +1577,47 @@ func TestEscapeSetErrorsNotIgnorable(t *testing.T) {
 	defer expectExecuteFailure(t, &b)
 	s.Execute(&b, "t", nil)
 }
+
+func TestRedundantFuncs(t *testing.T) {
+	inputs := []interface{}{
+		"\x00\x01\x02\x03\x04\x05\x06\x07\x08\t\n\x0b\x0c\r\x0e\x0f" +
+			"\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f" +
+			` !"#$%&'()*+,-./` +
+			`0123456789:;<=>?` +
+			`@ABCDEFGHIJKLMNO` +
+			`PQRSTUVWXYZ[\]^_` +
+			"`abcdefghijklmno" +
+			"pqrstuvwxyz{|}~\x7f" +
+			"\u00A0\u0100\u2028\u2029\ufeff\ufdec\ufffd\uffff\U0001D11E" +
+			"&amp;%22\\",
+		CSS(`a[href =~ "//example.com"]#foo`),
+		HTML(`Hello, <b>World</b> &amp;tc!`),
+		HTMLAttr(` dir="ltr"`),
+		JS(`c && alert("Hello, World!");`),
+		JSStr(`Hello, World & O'Reilly\x21`),
+		URL(`greeting=H%69&addressee=(World)`),
+	}
+
+	for n0, m := range redundantFuncs {
+		f0 := funcMap[n0].(func(...interface{}) string)
+		for n1, _ := range m {
+			f1 := funcMap[n1].(func(...interface{}) string)
+			for _, input := range inputs {
+				want := f0(input)
+				if got := f1(want); want != got {
+					t.Errorf("%s %s with %T %q: want\n\t%q,\ngot\n\t%q", n0, n1, input, input, want, got)
+				}
+			}
+		}
+	}
+}
+
+func BenchmarkEscapedExecute(b *testing.B) {
+	tmpl := template.Must(Escape(template.Must(template.New("t").Parse(`<a onclick="alert('{{.}}')">{{.}}</a>`))))
+	var buf bytes.Buffer
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		tmpl.Execute(&buf, "foo & 'bar' & baz")
+		buf.Reset()
+	}
+}