html/template: Ensure release of namespace mutex in Template.Execute()

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6727046

src/pkg/html/template/template.go

@@ -45,18 +45,24 @@ func (t *Template) Templates() []*Template {
 	return m
 }
 
-// Execute applies a parsed template to the specified data object,
-// writing the output to wr.
-func (t *Template) Execute(wr io.Writer, data interface{}) (err error) {
+// escape escapes all associated templates.
+func (t *Template) escape() error {
 	t.nameSpace.mu.Lock()
+	defer t.nameSpace.mu.Unlock()
 	if !t.escaped {
-		if err = escapeTemplates(t, t.Name()); err != nil {
-			t.escaped = true
+		if err := escapeTemplates(t, t.Name()); err != nil {
+			return err
 		}
+		t.escaped = true
 	}
-	t.nameSpace.mu.Unlock()
-	if err != nil {
-		return
+	return nil
+}
+
+// Execute applies a parsed template to the specified data object,
+// writing the output to wr.
+func (t *Template) Execute(wr io.Writer, data interface{}) error {
+	if err := t.escape(); err != nil {
+		return err
 	}
 	return t.text.Execute(wr, data)
 }