html/template: fix crash when escaping incomplete template

text/template turned this into an error but html/template crashed.
Refactor text/template.Execute to export a new function,
text/template.DefinedTemplates, so html/template can get the same
helpful error message in this case, and invoke it when there is no
definition for a template being escaped.

Fixes #10204.

Change-Id: I1d04e9e7ebca829bc08509caeb65e75da969711f
Reviewed-on: https://go-review.googlesource.com/7855Reviewed-by: Russ Cox <rsc@golang.org>

src/html/template/escape_test.go

@@ -1686,6 +1686,21 @@ func TestPipeToMethodIsEscaped(t *testing.T) {
 	}
 }
 
+// Unlike text/template, html/template crashed if given an incomplete
+// template, that is, a template that had been named but not given any content.
+// This is issue #10204.
+func TestErrorOnUndefined(t *testing.T) {
+	tmpl := New("undefined")
+
+	err := tmpl.Execute(nil, nil)
+	if err == nil {
+		t.Error("expected error")
+	}
+	if !strings.Contains(err.Error(), "incomplete") {
+		t.Errorf("expected error about incomplete template; got %s", err)
+	}
+}
+
 func BenchmarkEscapedExecute(b *testing.B) {
 	tmpl := Must(New("t").Parse(`<a onclick="alert('{{.}}')">{{.}}</a>`))
 	var buf bytes.Buffer

src/html/template/template.go

@@ -56,6 +56,9 @@ func (t *Template) escape() error {
 	t.nameSpace.mu.Lock()
 	defer t.nameSpace.mu.Unlock()
 	if t.escapeErr == nil {
+		if t.Tree == nil {
+			return fmt.Errorf("template: %q is an incomplete or empty template%s", t.Name(), t.text.DefinedTemplates())
+		}
 		if err := escapeTemplate(t, t.text.Root, t.Name()); err != nil {
 			return err
 		}

src/text/template/exec.go

@@ -136,6 +136,20 @@ func (t *Template) Execute(wr io.Writer, data interface{}) (err error) {
 	}
 	t.init()
 	if t.Tree == nil || t.Root == nil {
+		state.errorf("%q is an incomplete or empty template%s", t.Name(), t.DefinedTemplates())
+	}
+	state.walk(value, t.Root)
+	return
+}
+
+// DefinedTemplates returns a string listing the defined templates,
+// prefixed by the string "defined templates are: ". If there are none,
+// it returns the empty string. For generating an error message here
+// and in html/template.
+func (t *Template) DefinedTemplates() string {
+	if t.common == nil {
+		return ""
+	}
 	var b bytes.Buffer
 	for name, tmpl := range t.tmpl {
 		if tmpl.Tree == nil || tmpl.Root == nil {
@@ -150,10 +164,7 @@ func (t *Template) Execute(wr io.Writer, data interface{}) (err error) {
 	if b.Len() > 0 {
 		s = "; defined templates are: " + b.String()
 	}
-		state.errorf("%q is an incomplete or empty template%s", t.Name(), s)
-	}
-	state.walk(value, t.Root)
-	return
+	return s
 }
 
 // Walk functions step through the major pieces of the template structure,