text/template: improve error reporting for executing an empty template

Fixes #4522.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7502044

src/pkg/text/template/exec.go

@@ -5,6 +5,7 @@
 package template
 
 import (
+	"bytes"
 	"fmt"
 	"io"
 	"reflect"
@@ -125,8 +126,23 @@ func (t *Template) Execute(wr io.Writer, data interface{}) (err error) {
 		wr:   wr,
 		vars: []variable{{"$", value}},
 	}
+	t.init()
 	if t.Tree == nil || t.Root == nil {
-		state.errorf("%q is an incomplete or empty template", t.name)
+		var b bytes.Buffer
+		for name, tmpl := range t.tmpl {
+			if tmpl.Tree == nil || tmpl.Root == nil {
+				continue
+			}
+			if b.Len() > 0 {
+				b.WriteString(", ")
+			}
+			fmt.Fprintf(&b, "%q", name)
+		}
+		var s string
+		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

src/pkg/text/template/exec_test.go

@@ -816,3 +816,40 @@ func TestExecuteOnNewTemplate(t *testing.T) {
 	// This is issue 3872.
 	_ = New("Name").Templates()
 }
+
+const testTemplates = `{{define "one"}}one{{end}}{{define "two"}}two{{end}}`
+
+func TestMessageForExecuteEmpty(t *testing.T) {
+	// Test a truly empty template.
+	tmpl := New("empty")
+	var b bytes.Buffer
+	err := tmpl.Execute(&b, 0)
+	if err == nil {
+		t.Fatal("expected initial error")
+	}
+	got := err.Error()
+	want := `template: empty: "empty" is an incomplete or empty template`
+	if got != want {
+		t.Errorf("expected error %s got %s", want, got)
+	}
+	// Add a non-empty template to check that the error is helpful.
+	tests, err := New("").Parse(testTemplates)
+	if err != nil {
+		t.Fatal(err)
+	}
+	tmpl.AddParseTree("secondary", tests.Tree)
+	err = tmpl.Execute(&b, 0)
+	if err == nil {
+		t.Fatal("expected second error")
+	}
+	got = err.Error()
+	want = `template: empty: "empty" is an incomplete or empty template; defined templates are: "secondary"`
+	if got != want {
+		t.Errorf("expected error %s got %s", want, got)
+	}
+	// Make sure we can execute the secondary.
+	err = tmpl.ExecuteTemplate(&b, "secondary", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+}

src/pkg/text/template/template.go

@@ -40,6 +40,9 @@ func New(name string) *Template {
 
 // Name returns the name of the template.
 func (t *Template) Name() string {
+	if t.name == "" {
+		return "<unnamed>"
+	}
 	return t.name
 }