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

Fixes #4522.

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

diff --git a/src/pkg/text/template/exec.go b/src/pkg/text/template/exec.go
index b9c03d8f0b8b3a77632f6928f3347e8c46cc0c51..12c40b70f1d5f7cb2f8cc0843f1ea72b2bf5d6ea 100644 (file)
--- a/src/pkg/text/template/exec.go
+++ b/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

diff --git a/src/pkg/text/template/exec_test.go b/src/pkg/text/template/exec_test.go
index 683e9ac76b6d5255448301c076275bcdcc7d9a13..0f8beec5ed73252228ddbf4dd8544e21784fc5e3 100644 (file)
--- a/src/pkg/text/template/exec_test.go
+++ b/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)
+       }
+}

diff --git a/src/pkg/text/template/template.go b/src/pkg/text/template/template.go
index a2b9062ad1442006d5cbb5c2bffed945fc2c4160..d801c790ef0702f998149500940c856b66862f22 100644 (file)
--- a/src/pkg/text/template/template.go
+++ b/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
 }