html/template: fix multiple Clones of redefined template

This change redoes the fix for #16101 (CL 31092) in a different way by
making t.Clone return the template associated with the t.Name() while
allowing for the case that a template of the same name is define-d.

Fixes #17735.

Change-Id: I1e69672390a4c81aa611046a209008ae4a3bb723
Reviewed-on: https://go-review.googlesource.com/33210
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/html/template/clone_test.go

@@ -7,6 +7,7 @@ package template
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io/ioutil"
 	"sync"
 	"testing"
@@ -241,3 +242,23 @@ func TestCloneGrowth(t *testing.T) {
 		t.Fatalf("too many templates: %v", len(tmpl.DefinedTemplates()))
 	}
 }
+
+// https://golang.org/issue/17735
+func TestCloneRedefinedName(t *testing.T) {
+	const base = `
+{{ define "a" -}}<title>{{ template "b" . -}}</title>{{ end -}}
+{{ define "b" }}{{ end -}}
+`
+	const page = `{{ template "a" . }}`
+
+	t1 := Must(New("a").Parse(base))
+
+	for i := 0; i < 2; i++ {
+		t2 := Must(t1.Clone())
+		t2 = Must(t2.New(fmt.Sprintf("%d", i)).Parse(page))
+		err := t2.Execute(ioutil.Discard, nil)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}

src/html/template/template.go

@@ -259,9 +259,6 @@ func (t *Template) Clone() (*Template, error) {
 	ret.set[ret.Name()] = ret
 	for _, x := range textClone.Templates() {
 		name := x.Name()
-		if name == ret.Name() {
-			continue
-		}
 		src := t.set[name]
 		if src == nil || src.escapeErr != nil {
 			return nil, fmt.Errorf("html/template: cannot Clone %q after it has executed", t.Name())
@@ -274,7 +271,8 @@ func (t *Template) Clone() (*Template, error) {
 			ret.nameSpace,
 		}
 	}
-	return ret, nil
+	// Return the template associated with the name of this template.
+	return ret.set[ret.Name()], nil
 }
 
 // New allocates a new HTML template with the given name.