text/template: fix bug in Clone

Cloned template copied the root template incorrectly.
Add test of self-consistency.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5436063

src/pkg/text/template/multi_test.go

@@ -230,6 +230,15 @@ func TestClone(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	// Verify that the clone is self-consistent.
+	for k, v := range clone.tmpl {
+		if k == clone.name && v.tmpl[k] != clone {
+			t.Error("clone does not contain root")
+		}
+		if v != v.tmpl[v.name] {
+			t.Errorf("clone does not contain self for %q", k)
+		}
+	}
 	// Execute root.
 	var b bytes.Buffer
 	err = root.ExecuteTemplate(&b, "a", 0)

src/pkg/text/template/template.go

@@ -73,12 +73,15 @@ func (t *Template) init() {
 // common templates and use them with variant definitions for other templates by
 // adding the variants after the clone is made.
 func (t *Template) Clone() *Template {
-	nt := t.copy()
+	nt := t.copy(nil)
 	nt.init()
+	nt.tmpl[t.name] = nt
 	for k, v := range t.tmpl {
+		if k == t.name { // Already installed.
+			continue
+		}
 		// The associated templates share nt's common structure.
-		tmpl := v.copy()
-		tmpl.common = nt.common
+		tmpl := v.copy(nt.common)
 		nt.tmpl[k] = tmpl
 	}
 	for k, v := range t.parseFuncs {
@@ -90,10 +93,11 @@ func (t *Template) Clone() *Template {
 	return nt
 }
 
-// copy returns a shallow copy of t, with common set to nil.
-func (t *Template) copy() *Template {
+// copy returns a shallow copy of t, with common set to the argument.
+func (t *Template) copy(c *common) *Template {
 	nt := New(t.name)
 	nt.Tree = t.Tree
+	nt.common = c
 	nt.leftDelim = t.leftDelim
 	nt.rightDelim = t.rightDelim
 	return nt