[release-branch.go1.8] text/template: fix handling of empty blocks

This was a subtle bug introduced in the previous release's fix for
issue 16156.

The definition of empty template was broken, causing the answer
to depend on the order of templates in the map.

Fixes #16156 (for real).
Fixes #19294.
Fixes #19204.

Change-Id: I1cd915c94534cad3116d83bd158cbc28700510b9
Reviewed-on: https://go-review.googlesource.com/38420
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/39594Reviewed-by: Rob Pike <r@golang.org>

src/text/template/multi_test.go

@@ -363,7 +363,7 @@ func TestEmptyTemplate(t *testing.T) {
 		{[]string{"{{.}}", ""}, "twice", ""},
 	}
 
-	for _, c := range cases {
+	for i, c := range cases {
 		root := New("root")
 
 		var (
@@ -378,10 +378,43 @@ func TestEmptyTemplate(t *testing.T) {
 		}
 		buf := &bytes.Buffer{}
 		if err := m.Execute(buf, c.in); err != nil {
-			t.Fatal(err)
+			t.Error(i, err)
+			continue
 		}
 		if buf.String() != c.want {
 			t.Errorf("expected string %q: got %q", c.want, buf.String())
 		}
 	}
 }
+
+// Issue 19249 was a regression in 1.8 caused by the handling of empty
+// templates added in that release, which got different answers depending
+// on the order templates appeared in the internal map.
+func TestIssue19294(t *testing.T) {
+	// The empty block in "xhtml" should be replaced during execution
+	// by the contents of "stylesheet", but if the internal map associating
+	// names with templates is built in the wrong order, the empty block
+	// looks non-empty and this doesn't happen.
+	var inlined = map[string]string{
+		"stylesheet": `{{define "stylesheet"}}stylesheet{{end}}`,
+		"xhtml":      `{{block "stylesheet" .}}{{end}}`,
+	}
+	all := []string{"stylesheet", "xhtml"}
+	for i := 0; i < 100; i++ {
+		res, err := New("title.xhtml").Parse(`{{template "xhtml" .}}`)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, name := range all {
+			_, err := res.New(name).Parse(inlined[name])
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		var buf bytes.Buffer
+		res.Execute(&buf, 0)
+		if buf.String() != "stylesheet" {
+			t.Fatalf("iteration %d: got %q; expected %q", i, buf.String(), "stylesheet")
+		}
+	}
+}

src/text/template/template.go

@@ -127,7 +127,7 @@ func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error
 	// Even if nt == t, we need to install it in the common.tmpl map.
 	if replace, err := t.associate(nt, tree); err != nil {
 		return nil, err
-	} else if replace {
+	} else if replace || nt.Tree == nil {
 		nt.Tree = tree
 	}
 	return nt, nil
@@ -215,7 +215,7 @@ func (t *Template) associate(new *Template, tree *parse.Tree) (bool, error) {
 	if new.common != t.common {
 		panic("internal error: associate not common")
 	}
-	if t.tmpl[new.name] != nil && parse.IsEmptyTree(tree.Root) && t.Tree != nil {
+	if old := t.tmpl[new.name]; old != nil && parse.IsEmptyTree(tree.Root) && old.Tree != nil {
 		// If a template by that name exists,
 		// don't replace it with an empty template.
 		return false, nil