text/template: refactor code to accomodate bi-state requirement for templates

This is follow-up to CL10607

- Refactor AddParseTree() to use t.associate()
- Refactor Parse() to use AddParseTree() to put entries into common structure
- Clone() should not put entry in t.tmpl for undefined template
- Clarify documentation for Templates()
- Clarify documentation for AddParseTree() to include the error case

Updates #10910
Uodates #10926

Includes test cases for most of the above changes

Change-Id: I25b2fce6f9651272866f881acf44e4dbca04a4a8
Reviewed-on: https://go-review.googlesource.com/10622
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 85 insertions, 23 deletions

diff --git a/src/text/template/multi_test.go b/src/text/template/multi_test.go
index d79c20dc1a..ea01875e9c 100644
--- a/src/text/template/multi_test.go
+++ b/src/text/template/multi_test.go
@@ -296,3 +296,70 @@ func TestEmptyTemplateCloneCrash(t *testing.T) {
 	t1 := New("base")
 	t1.Clone() // used to panic
 }
+
+// Issue 10910, 10926
+func TestTemplateLookUp(t *testing.T) {
+	t1 := New("foo")
+	if t1.Lookup("foo") != nil {
+		t.Error("Lookup returned non-nil value for undefined template foo")
+	}
+	t1.New("bar")
+	if t1.Lookup("bar") != nil {
+		t.Error("Lookup returned non-nil value for undefined template bar")
+	}
+	t1.Parse(`{{define "foo"}}test{{end}}`)
+	if t1.Lookup("foo") == nil {
+		t.Error("Lookup returned nil value for defined template")
+	}
+}
+
+func TestNew(t *testing.T) {
+	// template with same name already exists
+	t1, _ := New("test").Parse(`{{define "test"}}foo{{end}}`)
+	t2 := t1.New("test")
+
+	if t1.common != t2.common {
+		t.Errorf("t1 & t2 didn't share common struct; got %v != %v", t1.common, t2.common)
+	}
+	if t1.Tree == nil {
+		t.Error("defined template got nil Tree")
+	}
+	if t2.Tree != nil {
+		t.Error("undefined template got non-nil Tree")
+	}
+
+	containsT1 := false
+	for _, tmpl := range t1.Templates() {
+		if tmpl == t2 {
+			t.Error("Templates included undefined template")
+		}
+		if tmpl == t1 {
+			containsT1 = true
+		}
+	}
+	if !containsT1 {
+		t.Error("Templates didn't include defined template")
+	}
+}
+
+func TestParse(t *testing.T) {
+	// In multiple calls to Parse with the same receiver template, only one call
+	// can contain text other than space, comments, and template definitions
+	var err error
+	t1 := New("test")
+	if _, err := t1.Parse(`{{define "test"}}{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+	if _, err := t1.Parse(`{{define "test"}}{{/* this is a comment */}}{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+	if _, err := t1.Parse(`{{define "test"}}foo{{end}}`); err != nil {
+		t.Fatalf("parsing test: %s", err)
+	}
+	if _, err = t1.Parse(`{{define "test"}}foo{{end}}`); err == nil {
+		t.Fatal("no error from redefining a template")
+	}
+	if !strings.Contains(err.Error(), "redefinition") {
+		t.Fatalf("expected redefinition error; got %v", err)
+	}
+}
diff --git a/src/text/template/template.go b/src/text/template/template.go
index f435cb6cca..de03a2e8a1 100644
--- a/src/text/template/template.go
+++ b/src/text/template/template.go
@@ -81,9 +81,9 @@ func (t *Template) init() {
 func (t *Template) Clone() (*Template, error) {
 	nt := t.copy(nil)
 	nt.init()
-	nt.tmpl[t.name] = nt
 	for k, v := range t.tmpl {
-		if k == t.name { // Already installed.
+		if k == t.name {
+			nt.tmpl[t.name] = nt
 			continue
 		}
 		// The associated templates share nt's common structure.
@@ -111,20 +111,26 @@ func (t *Template) copy(c *common) *Template {
 	return nt
 }
 
-// AddParseTree creates a new template with the name and parse tree
-// and associates it with t.
+// AddParseTree adds parse tree for template with given name and associates it with t.
+// If the template does not already exist, it will create a new one.
+// It is an error to reuse a name except to overwrite an empty template.
 func (t *Template) AddParseTree(name string, tree *parse.Tree) (*Template, error) {
-	if t.tmpl[name] != nil {
-		return nil, fmt.Errorf("template: redefinition of template %q", name)
+	// If the name is the name of this template, overwrite this template.
+	// The associate method checks it's not a redefinition.
+	nt := t
+	if name != t.name {
+		nt = t.New(name)
+	}
+	// 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 {
+		nt.Tree = tree
 	}
-	nt := t.New(name)
-	nt.Tree = tree
-	t.tmpl[name] = nt
 	return nt, nil
 }
 
-// Templates returns a slice of the templates associated with t, including t
-// itself.
+// Templates returns a slice of defined templates associated with t.
 func (t *Template) Templates() []*Template {
 	// Return a slice so we don't expose the map.
 	m := make([]*Template, 0, len(t.tmpl))
@@ -179,20 +185,9 @@ func (t *Template) Parse(text string) (*Template, error) {
 	}
 	// Add the newly parsed trees, including the one for t, into our common structure.
 	for name, tree := range trees {
-		// If the name we parsed is the name of this template, overwrite this template.
-		// The associate method checks it's not a redefinition.
-		tmpl := t
-		if name != t.name {
-			tmpl = t.New(name)
-		}
-		// Even if t == tmpl, we need to install it in the common.tmpl map.
-		if replace, err := t.associate(tmpl, tree); err != nil {
+		if _, err := t.AddParseTree(name, tree); err != nil {
 			return nil, err
-		} else if replace {
-			tmpl.Tree = tree
 		}
-		tmpl.leftDelim = t.leftDelim
-		tmpl.rightDelim = t.rightDelim
 	}
 	return t, nil
 }