templates: Fix base template block customization

Previously, kiln would error out if any blocks defined in the base
template were not defined in page or index templates. Instead, make
customizing base template blocks optional. This also allows base
template blocks to have a default value.

templates.go

@@ -9,50 +9,69 @@ import (
 	pathpkg "path"
 	"strings"
 	"text/template"
-	"text/template/parse"
 )
 
 // Template represents a template.
 type Template interface {
-	AddParseTree(*parse.Tree) error
+	Clone() (Template, error)
+	AddTemplates(Template) error
 	Execute(io.Writer, interface{}) error
-	Tree() *parse.Tree
 }
 
 type textTemplate struct {
 	tmpl *template.Template
 }
 
-func (t textTemplate) AddParseTree(tree *parse.Tree) error {
-	_, err := t.tmpl.AddParseTree(t.tmpl.Name(), tree)
+func (t textTemplate) Clone() (Template, error) {
+	clone, err := t.tmpl.Clone()
+	return textTemplate{clone}, err
+}
+
+func (t textTemplate) AddTemplates(other Template) error {
+	otherTmpl := other.(textTemplate).tmpl
+	for _, def := range otherTmpl.Templates() {
+		if def.Name() == otherTmpl.Name() {
+			continue
+		}
+		_, err := t.tmpl.AddParseTree(def.Name(), def.Tree)
+		if err != nil {
 			return err
+		}
+	}
+	return nil
 }
 
 func (t textTemplate) Execute(w io.Writer, data interface{}) error {
 	return t.tmpl.Execute(w, data)
 }
 
-func (t textTemplate) Tree() *parse.Tree {
-	return t.tmpl.Tree
-}
-
 type htmlTemplate struct {
 	tmpl *htemplate.Template
 }
 
-func (t htmlTemplate) AddParseTree(tree *parse.Tree) error {
-	_, err := t.tmpl.AddParseTree(t.tmpl.Name(), tree)
+func (t htmlTemplate) Clone() (Template, error) {
+	clone, err := t.tmpl.Clone()
+	return htmlTemplate{clone}, err
+}
+
+func (t htmlTemplate) AddTemplates(other Template) error {
+	otherTmpl := other.(htmlTemplate).tmpl
+	for _, def := range otherTmpl.Templates() {
+		if def.Name() == otherTmpl.Name() {
+			continue
+		}
+		_, err := t.tmpl.AddParseTree(def.Name(), def.Tree)
+		if err != nil {
 			return err
+		}
+	}
+	return nil
 }
 
 func (t htmlTemplate) Execute(w io.Writer, data interface{}) error {
 	return t.tmpl.Execute(w, data)
 }
 
-func (t htmlTemplate) Tree() *parse.Tree {
-	return t.tmpl.Tree
-}
-
 // Templates contains site templates.
 type Templates struct {
 	tmpls map[string]Template
@@ -129,12 +148,20 @@ func (t *Templates) Load(dir string, exts []string) error {
 		if _, ok := extsMap[ext]; !ok {
 			continue
 		}
-		base := pathpkg.Join(pathpkg.Dir(path), "base"+ext)
-		if tmpl, ok := t.tmpls[base]; ok {
-			err := t.tmpls[path].AddParseTree(tmpl.Tree())
+		basePath := pathpkg.Join(pathpkg.Dir(path), "base"+ext)
+		if path == basePath {
+			continue
+		}
+		if base, ok := t.tmpls[basePath]; ok {
+			tmpl, err := base.Clone()
 			if err != nil {
 				return err
 			}
+			// Load customized template definitions
+			if err := tmpl.AddTemplates(t.tmpls[path]); err != nil {
+				return err
+			}
+			t.tmpls[path] = tmpl
 		}
 	}
 	return nil