tpl: Rework to handle both text and HTML templates

Before this commit, Hugo used `html/template` for all Go templates.

While this is a fine choice for HTML and maybe also RSS feeds, it is painful for plain text formats such as CSV, JSON etc.

This commit fixes that by using the `IsPlainText` attribute on the output format to decide what to use.

A couple of notes:

* The above requires a nonambiguous template name to type mapping. I.e. `/layouts/_default/list.json` will only work if there is only one JSON output format, `/layouts/_default/list.mytype.json` will always work.
* Ambiguous types will fall back to HTML.
* Partials inherits the text vs HTML identificator of the container template. This also means that plain text templates can only include plain text partials.
* Shortcode templates are, by definition, currently HTML templates only.

Fixes #3221

tpl/template.go

@@ -1,28 +1,103 @@
+// Copyright 2017-present The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
 package tpl
 
 import (
-	"html/template"
 	"io"
+
+	"text/template/parse"
+
+	"html/template"
+	texttemplate "text/template"
+
+	bp "github.com/spf13/hugo/bufferpool"
 )
 
-// TODO(bep) make smaller
-type Template interface {
-	ExecuteTemplate(wr io.Writer, name string, data interface{}) error
-	ExecuteTemplateToHTML(context interface{}, layouts ...string) template.HTML
-	Lookup(name string) *template.Template
-	Templates() []*template.Template
-	New(name string) *template.Template
-	GetClone() *template.Template
-	RebuildClone() *template.Template
-	LoadTemplates(absPath string)
-	LoadTemplatesWithPrefix(absPath, prefix string)
+var (
+	_ TemplateExecutor = (*TemplateAdapter)(nil)
+)
+
+// TemplateHandler manages the collection of templates.
+type TemplateHandler interface {
+	TemplateFinder
 	AddTemplate(name, tpl string) error
-	AddTemplateFileWithMaster(name, overlayFilename, masterFilename string) error
-	AddAceTemplate(name, basePath, innerPath string, baseContent, innerContent []byte) error
-	AddInternalTemplate(prefix, name, tpl string) error
-	AddInternalShortcode(name, tpl string) error
-	Partial(name string, contextList ...interface{}) template.HTML
+	AddLateTemplate(name, tpl string) error
+	LoadTemplates(absPath, prefix string)
 	PrintErrors()
-	Funcs(funcMap template.FuncMap)
+
 	MarkReady()
+	RebuildClone()
+}
+
+// TemplateFinder finds templates.
+type TemplateFinder interface {
+	Lookup(name string) *TemplateAdapter
+}
+
+// Template is the common interface between text/template and html/template.
+type Template interface {
+	Execute(wr io.Writer, data interface{}) error
+	Name() string
+}
+
+// TemplateExecutor adds some extras to Template.
+type TemplateExecutor interface {
+	Template
+	ExecuteToString(data interface{}) (string, error)
+	Tree() string
+}
+
+// TemplateAdapter implements the TemplateExecutor interface.
+type TemplateAdapter struct {
+	Template
+}
+
+// ExecuteToString executes the current template and returns the result as a
+// string.
+func (t *TemplateAdapter) ExecuteToString(data interface{}) (string, error) {
+	b := bp.GetBuffer()
+	defer bp.PutBuffer(b)
+	if err := t.Execute(b, data); err != nil {
+		return "", err
+	}
+	return b.String(), nil
+}
+
+// Tree returns the template Parse tree as a string.
+// Note: this isn't safe for parallel execution on the same template
+// vs Lookup and Execute.
+func (t *TemplateAdapter) Tree() string {
+	var tree *parse.Tree
+	switch tt := t.Template.(type) {
+	case *template.Template:
+		tree = tt.Tree
+	case *texttemplate.Template:
+		tree = tt.Tree
+	default:
+		panic("Unknown template")
+	}
+
+	if tree.Root == nil {
+		return ""
+	}
+	s := tree.Root.String()
+
+	return s
+}
+
+// TemplateTestMocker adds a way to override some template funcs during tests.
+// The interface is named so it's not used in regular application code.
+type TemplateTestMocker interface {
+	SetFuncs(funcMap map[string]interface{})
 }