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

1 files changed, 18 insertions, 1 deletions

diff --git a/output/layout_base.go b/output/layout_base.go
index 2bb89c20d..a0d2bc4eb 100644
--- a/output/layout_base.go
+++ b/output/layout_base.go
@@ -29,7 +29,10 @@ var (
 )
 
 type TemplateNames struct {
-	Name            string
+	// The name used as key in the template map. Note that this will be
+	// prefixed with "_text/" if it should be parsed with text/template.
+	Name string
+
 	OverlayFilename string
 	MasterFilename  string
 }
@@ -51,6 +54,10 @@ type TemplateLookupDescriptor struct {
 	// The theme name if active.
 	Theme string
 
+	// All the output formats in play. This is used to decide if text/template or
+	// html/template.
+	OutputFormats Formats
+
 	FileExists  func(filename string) (bool, error)
 	ContainsAny func(filename string, subslices [][]byte) (bool, error)
 }
@@ -74,6 +81,12 @@ func CreateTemplateNames(d TemplateLookupDescriptor) (TemplateNames, error) {
 	// index.amp.html
 	// index.json
 	filename := filepath.Base(d.RelPath)
+	isPlainText := false
+	outputFormat, found := d.OutputFormats.FromFilename(filename)
+
+	if found && outputFormat.IsPlainText {
+		isPlainText = true
+	}
 
 	var ext, outFormat string
 
@@ -90,6 +103,10 @@ func CreateTemplateNames(d TemplateLookupDescriptor) (TemplateNames, error) {
 	id.OverlayFilename = fullPath
 	id.Name = name
 
+	if isPlainText {
+		id.Name = "_text/" + id.Name
+	}
+
 	// Ace and Go templates may have both a base and inner template.
 	pathDir := filepath.Dir(fullPath)