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, 24 insertions, 4 deletions

diff --git a/output/layout.go b/output/layout.go
index a2bfd7717..6dba7f3b4 100644
--- a/output/layout.go
+++ b/output/layout.go
@@ -152,9 +152,11 @@ func (l *LayoutHandler) For(d LayoutDescriptor, layoutOverride string, f Format)
 			}
 		}
 
-		return layoutsWithThemeLayouts, nil
+		layouts = layoutsWithThemeLayouts
 	}
 
+	layouts = prependTextPrefixIfNeeded(f, layouts...)
+
 	l.mu.Lock()
 	l.cache[key] = layouts
 	l.mu.Unlock()
@@ -184,10 +186,26 @@ func resolveListTemplate(d LayoutDescriptor, f Format,
 }
 
 func resolveTemplate(templ string, d LayoutDescriptor, f Format) []string {
-	return strings.Fields(replaceKeyValues(templ,
+	layouts := strings.Fields(replaceKeyValues(templ,
 		"SUFFIX", f.MediaType.Suffix,
 		"NAME", strings.ToLower(f.Name),
 		"SECTION", d.Section))
+
+	return layouts
+}
+
+func prependTextPrefixIfNeeded(f Format, layouts ...string) []string {
+	if !f.IsPlainText {
+		return layouts
+	}
+
+	newLayouts := make([]string, len(layouts))
+
+	for i, l := range layouts {
+		newLayouts[i] = "_text/" + l
+	}
+
+	return newLayouts
 }
 
 func replaceKeyValues(s string, oldNew ...string) string {
@@ -195,7 +213,9 @@ func replaceKeyValues(s string, oldNew ...string) string {
 	return replacer.Replace(s)
 }
 
-func regularPageLayouts(types string, layout string, f Format) (layouts []string) {
+func regularPageLayouts(types string, layout string, f Format) []string {
+	var layouts []string
+
 	if layout == "" {
 		layout = "single"
 	}
@@ -219,5 +239,5 @@ func regularPageLayouts(types string, layout string, f Format) (layouts []string
 	layouts = append(layouts, fmt.Sprintf("_default/%s.%s.%s", layout, name, suffix))
 	layouts = append(layouts, fmt.Sprintf("_default/%s.%s", layout, suffix))
 
-	return
+	return layouts
 }