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, 22 insertions, 2 deletions

diff --git a/hugolib/page_output.go b/hugolib/page_output.go
index f47343cb5..58d09d688 100644
--- a/hugolib/page_output.go
+++ b/hugolib/page_output.go
@@ -110,9 +110,29 @@ func (p *PageOutput) Render(layout ...string) template.HTML {
 	l, err := p.layouts(layout...)
 	if err != nil {
 		helpers.DistinctErrorLog.Printf("in .Render: Failed to resolve layout %q for page %q", layout, p.pathOrTitle())
-		return template.HTML("")
+		return ""
 	}
-	return p.s.Tmpl.ExecuteTemplateToHTML(p, l...)
+
+	for _, layout := range l {
+		templ := p.s.Tmpl.Lookup(layout)
+		if templ == nil {
+			// This is legacy from when we had only one output format and
+			// HTML templates only. Some have references to layouts without suffix.
+			// We default to good old HTML.
+			templ = p.s.Tmpl.Lookup(layout + ".html")
+		}
+		if templ != nil {
+			res, err := templ.ExecuteToString(p)
+			if err != nil {
+				helpers.DistinctErrorLog.Printf("in .Render: Failed to execute template %q for page %q", layout, p.pathOrTitle())
+				return template.HTML("")
+			}
+			return template.HTML(res)
+		}
+	}
+
+	return ""
+
 }
 
 func (p *Page) Render(layout ...string) template.HTML {