Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

1 files changed, 56 insertions, 0 deletions

diff --git a/hugolib/shortcode_page.go b/hugolib/shortcode_page.go
new file mode 100644
index 000000000..e8a3a37e1
--- /dev/null
+++ b/hugolib/shortcode_page.go
@@ -0,0 +1,56 @@
+// Copyright 2019 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 hugolib
+
+import (
+	"html/template"
+
+	"github.com/gohugoio/hugo/resources/page"
+)
+
+var tocShortcodePlaceholder = createShortcodePlaceholder("TOC", 0)
+
+// This is sent to the shortcodes. They cannot access the content
+// they're a part of. It would cause an infinite regress.
+//
+// Go doesn't support virtual methods, so this careful dance is currently (I think)
+// the best we can do.
+type pageForShortcode struct {
+	page.PageWithoutContent
+	page.ContentProvider
+
+	// We need to replace it after we have rendered it, so provide a
+	// temporary placeholder.
+	toc template.HTML
+
+	p *pageState
+}
+
+func newPageForShortcode(p *pageState) page.Page {
+	return &pageForShortcode{
+		PageWithoutContent: p,
+		ContentProvider:    page.NopPage,
+		toc:                template.HTML(tocShortcodePlaceholder),
+		p:                  p,
+	}
+}
+
+func (p *pageForShortcode) page() page.Page {
+	return p.PageWithoutContent.(page.Page)
+}
+
+func (p *pageForShortcode) TableOfContents() template.HTML {
+	p.p.enablePlaceholders()
+	return p.toc
+}