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, 50 insertions, 0 deletions

diff --git a/hugolib/page_unwrap.go b/hugolib/page_unwrap.go
new file mode 100644
index 000000000..eda6636d1
--- /dev/null
+++ b/hugolib/page_unwrap.go
@@ -0,0 +1,50 @@
+// 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 (
+	"github.com/pkg/errors"
+
+	"github.com/gohugoio/hugo/resources/page"
+)
+
+// Wraps a Page.
+type pageWrapper interface {
+	page() page.Page
+}
+
+// unwrapPage is used in equality checks and similar.
+func unwrapPage(in interface{}) (page.Page, error) {
+	switch v := in.(type) {
+	case *pageState:
+		return v, nil
+	case pageWrapper:
+		return v.page(), nil
+	case page.Page:
+		return v, nil
+	case nil:
+		return nil, nil
+	default:
+		return nil, errors.Errorf("unwrapPage: %T not supported", in)
+	}
+}
+
+func mustUnwrapPage(in interface{}) page.Page {
+	p, err := unwrapPage(in)
+	if err != nil {
+		panic(err)
+	}
+
+	return p
+}