Add render template hooks for links and images

This commit also

* revises the change detection for templates used by content files in server mode.
* Adds a Page.RenderString method

Fixes #6545
Fixes #4663
Closes #6043

markup/converter/converter.go

@@ -16,6 +16,8 @@ package converter
 import (
 	"github.com/gohugoio/hugo/common/loggers"
 	"github.com/gohugoio/hugo/config"
+	"github.com/gohugoio/hugo/identity"
+	"github.com/gohugoio/hugo/markup/converter/hooks"
 	"github.com/gohugoio/hugo/markup/markup_config"
 	"github.com/gohugoio/hugo/markup/tableofcontents"
 	"github.com/spf13/afero"
@@ -67,6 +69,7 @@ func (n newConverter) Name() string {
 // another format, e.g. Markdown to HTML.
 type Converter interface {
 	Convert(ctx RenderContext) (Result, error)
+	Supports(feature identity.Identity) bool
 }
 
 // Result represents the minimum returned from Convert.
@@ -94,6 +97,7 @@ func (b Bytes) Bytes() []byte {
 
 // DocumentContext holds contextual information about the document to convert.
 type DocumentContext struct {
+	Document        interface{} // May be nil. Usually a page.Page
 	DocumentID      string
 	DocumentName    string
 	ConfigOverrides map[string]interface{}
@@ -101,6 +105,11 @@ type DocumentContext struct {
 
 // RenderContext holds contextual information about the content to render.
 type RenderContext struct {
-	Src       []byte
-	RenderTOC bool
+	Src         []byte
+	RenderTOC   bool
+	RenderHooks *hooks.Render
 }
+
+var (
+	FeatureRenderHooks = identity.NewPathIdentity("markup", "renderingHooks")
+)

markup/converter/hooks/hooks.go

@@ -0,0 +1,57 @@
+// 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 hooks
+
+import (
+	"io"
+
+	"github.com/gohugoio/hugo/identity"
+)
+
+type LinkContext interface {
+	Page() interface{}
+	Destination() string
+	Title() string
+	Text() string
+}
+
+type Render struct {
+	LinkRenderer  LinkRenderer
+	ImageRenderer LinkRenderer
+}
+
+func (r *Render) Eq(other interface{}) bool {
+	ro, ok := other.(*Render)
+	if !ok {
+		return false
+	}
+	if r == nil || ro == nil {
+		return r == nil
+	}
+
+	if r.ImageRenderer.GetIdentity() != ro.ImageRenderer.GetIdentity() {
+		return false
+	}
+
+	if r.LinkRenderer.GetIdentity() != ro.LinkRenderer.GetIdentity() {
+		return false
+	}
+
+	return true
+}
+
+type LinkRenderer interface {
+	Render(w io.Writer, ctx LinkContext) error
+	identity.Provider
+}