Support canonical links for section chunks

By adding a `canonical-url` attribute to a section, the chunked HTML
page for that section will contain a `<link rel="canonical" ...>` tag in
the page header. This tag does not get included in later chunks of the
same input document.

Fixes #2357

Author: gtback

resources/asciidoctor/lib/chunker/extension.rb

@@ -132,6 +132,7 @@ def subdoc_attrs(doc, section)
       attrs['subdoc'] = true # Mark the subdoc so we don't try and chunk it
       attrs['noheader'] = true
       attrs['title-separator'] = ''
+      attrs['canonical-url'] = section.attributes['canonical-url']
       attrs.merge! find_related(section)
       attrs
     end

resources/asciidoctor/lib/chunker/extra_docinfo.rb

@@ -24,17 +24,22 @@ def extra_chunker_head
         link_rel('up', attributes['up_section']),
         link_rel('prev', attributes['prev_section']),
         link_rel('next', attributes['next_section']),
+        link_rel('canonical', attributes['canonical-url'])
       ].compact.join "\n"
     end
 
     def link_rel(rel, related)
       return unless related
 
-      extra = related.context == :document ? related.attr('title-extra') : ''
-      title = "#{strip_tags(link_text(related))}#{extra}"
-      # We're in an attribute so escape quotes too!
-      title = title.gsub '"', '"'
-      %(<link rel="#{rel}" #{link_href related} title="#{title}"/>)
+      if related.is_a?(String)
+        %(<link rel="#{rel}" href="#{related}"/>)
+      else
+        extra = related.context == :document ? related.attr('title-extra') : ''
+        title = "#{strip_tags(link_text(related))}#{extra}"
+        # We're in an attribute so escape quotes too!
+        title = title.gsub '"', '&quot;'
+        %(<link rel="#{rel}" #{link_href related} title="#{title}"/>)
+      end
     end
   end
 end

resources/asciidoctor/spec/chunker_spec.rb

@@ -529,6 +529,38 @@ def dest_file(file)
           end
         end
       end
+      context 'the section has a canonical link' do
+        let(:input) do
+          <<~ASCIIDOC
+            = Title
+
+            [id="otd",canonical-url="bazbar"]
+            == Outdated
+
+            [[current]]
+            == Current
+
+            Words.
+          ASCIIDOC
+        end
+        file_context 'first subpage', 'otd.html' do
+          let(:next_title) { "Current" }
+          include_examples 'standard page', 'index', 'current'
+          it 'contains a <link rel="canonical" ...> header tag' do
+            expect(contents).to include <<~HTML
+              <link rel="canonical" href="bazbar"/>
+            HTML
+          end
+        end
+        file_context 'second subpage', 'current.html' do
+          let(:prev_title) { "Outdated" }
+          include_examples 'standard page', 'otd', nil
+          it 'does not contains a canonical header tag' do
+            expect(contents).not_to include "canonical"
+            expect(contents).not_to include "bazbar"
+          end
+        end
+      end
     end
     context 'when chunk level is 2' do
       let(:convert_attributes) do