Respect that dirhtml builder does not have .html in canonical URLs

Author: benjaoming

sphinx_rtd_theme/layout.html

@@ -51,12 +51,22 @@
 
   {#- CANONICAL URL (deprecated) #}
   {%- if theme_canonical_url and not pageurl %}
-    <link rel="canonical" href="{{ theme_canonical_url }}{{ pagename }}.html"/>
+    <link rel="canonical" href="{{ theme_canonical_url }}{{ pagename }}{% if builder == 'dirhtml' %}/{% else %}.html{% endif %}"/>
   {%- endif -%}
 
   {#- CANONICAL URL #}
+  {# NB! pageurl is currently a non-documented template context variable! #}
+  {# pageurl implementation: https://www.sphinx-doc.org/en/master/_modules/sphinx/builders/html.html #}
   {%- if pageurl %}
-    <link rel="canonical" href="{{ pageurl|e }}" />
+    {# pageurl implementation wrongly: adds .html for the dirhtml builder's pageurl #}
+    {# Workaround for: https://github.com/sphinx-doc/sphinx/issues/9730 #}
+    {# Once a fix is released in Sphinx, put an upper bound on the Sphinx version for the workaround #}
+    {% if builder == 'dirhtml' and pageurl.endswith('.html') %}
+      {# This expression trims away .html and adds a / #}
+      <link rel="canonical" href="{{ (pageurl|e)[:-5] }}/" />
+    {% else %}
+      <link rel="canonical" href="{{ pageurl|e }}" />
+    {% endif %}
   {%- endif -%}
 
   {#- JAVASCRIPTS #}