Add HTML.Noopener to add a noopener rel to every external link

This has performance benefits https://jakearchibald.com/2016/performance-benefits-of-rel-noopener/ but most importantly also security benefits https://mathiasbynens.github.io/rel-noopener/

Adresses https://github.com/ezyang/htmlpurifier/issues/96

library/HTMLPurifier.includes.php

@@ -132,6 +132,7 @@ require 'HTMLPurifier/AttrTransform/Length.php';
 require 'HTMLPurifier/AttrTransform/Name.php';
 require 'HTMLPurifier/AttrTransform/NameSync.php';
 require 'HTMLPurifier/AttrTransform/Nofollow.php';
+require 'HTMLPurifier/AttrTransform/Noopener.php';
 require 'HTMLPurifier/AttrTransform/SafeEmbed.php';
 require 'HTMLPurifier/AttrTransform/SafeObject.php';
 require 'HTMLPurifier/AttrTransform/SafeParam.php';
@@ -163,6 +164,7 @@ require 'HTMLPurifier/HTMLModule/Legacy.php';
 require 'HTMLPurifier/HTMLModule/List.php';
 require 'HTMLPurifier/HTMLModule/Name.php';
 require 'HTMLPurifier/HTMLModule/Nofollow.php';
+require 'HTMLPurifier/HTMLModule/Noopener.php';
 require 'HTMLPurifier/HTMLModule/NonXMLCommonAttributes.php';
 require 'HTMLPurifier/HTMLModule/Object.php';
 require 'HTMLPurifier/HTMLModule/Presentation.php';

library/HTMLPurifier.safe-includes.php

@@ -126,6 +126,7 @@ require_once $__dir . '/HTMLPurifier/AttrTransform/Length.php';
 require_once $__dir . '/HTMLPurifier/AttrTransform/Name.php';
 require_once $__dir . '/HTMLPurifier/AttrTransform/NameSync.php';
 require_once $__dir . '/HTMLPurifier/AttrTransform/Nofollow.php';
+require_once $__dir . '/HTMLPurifier/AttrTransform/Noopener.php';
 require_once $__dir . '/HTMLPurifier/AttrTransform/SafeEmbed.php';
 require_once $__dir . '/HTMLPurifier/AttrTransform/SafeObject.php';
 require_once $__dir . '/HTMLPurifier/AttrTransform/SafeParam.php';
@@ -157,6 +158,7 @@ require_once $__dir . '/HTMLPurifier/HTMLModule/Legacy.php';
 require_once $__dir . '/HTMLPurifier/HTMLModule/List.php';
 require_once $__dir . '/HTMLPurifier/HTMLModule/Name.php';
 require_once $__dir . '/HTMLPurifier/HTMLModule/Nofollow.php';
+require_once $__dir . '/HTMLPurifier/HTMLModule/Noopener.php';
 require_once $__dir . '/HTMLPurifier/HTMLModule/NonXMLCommonAttributes.php';
 require_once $__dir . '/HTMLPurifier/HTMLModule/Object.php';
 require_once $__dir . '/HTMLPurifier/HTMLModule/Presentation.php';

library/HTMLPurifier/AttrTransform/Noopener.php

@@ -0,0 +1,52 @@
+<?php
+
+// must be called POST validation
+
+/**
+ * Adds rel="noopener" to all outbound links.  This transform is
+ * only attached if Attr.Noopener is TRUE.
+ */
+class HTMLPurifier_AttrTransform_Noopener extends HTMLPurifier_AttrTransform
+{
+    /**
+     * @type HTMLPurifier_URIParser
+     */
+    private $parser;
+
+    public function __construct()
+    {
+        $this->parser = new HTMLPurifier_URIParser();
+    }
+
+    /**
+     * @param array $attr
+     * @param HTMLPurifier_Config $config
+     * @param HTMLPurifier_Context $context
+     * @return array
+     */
+    public function transform($attr, $config, $context)
+    {
+        if (!isset($attr['href'])) {
+            return $attr;
+        }
+
+        // XXX Kind of inefficient
+        $url = $this->parser->parse($attr['href']);
+        $scheme = $url->getSchemeObj($config, $context);
+
+        if ($scheme->browsable && !$url->isLocal($config, $context)) {
+            if (isset($attr['rel'])) {
+                $rels = explode(' ', $attr['rel']);
+                if (!in_array('noopener', $rels)) {
+                    $rels[] = 'noopener';
+                }
+                $attr['rel'] = implode(' ', $rels);
+            } else {
+                $attr['rel'] = 'noopener';
+            }
+        }
+        return $attr;
+    }
+}
+
+// vim: et sw=4 sts=4

library/HTMLPurifier/ConfigSchema/schema/HTML.Noopener.txt

@@ -0,0 +1,7 @@
+HTML.Noopener
+TYPE: bool
+VERSION: 4.9.0
+DEFAULT: FALSE
+--DESCRIPTION--
+If enabled, noopener rel attributes are added to all outgoing links.
+--# vim: et sw=4 sts=4

library/HTMLPurifier/HTMLModule/Noopener.php

@@ -0,0 +1,25 @@
+<?php
+
+/**
+ * Module adds the noopener attribute transformation to a tags.  It
+ * is enabled by HTML.Noopener
+ */
+class HTMLPurifier_HTMLModule_Noopener extends HTMLPurifier_HTMLModule
+{
+
+    /**
+     * @type string
+     */
+    public $name = 'Noopener';
+
+    /**
+     * @param HTMLPurifier_Config $config
+     */
+    public function setup($config)
+    {
+        $a = $this->addBlankElement('a');
+        $a->attr_transform_post[] = new HTMLPurifier_AttrTransform_Noopener();
+    }
+}
+
+// vim: et sw=4 sts=4

library/HTMLPurifier/HTMLModuleManager.php

@@ -268,6 +268,9 @@ class HTMLPurifier_HTMLModuleManager
         if ($config->get('HTML.Nofollow')) {
             $modules[] = 'Nofollow';
         }
+        if ($config->get('HTML.Noopener')) {
+            $modules[] = 'Noopener';
+        }
         if ($config->get('HTML.TargetBlank')) {
             $modules[] = 'TargetBlank';
         }