Fix the leak mentioned in 504a3b5e by another way

The previous fix introduced another leak; as it made
Banzai::Filter::SanitizationFiler#customized? always return false, so we
were always appending two elements to
HTML::Pipeline::SanitizationFilter::WHITELIST[:elements]. This growth in
the elements array would slow the sanitization process over time.

CHANGELOG

@@ -4,6 +4,7 @@ v 8.13.0 (unreleased)
   - Speed-up group milestones show page
 v 8.12.1 (unreleased)
+  - Fix a memory leak in HTML::Pipeline::SanitizationFilter::WHITELIST
 v 8.12.0
   - Update the rouge gem to 2.0.6, which adds highlighting support for JSX, Prometheus, and others. !6251

lib/banzai/filter/sanitization_filter.rb

@@ -43,55 +43,57 @@ module Banzai
         whitelist[:protocols].delete('a')
         # ...but then remove links with unsafe protocols
-        whitelist[:transformers].push(remove_unsafe_links)
+        whitelist[:transformers].push(self.class.remove_unsafe_links)
         # Remove `rel` attribute from `a` elements
-        whitelist[:transformers].push(remove_rel)
+        whitelist[:transformers].push(self.class.remove_rel)
         # Remove `class` attribute from non-highlight spans
-        whitelist[:transformers].push(clean_spans)
+        whitelist[:transformers].push(self.class.clean_spans)
         whitelist
       end
-      def remove_unsafe_links
-        lambda do |env|
-          node = env[:node]
-          return unless node.name == 'a'
-          return unless node.has_attribute?('href')
-          begin
-            uri = Addressable::URI.parse(node['href'])
-            uri.scheme = uri.scheme.strip.downcase if uri.scheme
-            node.remove_attribute('href') if UNSAFE_PROTOCOLS.include?(uri.scheme)
-          rescue Addressable::URI::InvalidURIError
-            node.remove_attribute('href')
+      class << self
+        def remove_unsafe_links
+          lambda do |env|
+            node = env[:node]
+            return unless node.name == 'a'
+            return unless node.has_attribute?('href')
+            begin
+              uri = Addressable::URI.parse(node['href'])
+              uri.scheme = uri.scheme.strip.downcase if uri.scheme
+              node.remove_attribute('href') if UNSAFE_PROTOCOLS.include?(uri.scheme)
+            rescue Addressable::URI::InvalidURIError
+              node.remove_attribute('href')
+            end
           end
         end
-      end
-      def remove_rel
-        lambda do |env|
-          if env[:node_name] == 'a'
-            env[:node].remove_attribute('rel')
+        def remove_rel
+          lambda do |env|
+            if env[:node_name] == 'a'
+              env[:node].remove_attribute('rel')
+            end
           end
         end
-      end
-      def clean_spans
-        lambda do |env|
-          node = env[:node]
-          return unless node.name == 'span'
-          return unless node.has_attribute?('class')
-          unless has_ancestor?(node, 'pre')
-            node.remove_attribute('class')
-          end
-          { node_whitelist: [node] }
+        def clean_spans
+          lambda do |env|
+            node = env[:node]
+            return unless node.name == 'span'
+            return unless node.has_attribute?('class')
+            unless node.ancestors.any? { |n| n.name.casecmp('pre').zero? }
+              node.remove_attribute('class')
+            end
+            { node_whitelist: [node] }
+          end
         end
       end
     end