Fix serious performance bug with rendering Markdown with InlineDiffFilter

CHANGELOG

@@ -30,6 +30,7 @@ v 8.9.0 (unreleased)
 v 8.8.3
   - Fix incorrect links on pipeline page when merge request created from fork
   - Fix gitlab importer failing to import new projects due to missing credentials
+  - Fix serious performance bug with rendering Markdown with InlineDiffFilter
   - Fix import URL migration not rescuing with the correct Error
   - In search results, only show notes on confidential issues that the user has access to
   - Fix health check access token changing due to old application settings being used

lib/banzai/filter/inline_diff_filter.rb

@@ -8,15 +8,19 @@ module Banzai
           next if has_ancestor?(node, IGNORED_ANCESTOR_TAGS)
 
           content = node.to_html
-          content = content.gsub(/(?:\[\-(.*?)\-\]|\{\-(.*?)\-\})/, '<span class="idiff left right deletion">\1\2</span>')
-          content = content.gsub(/(?:\[\+(.*?)\+\]|\{\+(.*?)\+\})/, '<span class="idiff left right addition">\1\2</span>')
+          html_content = inline_diff_filter(content)
 
-          next if html == content
+          next if content == html_content
 
-          node.replace(content)
+          node.replace(html_content)
         end
         doc
       end
+
+      def inline_diff_filter(text)
+        html_content = text.gsub(/(?:\[\-(.*?)\-\]|\{\-(.*?)\-\})/, '<span class="idiff left right deletion">\1\2</span>')
+        html_content.gsub(/(?:\[\+(.*?)\+\]|\{\+(.*?)\+\})/, '<span class="idiff left right addition">\1\2</span>')
+      end
     end
   end
 end