Merge branch 'syntax-highlight-filter-performance' into 'master'

Improve performance of SyntaxHighlightFilter

## What does this MR do?

This MR improves the performance of `Banzai::Filter::SyntaxHighlightFilter`. See e9bacc6575d0002c6cab620075dea3dc7f93f100 for more information.

## Are there points in the code the reviewer needs to double check?

Styling mostly.

## Why was this MR needed?

Syntax highlighting is rather slow.

## What are the relevant issue numbers?

#18592 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] ~~[Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
- [x] ~~API support added~~
- Tests
  - [x] ~~Added for this feature/bug~~
  - [ ] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5643

CHANGELOG

@@ -22,6 +22,7 @@ v 8.11.0 (unreleased)
   - Limit git rev-list output count to one in forced push check
   - Clean up unused routes (Josef Strzibny)
   - Add green outline to New Branch button. !5447 (winniehell)
+  - Improve performance of syntax highlighting Markdown code blocks
   - Update to gitlab_git 10.4.1 and take advantage of preserved Ref objects
   - Remove delay when hitting "Reply..." button on page with a lot of discussions
   - Retrieve rendered HTML from cache in one request

lib/banzai/filter/syntax_highlight_filter.rb

@@ -18,14 +18,11 @@ module Banzai
       def highlight_node(node)
         language = node.attr('class')
         code = node.text
-
         css_classes = "code highlight"
-
-        lexer = Rouge::Lexer.find_fancy(language) || Rouge::Lexers::PlainText
-        formatter = Rouge::Formatters::HTML.new
+        lexer = lexer_for(language)
 
         begin
-          code = formatter.format(lexer.lex(code))
+          code = format(lex(lexer, code))
 
           css_classes << " js-syntax-highlight #{lexer.tag}"
         rescue
@@ -41,14 +38,27 @@ module Banzai
 
       private
 
+      # Separate method so it can be instrumented.
+      def lex(lexer, code)
+        lexer.lex(code)
+      end
+
+      def format(tokens)
+        rouge_formatter.format(tokens)
+      end
+
+      def lexer_for(language)
+        (Rouge::Lexer.find(language) || Rouge::Lexers::PlainText).new
+      end
+
       def replace_parent_pre_element(node, highlighted)
         # Replace the parent `pre` element with the entire highlighted block
         node.parent.replace(highlighted)
       end
 
       # Override Rouge::Plugins::Redcarpet#rouge_formatter
-      def rouge_formatter(lexer)
-        Rouge::Formatters::HTML.new
+      def rouge_formatter(lexer = nil)
+        @rouge_formatter ||= Rouge::Formatters::HTML.new
       end
     end
   end