Gracefully handle errors in syntax highlighting by leaving the block unformatted

Closes #2433

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.0.0 (unreleased)
+  - Gracefully handle errors in syntax highlighting by leaving the block unformatted (Stan Hu)
   - Fix URL construction for merge requests, issues, notes, and commits for relative URL config (Stan Hu)
   - Fix emoji URLs in Markdown when relative_url_root is used (Stan Hu)
   - Omit filename in Content-Disposition header in raw file download to avoid RFC 6266 encoding issues (Stan HU)

lib/gitlab/markdown/syntax_highlight_filter.rb

@@ -21,7 +21,11 @@ module Gitlab
         language = node.attr('class')
         code     = node.text
 
-        highlighted = block_code(code, language)
+        begin
+          highlighted = block_code(code, language)
+        rescue
+          highlighted = "<pre>#{code}</pre>"
+        end
 
         # Replace the parent `pre` element with the entire highlighted block
         node.parent.replace(highlighted)

spec/lib/gitlab/markdown/syntax_highlight_filter_spec.rb

+require 'spec_helper'
+
+module Gitlab::Markdown
+  describe SyntaxHighlightFilter do
+    include FilterSpecHelper
+
+    let(:project)   { create(:empty_project) }
+    let(:reference) { snippet.to_reference }
+
+    it 'highlights valid code blocks' do
+      result = filter('<pre><code>def fun end</code>')
+      expect(result.to_html).to eq("<pre class=\"code highlight js-syntax-highlight plaintext\"><code>def fun end</code></pre>\n")
+    end
+
+    it 'passes through invalid code blocks' do
+      allow_any_instance_of(SyntaxHighlightFilter).to receive(:block_code).and_raise(StandardError)
+
+      result = filter('<pre><code>This is a test</code></pre>')
+      expect(result.to_html).to eq('<pre>This is a test</pre>')
+    end
+  end
+end