Use Nokogiri as the ActiveSupport XML backend

This significantly improves performance and reduces memory consumption
when parsing XML files. On a test with 124 JUnit files from a CE build,
there was about a 4x reduction in processing time.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54068

changelogs/unreleased/sh-use-nokogiri-xml-backend.yml

+---
+title: Use Nokogiri as the ActiveSupport XML backend
+merge_request: 23136
+author:
+type: performance

config/application.rb

@@ -158,6 +158,9 @@ module Gitlab
 
     config.action_view.sanitized_allowed_protocols = %w(smb)
 
+    # Nokogiri is significantly faster and uses less memory than REXML
+    ActiveSupport::XmlMini.backend = 'Nokogiri'
+
     # This middleware needs to precede ActiveRecord::QueryCache and other middlewares that
     # connect to the database.
     config.middleware.insert_after Rails::Rack::Logger, ::Gitlab::Middleware::BasicHealthCheck

lib/gitlab/ci/parsers/test/junit.rb

@@ -14,7 +14,7 @@ module Gitlab
               test_case = create_test_case(test_case)
               test_suite.add_test_case(test_case)
             end
-          rescue REXML::ParseException
+          rescue Nokogiri::XML::SyntaxError
             raise JunitParserError, "XML parsing failed"
           rescue
             raise JunitParserError, "JUnit parsing failed"

spec/fast_spec_helper.rb

@@ -9,3 +9,4 @@ require 'active_support/all'
 
 ActiveSupport::Dependencies.autoload_paths << 'lib'
 ActiveSupport::Dependencies.autoload_paths << 'ee/lib'
+ActiveSupport::XmlMini.backend = 'Nokogiri'