Add eager load paths to help prevent dependency load issues with Sidekiq workers

Attempts to address #13521, #14825
Signed-off-by: Rémy Coutable <remy@rymai.me>

CHANGELOG

@@ -91,6 +91,8 @@ v 8.7.0
   - Fix `signed_in_ip` being set to 127.0.0.1 when using a reverse proxy !3524
   - Improved Markdown rendering performance !3389
   - Make shared runners text in box configurable
+  - Add eager load paths to help prevent dependency load issues with Sidekiq workers (Stan Hu)
+  - Improved Markdown rendering performance !3389 (Yorick Peterse)
   - Don't attempt to look up an avatar in repo if repo directory does not exist (Stan Hu)
   - API: Ability to subscribe and unsubscribe from issues and merge requests (Robert Schilling)
   - Expose project badges in project settings

config/application.rb

@@ -12,8 +12,15 @@ module Gitlab
     # Application configuration should go into files in config/initializers
     # -- all .rb files in that directory are automatically loaded.
 
-    # Custom directories with classes and modules you want to be autoloadable.
-    config.autoload_paths.push(*%W(#{config.root}/lib
+    # Sidekiq uses eager loading, but directories not in the standard Rails
+    # directories must be added to the eager load paths:
+    # https://github.com/mperham/sidekiq/wiki/FAQ#why-doesnt-sidekiq-autoload-my-rails-application-code
+    # Also, there is no need to add `lib` to autoload_paths since autoloading is
+    # configured to check for eager loaded paths:
+    # https://github.com/rails/rails/blob/v4.2.6/railties/lib/rails/engine.rb#L687
+    # This is a nice reference article on autoloading/eager loading:
+    # http://blog.arkency.com/2014/11/dont-forget-about-eager-load-when-extending-autoload
+    config.eager_load_paths.push(*%W(#{config.root}/lib
                                      #{config.root}/app/models/hooks
                                      #{config.root}/app/models/concerns
                                      #{config.root}/app/models/project_services