Merge branch 'rails5-autoload' into 'master'

[Rails5] Permit concurrent loads

Closes #48263

See merge request gitlab-org/gitlab-ce!20473

lib/declarative_policy.rb

@@ -21,7 +21,17 @@ module DeclarativePolicy
       cache = opts[:cache] || {}
       key = Cache.policy_key(user, subject)
 
-      cache[key] ||= class_for(subject).new(user, subject, opts)
+      cache[key] ||=
+        if Gitlab.rails5?
+          # to avoid deadlocks in multi-threaded environment when
+          # autoloading is enabled, we allow concurrent loads,
+          # https://gitlab.com/gitlab-org/gitlab-ce/issues/48263
+          ActiveSupport::Dependencies.interlock.permit_concurrent_loads do
+            class_for(subject).new(user, subject, opts)
+          end
+        else
+          class_for(subject).new(user, subject, opts)
+        end
     end
 
     def class_for(subject)