[Rails5] Permit concurrent loads

Rails 5 introduced a new (and safer) autoload mechanism which
uses locking when loading missing constants. Under some circumstances
it's possible that a deadlock occurs in multi-threaded environment
(especially when running capybara tests)

To avoid these deadlocks, concurrent load is enabled when loading
policies - we ere hitting timeout in this case:
https://gitlab.com/gitlab-org/gitlab-ce/issues/48263

Note that autoloading is enabled only in development or test envs
(not in production).

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)