Use a ConnectionPool for Rails.cache on Sidekiq servers

CHANGELOG

@@ -154,6 +154,7 @@ v 8.12.0 (unreleased)
   - Add notification_settings API calls !5632 (mahcsig)
   - Remove duplication between project builds and admin builds view !5680 (Katarzyna Kobierska Ula Budziszewska)
   - Fix URLs with anchors in wiki !6300 (houqp)
+  - Use a ConnectionPool for Rails.cache on Sidekiq servers
   - Deleting source project with existing fork link will close all related merge requests !6177 (Katarzyna Kobierska Ula Budziszeska)
   - Return 204 instead of 404 for /ci/api/v1/builds/register.json if no builds are scheduled for a runner !6225
   - Fix Gitlab::Popen.popen thread-safety issue

config/application.rb

@@ -116,6 +116,10 @@ module Gitlab
     redis_config_hash = Gitlab::Redis.params
     redis_config_hash[:namespace] = Gitlab::Redis::CACHE_NAMESPACE
     redis_config_hash[:expires_in] = 2.weeks # Cache should not grow forever
+    if Sidekiq.server? # threaded context
+      redis_config_hash[:pool_size] = Sidekiq.options[:concurrency] + 5
+      redis_config_hash[:pool_timeout] = 1
+    end
     config.cache_store = :redis_store, redis_config_hash
 
     config.active_record.raise_in_transactional_callbacks = true