Use only one Redis cache connection pool

We're currently using 2 connection pools when connecting to the
Redis cache instance. This makes Rails.cache and Gitlab::Redis::Cache
use the same pool so that we can reuse connections more efficiently.

changelogs/unreleased/share-redis-cache-connection-pool.yml

+---
+title: Make Rails.cache and Gitlab::Redis::Cache share the same Redis connection pool
+merge_request: 28074
+author:
+type: performance

config/application.rb

@@ -258,18 +258,11 @@ module Gitlab
     # Use caching across all environments
     # Full list of options:
     # https://api.rubyonrails.org/classes/ActiveSupport/Cache/RedisCacheStore.html#method-c-new
-    caching_config_hash = Gitlab::Redis::Cache.params
+    caching_config_hash = {}
+    caching_config_hash[:redis] = Gitlab::Redis::Cache.pool
     caching_config_hash[:compress] = Gitlab::Utils.to_boolean(ENV.fetch('ENABLE_REDIS_CACHE_COMPRESSION', '1'))
     caching_config_hash[:namespace] = Gitlab::Redis::Cache::CACHE_NAMESPACE
     caching_config_hash[:expires_in] = 2.weeks # Cache should not grow forever
-    if Gitlab::Runtime.multi_threaded?
-      caching_config_hash[:pool_size] = Gitlab::Redis::Cache.pool_size
-      caching_config_hash[:pool_timeout] = 1
-    end
-
-    # Overrides RedisCacheStore's default value of 0
-    # This makes the default value the same with Gitlab::Redis::Cache
-    caching_config_hash[:reconnect_attempts] ||= ::Redis::Client::DEFAULTS[:reconnect_attempts]
 
     config.cache_store = :redis_cache_store, caching_config_hash
 

lib/gitlab/redis/wrapper.rb

@@ -15,8 +15,11 @@ module Gitlab
         delegate :params, :url, to: :new
 
         def with
+          pool.with { |redis| yield redis }
+        end
+
+        def pool
           @pool ||= ConnectionPool.new(size: pool_size) { ::Redis.new(params) }
-          @pool.with { |redis| yield redis }
         end
 
         def pool_size