remove_stale_cached_threads!

# File activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb, line 231
      def remove_stale_cached_threads!(cache, &block)
        keys = Set.new(cache.keys)

        Thread.list.each do |thread|
          keys.delete(thread.object_id) if thread.alive?
        end
        keys.each do |key|
          next unless cache.has_key?(key)
          block.call(key, cache[key])
          cache.delete(key)
        end
      end