instance method mirror

# File activestorage/lib/active_storage/service/mirror_service.rb, line 67
    def mirror(key, checksum:)
      instrument :mirror, key: key, checksum: checksum do
        mirrors_in_need_of_mirroring = mirrors_needing_mirroring(key)
        if mirrors_in_need_of_mirroring.any?
          metadata = ActiveStorage::Blob.find_by(key: key)&.service_metadata || {}

          primary.open(key, checksum: checksum, verify: checksum.present?) do |io|
            io.rewind
            content = io.read.freeze
            tasks = mirrors_in_need_of_mirroring.map do |service|
              Concurrent::Promise.execute(executor: @executor) do
                service.upload key, StringIO.new(content), checksum: checksum, **metadata
              end
            end
            tasks.each(&:value!)
          end
        end
      end
    end