code: def push(id, body)
        mods = @entrance.push(id, body)
        return mods if @remotes.all.empty?
        mods.each do |m|
          next if @seen.include?(m)
          @mutex.synchronize { @seen << m }
          @modified.push(m)
          @log.debug("Spread-push scheduled for #{m}, queue size is #{@modified.size}")
        end
        mods
      end
docs: This method is thread-safe