readpartial

# File ext/openssl/lib/openssl/buffering.rb, line 110
  def readpartial(maxlen, buf=nil)
    if maxlen == 0
      if buf
        buf.clear
        return buf
      else
        return ""
      end
    end
    if @rbuffer.empty?
      begin
        return sysread(maxlen, buf)
      rescue Errno::EAGAIN
        retry
      end
    end
    ret = consume_rbuff(maxlen)
    if buf
      buf.replace(ret)
      ret = buf
    end
    raise EOFError if ret.empty?
    ret
  end