rbuf_fill

# File lib/net/protocol.rb, line 171
    def rbuf_fill
      case rv = @io.read_nonblock(BUFSIZE, exception: false)
      when String
        return @rbuf << rv
      when :wait_readable
        @io.to_io.wait_readable(@read_timeout) or raise Net::ReadTimeout
        # continue looping
      when :wait_writable
        # OpenSSL::Buffering#read_nonblock may fail with IO::WaitWritable.
        # http://www.openssl.org/support/faq.html#PROG10
        @io.to_io.wait_writable(@read_timeout) or raise Net::ReadTimeout
        # continue looping
      when nil
        # callers do not care about backtrace, so avoid allocating for it
        raise EOFError, 'end of file reached', []
      end while true
    end