/** Read a line and echo it back.

    This function is used to read a line ending in a carriage-return
    ("CR") from the stream, and then write it back. The function call
    blocks until one of the following conditions is true:

    @li A line was read in and sent back on the stream

    @li An error occurs.

    The implementation may read additional octets that lie past
    the end of the line being read. These octets are silently
    discarded.

    @param The stream to operate on. The type must meet the
    requirements of @b AsyncReadStream and @AsyncWriteStream

    @param ec Set to the error, if any occurred.
*/
template<class SyncStream>
void
echo(SyncStream& stream, boost::beast::error_code& ec)
{
    // This dynamic buffer will be used to hold the line of text
    boost::asio::streambuf buffer;

    // Read a line of text from the stream into the buffer
    auto const bytes_matched = boost::asio::read_until(stream, buffer, "\r", ec);
    if(ec)
        return;

    // Now echo the text. Since we could have read past the carriage
    // return, we use buffer_prefix to send only the text up to and
    // including the carriage return.
    boost::asio::write(stream, boost::beast::buffers_prefix(bytes_matched, buffer.data()), ec);
    if(ec)
        return;

    // If we get here then there was no error
}