C ++ Boost http程序，什么是“需要缓冲区”错误？

Question

有时，boost http客户端异步读取响应得到错误代码5，消息为“需要缓冲区”，我找不到有关此错误代码的任何信息，此错误代码是什么意思？

void
    do_read()
{
    // Make the request empty before reading,
    // otherwise the operation behavior is undefined.
    std::shared_ptr<boost::beast::http::response<boost::beast::http::buffer_body>> p_read_remote_server_response =  std::make_shared<boost::beast::http::response<boost::beast::http::buffer_body>>(); ;

    std::shared_ptr<boost::beast::flat_buffer> p_read_remote_server_buffer = std::make_shared<boost::beast::flat_buffer>(15000000);
    // Read a response
    boost::beast::http::async_read(*pClientToServerStream, *p_read_remote_server_buffer, *p_read_remote_server_response,
        boost::bind(&DCClient::on_read_remote_server,
            shared_from_this(), p_read_remote_server_buffer, p_read_remote_server_response, _1, _2));
}

void
    on_read_remote_server(
        std::shared_ptr<boost::beast::flat_buffer> p_read_remote_server_buffer, 
        std::shared_ptr<boost::beast::http::response<boost::beast::http::buffer_body>> p_read_remote_server_response, 
        boost::beast::error_code ec,
        std::size_t bytes_transferred)
{
    boost::ignore_unused(bytes_transferred);

    std::stringstream strstm;
    strstm.write((const char *)((*p_read_remote_server_buffer).data().data()), bytes_transferred);
    std::string s = strstm.str();

    // This means they closed the connection
    if (ec == boost::beast::http::error::end_of_stream)
        return do_close();

    if (ec)
        throw boost::system::system_error{ ec };
}

响应错误，方法on_read_remote_server参数bytes_transferred为124，我将响应转换为字符串，该字符串缺少http标头，仅正文：

std::stringstream strstm;
    strstm.write((const char *)((*p_read_remote_server_buffer).data().data()), bytes_transferred);
    std::string s = strstm.str();