beast: bug when reading chunked on "slow" stream

Version of Beast

master (v49) and v50

compiler

os: ubuntu 17.04 compiler: gcc (Ubuntu 6.3.0-12ubuntu2) 6.3.0 20170406

code for repeating the error

struct very_slow_stream {
  boost::asio::io_service& io_service;
  const std::string input =
      "HTTP/1.1 200 OK\r\n"
      "Transfer-Encoding: chunked\r\n"
      "Content-Type: application/octet-stream\r\n"
      "\r\n"
      "4\r\n"
      "abcd\r\n"
      "0\r\n"
      "\r\n";
  std::size_t index{};

  template <typename MutableBufferSequence>
  std::size_t read_some(const MutableBufferSequence& buffer,
                        boost::system::error_code& /*error*/) {
    auto write_to = boost::asio::buffer_cast<char*>(*buffer.begin());
    // std::size_t copy_count{input.size()}; // WORKS!
    std::size_t copy_count{1};  // FAILS, with bad chunk!
    for (std::size_t index_copy = 0; index_copy < copy_count; ++index_copy)
      *(write_to + index_copy) = input[index_copy + index];
    index += copy_count;
    return copy_count;
  }

  template <typename MutableBufferSequence>
  std::size_t read_some(const MutableBufferSequence& buffer) {
    boost::system::error_code error;
    const auto size = read_some(buffer, error);
    if (error) throw error;
    return size;
  }
  boost::asio::io_service& get_io_service() { return io_service; }
};

void test_bug() {
  boost::asio::io_service service;
  very_slow_stream stream_{service};
  beast::multi_buffer stream_buffer;
  beast::http::parser<false, beast::http::dynamic_body, beast::http::fields>
      parser;

  while (!parser.is_done()) {
    const auto read_count =
        beast::http::read_some(stream_, stream_buffer, parser);
    std::cout << parser.get() << std::endl;
    stream_buffer.consume(read_count);
  }
  std::cout << beast::buffers(parser.get().body.data()) << std::endl;
}

the issue

I’m trying to read a chunked http stream, !chunk by chunk!. So I’m using beast::http::read_some. !Sometimes!, I get the “bad chunk” error when beast tries to parse the chunk header (pasic_parser.ipp:546). After some hours of debugging, I found out the error occurs when the stream happens to be very slow. There seems to be a logic error with the \r\n of the previous body chunk.