nghttp2 multipart POST message

Question

I am currently trying to use nghttp2 to build a multipart message. The message should be strcuture as the following.

I should use the nghttp2_submit_request(here) function, with nva as my HTTP/2 header, and nghttp2_data_provider *data_prd for my data. However, I still dont understand how exactly can I create two message (with two message headers).

---

Update:

Is it possible for me to describe what I was think in my source code? Please see below. Here, I am using nghttp2_data_provider to open up an audio file, and write to the buffer.

ssize_t data_prd_read_callback(
    nghttp2_session *session, int32_t stream_id, uint8_t *buf, size_t length,
    uint32_t *data_flags, nghttp2_data_source *source, void *user_data) 
{

  printf("[INFO] C ----------------------------> S (DATA post body), length:%zu\n", length);

  int fd = source->fd;
  ssize_t r;
// writting my opened audio file into buffer
  while ((r = read(fd, buf, length)) == -1 && errno == EINTR); 
  printf("stream_id:%d, nread:%zu\r\n", stream_id, r);
  return nread;
}



void submit_postAudio(http2_session_data *session_data) {
  int32_t stream_id;
  http2_stream_data *stream_data = session_data->stream_data[STREAM_ID_REQUESTAUDIO];
  const char *uri = stream_data->uri;
  const struct http_parser_url *u = stream_data->u;
  char byLength = 0;

  const nghttp2_nv hdrs[] = {
  MAKE_NV(":method", "POST"),   MAKE_NV_CS(":path", stream_data->path),
  MAKE_NV(":scheme", "https"), MAKE_NV("accept", "*/*"),    
  MAKE_NV_CS("authorization", stream_data->access_token),
  MAKE_NV("content-type", "multipart/form-data; boundary=abcdefg123")
  };

  fprintf(stderr, "Request headers:\n");
  print_headers(stderr, hdrs, ARRLEN(hdrs));

  int fileDescriptor = open ("/my_audio.wmv", O_APPEND);  // open my audio file 
  nghttp2_data_provider data_prd;
  data_prd.source.fd = fileDescriptor   // set the file descriptor 
  data_prd.source.ptr = NULL;
  data_prd.read_callback = data_prd_read_callback;

  stream_id = nghttp2_submit_request(session_data->session, NULL, hdrs,
                                     ARRLEN(hdrs), &data_prd, stream_data);
  if (stream_id < 0) {
    errx(1, "Could not submit HTTP request: %s", nghttp2_strerror(stream_id));
  }

  stream_data->stream_id = stream_id;
}

What confused me is 1) how do I add a header (a message header to be more specific) to the audio. 2) how do I append it after my Json file.

Answer 1

A multi-part mime header is usually defined like this:

-----boundary_id
name: value  # header stuff here, ie. "name: value" per line. No spaces in name, all ASCII
CRLF         # End of header is defined with an extra linefeed, AKA a "null line", see ref 1.

content here, ends at next boundary id

-----boundary_id

Ref 1

If the content is binary you would usually encode it as base-64 with a fixed line length (you can send binary data too but have to take precautions as to extra white-spaces such as a final line-feed). A header field-name stating this can be included:

Content-Transfer-Encoding: Base64

(for binary data use value "binary" instead).

The boundary id must be unique enough to not risk being part of any other content. It does not use double-dashes when defined in the main header, but you need to prepend it with such double-dashes everywhere it acts as a separator.

So in this case the entire message would look something like this:

name1: value1
name2: value2
name3: value3
x-comment: "as required by the specs (ignore this line)"
content-type: multipart/form-data; boundary: my_unique_boundary_id

--my_unique_boundary_id
Content-Disposition: form-data; name="metadata"
Content-Type: application/json; charset=UTF-08

{JSON data here}

--my_unique_boundary_id
Content-Disposition: form-data; name="audio"
Content-Type: application/octet-stream
Content-Transfer-Encoding: Base64
x-comment: "binary audio encoded as base-64 follows next"
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... etc.

--my_unique_boundary_id