Is it possible to pause one stream on receive data from server

[nylon009]

The device has very limited memory, so we can not download all the response together, we need to pause some stream and resume it later.

For exampele, we play audio on device, we can not download all the audio, need to do it in a streaming way.

We set a ringbuffer to cache the data, if the ringbuffer is full, we need to pause the download of the audio stream, but we cannot block nghttp2_on_data_chunk_recv_callback function to avoid affect other streams.
if the ringbuff is not full, we notify the nghttp2 session to resume the stream.

[JackyYin]

Hi, how about check stream ID before pause it?
something like this:

 if (is_ringbuffer_full(stream_id)) {
    return nghttp2_error.NGHTTP2_ERR_PAUSE;
}

according to the document:

If the application uses nghttp2_session_mem_recv(), it can return nghttp2_error.NGHTTP2_ERR_PAUSE to make nghttp2_session_mem_recv() return without processing further input bytes. The memory by pointed by the data is retained until nghttp2_session_mem_recv() or nghttp2_session_recv() is called.

[JackyYin]

Hi, I have modified examples/libevent-client.c to test this and I think it works.
I am able to read BUFFER_LEN of data into my buffer and process it.
sleep(2) is just for simulate time-consuming processing.

readcb:

static void readcb(struct bufferevent *bev, void *ptr) {
  http2_session_data *session_data = (http2_session_data *)ptr;
  ssize_t readlen;
  struct evbuffer *input = bufferevent_get_input(bev);
  size_t datalen = evbuffer_get_length(input);
  unsigned char *data = evbuffer_pullup(input, -1);

  session_data->accureadlen = 0;
  while (session_data->accureadlen < datalen) {
    readlen = nghttp2_session_mem_recv(session_data->session, data, MIN(datalen, BUFFER_LEN));
    if (readlen < 0) {
      warnx("Fatal error: %s", nghttp2_strerror((int)readlen));
      delete_http2_session_data(session_data);
      return;
    }
    session_data->accureadlen += readlen;
    sleep(2);
  }
  if (evbuffer_drain(input, (size_t)datalen) != 0) {
    warnx("Fatal error: evbuffer_drain failed");
    delete_http2_session_data(session_data);
    return;
  }
  if (session_send(session_data) != 0) {
    delete_http2_session_data(session_data);
    return;
  }
}

on_data_chunk_recv_callback:

static int on_data_chunk_recv_callback(nghttp2_session *session, uint8_t flags,
                                       int32_t stream_id, const uint8_t *data,
                                       size_t len, void *user_data) {
  http2_session_data *session_data = (http2_session_data *)user_data;
  (void)session;
  (void)flags;

  if (session_data->stream_data->stream_id == stream_id) {
    if (len >= BUFFER_LEN) {
        memcpy(session_data->ring_buffer, &(data[session_data->accureadlen]), BUFFER_LEN);
        fwrite(session_data->ring_buffer, 1, BUFFER_LEN, stdout);
        return NGHTTP2_ERR_PAUSE;
    } else {
        memset(session_data->ring_buffer, 0, BUFFER_LEN);
        memcpy(session_data->ring_buffer, &(data[session_data->accureadlen]), len);
        fwrite(session_data->ring_buffer, 1, len, stdout);
    }
  }
  return 0;
}

[nylon009]

Thanks for your quick response!

But if there are more than one steam are transfering, if the nghttp2_session_mem_recv is return, will other streams been also paused?

If the application uses `nghttp2_session_mem_recv()`, it can return
 :enum:`nghttp2_error.NGHTTP2_ERR_PAUSE` to make
 `nghttp2_session_mem_recv()` return **without processing further
 input bytes**

[JackyYin]

Hi @nevinguo,
I have never tried this before, but I think on_data_chunk_recv_callback is always associated with a particular stream,
so an return of NGHTTP2_ERR_PAUSE should not affect other streams.

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