TL;DR: Your hunch is wrong because on send stream->upload_len is adjusted so that the bytes sent are subtracted. Can you give us verbose output, or a self-contained example that we can use to reproduce?

I will try to explain what is happening with upload_len since that may be confusing due to the callback. You are referring to this code in http2_send:

    stream->upload_mem = mem;
    stream->upload_len = len;
    nghttp2_session_resume_data(h2, stream->stream_id);
    rv = h2_session_send(conn->data, h2);
    if(nghttp2_is_fatal(rv)) {
      *err = CURLE_SEND_ERROR;
      return -1;
    }
    len -= stream->upload_len;

h2_session_send is basically a wrapper for nghttp2_session_send which uses a read callback function that libcurl set earlier called data_source_read_callback. In that function stream->upload_len is adjusted, which is this code:

  nread = MIN(stream->upload_len, length);
  if(nread > 0) {
    memcpy(buf, stream->upload_mem, nread);
    stream->upload_mem += nread;
    stream->upload_len -= nread;
    stream->upload_left -= nread;
  }

So for example len starts at 500 and therefore so does upload_len. nghttp2 asks and gets 300 bytes so upload_len is now 200 because that's what's left to upload. Then back in http2_send len is set to bytes to be sent minus bytes that weren't sent, or 500 - 200 = 300. Or in other words len becomes number of bytes that were sent, and that is what is returned.

Thanks for the detail explanation. My understand is wrong.

Now I have to figure it out why data can not be sent. (since my stream->upload_len is same as len after nghttp2_session_send).

If you think libcurl is at fault reopen this and give us more information, ideally a self contained example that we can use to reproduce.

Here is my new finding of this problem. The attached program is modified from libcurl example http2-upload.c. I just change the CURLOPT_UPLOAD to CURLOPT_POST. And CURLOPT_INFILESIZE_LARGE to CURLOPT_POSTFIELDSIZE and it works good.

The only thing makes different is the CURLOPT_POSTFIELDSIZE. Without the content length, http2 will not send the second block. In contrast, with old http program it can depend on the CURLOPT_READFUNCTION return value instead of content lenght.

What are my alternatives if I can not get the total length of data before POST?

utest2.zip

Sounds like a bug we should fix

What server are you running this against? Are you using HTTPS or HTTP? I've ran a few tests now against nghttpd and it works just fine for me.

I use HTTPS. The same binary works if it send to a non-http2 site.

So again: what server are you running this against? It is hard for me to work on this if I cannot reproduce the case so it is in your interest to help us repeat the problem.

Thanks for continue watch for this. Here are sites I use for test:
https://avs-alexa-na.amazon.com/v20160207/event --> it is a http2 site. it will said your are not authenticate
https://echo.getpostman.com/post --> this is a http1.1 site, it always works
https://http2.golang.org/ECH --> this is another http2 site, but it won't take POST, it only take PUT
I am not expect any of above site reply successfully or not, what I care is the content not been sent:

0000: POST /ECHO HTTP/1.1
0015: Host: http2.golang.org
002d: Accept: */*
003a: Content-type: multipart/form-data; boundary=MULTIPLEPART_BOUNDAR
007a: Y
007d: 
=> Send data, 0 bytes (0x0)
=> Send data, 0 bytes (0x0)
=> Send data, 0 bytes (0x0)
=> Send data, 0 bytes (0x0)
<= Recv header, 13 bytes (0xd)
0000: HTTP/2 400 

Above is the curl with CURLOPT_VERBOSE and CURLOPT_DEBUGFUNCTION output. You can see the header has been sent, but no data can be sent at all.

Ok, I can reproduce. I first didn't understand I had to modify the example to trigger it. Working on a fix.

@shihancheng thanks for your report. Please try out the commits I just merged just to make sure it works for you as it does for me!