quic: http/3 writer interface stream is closed with fin

[martenrichter]

While trying to do a sort of Webtransport client, I have issues with creating a bidirectional stream.
I do:

this.sessionInt
      .createBidirectionalStream({
        headers: {
          ':method': 'CONNECT',
          ':scheme': 'https',
          // this one depends on draft, draft14 says "webtransport", draft15 says "webtransport-h3"
          ':protocol': 'webtransport',
          ':path': path,
          ':authority': this.hostname + ':' + this.port
        },
        onheaders(headers) {
          headersReceivedResolve(headers)
        }
      })
      .then((stream) => {

where this.sessionInt is created through connect.
My plan was to later get a writer and write something.
However, I see on the non-node.js server immediately the arrival of a fin, signaling stream end.
Looking at the test mades so far:

const stream = await clientSession.createBidirectionalStream();
const w = stream.writer;

it seems to be a valid pattern to get the writer later.
After a few hours of debugging (I followed the handlers from nghttp3 up to the js side), I found the cause:

node/lib/internal/quic/quic.js

Line 3092 in 5a8882a

stream.sendHeaders(headers, { terminal: validatedBody === undefined });

The terminal flag , which send on nghttp3 level the fin, is sent depending on passed on body or not.
@jasnell
For me, it looks like an api design problem. As the opened stream can not know if it is planned to get a stream.writer. So I wonder why the tests worked? Probably a race condition.