John,
It is a difficult case as the issue is how do you handle a large body that needs to be processed as a stream?
As soon as the upgrade is accepted, the server has to respond that it is changing protocols and can begin sending the h2 response to the h1 request. We can read the request as h1 and write the response as h2 because the h2 output stream is actually 2 way and requires window updates to be sent. These updates can't just be sent after the request body as you can dead lock if the response consumes all the h2 flow control window it will block in a write and thus may not continue to consume the h1 request body!
The alternative is to buffer the entire h1 request body in memory before the upgrade, but you are opening yourself up for a DOS attack by doing that, unless you limit the size of the request body to something very small.
We need to think about this some more and discuss it to see if there is something better we can do.... but it is not simple.
regards
