HTTP2 sends spurious PRIORITY packet on duplicated handles #4303

I did this

1. I prepared a curl_easy_init()ed handle and stashed it away.
2. I duplicated that handle with curl_easy_duphandle()
3. I made an HTTP2 request to https://twitter.com/ using that duplicated handle

I expected the following

1. An SSL connection to establish
2. An HTTP2 session over that connection
3. Successful twittering

What happened instead

1. An SSL connection was established
2. An HTTP2 session began over that connection
3. An unexpected PRIORITY message was sent for stream -1
4. Twitter shut the connection down on an RST_STREAM for stream -1

curl/libcurl version

curl 7.64.0 (x86_64-pc-linux-gnu) libcurl/7.64.0 OpenSSL/1.1.1c zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2 (+libidn2/2.0.5) libssh2/1.8.0 nghttp2/1.36.0 librtmp/2.3

operating system

Debian Buster (amd64)

Diagnosis

I spent a few hours working through this before discovering the unexpected PRIORITY message. Once I had tracked this down, I pared back why it was occurring and discovered that in http2.c inside h2_session_send() where the new weight and/or dependency detection logic exists, the value of data->state.stream_weight was zero whereas the value of data->set.stream_weight was 16 (which I determined was the default weight from nghttp2).

Tracking through the places where state.stream_weight is set, only Curl_http2_init_state() and the duphandle() called by push_promise() look useful. (Both in http2.c)

Possible ameliorations

One approach is simply in h2_session_send() after the call to h2_pri_spec() to only queue the PRIORITY packet if stream_id is not -1. Using a patch to this effect, I was able to fetch twitter.com usefully in my program.

Another approach, and perhaps the preferable one, would be to find the right place for Curl_http2_init_state() to be called during the point at which the HTTP connection is upgraded to HTTP2. Perhaps during Curl_http2_setup_conn() ?

I am not well-enough versed in the core of the library to make the right call here, but certainly sending a PRIORITY message to stream -1 seems very broken to me, and also to twitter.com's web server 😁