quic: fix edge cases where stream FIN would be lost

[pimterry]

I found that test-quic-flow-control-concurrent-volume failed on my machine under load while doing some other QUIC dev, did some digging, and it turns out it's a real bug. Flow:

• A stream ends, and an empty FIN frame needs to be sent.
• When sending it, ngtcp2 doesn't have space in the packet due to other data (ACKs, retransmissions, the rest of the stream data, etc) so ngtcp2_conn_writev_stream sets nwrite > 0 (a packet was produced) but ndatalen == -1 (this stream's data wasn't included).
• Because a packet was produced, we skip the existing nwrite == 0 case which reschedules streams to try again later. This applies for data not sent due to congestion, but not data omitted due to packet space.
• For frames containing data, this doesn't matter, as they're always rescheduled for later here.
• Empty FIN has no data though - so the stream is never rescheduled, the FIN is lost, and from peer's POV stream never ends.

The fix here works by rescheduling the stream in all cases where the frame couldn't be included (ndatalen < 0) and there was no error (nwrite >= 0), not just congestion. The early exit below for nwrite == 0 remains, since we do still need to stop once we hit congestion regardless. Simplified the comment to explain this more clearly en route.

Reproduced in a standalone test here, which triggers this by sending a STREAM frame + FIN frame where the data is large enough that the FIN doesn't fit. The exact size to trigger this can vary, so this runs a full sweep over message sizes from 1000 - 1300, and makes sure they all deliver successfully (including clean FIN). Fails reliably on main, passes with this change. The original flaky test also no longer flakes 😄.

[nodejs-github-bot]

Review requested:

• @nodejs/quic

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.50%. Comparing base (12ce819) to head (9daf646).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66759      +/-   ##
==========================================
+ Coverage   90.48%   90.50%   +0.02%     
==========================================
  Files         797      797              
  Lines      277491   277503      +12     
  Branches    53318    53322       +4     
==========================================
+ Hits       251091   251161      +70     
+ Misses      16793    16752      -41     
+ Partials     9607     9590      -17     

see 37 files with indirect coverage changes

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[efekrskl]

Isn't this part still relevant?