The qlog confirmed the writer emits 1 STREAM frame per packet on the
live wire, while MultiplexingCoalescingTest + MultiplexingAioquicTpsTest
both show ~9 streams per packet under synchronous drain. So the bug is
in the live driver flow — concurrent send loop + parser feed +
real-socket interleaving — and not in buildApplicationPacket itself.
To localize: add an opt-in trace at the END of buildApplicationPacket
that dumps per-drain state when QUIC_INTEROP_DEBUG=1:
[writer.app] frames=N stream_frames=K streamsView=M active=A
packetBudget_remaining=R connBudget_initial=C
- frames vs stream_frames tells us if non-stream frames (ACK,
MAX_DATA, MAX_STREAM_DATA) are bloating the packet
- active vs streamsView tells us if isClosed filter dropped streams
- packetBudget_remaining tells us if we hit the 64-byte break early
- connBudget_initial tells us if conn flow control was zero
Wired three pieces:
1. WriterDebug.kt — a single @Volatile boolean owned by commonMain,
`writerDebugEnabled`. Off by default.
2. InteropClient.main flips it to true if QUIC_INTEROP_DEBUG=1 is set
in the env.
3. Dockerfile + Makefile accept --build-arg DEBUG=1 (or `make build
DEBUG=1`) to bake the env var into the image.
Usage:
cd quic/interop
make build DEBUG=1
cd ../..
./quic/interop/run-matrix.sh -s aioquic -t multiplexing
cat ../quic-interop-runner/logs/run-*/aioquic_amethyst/multiplexing/output.txt | grep '^\[writer'
When off, cost is one volatile read in the writer hot path — negligible.
https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT
Claude
9cd8d0a6ee