The single connection-wide `QuicConnection.lock` mutex serialised every
critical path: the read loop's `feedDatagram`, the send loop's
`drainOutbound`, and every public mutator (`openBidiStream`,
`streamById`, `flowControlSnapshot`, ...). The multiplexing testcase
opens hundreds of bidi streams in parallel and was capped at ~25
streams/sec by lock contention against the I/O loops.
Phase 1 of the lock split (see
`quic/plans/2026-05-08-lock-split-design.md`) introduces three
domain-specific mutexes:
- `streamsLock` — streams registry, datagram queues, stream-id
counters, connection-level flow-control bookkeeping, pending-
retransmit maps for control frames
- `LevelState.levelLock` (one per encryption level) — per-level
pnSpace / sentPackets / ackTracker / CRYPTO buffers
- `lifecycleLock` — status transitions, close reason/error code
Acquisition order: `lifecycleLock < streamsLock < levelLock`.
Per-stream `synchronized(this)` blocks inside SendBuffer/ReceiveBuffer
remain at the leaf — never acquire any QuicConnection mutex while
holding a per-stream lock.
The legacy `lock: Mutex` field is preserved as a deprecated alias of
`lifecycleLock` for source-compatibility with external test harnesses;
new code MUST use the appropriate domain lock.
Highlights:
- `feedDatagram` / `drainOutbound` now require the caller to hold
`streamsLock`; the driver wraps each call. Phase 1 keeps the whole
feed/drain inside `streamsLock` for safety; phase 2 (deferred) will
split frame-collection from encrypt + sentPackets-record so app
coroutines can intersperse during the encrypt window.
- `pendingPing`, `peerTransportParameters`, `status`,
`handshakeComplete` are now @Volatile so observers read them
without a lock.
- `markClosedExternally` no longer needs any lock (status is
@Volatile, signals are channel-thread-safe).
- Driver's PTO bookkeeping uses the volatile fields directly — no
lock needed.
- Tests that manually acquired `conn.lock` to call
`getOrCreatePeerStreamLocked` / `onTokensAcked` / `onTokensLost`
now acquire `streamsLock` (the domain those routines mutate).
- New `MultiplexingThroughputTest` locks in the contract: 1000
parallel `openBidiStream` calls must complete in <2 s.
Test plan:
- `:quic:jvmTest` — 294 tests pass (293 prior + 1 new throughput).
- `MultiplexingThroughputTest`: 1000 bidi streams in 52 ms
(~19,000 streams/sec on the in-memory pipe), well above the
250+/sec target.
- `:nestsClient:compileKotlinJvm` — clean, no API breaks.
- `./gradlew :quic:spotlessApply` — clean.
https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT
Claude
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