QuicConnectionDriver.sendLoop holds the connection mutex while it calls
SendBuffer.takeChunk via QuicConnectionWriter.drainOutbound, but
WtPeerStreamDemux's per-stream `send` callback calls
SendBuffer.enqueue from arbitrary application coroutines without the
connection lock. The two paths concurrently mutate
(chunks, pendingBytes, headOffset, finPending, finSent, sentEnd,
nextOffset). Under load this surfaced as
java.util.NoSuchElementException: ArrayDeque is empty.
at kotlin.collections.ArrayDeque.first(ArrayDeque.kt:102)
at com.vitorpamplona.quic.stream.SendBuffer.takeChunk(SendBuffer.kt:85)
at com.vitorpamplona.quic.connection.QuicConnectionWriterKt.buildApplicationPacket(...)
at com.vitorpamplona.quic.connection.QuicConnectionDriver.sendLoop(...)
The writer saw `pendingBytes > 0` (incremented by an in-flight
enqueue on another thread) before the matching `chunks.addLast`
became visible, fell into the head-peel branch, and tripped on
chunks.first().
Wrap every read and write of SendBuffer state in `synchronized(this)`,
including the cheap `readableBytes` / `sentOffset` / `finPending` /
`finSent` getters used by the writer's pre-flight checks (so they
can't read torn state either). The lock is uncontended in the common
case and short-held in the rare race; we already use synchronized
blocks elsewhere in commonMain (QuicConnectionDriver.kt).
Claude
2d45c6ff4a