audit-state-machine

Audit the cache's explicit state machines for illegal transitions, missed transitions (lost wakeups), and ABA across transitions. The snapshot audits trace fields and methods one at a time; this one builds the full transition table for each machine and asks whether every reachable interleaving keeps the machine legal. A missed transition wedges the cache (work buffered, never drained); an illegal transition resurrects a dead node or strands a future.

The drain/maintenance path was recently changed ("assist maintenance directly when the write buffer is full"), so Machine 1 is the priority.

Machine 1: Drain status (priority)

States: IDLE, REQUIRED, PROCESSING_TO_IDLE, PROCESSING_TO_REQUIRED. Transition sites: afterWrite, scheduleAfterWrite, scheduleDrainBuffers, maintenance, rescheduleCleanUpIfIncomplete, performCleanUp. Access via drainStatusOpaque/drainStatusAcquire, casDrainStatus, setDrainStatusOpaque/setDrainStatusRelease.

Build the table: for each (state, event) pair — a write arrives, a read arrives, maintenance starts/ends, the pacer fires, the executor rejects, the buffer-full inline-assist path runs — what is the next state and who drives it? Then attack:

1. Lost wakeup: can the machine settle in IDLE while work remains buffered? Trace the maintenance-exit CAS (PROCESSING_TO_IDLE → IDLE) against a concurrent scheduleAfterWrite that observed PROCESSING_TO_IDLE and CAS'd it to PROCESSING_TO_REQUIRED. Which write loses, and does the fallback (setDrainStatusOpaque(REQUIRED)) re-arm it?
2. Double schedule: can two threads both schedule maintenance for the same epoch, or the inline-assist path run concurrently with an executor-scheduled drain?
3. Opaque vs CAS staleness: reads are opaque, transitions are CAS/release. For every decision that gates scheduling, can the opaque read be stale in a way that drops a reschedule? Verify the PROCESSING_TO_IDLE → PROCESSING_TO_REQUIRED CAS and the maintenance-exit re-check close the window on all paths.
4. Pacer coupling: rescheduleCleanUpIfIncomplete gates on REQUIRED && !pacer.isScheduled(). Can REQUIRED coexist with no scheduled pacer and no in-flight maintenance — i.e. the cache wedged until the next user operation happens to drive it?

Machine 2: Node lifecycle

States: alive (has value) → retired (marked) → dead (unlinked). Strictly unidirectional. Sites: makeDead, the retire paths, isAlive/isRetired/isDead (on the generated Node), and the resurrect path in remap/compute.

1. Can any path move dead → retired, dead → alive, or retired → alive except the sanctioned resurrection (which re-creates within the same synchronized(node))? Resurrection that observes a node already made dead is the bug to hunt.
2. On every exception or early-return in the compute and eviction paths, does the node land in a legal terminal state — never stuck retired with no one left to finish makeDead?
3. Is weight / region accounting applied exactly once per transition — not twice on a retried path, not zero on an exception path?

Machine 3: Async-value lifecycle

An async entry's value is an incomplete future → completes (value | null | exception). Sites: isComputingAsync, ASYNC_EXPIRY, refreshes(), the refresh bit in writeTime (& 1L).

1. Can an entry be treated as both computing-async and expired/evicted in a way that strands the future or the ASYNC_EXPIRY timestamp? (Historical: timestamp stuck after executor rejection.)
2. The refresh-in-progress bit in writeTime and the refreshes() map: can they disagree — bit set but map entry gone, or vice versa — so a refresh is double-started or never cleared?

Output

For each finding: the interleaving (thread-by-thread), the illegal or missed transition, the observable consequence (wedged cache, lost notification, stranded future, resurrected dead node), and a Verification. Verify each interleaving is JMM-legal, not merely sequentially consistent. If a transition cannot be resolved statically, ESCALATE with a Fray skeleton — the drain machine is a prime Fray target.