The old ColdMutex (interprocess_mutex) was in process-local anonymous
namespace storage, so each process had its own copy — no actual
cross-process exclusion. Even if moved to SHM, interprocess_mutex is
not robust: a crash while holding the lock would deadlock on restart.
New design:
- ShmSpinLock: atomic<pid_t> in shared memory, kill(pid,0) detects
dead owners (ESRCH → takeover), crash-safe by construction
- std::mutex: process-local, handles intra-process thread contention
without burning CPU on the SHM spinlock
- DualLock: locks local first, then shm; unlocks in reverse
9 lock sites in MapRuleStat upgraded to std::lock_guard<DualLock>.
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