The Swift Actor Pitfall: Understanding and Managing Reentrancy
summary
Actors prevent data races but allow logic bugs when an awaited call creates a suspension point and a second task re-enters, exemplified by two concurrent withdrawals of $800 and $500 against an $11,000 balance that nevertheless ends negative. Concrete fixes use Swift async actors: avoid mutable-state access across suspension points, authorize before checking or recheck after suspension, and treat suspension points as the place to audit or serialize state changes rather than assume sequential safety.
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