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a73dad00e2
Add a lock to `StateMachineState`, allowing every flow to lock themselves when performing a transition or when an external thread (such as `killFlow`) tries to interact with a flow from occurring at the same time. Doing this prevents race-conditions where the external threads mutate the database or the flow's state causing an in-flight transition to fail. A `Semaphore` is used to acquire and release the lock. A `ReentrantLock` is not used as it is possible for a flow to suspend while locked, and resume on a different thread. This causes a `ReentrantLock` to fail when releasing the lock because the thread doing so is not the thread holding the lock. `Semaphore`s can be used across threads, therefore bypassing this issue. The lock is copied across when a flow is retried. This is to prevent another thread from interacting with a flow just after it has been retried. Without copying the lock, the external thread would acquire the old lock and execute, while the fiber thread acquires the new lock and also executes. |
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