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Quiescence precondition, stated in the same words flush's is: the feeder must not be between a submit call's start and its return when this runs. Confinement alone does not give that, because submit runs on the feed worker rather than the session owner; what gives it is the engine joining the feeder's job before teardown reaches this call. A submit that races a close anyway reads the cleared field under the lock and fails loudly instead of touching freed memory.