closingFor

Whom this zone is closing for, or null when it is open to whoever gets there first.

A zone is closing between the moment something asks for it and the moment it can be given: no new claim and no new admission succeeds, while whatever is already there finishes and leaves. Draining rather than evicting is the whole of the discipline -- "the aisle is closed now, vehicles must leave" is evacuation, which needs somewhere for them to go and a policy for choosing it, and is a different problem.

Reserving for a named holder rather than setting a flag is what makes the drain terminate. A closed zone that merely refused everyone would be given to whichever waiting vehicle asked next, and on a busy aisle the request would never be satisfied at all.

Null in every model that does not close a zone, which is why the hot path pays a single null test for it.