tryRequestZones

Asks for a set of zones, and answers null when some zone of it is already promised.

requestZones with the one refusable condition turned into an answer. A zone carries one promise at a time, and by default a second asker is refused rather than put in a queue behind the first -- ZoneOverlap.QUEUE is the opt-in for waiting instead. So a model whose closures land where they land -- spills, most obviously -- has to say what an overlap means. This is how it says it, in one call that cannot be got wrong.

The hand-written alternative is the reason this exists. The test that matters is a promise, not a hold: asking for a zone another holder already holds is perfectly ordinary and simply waits for the hold to end. A guard written by hand tends to test the hold as well, and then refuses closures that would have worked.

// A spill landing where one is already being dealt with is part of that spill.
val request = space.tryRequestZones(crew, extent, action)
if (request == null) {
    absorbed.increment()
    return
}

Everything else a request must satisfy still raises: an empty set, a repeated zone, a zone of another guide path, or a holder that already has a request are programming errors rather than conditions of the guide path, and answering null to those would hide a defect.

tryRequestZone is the single-zone spelling. It was left out at first, on the argument that what matters is handling the null rather than how the call is written -- true, but it left requestZone with no try counterpart while requestZones had one, and an asymmetry a reader has to remember is worth more than the argument against it.

Parameters