Enum ReadOutcome 

pub enum ReadOutcome<T> {
    Value(T),
    Scheduled(EventListener),
    InProgress(EventListener),
}

What a read of a task’s output or cell found.

The two “not available yet” cases are distinguished because the reader can act on them differently: a task that is only scheduled can be taken over and executed by the reader, while one that some worker is already executing can only be waited for — and finding that out without taking the scheduler’s queue lock is the point of telling them apart.

The state is a hint: it is a snapshot, and a task can be picked up by a worker right after the read looked (a worker even pops a task off the queue before it marks it as started). Acting on a stale hint costs at most a failed claim, never correctness.

Variants

Value(T)

The value is available.

Scheduled(EventListener)

The task is queued but no worker has started it, so the reader may take it out of the queue and execute it itself. The listener fires when the task is done.

InProgress(EventListener)

A worker is already executing the task; there is nothing to take over. The listener fires when the task is done.