Struct Step

pub struct Step<'exec, 'fut, F, P> { /* private fields */ }

A non-blocking executor-future-park state machine.

All executor operations are ultimately implemented with Step. It allows fine-grained control over execution and control flow. Step can only perform one poll step at a time and requires a pinned future. A Step object is created by calling Executor::step().

See Executor::block_on() for a blocking runner.

Implementations

impl<F, P> Step<'_, '_, F, P>

where F: Future, P: FnMut(Park<'_>) -> Parked<'_>,

pub fn poll(&mut self) -> Poll<F::Output>

Perform at most one non-blocking polling attempt.

This method never blocks unless the park function does (or the future, but that’s totally wrong anyway). The future will be polled given any of these:

• The wakeup signal set is zero. This is intended to provide a reset or “full scan” mechanism. In this case, the raised signal set will be hard-coded to all-ones, regardless of the current value of the event mask. Note: A side effect of this special case is that core::future::pending() and similar constructs might busy-loop depending on the particular park function.

• Any bit in the wakeup signal set matches the event mask. The event mask is atomically frozen at the same time that this condition is check, creating the raised signal set.

The wakeup signal set is cleared just before polling the future (edge-triggering). Regardless of the poll result or if the future is polled at all, a park operation may be triggered under implementation-defined conditions. If parking occurs, it will be the last observable operation before poll() returns. The return value will be Poll::Pending if the future is not polled. Do not call call poll() again after it returns Poll::Ready.

On first call, the wakeup signal set is zero. This ensures that all driven poll functions are attempted at least once.