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//! Executors.
if_std! {
use std::boxed::Box;
use Future;
use never::Never;
/// A task executor.
///
/// A *task* is a `()`-producing future that runs at the top level, and will
/// be `poll`ed until completion. It's also the unit at which wake-up
/// notifications occur. Executors, such as thread pools, allow tasks to be
/// spawned and are responsible for putting tasks onto ready queues when
/// they are woken up, and polling them when they are ready.
pub trait Executor {
/// Spawn the given task, polling it until completion.
///
/// Tasks must be infallible, as the type suggests; it is the
/// client's reponsibility to route any errors elsewhere via a channel
/// or some other means of communication.
///
/// # Errors
///
/// The executor may be unable to spawn tasks, either because it has
/// been shut down or is resource-constrained.
fn spawn(&mut self, f: Box<Future<Item = (), Error = Never> + Send>) -> Result<(), SpawnError>;
/// Determine whether the executor is able to spawn new tasks.
///
/// # Returns
///
/// An `Ok` return means the executor is *likely* (but not guaranteed)
/// to accept a subsequent spawn attempt. Likewise, an `Err` return
/// means that `spawn` is likely, but not guaranteed, to yield an error.
fn status(&self) -> Result<(), SpawnError> {
Ok(())
}
// TODO: downcasting hooks
}
/// Provides the reason that an executor was unable to spawn.
#[derive(Debug)]
pub struct SpawnError {
_a: ()
}
impl SpawnError {
/// Spawning is failing because the executor has been shut down.
pub fn shutdown() -> SpawnError {
SpawnError { _a: () }
}
/// Check whether this error is the `shutdown` error.
pub fn is_shutdown() -> bool {
true
}
}
}
#[cfg(not(feature = "std"))]
pub(crate) trait Executor {}