109 lines
3.6 KiB
Rust
109 lines
3.6 KiB
Rust
//! Provides utilities to deal with futures, such as a `Cancellable` future.
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use std::sync::{Arc, Mutex};
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use failure::Error;
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use futures::{Async, Future, Poll};
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use crate::tools::async_mutex::{AsyncMutex, AsyncLockGuard, LockFuture};
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/// Make a future cancellable.
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///
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/// This simply performs a `select()` on the future and something waiting for a signal. If the
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/// future finishes successfully, it yields `Some(T::Item)`. If it was cancelled, it'll yield
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/// `None`.
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///
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/// In order to cancel the future, a `Canceller` is used.
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///
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/// ```no_run
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/// # use failure::Error;
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/// # use futures::Future;
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/// # use proxmox_backup::tools::futures::Cancellable;
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/// # fn doc<T>(future: T) -> Result<(), Error>
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/// # where
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/// # T: Future<Item = i32, Error = ()> + Send + Sync + 'static,
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/// # {
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/// let (future, canceller) = Cancellable::new(future)?;
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/// tokio::spawn(future.and_then(|res| {
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/// match res {
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/// Some(value) => println!("Future finished with {}", value),
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/// None => println!("Future was cancelled"),
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/// }
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/// Ok(())
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/// }));
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/// // Do something
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/// canceller.cancel();
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/// # Ok(())
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/// # }
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/// ```
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pub struct Cancellable<T: Future> {
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/// Our core: we're waiting on a future, on on a lock. The cancel method just unlocks the
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/// lock, so that our LockFuture finishes.
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inner: futures::future::Select2<T, LockFuture<(), <T as Future>::Error>>,
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/// When this future is created, this holds a guard. When a `Canceller` wants to cancel the
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/// future, it'll drop this guard, causing our inner future to resolve to `None`.
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guard: Arc<Mutex<Option<AsyncLockGuard<()>>>>,
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}
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/// Reference to a cancellable future. Multiple instances may exist simultaneously.
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///
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/// This allows cancelling another future. If the future already finished, nothing happens.
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///
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/// This can be cloned to be used in multiple places.
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#[derive(Clone)]
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pub struct Canceller(Arc<Mutex<Option<AsyncLockGuard<()>>>>);
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impl Canceller {
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/// Cancel the associated future.
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///
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/// This does nothing if the future already finished successfully.
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pub fn cancel(&self) {
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*self.0.lock().unwrap() = None;
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}
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}
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impl<T: Future> Cancellable<T> {
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/// Make a future cancellable.
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///
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/// Returns a future and a `Canceller` which can be cloned and used later to cancel the future.
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pub fn new(inner: T) -> Result<(Self, Canceller), Error> {
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// we don't even need to sture the mutex...
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let (mutex, guard) = AsyncMutex::new_locked(())?;
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let this = Self {
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inner: inner.select2(mutex.lock()),
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guard: Arc::new(Mutex::new(Some(guard))),
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};
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let canceller = this.canceller();
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Ok((this, canceller))
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}
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/// Create another `Canceller` for this future.
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pub fn canceller(&self) -> Canceller {
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Canceller(self.guard.clone())
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}
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}
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/// Make a future cancellable.
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///
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/// This is a shortcut for `Cancellable::new`
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pub fn cancellable<T: Future>(future: T) -> Result<(Cancellable<T>, Canceller), Error> {
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Cancellable::new(future)
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}
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impl<T: Future> Future for Cancellable<T> {
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type Item = Option<<T as Future>::Item>;
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type Error = <T as Future>::Error;
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fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
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use futures::future::Either;
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match self.inner.poll() {
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Ok(Async::Ready(Either::A((item, _)))) => Ok(Async::Ready(Some(item))),
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Ok(Async::Ready(Either::B(_))) => Ok(Async::Ready(None)),
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Ok(Async::NotReady) => Ok(Async::NotReady),
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Err(Either::A((err, _))) => Err(err),
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Err(Either::B((err, _))) => Err(err),
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}
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}
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}
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