Abortable streams

futures-util/src/abortable.rs

@@ -0,0 +1,185 @@
+use crate::task::AtomicWaker;
+use alloc::sync::Arc;
+use core::fmt;
+use core::pin::Pin;
+use core::sync::atomic::{AtomicBool, Ordering};
+use futures_core::future::Future;
+use futures_core::task::{Context, Poll};
+use futures_core::Stream;
+use pin_project_lite::pin_project;
+
+pin_project! {
+    /// A future/stream which can be remotely short-circuited using an `AbortHandle`.
+    #[derive(Debug, Clone)]
+    #[must_use = "futures/streams do nothing unless you poll them"]
+    pub struct Abortable<T> {
+        #[pin]
+        task: T,
+        inner: Arc<AbortInner>,
+    }
+}
+
+impl<T> Abortable<T> {
+    /// Creates a new `Abortable` future/stream using an existing `AbortRegistration`.
+    /// `AbortRegistration`s can be acquired through `AbortHandle::new`.
+    ///
+    /// When `abort` is called on the handle tied to `reg` or if `abort` has
+    /// already been called, the future/stream will complete immediately without making
+    /// any further progress.
+    ///
+    /// # Examples:
+    ///
+    /// Usage with futures:
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::future::{Abortable, AbortHandle, Aborted};
+    ///
+    /// let (abort_handle, abort_registration) = AbortHandle::new_pair();
+    /// let future = Abortable::new(async { 2 }, abort_registration);
+    /// abort_handle.abort();
+    /// assert_eq!(future.await, Err(Aborted));
+    /// # });
+    /// ```
+    ///
+    /// Usage with streams:
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// # use futures::future::{Abortable, AbortHandle};
+    /// # use futures::stream::{self, StreamExt};
+    ///
+    /// let (abort_handle, abort_registration) = AbortHandle::new_pair();
+    /// let mut stream = Abortable::new(stream::iter(vec![1, 2, 3]), abort_registration);
+    /// abort_handle.abort();
+    /// assert_eq!(stream.next().await, None);
+    /// # });
+    /// ```
+    pub fn new(task: T, reg: AbortRegistration) -> Self {
+        Self { task, inner: reg.inner }
+    }
+
+    /// Checks whether the task has been aborted. Note that all this
+    /// method indicates is whether [`AbortHandle::abort`] was *called*.
+    /// This means that it will return `true` even if:
+    /// * `abort` was called after the task had completed.
+    /// * `abort` was called while the task was being polled - the task may still be running and
+    /// will not be stopped until `poll` returns.
+    pub fn is_aborted(&self) -> bool {
+        self.inner.aborted.load(Ordering::Relaxed)
+    }
+}
+
+/// A registration handle for an `Abortable` task.
+/// Values of this type can be acquired from `AbortHandle::new` and are used
+/// in calls to `Abortable::new`.
+#[derive(Debug)]
+pub struct AbortRegistration {
+    inner: Arc<AbortInner>,
+}
+
+/// A handle to an `Abortable` task.
+#[derive(Debug, Clone)]
+pub struct AbortHandle {
+    inner: Arc<AbortInner>,
+}
+
+impl AbortHandle {
+    /// Creates an (`AbortHandle`, `AbortRegistration`) pair which can be used
+    /// to abort a running future or stream.
+    ///
+    /// This function is usually paired with a call to [`Abortable::new`].
+    pub fn new_pair() -> (Self, AbortRegistration) {
+        let inner =
+            Arc::new(AbortInner { waker: AtomicWaker::new(), aborted: AtomicBool::new(false) });
+
+        (Self { inner: inner.clone() }, AbortRegistration { inner })
+    }
+}
+
+// Inner type storing the waker to awaken and a bool indicating that it
+// should be aborted.
+#[derive(Debug)]
+struct AbortInner {
+    waker: AtomicWaker,
+    aborted: AtomicBool,
+}
+
+/// Indicator that the `Abortable` task was aborted.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct Aborted;
+
+impl fmt::Display for Aborted {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "`Abortable` future has been aborted")
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for Aborted {}
+
+impl<T> Abortable<T> {
+    fn try_poll<I>(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        poll: impl Fn(Pin<&mut T>, &mut Context<'_>) -> Poll<I>,
+    ) -> Poll<Result<I, Aborted>> {
+        // Check if the task has been aborted
+        if self.is_aborted() {
+            return Poll::Ready(Err(Aborted));
+        }
+
+        // attempt to complete the task
+        if let Poll::Ready(x) = poll(self.as_mut().project().task, cx) {
+            return Poll::Ready(Ok(x));
+        }
+
+        // Register to receive a wakeup if the task is aborted in the future
+        self.inner.waker.register(cx.waker());
+
+        // Check to see if the task was aborted between the first check and
+        // registration.
+        // Checking with `is_aborted` which uses `Relaxed` is sufficient because
+        // `register` introduces an `AcqRel` barrier.
+        if self.is_aborted() {
+            return Poll::Ready(Err(Aborted));
+        }
+
+        Poll::Pending
+    }
+}
+
+impl<Fut> Future for Abortable<Fut>
+where
+    Fut: Future,
+{
+    type Output = Result<Fut::Output, Aborted>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.try_poll(cx, |fut, cx| fut.poll(cx))
+    }
+}
+
+impl<St> Stream for Abortable<St>
+where
+    St: Stream,
+{
+    type Item = St::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        self.try_poll(cx, |stream, cx| stream.poll_next(cx)).map(Result::ok).map(Option::flatten)
+    }
+}
+
+impl AbortHandle {
+    /// Abort the `Abortable` stream/future associated with this handle.
+    ///
+    /// Notifies the Abortable task associated with this handle that it
+    /// should abort. Note that if the task is currently being polled on
+    /// another thread, it will not immediately stop running. Instead, it will
+    /// continue to run until its poll method returns.
+    pub fn abort(&self) {
+        self.inner.aborted.store(true, Ordering::Relaxed);
+        self.inner.waker.wake();
+    }
+}

futures-util/src/future/abortable.rs

@@ -1,101 +1,8 @@
 use super::assert_future;
-use crate::task::AtomicWaker;
-use alloc::sync::Arc;
-use core::fmt;
-use core::pin::Pin;
-use core::sync::atomic::{AtomicBool, Ordering};
+use crate::future::{AbortHandle, Abortable, Aborted};
 use futures_core::future::Future;
-use futures_core::task::{Context, Poll};
-use pin_project_lite::pin_project;
 
-pin_project! {
-    /// A future which can be remotely short-circuited using an `AbortHandle`.
-    #[derive(Debug, Clone)]
-    #[must_use = "futures do nothing unless you `.await` or poll them"]
-    pub struct Abortable<Fut> {
-        #[pin]
-        future: Fut,
-        inner: Arc<AbortInner>,
-    }
-}
-
-impl<Fut> Abortable<Fut>
-where
-    Fut: Future,
-{
-    /// Creates a new `Abortable` future using an existing `AbortRegistration`.
-    /// `AbortRegistration`s can be acquired through `AbortHandle::new`.
-    ///
-    /// When `abort` is called on the handle tied to `reg` or if `abort` has
-    /// already been called, the future will complete immediately without making
-    /// any further progress.
-    ///
-    /// Example:
-    ///
-    /// ```
-    /// # futures::executor::block_on(async {
-    /// use futures::future::{Abortable, AbortHandle, Aborted};
-    ///
-    /// let (abort_handle, abort_registration) = AbortHandle::new_pair();
-    /// let future = Abortable::new(async { 2 }, abort_registration);
-    /// abort_handle.abort();
-    /// assert_eq!(future.await, Err(Aborted));
-    /// # });
-    /// ```
-    pub fn new(future: Fut, reg: AbortRegistration) -> Self {
-        assert_future::<Result<Fut::Output, Aborted>, _>(Self { future, inner: reg.inner })
-    }
-}
-
-/// A registration handle for a `Abortable` future.
-/// Values of this type can be acquired from `AbortHandle::new` and are used
-/// in calls to `Abortable::new`.
-#[derive(Debug)]
-pub struct AbortRegistration {
-    inner: Arc<AbortInner>,
-}
-
-/// A handle to a `Abortable` future.
-#[derive(Debug, Clone)]
-pub struct AbortHandle {
-    inner: Arc<AbortInner>,
-}
-
-impl AbortHandle {
-    /// Creates an (`AbortHandle`, `AbortRegistration`) pair which can be used
-    /// to abort a running future.
-    ///
-    /// This function is usually paired with a call to `Abortable::new`.
-    ///
-    /// Example:
-    ///
-    /// ```
-    /// # futures::executor::block_on(async {
-    /// use futures::future::{Abortable, AbortHandle, Aborted};
-    ///
-    /// let (abort_handle, abort_registration) = AbortHandle::new_pair();
-    /// let future = Abortable::new(async { 2 }, abort_registration);
-    /// abort_handle.abort();
-    /// assert_eq!(future.await, Err(Aborted));
-    /// # });
-    /// ```
-    pub fn new_pair() -> (Self, AbortRegistration) {
-        let inner =
-            Arc::new(AbortInner { waker: AtomicWaker::new(), cancel: AtomicBool::new(false) });
-
-        (Self { inner: inner.clone() }, AbortRegistration { inner })
-    }
-}
-
-// Inner type storing the waker to awaken and a bool indicating that it
-// should be cancelled.
-#[derive(Debug)]
-struct AbortInner {
-    waker: AtomicWaker,
-    cancel: AtomicBool,
-}
-
-/// Creates a new `Abortable` future and a `AbortHandle` which can be used to stop it.
+/// Creates a new `Abortable` future and an `AbortHandle` which can be used to stop it.
 ///
 /// This function is a convenient (but less flexible) alternative to calling
 /// `AbortHandle::new` and `Abortable::new` manually.
@@ -107,63 +14,6 @@ where
     Fut: Future,
 {
     let (handle, reg) = AbortHandle::new_pair();
-    (Abortable::new(future, reg), handle)
-}
-
-/// Indicator that the `Abortable` future was aborted.
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
-pub struct Aborted;
-
-impl fmt::Display for Aborted {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "`Abortable` future has been aborted")
-    }
-}
-
-#[cfg(feature = "std")]
-impl std::error::Error for Aborted {}
-
-impl<Fut> Future for Abortable<Fut>
-where
-    Fut: Future,
-{
-    type Output = Result<Fut::Output, Aborted>;
-
-    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        // Check if the future has been aborted
-        if self.inner.cancel.load(Ordering::Relaxed) {
-            return Poll::Ready(Err(Aborted));
-        }
-
-        // attempt to complete the future
-        if let Poll::Ready(x) = self.as_mut().project().future.poll(cx) {
-            return Poll::Ready(Ok(x));
-        }
-
-        // Register to receive a wakeup if the future is aborted in the... future
-        self.inner.waker.register(cx.waker());
-
-        // Check to see if the future was aborted between the first check and
-        // registration.
-        // Checking with `Relaxed` is sufficient because `register` introduces an
-        // `AcqRel` barrier.
-        if self.inner.cancel.load(Ordering::Relaxed) {
-            return Poll::Ready(Err(Aborted));
-        }
-
-        Poll::Pending
-    }
-}
-
-impl AbortHandle {
-    /// Abort the `Abortable` future associated with this handle.
-    ///
-    /// Notifies the Abortable future associated with this handle that it
-    /// should abort. Note that if the future is currently being polled on
-    /// another thread, it will not immediately stop running. Instead, it will
-    /// continue to run until its poll method returns.
-    pub fn abort(&self) {
-        self.inner.cancel.store(true, Ordering::Relaxed);
-        self.inner.waker.wake();
-    }
+    let abortable = assert_future::<Result<Fut::Output, Aborted>, _>(Abortable::new(future, reg));
+    (abortable, handle)
 }

futures-util/src/future/mod.rs

@@ -110,7 +110,9 @@ cfg_target_has_atomic! {
     #[cfg(feature = "alloc")]
     mod abortable;
     #[cfg(feature = "alloc")]
-    pub use self::abortable::{abortable, Abortable, AbortHandle, AbortRegistration, Aborted};
+    pub use crate::abortable::{Abortable, AbortHandle, AbortRegistration, Aborted};
+    #[cfg(feature = "alloc")]
+    pub use abortable::abortable;
 }
 
 // Just a helper function to ensure the futures we're returning all have the

futures-util/src/lib.rs

@@ -334,5 +334,10 @@ pub use crate::io::{
 #[cfg(feature = "alloc")]
 pub mod lock;
 
+cfg_target_has_atomic! {
+    #[cfg(feature = "alloc")]
+    mod abortable;
+}
+
 mod fns;
 mod unfold_state;

futures-util/src/stream/abortable.rs

@@ -0,0 +1,19 @@
+use super::assert_stream;
+use crate::stream::{AbortHandle, Abortable};
+use crate::Stream;
+
+/// Creates a new `Abortable` stream and an `AbortHandle` which can be used to stop it.
+///
+/// This function is a convenient (but less flexible) alternative to calling
+/// `AbortHandle::new` and `Abortable::new` manually.
+///
+/// This function is only available when the `std` or `alloc` feature of this
+/// library is activated, and it is activated by default.
+pub fn abortable<St>(stream: St) -> (Abortable<St>, AbortHandle)
+where
+    St: Stream,
+{
+    let (handle, reg) = AbortHandle::new_pair();
+    let abortable = assert_stream::<St::Item, _>(Abortable::new(stream, reg));
+    (abortable, handle)
+}