Add select_biased, document how it differs from select

Author: Xaeroxe

futures-util/src/future/mod.rs

@@ -85,6 +85,9 @@ pub use self::join_all::{join_all, JoinAll};
 mod select;
 pub use self::select::{select, Select};
 
+mod select_biased;
+pub use self::select_biased::{select_biased, SelectBiased};
+
 #[cfg(feature = "alloc")]
 mod select_all;
 #[cfg(feature = "alloc")]

futures-util/src/future/select.rs

@@ -22,8 +22,9 @@ impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
 /// Note that this function consumes the receiving futures and returns a
 /// wrapped version of them.
 ///
-/// If both futures are ready when this is polled, the winner will be pseudo-randomly
-/// selected.
+/// **If both futures are ready when this is polled, the winner will be pseudo-randomly
+/// selected, unless this crate is compiled without the `std` feature. Without `std`,
+/// the first argument will always be selected.**
 ///
 /// Also note that if both this and the second future have the same
 /// output type you can use the `Either::factor_first` method to

futures-util/src/future/select_biased.rs

@@ -0,0 +1,138 @@
+use super::assert_future;
+use crate::future::{Either, FutureExt};
+use core::pin::Pin;
+use futures_core::future::{FusedFuture, Future};
+use futures_core::task::{Context, Poll};
+
+/// Future for the [`select_biased()`] function.
+#[must_use = "futures do nothing unless you `.await` or poll them"]
+#[derive(Debug)]
+pub struct SelectBiased<A, B> {
+    inner: Option<(A, B)>,
+}
+
+impl<A: Unpin, B: Unpin> Unpin for SelectBiased<A, B> {}
+
+/// Waits for either one of two differently-typed futures to complete, giving preferential treatment to the first argument.
+///
+/// This function will return a new future which awaits for either one of both
+/// futures to complete. The returned future will finish with both the value
+/// resolved and a future representing the completion of the other work.
+///
+/// Note that this function consumes the receiving futures and returns a
+/// wrapped version of them.
+///
+/// **If both futures are ready when this is polled, the first argument will always be selected.**
+///
+/// Also note that if both this and the second future have the same
+/// output type you can use the `Either::factor_first` method to
+/// conveniently extract out the value at the end.
+///
+/// # Examples
+///
+/// A simple example
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::{
+///     pin_mut,
+///     future::Either,
+///     future::self,
+/// };
+///
+/// // These two futures have different types even though their outputs have the same type.
+/// let future1 = async {
+///     future::pending::<()>().await; // will never finish
+///     1
+/// };
+/// let future2 = async {
+///     future::ready(2).await
+/// };
+///
+/// // 'select_biased' requires Future + Unpin bounds
+/// pin_mut!(future1);
+/// pin_mut!(future2);
+///
+/// let value = match future::select_biased(future1, future2).await {
+///     Either::Left((value1, _)) => value1,  // `value1` is resolved from `future1`
+///                                           // `_` represents `future2`
+///     Either::Right((value2, _)) => value2, // `value2` is resolved from `future2`
+///                                           // `_` represents `future1`
+/// };
+///
+/// assert!(value == 2);
+/// # });
+/// ```
+///
+/// A more complex example
+///
+/// ```
+/// use futures::future::{self, Either, Future, FutureExt};
+///
+/// // A poor-man's join implemented on top of select_biased
+///
+/// fn join<A, B>(a: A, b: B) -> impl Future<Output=(A::Output, B::Output)>
+///     where A: Future + Unpin,
+///           B: Future + Unpin,
+/// {
+///     future::select_biased(a, b).then(|either| {
+///         match either {
+///             Either::Left((x, b)) => b.map(move |y| (x, y)).left_future(),
+///             Either::Right((y, a)) => a.map(move |x| (x, y)).right_future(),
+///         }
+///     })
+/// }
+/// ```
+pub fn select_biased<A, B>(future1: A, future2: B) -> SelectBiased<A, B>
+where
+    A: Future + Unpin,
+    B: Future + Unpin,
+{
+    assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(SelectBiased {
+        inner: Some((future1, future2)),
+    })
+}
+
+impl<A, B> Future for SelectBiased<A, B>
+where
+    A: Future + Unpin,
+    B: Future + Unpin,
+{
+    type Output = Either<(A::Output, B), (B::Output, A)>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        /// When compiled with `-C opt-level=z`, this function will help the compiler eliminate the `None` branch, where
+        /// `Option::unwrap` does not.
+        #[inline(always)]
+        fn unwrap_option<T>(value: Option<T>) -> T {
+            match value {
+                None => unreachable!(),
+                Some(value) => value,
+            }
+        }
+
+        let (a, b) = self.inner.as_mut().expect("cannot poll Select twice");
+
+        macro_rules! poll_wrap {
+            ($to_poll:expr, $unpolled:expr, $wrap:expr) => {
+                if let Poll::Ready(val) = $to_poll.poll_unpin(cx) {
+                    return Poll::Ready($wrap((val, $unpolled)));
+                }
+            };
+        }
+
+        poll_wrap!(a, unwrap_option(self.inner.take()).1, Either::Left);
+        poll_wrap!(b, unwrap_option(self.inner.take()).0, Either::Right);
+        Poll::Pending
+    }
+}
+
+impl<A, B> FusedFuture for SelectBiased<A, B>
+where
+    A: Future + Unpin,
+    B: Future + Unpin,
+{
+    fn is_terminated(&self) -> bool {
+        self.inner.is_none()
+    }
+}