Add biased variants for newly fair select functions

Author: Xaeroxe

futures-util/src/future/mod.rs

@@ -86,7 +86,7 @@ mod join_all;
 pub use self::join_all::{join_all, JoinAll};
 
 mod select;
-pub use self::select::{select, Select};
+pub use self::select::{select, select_biased, Select};
 
 #[cfg(feature = "alloc")]
 mod select_all;
@@ -102,12 +102,12 @@ mod try_join_all;
 pub use self::try_join_all::{try_join_all, TryJoinAll};
 
 mod try_select;
-pub use self::try_select::{try_select, TrySelect};
+pub use self::try_select::{try_select, try_select_biased, TrySelect};
 
 #[cfg(feature = "alloc")]
 mod select_ok;
 #[cfg(feature = "alloc")]
-pub use self::select_ok::{select_ok, SelectOk};
+pub use self::select_ok::{select_ok, select_ok_biased, SelectOk};
 
 mod either;
 pub use self::either::Either;

futures-util/src/future/select.rs

@@ -9,6 +9,7 @@ use futures_core::task::{Context, Poll};
 #[derive(Debug)]
 pub struct Select<A, B> {
     inner: Option<(A, B)>,
+    _biased: bool,
 }
 
 impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
@@ -23,7 +24,8 @@ impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
 /// wrapped version of them.
 ///
 /// If both futures are ready when this is polled, the winner will be pseudo-randomly
-/// selected.
+/// selected, unless the std feature is not enabled. If std is enabled, the first
+/// argument will always win.
 ///
 /// Also note that if both this and the second future have the same
 /// output type you can use the `Either::factor_first` method to
@@ -91,6 +93,88 @@ where
 {
     assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select {
         inner: Some((future1, future2)),
+        _biased: false,
+    })
+}
+
+/// Waits for either one of two differently-typed futures to complete, giving preferential treatment to the first one.
+///
+/// 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 winner will always be the first argument.
+///
+/// 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
+///
+/// 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) -> Select<A, B>
+where
+    A: Future + Unpin,
+    B: Future + Unpin,
+{
+    assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select {
+        inner: Some((future1, future2)),
+        _biased: true,
     })
 }
 
@@ -111,6 +195,7 @@ where
                 Some(value) => value,
             }
         }
+        let _biased = self._biased;
 
         let (a, b) = self.inner.as_mut().expect("cannot poll Select twice");
 
@@ -123,7 +208,7 @@ where
         }
 
         #[cfg(feature = "std")]
-        if crate::gen_index(2) == 0 {
+        if _biased || crate::gen_index(2) == 0 {
             poll_wrap!(a, unwrap_option(self.inner.take()).1, Either::Left);
             poll_wrap!(b, unwrap_option(self.inner.take()).0, Either::Right);
         } else {

futures-util/src/future/select_ok.rs

@@ -12,6 +12,7 @@ use futures_core::task::{Context, Poll};
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct SelectOk<Fut> {
     inner: Vec<Fut>,
+    _biased: bool,
 }
 
 impl<Fut: Unpin> Unpin for SelectOk<Fut> {}
@@ -35,7 +36,7 @@ impl<Fut: Unpin> Unpin for SelectOk<Fut> {}
 /// Some futures that would have been polled and had errors get dropped, may now instead
 /// remain in the collection without being polled.
 ///
-/// If you were relying on this biased behavior, consider switching to the [`select_biased!`](crate::select_biased) macro.
+/// If you were relying on this biased behavior, consider switching to the [`select_ok_biased`] function.
 ///
 /// # Panics
 ///
@@ -45,7 +46,36 @@ where
     I: IntoIterator,
     I::Item: TryFuture + Unpin,
 {
-    let ret = SelectOk { inner: iter.into_iter().collect() };
+    let ret = SelectOk { inner: iter.into_iter().collect(), _biased: false };
+    assert!(!ret.inner.is_empty(), "iterator provided to select_ok was empty");
+    assert_future::<
+        Result<(<I::Item as TryFuture>::Ok, Vec<I::Item>), <I::Item as TryFuture>::Error>,
+        _,
+    >(ret)
+}
+
+/// Creates a new future which will select the first successful future over a list of futures.
+///
+/// The returned future will wait for any future within `iter` to be ready and Ok. Unlike
+/// `select_all`, this will only return the first successful completion, or the last
+/// failure. This is useful in contexts where any success is desired and failures
+/// are ignored, unless all the futures fail.
+///
+///  This function is only available when the `std` or `alloc` feature of this
+/// library is activated, and it is activated by default.
+///
+/// If multiple futures are ready at the same time this function is biased towards
+/// entries that are earlier in the list.
+///
+/// # Panics
+///
+/// This function will panic if the iterator specified contains no items.
+pub fn select_ok_biased<I>(iter: I) -> SelectOk<I::Item>
+where
+    I: IntoIterator,
+    I::Item: TryFuture + Unpin,
+{
+    let ret = SelectOk { inner: iter.into_iter().collect(), _biased: true };
     assert!(!ret.inner.is_empty(), "iterator provided to select_ok was empty");
     assert_future::<
         Result<(<I::Item as TryFuture>::Ok, Vec<I::Item>), <I::Item as TryFuture>::Error>,
@@ -57,10 +87,12 @@ impl<Fut: TryFuture + Unpin> Future for SelectOk<Fut> {
     type Output = Result<(Fut::Ok, Vec<Fut>), Fut::Error>;
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let Self { inner } = &mut *self;
+        let Self { inner, _biased } = &mut *self;
         #[cfg(feature = "std")]
         {
-            crate::shuffle(inner);
+            if !*_biased {
+                crate::shuffle(inner);
+            }
         }
         // loop until we've either exhausted all errors, a success was hit, or nothing is ready
         loop {

futures-util/src/future/try_select.rs

@@ -8,6 +8,7 @@ use futures_core::task::{Context, Poll};
 #[derive(Debug)]
 pub struct TrySelect<A, B> {
     inner: Option<(A, B)>,
+    _biased: bool,
 }
 
 impl<A: Unpin, B: Unpin> Unpin for TrySelect<A, B> {}
@@ -25,7 +26,8 @@ type EitherErr<A, B> = Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::E
 /// wrapped version of them.
 ///
 /// If both futures are ready when this is polled, the winner will be pseudo-randomly
-/// selected.
+/// selected, unless the `std` feature is disabled. If the std feature is disabled,
+/// the first argument will always win.
 ///
 /// Also note that if both this and the second future have the same
 /// success/error type you can use the `Either::factor_first` method to
@@ -60,6 +62,55 @@ where
 {
     super::assert_future::<Result<EitherOk<A, B>, EitherErr<A, B>>, _>(TrySelect {
         inner: Some((future1, future2)),
+        _biased: false,
+    })
+}
+
+/// Waits for either one of two differently-typed futures to complete, giving preferential treatment to the first one.
+///
+/// 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 winner will always be the first one.
+///
+/// Also note that if both this and the second future have the same
+/// success/error type you can use the `Either::factor_first` method to
+/// conveniently extract out the value at the end.
+///
+/// # Examples
+///
+/// ```
+/// use futures::future::{self, Either, Future, FutureExt, TryFuture, TryFutureExt};
+///
+/// // A poor-man's try_join implemented on top of select
+///
+/// fn try_join<A, B, E>(a: A, b: B) -> impl TryFuture<Ok=(A::Ok, B::Ok), Error=E>
+///      where A: TryFuture<Error = E> + Unpin + 'static,
+///            B: TryFuture<Error = E> + Unpin + 'static,
+///            E: 'static,
+/// {
+///     future::try_select_biased(a, b).then(|res| -> Box<dyn Future<Output = Result<_, _>> + Unpin> {
+///         match res {
+///             Ok(Either::Left((x, b))) => Box::new(b.map_ok(move |y| (x, y))),
+///             Ok(Either::Right((y, a))) => Box::new(a.map_ok(move |x| (x, y))),
+///             Err(Either::Left((e, _))) => Box::new(future::err(e)),
+///             Err(Either::Right((e, _))) => Box::new(future::err(e)),
+///         }
+///     })
+/// }
+/// ```
+pub fn try_select_biased<A, B>(future1: A, future2: B) -> TrySelect<A, B>
+where
+    A: TryFuture + Unpin,
+    B: TryFuture + Unpin,
+{
+    super::assert_future::<Result<EitherOk<A, B>, EitherErr<A, B>>, _>(TrySelect {
+        inner: Some((future1, future2)),
+        _biased: true,
     })
 }
 
@@ -91,7 +142,7 @@ where
 
         #[cfg(feature = "std")]
         {
-            if crate::gen_index(2) == 0 {
+            if self._biased || crate::gen_index(2) == 0 {
                 poll_wrap!(a, b, Either::Left, Either::Right)
             } else {
                 poll_wrap!(b, a, Either::Right, Either::Left)

futures/tests/future_select.rs

@@ -1,6 +1,6 @@
 use std::future::ready;
 
-use futures::future::select;
+use futures::future::{select, select_biased};
 use futures_executor::block_on;
 
 #[test]
@@ -14,3 +14,13 @@ fn is_fair() {
     assert_eq!(results.iter().filter(|i| **i == 0).take(THRESHOLD).count(), THRESHOLD);
     assert_eq!(results.iter().filter(|i| **i == 1).take(THRESHOLD).count(), THRESHOLD)
 }
+
+#[test]
+fn is_biased() {
+    let mut results = Vec::with_capacity(100);
+    for _ in 0..100 {
+        let (i, _) = block_on(select_biased(ready(0), ready(1))).factor_first();
+        results.push(i);
+    }
+    assert!(results.iter().all(|i| *i == 0));
+}

futures/tests/future_select_ok.rs

@@ -2,7 +2,7 @@ use std::fmt::Debug;
 use std::time::Duration;
 
 use futures::executor::block_on;
-use futures::future::{err, ok, select_ok, Future};
+use futures::future::{err, ok, select_ok, select_ok_biased, Future};
 use futures_channel::oneshot;
 use std::thread;
 
@@ -65,3 +65,15 @@ fn is_fair() {
     assert_eq!(results.iter().filter(|i| **i == 3).take(THRESHOLD).count(), THRESHOLD);
     assert_eq!(results.iter().filter(|i| **i == 4).take(THRESHOLD).count(), THRESHOLD);
 }
+
+#[test]
+fn is_biased() {
+    let mut results = Vec::with_capacity(100);
+    for _ in 0..100 {
+        let v = vec![err(1), err(2), ok(3), ok(4)];
+
+        let (i, _v) = block_on(select_ok_biased(v)).ok().unwrap();
+        results.push(i);
+    }
+    assert!(results.iter().all(|i| *i == 3));
+}

futures/tests/future_try_select.rs

@@ -1,4 +1,4 @@
-use futures::future::{ok, try_select};
+use futures::future::{ok, try_select, try_select_biased};
 use futures_executor::block_on;
 
 #[test]
@@ -12,3 +12,14 @@ fn is_fair() {
     assert_eq!(results.iter().filter(|i| **i == 0).take(THRESHOLD).count(), THRESHOLD);
     assert_eq!(results.iter().filter(|i| **i == 1).take(THRESHOLD).count(), THRESHOLD)
 }
+
+#[test]
+fn is_biased() {
+    let mut results = Vec::with_capacity(100);
+    for _ in 0..100 {
+        let (i, _) =
+            block_on(try_select_biased(ok::<_, ()>(0), ok::<_, ()>(1))).unwrap().factor_first();
+        results.push(i);
+    }
+    assert!(results.iter().all(|i| *i == 0));
+}