Add the ability to spawn futures

Add a way to spawn tasks with a returned `Future`. The task is
immediately queued for the thread pool to execute.

rayon-core/src/future.rs

@@ -0,0 +1,138 @@
+#![allow(missing_docs)]
+
+use crate::job::JobResult;
+use crate::unwind;
+use crate::ThreadPool;
+use crate::{spawn, spawn_fifo};
+use crate::{Scope, ScopeFifo};
+
+use std::future::Future;
+use std::mem;
+use std::pin::Pin;
+use std::sync::{Arc, Mutex};
+use std::task::{Context, Poll, Waker};
+
+struct RayonFuture<T> {
+    state: Arc<Mutex<State<T>>>,
+}
+
+struct RayonFutureJob<T> {
+    state: Arc<Mutex<State<T>>>,
+}
+
+struct State<T> {
+    result: JobResult<T>,
+    waker: Option<Waker>,
+}
+
+fn new<T>() -> (RayonFuture<T>, RayonFutureJob<T>) {
+    let state = Arc::new(Mutex::new(State {
+        result: JobResult::None,
+        waker: None,
+    }));
+    (
+        RayonFuture {
+            state: state.clone(),
+        },
+        RayonFutureJob { state },
+    )
+}
+
+impl<T> Future for RayonFuture<T> {
+    type Output = T;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let mut guard = self.state.lock().expect("rayon future lock");
+        match mem::replace(&mut guard.result, JobResult::None) {
+            JobResult::None => {
+                guard.waker = Some(cx.waker().clone());
+                Poll::Pending
+            }
+            JobResult::Ok(x) => Poll::Ready(x),
+            JobResult::Panic(p) => {
+                drop(guard); // don't poison the lock
+                unwind::resume_unwinding(p);
+            }
+        }
+    }
+}
+
+impl<T> RayonFutureJob<T> {
+    fn execute(self, func: impl FnOnce() -> T) {
+        let result = unwind::halt_unwinding(func);
+        let mut guard = self.state.lock().expect("rayon future lock");
+        guard.result = match result {
+            Ok(x) => JobResult::Ok(x),
+            Err(p) => JobResult::Panic(p),
+        };
+        if let Some(waker) = guard.waker.take() {
+            waker.wake();
+        }
+    }
+}
+
+pub fn spawn_future<F, T>(func: F) -> impl Future<Output = T>
+where
+    F: FnOnce() -> T + Send + 'static,
+    T: Send + 'static,
+{
+    let (future, job) = new();
+    spawn(move || job.execute(func));
+    future
+}
+
+pub fn spawn_fifo_future<F, T>(func: F) -> impl Future<Output = T>
+where
+    F: FnOnce() -> T + Send + 'static,
+    T: Send + 'static,
+{
+    let (future, job) = new();
+    spawn_fifo(move || job.execute(func));
+    future
+}
+
+impl ThreadPool {
+    pub fn spawn_future<F, T>(&self, func: F) -> impl Future<Output = T>
+    where
+        F: FnOnce() -> T + Send + 'static,
+        T: Send + 'static,
+    {
+        let (future, job) = new();
+        self.spawn(move || job.execute(func));
+        future
+    }
+
+    pub fn spawn_fifo_future<F, T>(&self, func: F) -> impl Future<Output = T>
+    where
+        F: FnOnce() -> T + Send + 'static,
+        T: Send + 'static,
+    {
+        let (future, job) = new();
+        self.spawn_fifo(move || job.execute(func));
+        future
+    }
+}
+
+impl<'scope> Scope<'scope> {
+    pub fn spawn_future<F, T>(&self, func: F) -> impl Future<Output = T>
+    where
+        F: FnOnce(&Self) -> T + Send + 'scope,
+        T: Send + 'scope,
+    {
+        let (future, job) = new();
+        self.spawn(|scope| job.execute(move || func(scope)));
+        future
+    }
+}
+
+impl<'scope> ScopeFifo<'scope> {
+    pub fn spawn_fifo_future<F, T>(&self, func: F) -> impl Future<Output = T>
+    where
+        F: FnOnce(&Self) -> T + Send + 'scope,
+        T: Send + 'scope,
+    {
+        let (future, job) = new();
+        self.spawn_fifo(|scope| job.execute(move || func(scope)));
+        future
+    }
+}

rayon-core/src/lib.rs

@@ -80,6 +80,7 @@ mod log;
 mod private;
 
 mod broadcast;
+mod future;
 mod job;
 mod join;
 mod latch;
@@ -94,6 +95,7 @@ mod compile_fail;
 mod test;
 
 pub use self::broadcast::{broadcast, spawn_broadcast, BroadcastContext};
+pub use self::future::{spawn_fifo_future, spawn_future};
 pub use self::join::{join, join_context};
 pub use self::registry::ThreadBuilder;
 pub use self::scope::{in_place_scope, scope, Scope};