Add a method to learn about cancel on Complete

This commits adds a `poll_cancel` method to learn about when the `Oneshot` half
of a complete/oneshot pair has gone away. This can then be used to detect when a
computation is no longer wanted.

Closes rust-lang#63

src/oneshot.rs

@@ -1,8 +1,10 @@
 use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, Ordering};
 
 use {Future, Poll};
 use slot::{Slot, Token};
-use task;
+use lock::Lock;
+use task::{self, TaskHandle};
 
 /// A future representing the completion of a computation happening elsewhere in
 /// memory.
@@ -24,6 +26,8 @@ pub struct Complete<T> {
 
 struct Inner<T> {
     slot: Slot<Option<T>>,
+    oneshot_gone: AtomicBool,
+    notify_cancel: Lock<Option<TaskHandle>>,
 }
 
 /// Creates a new in-memory oneshot used to represent completing a computation.
@@ -55,6 +59,8 @@ struct Inner<T> {
 pub fn oneshot<T>() -> (Complete<T>, Oneshot<T>) {
     let inner = Arc::new(Inner {
         slot: Slot::new(None),
+        oneshot_gone: AtomicBool::new(false),
+        notify_cancel: Lock::new(None),
     });
     let oneshot = Oneshot {
         inner: inner.clone(),
@@ -78,6 +84,57 @@ impl<T> Complete<T> {
         self.send(Some(t))
     }
 
+    /// Polls this `Complete` half to detect whether the `Oneshot` this has
+    /// paired with has gone away.
+    ///
+    /// This function can be used to learn about when the `Oneshot` (consumer)
+    /// half has gone away and nothing will be able to receive a message sent
+    /// from `complete`.
+    ///
+    /// Like `Future::poll`, this function will panic if it's not called from
+    /// within the context of a task. In otherwords, this should only ever be
+    /// called from inside another future.
+    ///
+    /// If `Poll::Ok` is returned then it means that the `Oneshot` has
+    /// disappeared and the result this `Complete` would otherwise produce
+    /// should no longer be produced.
+    ///
+    /// If `Poll::NotReady` is returned then the `Oneshot` is still alive and
+    /// may be able to receive a message if sent. The current task, however,
+    /// is scheduled to receive a notification if the corresponding `Oneshot`
+    /// goes away.
+    pub fn poll_cancel(&mut self) -> Poll<(), ()> {
+        // Fast path up first, just read the flag and see if our other half is
+        // gone.
+        if self.inner.oneshot_gone.load(Ordering::SeqCst) {
+            return Poll::Ok(())
+        }
+
+        // If our other half is not gone then we need to park our current task
+        // and move it into the `notify_cancel` slot to get notified when it's
+        // actually gone.
+        //
+        // If `try_lock` fails, then the `Oneshot` is in the process of using
+        // it, so we can deduce that it's now in the process of going away and
+        // hence we're canceled. If it succeeds then we just store our handle.
+        //
+        // Crucially we then check `oneshot_gone` *again* before we return.
+        // While we were storing our handle inside `notify_cancel` the `Oneshot`
+        // may have been dropped. The first thing it does is set the flag, and
+        // if it fails to acquire the lock it assumes that we'll see the flag
+        // later on. So... we then try to see the flag later on!
+        let handle = task::park();
+        match self.inner.notify_cancel.try_lock() {
+            Some(mut p) => *p = Some(handle),
+            None => return Poll::Ok(()),
+        }
+        if self.inner.oneshot_gone.load(Ordering::SeqCst) {
+            Poll::Ok(())
+        } else {
+            Poll::NotReady
+        }
+    }
+
     fn send(&mut self, t: Option<T>) {
         if let Err(e) = self.inner.slot.try_produce(t) {
             self.inner.slot.on_empty(Some(e.into_inner()), |slot, item| {
@@ -125,8 +182,27 @@ impl<T> Future for Oneshot<T> {
 
 impl<T> Drop for Oneshot<T> {
     fn drop(&mut self) {
+        // First up, if we squirreled away a task to get notified once the
+        // oneshot was filled in, we cancel that notification. We'll never end
+        // up actually receiving data (as we're being dropped) so no need to
+        // hold onto the task.
         if let Some(cancel_token) = self.cancel_token.take() {
             self.inner.slot.cancel(cancel_token)
         }
+
+        // Next up, inform the `Complete` half that we're going away. First up
+        // we flag ourselves as gone, and next we'll attempt to wake up any
+        // handle that was stored.
+        //
+        // If we fail to acquire the lock on the handle, that means that a
+        // `Complete` is in the process of storing one, and it'll check
+        // `oneshot_gone` on its way out to see our write here.
+        self.inner.oneshot_gone.store(true, Ordering::SeqCst);
+        if let Some(mut handle) = self.inner.notify_cancel.try_lock() {
+            if let Some(task) = handle.take() {
+                drop(handle);
+                task.unpark()
+            }
+        }
     }
 }

tests/oneshot.rs

@@ -0,0 +1,64 @@
+extern crate futures;
+
+use std::sync::mpsc::{channel, Sender};
+use std::thread;
+
+use futures::{oneshot, Complete, Future, Poll};
+use futures::task::Task;
+
+#[test]
+fn smoke_poll() {
+    let (mut tx, rx) = oneshot::<u32>();
+    Task::new().enter(|| {
+        assert!(tx.poll_cancel().is_not_ready());
+        assert!(tx.poll_cancel().is_not_ready());
+        drop(rx);
+        assert!(tx.poll_cancel().is_ready());
+        assert!(tx.poll_cancel().is_ready());
+    })
+}
+
+#[test]
+fn cancel_notifies() {
+    let (tx, rx) = oneshot::<u32>();
+    let (tx2, rx2) = channel();
+
+    WaitForCancel { tx: tx }.then(move |v| tx2.send(v)).forget();
+    drop(rx);
+    rx2.recv().unwrap().unwrap();
+}
+
+struct WaitForCancel {
+    tx: Complete<u32>,
+}
+
+impl Future for WaitForCancel {
+    type Item = ();
+    type Error = ();
+
+    fn poll(&mut self) -> Poll<(), ()> {
+        self.tx.poll_cancel()
+    }
+}
+
+#[test]
+fn cancel_lots() {
+    let (tx, rx) = channel::<(Complete<_>, Sender<_>)>();
+    let t = thread::spawn(move || {
+        for (tx, tx2) in rx {
+            WaitForCancel { tx: tx }.then(move |v| tx2.send(v)).forget();
+        }
+
+    });
+
+    for _ in 0..20000 {
+        let (otx, orx) = oneshot::<u32>();
+        let (tx2, rx2) = channel();
+        tx.send((otx, tx2)).unwrap();
+        drop(orx);
+        rx2.recv().unwrap().unwrap();
+    }
+    drop(tx);
+
+    t.join().unwrap();
+}