[WIP] WakeOnWrite

Cargo.toml

@@ -16,5 +16,8 @@ authors = [
 [features]
 
 [dependencies]
+async-std = "1"
 
 [dev-dependencies]
+async-std = { version = "1", features = ["attributes", "unstable"] }
+pin-utils = "0.1.0-alpha.4"

rustfmt.toml

@@ -0,0 +1 @@
+version = "Two"

src/lib.rs

@@ -9,3 +9,5 @@
 #![forbid(unsafe_code, future_incompatible, rust_2018_idioms)]
 #![deny(missing_debug_implementations, nonstandard_style)]
 #![warn(missing_docs, missing_doc_code_examples, unreachable_pub)]
+
+pub mod wake_on;

src/wake_on.rs

@@ -0,0 +1,117 @@
+//! Smart pointers to wake tasks on access
+use async_std::task::Waker;
+use std::ops::{Deref, DerefMut};
+
+/// A wrapper type which wakes tasks whenever the wrapped value is accessed
+/// through an `&mut` reference.
+///
+/// `T` is the type of the value being wrapped. This struct is `Deref` and
+/// `DerefMut` for that type, giving `&T` and `&mut T` respectively.
+/// When a `Waker` is registered with `set_waker`, that `Waker` is woken
+/// whenever the wrapped value is accessed through an `&mut` reference
+/// and therefore potentially mutated.
+///
+/// This is useful when there is a future polling the state of the wrapped
+/// value. It needs to be awoken whenever that value changes so that they
+/// can check whether or not its value is in a state that will let the
+/// future make progress. That future can register the `Waker` from the
+/// `Context` it is passed with the `WakeOnWrite` wrapping the value it is
+/// interested in so that all mutations cause it to be woken going forward.
+///
+/// This type isn't effective for observing changes on values with interior
+/// mutablity, because it only wakes on `&mut` access.
+#[derive(Default, Debug, Clone)]
+pub struct WakeOnWrite<T> {
+    inner: T,
+    waker: Option<Waker>,
+}
+
+impl<T> WakeOnWrite<T> {
+    /// Create a new `WakeOnWrite` with the given value.
+    pub fn new(value: T) -> Self {
+        Self {
+            inner: value,
+            waker: None,
+        }
+    }
+
+    /// Set the `Waker` to be awoken when this value is mutated.
+    ///
+    /// Returns the currently registered `Waker`, if there is one.
+    pub fn set_waker(wow: &mut Self, waker: Waker) -> Option<Waker> {
+        wow.waker.replace(waker)
+    }
+
+    /// Removes and returns the currently registered `Waker`, if there is one.
+    pub fn take_waker(wow: &mut Self) -> Option<Waker> {
+        wow.waker.take()
+    }
+
+    /// Returns the currently registered `Waker`, leaving it registered, if
+    /// there is one.
+    pub fn waker(wow: &Self) -> Option<&Waker> {
+        wow.waker.as_ref()
+    }
+}
+
+impl<T> Deref for WakeOnWrite<T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl<T> DerefMut for WakeOnWrite<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.waker.as_ref().map(|w| w.wake_by_ref());
+        &mut self.inner
+    }
+}
+
+#[async_std::test]
+async fn wow_wakes_target_on_mut_access() {
+    use async_std::future::poll_fn;
+    use async_std::prelude::*;
+    use async_std::sync::Arc;
+    use async_std::sync::Mutex;
+    use async_std::task::Poll;
+    use pin_utils::pin_mut;
+    use std::future::Future;
+
+    let data: Arc<Mutex<WakeOnWrite<u8>>> = Default::default();
+    let data_checker = {
+        let data_ref = data.clone();
+        poll_fn(move |ctx| {
+            // This is an inefficient use of futures, but it does work in this
+            // case.
+            let data_lock_future = data_ref.lock();
+            pin_mut!(data_lock_future);
+            match data_lock_future.poll(ctx) {
+                Poll::Ready(mut lock) => match **lock {
+                    10 => Poll::Ready(()),
+                    _ => {
+                        WakeOnWrite::set_waker(&mut lock, ctx.waker().clone());
+                        Poll::Pending
+                    }
+                },
+                Poll::Pending => Poll::Pending,
+            }
+        })
+    };
+
+    let data_incrementor = {
+        let data_ref = data.clone();
+        async move {
+            for _ in 0..10u8 {
+                let mut lock = data_ref.lock().await;
+                **lock += 1;
+            }
+        }
+    };
+
+    data_checker
+        .join(data_incrementor)
+        .timeout(core::time::Duration::new(1, 0))
+        .await
+        .unwrap();
+}