Clean up atomics/futures + polyfill

* Remove now-unneeded `State` enum
* Remove timeout argument from polyfill since we don't need it
* Call `Atomics.waitAsync` if it's available instead of using our polyfill
* Remove some extraneous dead code from the polyfill
* Add a `val: i32` argument to the polyfill
* Simplify the flow of futures with `Package` since `waitAsync` handles
  all the heavy lifting for us.
* Remove `Arc<Package>` and just use `Package`
* Remove `RefCell` from inside of `Package` now that it is no longer
  needed.

Author: alexcrichton

crates/futures/src/atomics.rs

@@ -1,4 +1,4 @@
-use std::cell::{Cell, RefCell};
+use std::cell::RefCell;
 use std::fmt;
 use std::rc::Rc;
 use std::sync::atomic::{AtomicI32, Ordering};
@@ -8,8 +8,9 @@ use futures::executor::{self, Notify, Spawn};
 use futures::future;
 use futures::prelude::*;
 use futures::sync::oneshot;
-use js_sys::{Function, Promise};
+use js_sys::Function;
 use wasm_bindgen::prelude::*;
+use wasm_bindgen::JsCast;
 
 /// A Rust `Future` backed by a JavaScript `Promise`.
 ///
@@ -23,14 +24,28 @@ pub struct JsFuture {
     rx: oneshot::Receiver<Result<JsValue, JsValue>>,
 }
 
+// Duplicate a bit here because `then` takes a `JsValue` instead of a `Closure`.
+#[wasm_bindgen]
+extern "C" {
+    type Promise;
+    #[wasm_bindgen(method)]
+    fn then(this: &Promise, cb: &JsValue) -> Promise;
+
+    type Atomics;
+    #[wasm_bindgen(static_method_of = Atomics, js_name = waitAsync)]
+    fn wait_async(buf: &JsValue, index: i32, value: i32) -> js_sys::Promise;
+    #[wasm_bindgen(static_method_of = Atomics, js_name = waitAsync, getter)]
+    fn get_wait_async() -> JsValue;
+}
+
 impl fmt::Debug for JsFuture {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "JsFuture {{ ... }}")
     }
 }
 
-impl From<Promise> for JsFuture {
-    fn from(js: Promise) -> JsFuture {
+impl From<js_sys::Promise> for JsFuture {
+    fn from(js: js_sys::Promise) -> JsFuture {
         // Use the `then` method to schedule two callbacks, one for the
         // resolved value and one for the rejected value. We're currently
         // assuming that JS engines will unconditionally invoke precisely one of
@@ -112,205 +127,132 @@ impl Future for JsFuture {
 /// If the `future` provided panics then the returned `Promise` **will not
 /// resolve**. Instead it will be a leaked promise. This is an unfortunate
 /// limitation of wasm currently that's hoped to be fixed one day!
-pub fn future_to_promise<F>(future: F) -> Promise
-    where
-        F: Future<Item = JsValue, Error = JsValue> + 'static,
+pub fn future_to_promise<F>(future: F) -> js_sys::Promise
+where
+    F: Future<Item = JsValue, Error = JsValue> + 'static,
 {
     _future_to_promise(Box::new(future))
 }
 
 // Implementation of actually transforming a future into a JavaScript `Promise`.
 //
-// The only primitive we have to work with here is `Promise::new`, which gives
-// us two callbacks that we can use to either reject or resolve the promise.
-// It's our job to ensure that one of those callbacks is called at the
-// appropriate time.
-//
-// Now we know that JavaScript (in general) can't block and is largely
-// notification/callback driven. That means that our future must either have
-// synchronous computational work to do, or it's "scheduled a notification" to
-// happen. These notifications are likely callbacks to get executed when things
-// finish (like a different promise or something like `setTimeout`). The general
-// idea here is thus to do as much synchronous work as we can and then otherwise
-// translate notifications of a future's task into "let's poll the future!"
+// The main primitives used here are `Promise::new` to actually create a JS
+// promise to return as well as `Atomics.waitAsync` to create a promise that we
+// can asynchronously wait on. The general idea here is that we'll create a
+// promise to return and schedule work to happen in `Atomics.waitAsync`
+// callbacks.
 //
-// This isn't necessarily the greatest future executor in the world, but it
-// should get the job done for now hopefully.
-fn _future_to_promise(future: Box<dyn Future<Item = JsValue, Error = JsValue>>) -> Promise {
+// After we've created a promise we start polling a future, and whenever it's
+// not ready we'll execute `Atomics.waitAsync`. When that resolves we'll keep
+// polling the future, and this happens until the future is done. Finally
+// when it's all finished we call either resolver or reject depending on the
+// result of the future.
+fn _future_to_promise(future: Box<dyn Future<Item = JsValue, Error = JsValue>>) -> js_sys::Promise {
     let mut future = Some(executor::spawn(future));
-    return Promise::new(&mut |resolve, reject| {
-        Package::poll(&Arc::new(Package {
-            spawn: RefCell::new(future.take().unwrap()),
+    return js_sys::Promise::new(&mut |resolve, reject| {
+        Package {
+            spawn: future.take().unwrap(),
             resolve,
             reject,
-            notified: Cell::new(State::Notified),
-            waker: Arc::new(Waker::default()),
-        }));
+            waker: Arc::new(Waker {
+                value: AtomicI32::new(1), // 1 == "notified, ready to poll"
+            }),
+        }
+        .poll();
     });
 
     struct Package {
         // Our "spawned future". This'll have everything we need to poll the
         // future and continue to move it forward.
-        spawn: RefCell<Spawn<Box<dyn Future<Item = JsValue, Error = JsValue>>>>,
-
-        // The current state of this future, expressed in an enum below. This
-        // indicates whether we're currently polling the future, received a
-        // notification and need to keep polling, or if we're waiting for a
-        // notification to come in (and no one is polling).
-        notified: Cell<State>,
+        spawn: Spawn<Box<dyn Future<Item = JsValue, Error = JsValue>>>,
 
         // Our two callbacks connected to the `Promise` that we returned to
         // JavaScript.  We'll be invoking one of these at the end.
         resolve: Function,
         reject: Function,
 
-        // Struct to wake a future
+        // Shared state used to communicate waking up this future, this is the
+        // `Send + Sync` piece needed by the async task system.
         waker: Arc<Waker>,
     }
 
-    // The possible states our `Package` (future) can be in, tracked internally
-    // and used to guide what happens when polling a future.
-    enum State {
-        // This future is currently and actively being polled. Attempting to
-        // access the future will result in a runtime panic and is considered a
-        // bug.
-        Polling,
-
-        // This future has been notified, while it was being polled. This marker
-        // is used in the `Notify` implementation below, and indicates that a
-        // notification was received that the future is ready to make progress.
-        // If seen, however, it probably means that the future is also currently
-        // being polled.
-        Notified,
-
-        // The future is blocked, waiting for something to happen. Stored here
-        // is a self-reference to the future itself so we can pull it out in
-        // `Notify` and continue polling.
-        //
-        // Note that the self-reference here is an Arc-cycle that will leak
-        // memory unless the future completes, but currently that should be ok
-        // as we'll have to stick around anyway while the future is executing!
-        //
-        // This state is removed as soon as a notification comes in, so the leak
-        // should only be "temporary"
-        Waiting(Arc<Package>),
-    }
-
-    #[derive(Default)]
     struct Waker {
-        // worker will be waiting on this value
-        // 0 by default, which means not notified
         value: AtomicI32,
     };
 
     impl Notify for Waker {
         fn notify(&self, _id: usize) {
-            // since we have only value field here
-            // let it be 1 if notified, 0 if not
-            if self.value.swap(1, Ordering::SeqCst) == 0 {
-                let _ = unsafe {
-                    core::arch::wasm32::atomic_notify(
-                        &self.value as *const AtomicI32 as *mut i32,
-                        std::u32::MAX, // number of threads to notify
-                    )
-                };
-            }
-        }
-    }
-
-    fn poll_again(package: Arc<Package>) {
-        let me = match package.notified.replace(State::Notified) {
-            // we need to schedule polling to resume, so keep going
-            State::Waiting(me) => {
-                me
-            }
-
-            // we were already notified, and were just notified again;
-            // having now coalesced the notifications we return as it's
-            // still someone else's job to process this
-            State::Notified => {
+            // Attempt to notify us by storing 1. If we're already 1 then we
+            // were previously notified and there's nothing to do. Otherwise
+            // we execute the native `notify` instruction to wake up the
+            // corresponding `waitAsync` that was waiting for the transition
+            // from 0 to 1.
+            let prev = self.value.swap(1, Ordering::SeqCst);
+            if prev == 1 {
                 return;
             }
-
-            // the future was previously being polled, and we've just
-            // switched it to the "you're notified" state. We don't have
-            // access to the future as it's being polled, so the future
-            // polling process later sees this notification and will
-            // continue polling. For us, though, there's nothing else to do,
-            // so we bail out.
-            // later see
-            State::Polling => {
-                return;
+            debug_assert_eq!(prev, 0);
+            unsafe {
+                core::arch::wasm32::atomic_notify(
+                    &self.value as *const AtomicI32 as *mut i32,
+                    1, // number of threads to notify
+                );
             }
-        };
-
-        // Use `Promise.then` on a resolved promise to place our execution
-        // onto the next turn of the microtask queue, enqueueing our poll
-        // operation. We don't currently poll immediately as it turns out
-        // `futures` crate adapters aren't compatible with it and it also
-        // helps avoid blowing the stack by accident.
-        let promise =
-            crate::polyfill::wait_async(&package.waker.value).expect("Should create a Promise");
-        let closure = Closure::once(move |_| {
-            Package::poll(&me);
-        });
-        promise.then(&closure);
-        closure.forget();
+        }
     }
 
     impl Package {
-        // Move the future contained in `me` as far forward as we can. This will
-        // do as much synchronous work as possible to complete the future,
-        // ensuring that when it blocks we're scheduled to get notified via some
-        // callback somewhere at some point (vague, right?)
-        //
-        // TODO: this probably shouldn't do as much synchronous work as possible
-        //       as it can starve other computations. Rather it should instead
-        //       yield every so often with something like `setTimeout` with the
-        //       timeout set to zero.
-        fn poll(me: &Arc<Package>) {
-            loop {
-                match me.notified.replace(State::Polling) {
-                    // We received a notification while previously polling, or
-                    // this is the initial poll. We've got work to do below!
-                    State::Notified => {}
-
-                    // We've gone through this loop once and no notification was
-                    // received while we were executing work. That means we got
-                    // `NotReady` below and we're scheduled to receive a
-                    // notification. Block ourselves and wait for later.
-                    //
-                    // When the notification comes in it'll notify our task, see
-                    // our `Waiting` state, and resume the polling process
-                    State::Polling => {
-                        me.notified.set(State::Waiting(me.clone()));
-
-                        poll_again(me.clone());
-
-                        break;
-                    }
-
-                    State::Waiting(_) => panic!("shouldn't see waiting state!"),
-                }
-
-                let (val, f) = match me.spawn.borrow_mut().poll_future_notify(&me.waker, 0) {
+        fn poll(mut self) {
+            // Poll in a loop waiting for the future to become ready. Note that
+            // we probably shouldn't maximize synchronous work here but rather
+            // we should occasionally yield back to the runtime and schedule
+            // ourselves to resume this future later on.
+            //
+            // Note that 0 here means "need a notification" and 1 means "we got
+            // a notification". That means we're storing 0 into the `notified`
+            // slot and we're trying to read 1 to keep on going.
+            while self.waker.value.swap(0, Ordering::SeqCst) == 1 {
+                let (val, f) = match self.spawn.poll_future_notify(&self.waker, 0) {
                     // If the future is ready, immediately call the
                     // resolve/reject callback and then return as we're done.
-                    Ok(Async::Ready(value)) => (value, &me.resolve),
-                    Err(value) => (value, &me.reject),
+                    Ok(Async::Ready(value)) => (value, &self.resolve),
+                    Err(value) => (value, &self.reject),
 
-                    // Otherwise keep going in our loop, if we weren't notified
-                    // we'll break out and start waiting.
-                    Ok(Async::NotReady) => continue,
+                    // ... otherwise let's break out and wait
+                    Ok(Async::NotReady) => break,
                 };
 
+                // Call the resolution function, and then when we're done
+                // destroy ourselves through `drop` since our future is no
+                // longer needed.
                 drop(f.call1(&JsValue::undefined(), &val));
-                break;
+                return;
             }
+
+            // Create a `js_sys::Promise` using `Atomics.waitAsync` (or our
+            // polyfill) and then register its completion callback as simply
+            // calling this function again.
+            let promise = wait_async(&self.waker.value, 0).unchecked_into::<Promise>();
+            let closure = Closure::once_into_js(move || {
+                self.poll();
+            });
+            promise.then(&closure);
         }
     }
 }
 
+fn wait_async(ptr: &AtomicI32, val: i32) -> js_sys::Promise {
+    // If `Atomics.waitAsync` isn't defined (as it isn't defined anywhere today)
+    // then we use our fallback, otherwise we use the native function.
+    if Atomics::get_wait_async().is_undefined() {
+        crate::polyfill::wait_async(ptr, val)
+    } else {
+        let mem = wasm_bindgen::memory().unchecked_into::<js_sys::WebAssembly::Memory>();
+        Atomics::wait_async(&mem.buffer(), ptr as *const AtomicI32 as i32 / 4, val)
+    }
+
+}
+
 /// Converts a Rust `Future` on a local task queue.
 ///
 /// The `future` provided must adhere to `'static` because it'll be scheduled
@@ -320,8 +262,8 @@ fn _future_to_promise(future: Box<dyn Future<Item = JsValue, Error = JsValue>>)
 ///
 /// This function has the same panic behavior as `future_to_promise`.
 pub fn spawn_local<F>(future: F)
-    where
-        F: Future<Item = (), Error = ()> + 'static,
+where
+    F: Future<Item = (), Error = ()> + 'static,
 {
     future_to_promise(
         future