Merge pull request #1514 from ibaryshnikov/threadsafe-futures

Threadsafe futures

Author: alexcrichton

crates/futures/Cargo.toml

@@ -11,13 +11,22 @@ version = "0.3.25"
 edition = "2018"
 
 [dependencies]
+cfg-if = "0.1.9"
 futures = "0.1.20"
 js-sys = { path = "../js-sys", version = '0.3.25' }
 wasm-bindgen = { path = "../..", version = '0.2.48' }
 futures-util-preview = { version = "0.3.0-alpha.15", optional = true }
 futures-channel-preview = { version = "0.3.0-alpha.15", optional = true }
 lazy_static = { version = "1.3.0", optional = true }
 
+[target.'cfg(target_feature = "atomics")'.dependencies.web-sys]
+path = "../web-sys"
+version = "0.3.24"
+features = [
+    "MessageEvent",
+    "Worker",
+]
+
 [target.'cfg(target_arch = "wasm32")'.dev-dependencies]
 wasm-bindgen-test = { path = '../test', version = '0.2.48' }
 

crates/futures/src/legacy.rs

@@ -0,0 +1,204 @@
+use futures::executor::{self, Notify, Spawn};
+use futures::prelude::*;
+use js_sys::{Function, Promise};
+use std::cell::{Cell, RefCell};
+use std::rc::Rc;
+use std::sync::Arc;
+use wasm_bindgen::prelude::*;
+
+/// Converts a Rust `Future` into a JavaScript `Promise`.
+///
+/// This function will take any future in Rust and schedule it to be executed,
+/// returning a JavaScript `Promise` which can then be passed back to JavaScript
+/// to get plumbed into the rest of a system.
+///
+/// The `future` provided must adhere to `'static` because it'll be scheduled
+/// to run in the background and cannot contain any stack references. The
+/// returned `Promise` will be resolved or rejected when the future completes,
+/// depending on whether it finishes with `Ok` or `Err`.
+///
+/// # Panics
+///
+/// Note that in wasm panics are currently translated to aborts, but "abort" in
+/// this case means that a JavaScript exception is thrown. The wasm module is
+/// still usable (likely erroneously) after Rust panics.
+///
+/// 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,
+{
+    _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!"
+//
+// 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 {
+    let mut future = Some(executor::spawn(future));
+    return Promise::new(&mut |resolve, reject| {
+        Package::poll(&Arc::new(Package {
+            spawn: RefCell::new(future.take().unwrap()),
+            resolve,
+            reject,
+            notified: Cell::new(State::Notified),
+        }));
+    });
+
+    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>,
+
+        // 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,
+    }
+
+    // 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>),
+    }
+
+    // No shared memory right now, wasm is single threaded, no need to worry
+    // about this!
+    unsafe impl Send for Package {}
+    unsafe impl Sync for Package {}
+
+    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()));
+                        break;
+                    }
+
+                    State::Waiting(_) => panic!("shouldn't see waiting state!"),
+                }
+
+                let (val, f) = match me.spawn.borrow_mut().poll_future_notify(me, 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),
+
+                    // Otherwise keep going in our loop, if we weren't notified
+                    // we'll break out and start waiting.
+                    Ok(Async::NotReady) => continue,
+                };
+
+                drop(f.call1(&JsValue::undefined(), &val));
+                break;
+            }
+        }
+    }
+
+    impl Notify for Package {
+        fn notify(&self, _id: usize) {
+            let me = match self.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 => 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,
+            };
+
+            // 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.
+            //
+            // Note that the `Rc`/`RefCell` trick here is basically to just
+            // ensure that our `Closure` gets cleaned up appropriately.
+            let promise = Promise::resolve(&JsValue::undefined());
+            let slot = Rc::new(RefCell::new(None));
+            let slot2 = slot.clone();
+            let closure = Closure::wrap(Box::new(move |_| {
+                let myself = slot2.borrow_mut().take();
+                debug_assert!(myself.is_some());
+                Package::poll(&me);
+            }) as Box<dyn FnMut(JsValue)>);
+            promise.then(&closure);
+            *slot.borrow_mut() = Some(closure);
+        }
+    }
+}

crates/futures/src/legacy_atomics.rs

@@ -0,0 +1,166 @@
+use futures::executor::{self, Notify, Spawn};
+use futures::prelude::*;
+use js_sys::Function;
+use std::sync::atomic::{AtomicI32, Ordering};
+use std::sync::Arc;
+use wasm_bindgen::prelude::*;
+use wasm_bindgen::JsCast;
+
+// 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;
+}
+
+/// Converts a Rust `Future` into a JavaScript `Promise`.
+///
+/// This function will take any future in Rust and schedule it to be executed,
+/// returning a JavaScript `Promise` which can then be passed back to JavaScript
+/// to get plumbed into the rest of a system.
+///
+/// The `future` provided must adhere to `'static` because it'll be scheduled
+/// to run in the background and cannot contain any stack references. The
+/// returned `Promise` will be resolved or rejected when the future completes,
+/// depending on whether it finishes with `Ok` or `Err`.
+///
+/// # Panics
+///
+/// Note that in wasm panics are currently translated to aborts, but "abort" in
+/// this case means that a JavaScript exception is thrown. The wasm module is
+/// still usable (likely erroneously) after Rust panics.
+///
+/// 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) -> 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 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.
+//
+// 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 js_sys::Promise::new(&mut |resolve, reject| {
+        Package {
+            spawn: future.take().unwrap(),
+            resolve,
+            reject,
+            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: 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,
+
+        // Shared state used to communicate waking up this future, this is the
+        // `Send + Sync` piece needed by the async task system.
+        waker: Arc<Waker>,
+    }
+
+    struct Waker {
+        value: AtomicI32,
+    };
+
+    impl Notify for Waker {
+        fn notify(&self, _id: usize) {
+            // 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;
+            }
+            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
+                );
+            }
+        }
+    }
+
+    impl Package {
+        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, &self.resolve),
+                    Err(value) => (value, &self.reject),
+
+                    // ... 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));
+                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::wait_async_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)
+    }
+}