Add WASM support for executor

* Adds an executor for WASM runtimes based on wasm_bindgen.
* Add time driver based on JS time handling.
* Add example that can run in browser locally.
* Update to critical-section version that supports 'std' flag

embassy/Cargo.toml

@@ -8,6 +8,7 @@ resolver = "2"
 [features]
 default = []
 std = ["futures/std", "embassy-traits/std", "time", "time-tick-1mhz", "embassy-macros/std"]
+wasm = ["wasm-bindgen", "js-sys", "embassy-macros/wasm", "wasm-timer", "time", "time-tick-1mhz"]
 
 # Enable `embassy::time` module. 
 # NOTE: This feature is only intended to be enabled by crates providing the time driver implementation.
@@ -40,10 +41,15 @@ pin-project = { version = "1.0.8", default-features = false }
 embassy-macros  = { version = "0.1.0", path = "../embassy-macros"}
 embassy-traits  = { version = "0.1.0", path = "../embassy-traits"}
 atomic-polyfill  = "0.1.3"
-critical-section = "0.2.1"
+critical-section = "0.2.2"
 embedded-hal = "0.2.6"
 heapless = "0.7.5"
 
+# WASM dependencies
+wasm-bindgen = { version = "0.2.76", features = ["nightly"], optional = true }
+js-sys = { version = "0.3", optional = true }
+wasm-timer = { version = "0.2.5", optional = true }
+
 [dev-dependencies]
 embassy = { path = ".", features = ["executor-agnostic"] }
 futures-executor = { version = "0.3.17", features = [ "thread-pool" ] }

embassy/src/executor/arch/wasm.rs

@@ -0,0 +1,74 @@
+use core::marker::PhantomData;
+use js_sys::Promise;
+use wasm_bindgen::prelude::*;
+
+use super::{
+    raw::{self, util::UninitCell},
+    Spawner,
+};
+
+/// WASM executor, wasm_bindgen to schedule tasks on the JS event loop.
+pub struct Executor {
+    inner: raw::Executor,
+    ctx: &'static WasmContext,
+    not_send: PhantomData<*mut ()>,
+}
+
+pub(crate) struct WasmContext {
+    promise: Promise,
+    closure: UninitCell<Closure<dyn FnMut(JsValue)>>,
+}
+
+impl WasmContext {
+    pub fn new() -> Self {
+        Self {
+            promise: Promise::resolve(&JsValue::undefined()),
+            closure: UninitCell::uninit(),
+        }
+    }
+}
+
+impl Executor {
+    /// Create a new Executor.
+    pub fn new() -> Self {
+        let ctx = &*Box::leak(Box::new(WasmContext::new()));
+        let inner = raw::Executor::new(
+            |p| unsafe {
+                let ctx = &*(p as *const () as *const WasmContext);
+                let _ = ctx.promise.then(ctx.closure.as_mut());
+            },
+            ctx as *const _ as _,
+        );
+        Self {
+            inner,
+            not_send: PhantomData,
+            ctx,
+        }
+    }
+
+    /// Run the executor.
+    ///
+    /// The `init` closure is called with a [`Spawner`] that spawns tasks on
+    /// this executor. Use it to spawn the initial task(s). After `init` returns,
+    /// the executor starts running the tasks.
+    ///
+    /// To spawn more tasks later, you may keep copies of the [`Spawner`] (it is `Copy`),
+    /// for example by passing it as an argument to the initial tasks.
+    ///
+    /// This function requires `&'static mut self`. This means you have to store the
+    /// Executor instance in a place where it'll live forever and grants you mutable
+    /// access. There's a few ways to do this:
+    ///
+    /// - a [Forever](crate::util::Forever) (safe)
+    /// - a `static mut` (unsafe)
+    /// - a local variable in a function you know never returns (like `fn main() -> !`), upgrading its lifetime with `transmute`. (unsafe)
+    pub fn start(&'static mut self, init: impl FnOnce(Spawner)) {
+        unsafe {
+            let executor = &self.inner;
+            self.ctx.closure.write(Closure::new(move |_| {
+                executor.poll();
+            }));
+            init(self.inner.spawner());
+        }
+    }
+}

embassy/src/executor/mod.rs

@@ -3,7 +3,8 @@
 #![deny(missing_docs)]
 
 #[cfg_attr(feature = "std", path = "arch/std.rs")]
-#[cfg_attr(not(feature = "std"), path = "arch/arm.rs")]
+#[cfg_attr(feature = "wasm", path = "arch/wasm.rs")]
+#[cfg_attr(not(any(feature = "std", feature = "wasm")), path = "arch/arm.rs")]
 mod arch;
 pub mod raw;
 mod spawner;

embassy/src/executor/raw/mod.rs

@@ -10,7 +10,7 @@
 mod run_queue;
 #[cfg(feature = "time")]
 mod timer_queue;
-mod util;
+pub(crate) mod util;
 mod waker;
 
 use atomic_polyfill::{AtomicU32, Ordering};

embassy/src/lib.rs

@@ -1,4 +1,4 @@
-#![cfg_attr(not(feature = "std"), no_std)]
+#![cfg_attr(not(any(feature = "std", feature = "wasm")), no_std)]
 #![feature(generic_associated_types)]
 #![feature(const_fn_trait_bound)]
 #![feature(const_fn_fn_ptr_basics)]

embassy/src/time/driver_wasm.rs

@@ -0,0 +1,135 @@
+use atomic_polyfill::{AtomicU8, Ordering};
+use std::cell::UnsafeCell;
+use std::mem::MaybeUninit;
+use std::ptr;
+use std::sync::{Mutex, Once};
+use wasm_bindgen::prelude::*;
+use wasm_timer::Instant as StdInstant;
+
+use crate::time::driver::{AlarmHandle, Driver};
+
+const ALARM_COUNT: usize = 4;
+
+struct AlarmState {
+    token: Option<f64>,
+    closure: Option<Closure<dyn FnMut() + 'static>>,
+}
+
+unsafe impl Send for AlarmState {}
+
+impl AlarmState {
+    const fn new() -> Self {
+        Self {
+            token: None,
+            closure: None,
+        }
+    }
+}
+
+#[wasm_bindgen]
+extern "C" {
+    fn setTimeout(closure: &Closure<dyn FnMut()>, millis: u32) -> f64;
+    fn clearTimeout(token: f64);
+}
+
+struct TimeDriver {
+    alarm_count: AtomicU8,
+
+    once: Once,
+    alarms: UninitCell<Mutex<[AlarmState; ALARM_COUNT]>>,
+    zero_instant: UninitCell<StdInstant>,
+}
+
+const ALARM_NEW: AlarmState = AlarmState::new();
+crate::time_driver_impl!(static DRIVER: TimeDriver = TimeDriver {
+    alarm_count: AtomicU8::new(0),
+    once: Once::new(),
+    alarms: UninitCell::uninit(),
+    zero_instant: UninitCell::uninit(),
+});
+
+impl TimeDriver {
+    fn init(&self) {
+        self.once.call_once(|| unsafe {
+            self.alarms.write(Mutex::new([ALARM_NEW; ALARM_COUNT]));
+            self.zero_instant.write(StdInstant::now());
+        });
+    }
+}
+
+impl Driver for TimeDriver {
+    fn now(&self) -> u64 {
+        self.init();
+
+        let zero = unsafe { self.zero_instant.read() };
+        StdInstant::now().duration_since(zero).as_micros() as u64
+    }
+
+    unsafe fn allocate_alarm(&self) -> Option<AlarmHandle> {
+        let id = self
+            .alarm_count
+            .fetch_update(Ordering::AcqRel, Ordering::Acquire, |x| {
+                if x < ALARM_COUNT as u8 {
+                    Some(x + 1)
+                } else {
+                    None
+                }
+            });
+
+        match id {
+            Ok(id) => Some(AlarmHandle::new(id)),
+            Err(_) => None,
+        }
+    }
+
+    fn set_alarm_callback(&self, alarm: AlarmHandle, callback: fn(*mut ()), ctx: *mut ()) {
+        self.init();
+        let mut alarms = unsafe { self.alarms.as_ref() }.lock().unwrap();
+        let alarm = &mut alarms[alarm.id() as usize];
+        alarm.closure.replace(Closure::new(move || {
+            callback(ctx);
+        }));
+    }
+
+    fn set_alarm(&self, alarm: AlarmHandle, timestamp: u64) {
+        self.init();
+        let mut alarms = unsafe { self.alarms.as_ref() }.lock().unwrap();
+        let alarm = &mut alarms[alarm.id() as usize];
+        let timeout = (timestamp - self.now()) as u32;
+        if let Some(token) = alarm.token {
+            clearTimeout(token);
+        }
+        alarm.token = Some(setTimeout(alarm.closure.as_ref().unwrap(), timeout / 1000));
+    }
+}
+
+pub(crate) struct UninitCell<T>(MaybeUninit<UnsafeCell<T>>);
+unsafe impl<T> Send for UninitCell<T> {}
+unsafe impl<T> Sync for UninitCell<T> {}
+
+impl<T> UninitCell<T> {
+    pub const fn uninit() -> Self {
+        Self(MaybeUninit::uninit())
+    }
+    unsafe fn as_ptr(&self) -> *const T {
+        (*self.0.as_ptr()).get()
+    }
+
+    pub unsafe fn as_mut_ptr(&self) -> *mut T {
+        (*self.0.as_ptr()).get()
+    }
+
+    pub unsafe fn as_ref(&self) -> &T {
+        &*self.as_ptr()
+    }
+
+    pub unsafe fn write(&self, val: T) {
+        ptr::write(self.as_mut_ptr(), val)
+    }
+}
+
+impl<T: Copy> UninitCell<T> {
+    pub unsafe fn read(&self) -> T {
+        ptr::read(self.as_mut_ptr())
+    }
+}

embassy/src/time/mod.rs

@@ -51,6 +51,9 @@ mod timer;
 #[cfg(feature = "std")]
 mod driver_std;
 
+#[cfg(feature = "wasm")]
+mod driver_wasm;
+
 pub use delay::{block_for, Delay};
 pub use duration::Duration;
 pub use instant::Instant;