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

18 files changed, 414 insertions, 8 deletions

diff --git a/.github/workflows/rust.yml b/.github/workflows/rust.yml
index 3ddda1a34..79a5ad796 100644
--- a/.github/workflows/rust.yml
+++ b/.github/workflows/rust.yml
@@ -94,6 +94,8 @@ jobs:
             target: thumbv6m-none-eabi
           - package: examples/stm32g0
             target: thumbv6m-none-eabi
+          - package: examples/wasm
+            target: wasm32-unknown-unknown
     steps:
       - uses: actions/checkout@v2
         with:
diff --git a/embassy-macros/Cargo.toml b/embassy-macros/Cargo.toml
index b56f0f72f..955121230 100644
--- a/embassy-macros/Cargo.toml
+++ b/embassy-macros/Cargo.toml
@@ -18,3 +18,4 @@ nrf = []
 stm32 = []
 rp = []
 std = []
+wasm = []
diff --git a/embassy-macros/src/lib.rs b/embassy-macros/src/lib.rs
index ab327aced..708eed4cd 100644
--- a/embassy-macros/src/lib.rs
+++ b/embassy-macros/src/lib.rs
@@ -450,3 +450,82 @@ pub fn main(args: TokenStream, item: TokenStream) -> TokenStream {
     };
     result.into()
 }
+
+#[cfg(feature = "wasm")]
+#[proc_macro_attribute]
+pub fn main(args: TokenStream, item: TokenStream) -> TokenStream {
+    let macro_args = syn::parse_macro_input!(args as syn::AttributeArgs);
+    let task_fn = syn::parse_macro_input!(item as syn::ItemFn);
+
+    let macro_args = match MainArgs::from_list(&macro_args) {
+        Ok(v) => v,
+        Err(e) => {
+            return TokenStream::from(e.write_errors());
+        }
+    };
+
+    let embassy_path = macro_args.embassy_prefix.append("embassy");
+
+    let mut fail = false;
+    if task_fn.sig.asyncness.is_none() {
+        task_fn
+            .sig
+            .span()
+            .unwrap()
+            .error("task functions must be async")
+            .emit();
+        fail = true;
+    }
+    if !task_fn.sig.generics.params.is_empty() {
+        task_fn
+            .sig
+            .span()
+            .unwrap()
+            .error("main function must not be generic")
+            .emit();
+        fail = true;
+    }
+
+    let args = task_fn.sig.inputs.clone();
+
+    if args.len() != 1 {
+        task_fn
+            .sig
+            .span()
+            .unwrap()
+            .error("main function must have one argument")
+            .emit();
+        fail = true;
+    }
+
+    if fail {
+        return TokenStream::new();
+    }
+
+    let task_fn_body = task_fn.block.clone();
+
+    let embassy_path = embassy_path.path();
+    let embassy_prefix_lit = macro_args.embassy_prefix.literal();
+
+    let result = quote! {
+        #[#embassy_path::task(embassy_prefix = #embassy_prefix_lit)]
+        async fn __embassy_main(#args) {
+            #task_fn_body
+        }
+
+        use wasm_bindgen::prelude::*;
+
+        #[wasm_bindgen(start)]
+        pub fn main() -> Result<(), JsValue> {
+            static EXECUTOR: #embassy_path::util::Forever<#embassy_path::executor::Executor> = #embassy_path::util::Forever::new();
+            let executor = EXECUTOR.put(#embassy_path::executor::Executor::new());
+
+            executor.start(|spawner| {
+                spawner.spawn(__embassy_main(spawner)).unwrap();
+            });
+
+            Ok(())
+        }
+    };
+    result.into()
+}
diff --git a/embassy-nrf/Cargo.toml b/embassy-nrf/Cargo.toml
index f42840133..edc1e22a3 100644
--- a/embassy-nrf/Cargo.toml
+++ b/embassy-nrf/Cargo.toml
@@ -45,7 +45,7 @@ cortex-m = "0.7.3"
 embedded-hal    = "0.2.6"
 embedded-dma    = "0.1.2"
 futures     = { version = "0.3.17", default-features = false }
-critical-section = "0.2.1"
+critical-section = "0.2.2"
 rand_core = "0.6.3"
 
 nrf52805-pac  = { version = "0.10.1", optional = true, features = [ "rt" ] }
diff --git a/embassy-rp/Cargo.toml b/embassy-rp/Cargo.toml
index 770005119..1702e0df8 100644
--- a/embassy-rp/Cargo.toml
+++ b/embassy-rp/Cargo.toml
@@ -27,7 +27,7 @@ defmt = { version = "0.2.3", optional = true }
 log = { version = "0.4.14", optional = true }
 cortex-m-rt = ">=0.6.15,<0.8"
 cortex-m = "0.7.3"
-critical-section = "0.2.1"
+critical-section = "0.2.2"
 
 rp2040-pac2 = { git = "https://github.com/embassy-rs/rp2040-pac2", rev="9ad7223a48a065e612bc7dc7be5bf5bd0b41cfc4", features = ["rt"] }
 #rp2040-pac2 = { path = "../../rp/rp2040-pac2", features = ["rt"] }
diff --git a/embassy-stm32/Cargo.toml b/embassy-stm32/Cargo.toml
index b34d56854..3758c0626 100644
--- a/embassy-stm32/Cargo.toml
+++ b/embassy-stm32/Cargo.toml
@@ -21,7 +21,7 @@ futures = { version = "0.3.17", default-features = false, features = ["async-awa
 rand_core = "0.6.3"
 sdio-host = "0.5.0"
 embedded-sdmmc = { git = "https://github.com/thalesfragoso/embedded-sdmmc-rs", branch = "async", optional = true }
-critical-section = "0.2.1"
+critical-section = "0.2.2"
 bare-metal = "1.0.0"
 atomic-polyfill = "0.1.3"
 stm32-metapac = { version = "0.1.0", path = "../stm32-metapac", features = ["rt"] }
diff --git a/embassy/Cargo.toml b/embassy/Cargo.toml
index ae06bc198..8aac861e3 100644
--- a/embassy/Cargo.toml
+++ b/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" ] }
diff --git a/embassy/src/executor/arch/wasm.rs b/embassy/src/executor/arch/wasm.rs
new file mode 100644
index 000000000..f069ebc3d
--- /dev/null
+++ b/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());
+        }
+    }
+}
diff --git a/embassy/src/executor/mod.rs b/embassy/src/executor/mod.rs
index 1f6bdd277..3ec24c13c 100644
--- a/embassy/src/executor/mod.rs
+++ b/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;
diff --git a/embassy/src/executor/raw/mod.rs b/embassy/src/executor/raw/mod.rs
index 05fb29758..08de77739 100644
--- a/embassy/src/executor/raw/mod.rs
+++ b/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};
diff --git a/embassy/src/lib.rs b/embassy/src/lib.rs
index 847142285..2eadefb08 100644
--- a/embassy/src/lib.rs
+++ b/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)]
diff --git a/embassy/src/time/driver_wasm.rs b/embassy/src/time/driver_wasm.rs
new file mode 100644
index 000000000..0a9270823
--- /dev/null
+++ b/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())
+    }
+}
diff --git a/embassy/src/time/mod.rs b/embassy/src/time/mod.rs
index 6ce18d475..c8971bd13 100644
--- a/embassy/src/time/mod.rs
+++ b/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;
diff --git a/examples/stm32h7/Cargo.toml b/examples/stm32h7/Cargo.toml
index 94586b8ac..c581e212a 100644
--- a/examples/stm32h7/Cargo.toml
+++ b/examples/stm32h7/Cargo.toml
@@ -35,7 +35,7 @@ futures = { version = "0.3.17", default-features = false, features = ["async-awa
 rtt-target = { version = "0.3.1", features = ["cortex-m"] }
 heapless = { version = "0.7.5", default-features = false }
 rand_core = "0.6.3"
-critical-section = "0.2.1"
+critical-section = "0.2.2"
 
 micromath = "2.0.0"
 
diff --git a/examples/wasm/Cargo.toml b/examples/wasm/Cargo.toml
new file mode 100644
index 000000000..77f513ffa
--- /dev/null
+++ b/examples/wasm/Cargo.toml
@@ -0,0 +1,17 @@
+[package]
+authors = ["Ulf Lilleengen <[email protected]>"]
+edition = "2018"
+name = "embassy-wasm-example"
+version = "0.1.0"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+embassy = { version = "0.1.0", path = "../../embassy", features = ["log", "wasm"] }
+
+wasm-logger = "0.2.0"
+wasm-bindgen = "0.2"
+web-sys = { version = "0.3", features = ["Document", "Element", "HtmlElement", "Node", "Window" ] }
+log = "0.4.11"
+critical-section = "0.2.2"
diff --git a/examples/wasm/README.md b/examples/wasm/README.md
new file mode 100644
index 000000000..4bed4a797
--- /dev/null
+++ b/examples/wasm/README.md
@@ -0,0 +1,26 @@
+# WASM example
+
+Examples use a CLI tool named `wasm-pack` to build this example:
+
+```
+cargo install wasm-pack
+```
+
+## Building
+
+To build the example, run:
+
+```
+wasm-pack build --target web
+```
+
+## Running
+
+To run the example, start a webserver server the local folder:
+
+
+```
+python -m http.server
+```
+
+Then, open a browser at https://127.0.0.1:8000 and watch the ticker print entries to the window.
diff --git a/examples/wasm/index.html b/examples/wasm/index.html
new file mode 100644
index 000000000..05e8b29f8
--- /dev/null
+++ b/examples/wasm/index.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <meta content="text/html;charset=utf-8" http-equiv="Content-Type"/>
+  </head>
+  <body>
+    <!-- Note the usage of `type=module` here as this is an ES6 module -->
+    <script type="module">
+      // Use ES module import syntax to import functionality from the module
+      // that we have compiled.
+      //
+      // Note that the `default` import is an initialization function which
+      // will "boot" the module and make it ready to use. Currently browsers
+      // don't support natively imported WebAssembly as an ES module, but
+      // eventually the manual initialization won't be required!
+      import init from './pkg/embassy_wasm_example.js';
+      await init();
+    </script>
+    <h1>Log</h1>
+    <div>
+        <ul id="log">
+        </ul>
+    </div>
+  </body>
+</html>
diff --git a/examples/wasm/src/lib.rs b/examples/wasm/src/lib.rs
new file mode 100644
index 000000000..0aa32a70b
--- /dev/null
+++ b/examples/wasm/src/lib.rs
@@ -0,0 +1,37 @@
+#![feature(type_alias_impl_trait)]
+#![allow(incomplete_features)]
+
+use embassy::{
+    executor::Spawner,
+    time::{Duration, Timer},
+};
+
+#[embassy::task]
+async fn ticker() {
+    let window = web_sys::window().expect("no global `window` exists");
+
+    let mut counter = 0;
+    loop {
+        let document = window.document().expect("should have a document on window");
+        let list = document
+            .get_element_by_id("log")
+            .expect("should have a log element");
+
+        let li = document
+            .create_element("li")
+            .expect("error creating list item element");
+        li.set_text_content(Some(&format!("tick {}", counter)));
+
+        list.append_child(&li).expect("error appending list item");
+        log::info!("tick {}", counter);
+        counter += 1;
+
+        Timer::after(Duration::from_secs(1)).await;
+    }
+}
+
+#[embassy::main]
+async fn main(spawner: Spawner) {
+    wasm_logger::init(wasm_logger::Config::default());
+    spawner.spawn(ticker()).unwrap();
+}