# wasm-futures-executor

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This crate provides an executor for asynchronous task with the same
API as [`futures_executor::ThreadPool`] targeting the web browser
environment. Instead of using spawning threads via `std::thread`, web
workers are created. This crate tries hard to make this process as
seamless and painless as possible.

[`futures_executor::ThreadPool`]: https://docs.rs/futures-executor/0.3.16/futures_executor/struct.ThreadPool.html

## Sample Usage
```rust
use futures::channel::mpsc;
use futures::StreamExt;
use js_sys::Promise;
use wasm_bindgen::prelude::*;
use wasm_futures_executor::ThreadPool;

#[wasm_bindgen]
pub async fn start() -> Result<JsValue, JsValue> {
    let pool = ThreadPool::max_threads().await?;
    let (tx, mut rx) = mpsc::channel(10);
    for i in 0..20 {
        let mut tx_c = tx.clone();
        pool.spawn_ok(async move {
            tx_c.start_send(i * i).unwrap();
        });
    }
    drop(tx);
    let mut i = 0;
    while let Some(x) = rx.next().await {
        i += x;
    }
    Ok(i.into())
}
```
.. and using it:
```javascript
import init, { start } from './sample.js';

async function run() {
  await init();

  const res = await start();
  console.log("result", res);
}
run();
```

And build your project with:
```sh
RUSTFLAGS='-C target-feature=+atomics,+bulk-memory,+mutable-globals' \
  cargo +nightly build --target wasm32-unknown-unknown --release -Z build-std=std,panic_abort

wasm-bindgen \
  ./target/wasm32-unknown-unknown/release/sample.wasm \
  --out-dir . \
  --target web \
  --weak-refs
``` 
.. or if you want to use `wasm-pack build -t web`, make sure to set up
the nightly toolchain and the proper `RUSTFLAGS` (for example by
providing creating the `rust-toolchain.toml` and `.cargo/config` files
like done in this repo).

Please have a look at the [sample](./sample) for a complete end-to-end
example project without bundlers, and
[sample-webpack](./sample-webpack) using Webpack 5.

Note: This crate requires usage of the `web` target of
`wasm-bindgen`/`wasm-pack`. Given the broad standardization on ES
Modules, this should be mostly fine.

## How it works

Similar to the async executor provided by
[`futures-executor`](https://crates.io/crates/futures-executor),
multiple worker "threads" are spawned upon instantation of the
`ThreadPool`. Each thread is a web worker, which loads some js glue
code. The glue code is provided as a js snippet, which is linked from
the wasm-bindgen generated js glue code. This way, it's transparent to
any bundlers.
Each web worker is constructed with the following arguments:
1. WebAssembly module initialization and its shared memory
([`SharedArrayBuffer`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer)).
2. The third one is a pointer to some shared state, including a channel,
where the async tasks are passed in. The library provides the 
`worker_entry_point` function for this purpose.

Once the `ThreadPool` is dropped, all channels are closed and the web
workers are terminated.

Unfortunately, this requires a nightly compilers because Rust's
standard library needs to be recompiled with the following unstable
features:
* `atomics`: (rust feature) supports for wasm atomics, see
  https://github.com/rust-lang/rust/issues/77839
* `bulk-memory`: (llvm feature) generation of atomic instruction,
  shared memory, passive segments, etc.
* `mutable-globals`: (llvm feature)

A note: When workers are destroyed, some memory might be leaked (for
example thread local storage or the thead's stack). I recommend
passing the `--weak-ref` option to `wasm-bindgen` in order to let
wasm-bindgen create user defined finalizers according to the [WeakRef
proposal](https://github.com/tc39/proposal-weakrefs).

In general, implementing wasm threads in Rust seems to have lost some
of its traction ..


## Browser support

Sharing memory via
[`SharedArrayBuffer`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer)
will soon (starting with Chrome 92) require setting the right headers
to mark the website as "cross-origin isolated", meaning that the
following headers need to be sent with the main document:
```
Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin
```
For more information check out [this](https://web.dev/coop-coep/)
link.

Loading modules in web workers is currently only supported in
Chromium. There seems the be finally some
[progress](https://bugzilla.mozilla.org/show_bug.cgi?id=1247687) in
Firefox. As a workaround, you can include a
[workers-polyfill](https://unpkg.com/module-workers-polyfill).

If you're targeting older browser, you might want to do some feature
detection and fall back gracefully -- but that's out of scope for this
documentation.

## Is it worth it?

There is a significant overhead of sending and spawning futures across the
thread boundary. It makes most sense for long-lived tasks (check out the
[factorial](./factorial) demo, which is a rough 3x performance increase).
As always, make sure to profile your use case.

## Related crates and further information

* The first and foremost resource is the great raytracing demo of the
[wasm-bindgen
docs](https://rustwasm.github.io/wasm-bindgen/examples/raytrace.html)
and its background [blog
post](https://rustwasm.github.io/2018/10/24/multithreading-rust-and-wasm.html).

* The
[wasm-bindgen-rayon](https://github.com/GoogleChromeLabs/wasm-bindgen-rayon)
crate provides extensive documentation and a nice end-to-end example.

* The [wasm_thread](https://github.com/chemicstry/wasm_thread) crate
  is quite similar in its approach as this crate. The main difference
  is the usage of ES modules.

## License

Licensed under either of

 * Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

#### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.