broken compatibility with Rust async machinery

[rvolosatovs]

Not sure if this is a in issue in wit-bindgen, wasmtime or both, but it appears that the Waker functionality does not function correctly in Rust guests when using async exports.

For example:

use futures::channel::mpsc;
use futures::join;
use futures::{SinkExt as _, StreamExt as _};

struct Component;

wasi::cli::command::export!(Component);

impl wasi::exports::cli::run::Guest for Component {
    async fn run() -> Result<(), ()> {
        let (mut tx, mut rx) = mpsc::channel(1);
        join!(
            async {
                _ = rx.next().await;
            },
            async {
                _ = tx.send(()).await;
            }
        );
        Ok(())
    }
}

with Cargo.toml:

[package]
name = "p3-test"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["cdylib"]

[dependencies]
futures = "0.3"
wasi = { git = "https://github.com/rvolosatovs/wasi", branch = "feat/p3" }

wasi repo is pointing to commit rvolosatovs/wasi@0792066

executed via:

cargo build --release --target wasm32-unknown-unknown && ~/.cargo/bin/wasm-tools component new --skip-validation target/wasm32-unknown-unknown/release/p3_test.wasm -o component.wasm && ~/src/github.com/bytecodealliance/wasmtime/target/debug/wasmtime run component.wasm 

Produces:

Error: failed to run main module `component.wasm`

Caused by:
    wasm trap: async-lifted export failed to produce a result

That's using Wasmtime from #60

Interestingly, causing a task allocation from the guest turns the error into a deadlock (which is how I first encountered the issue and is the reason for the title), for example:

use futures::channel::mpsc;
use futures::join;
use futures::{SinkExt as _, StreamExt as _};
use wasi::wit_stream;

struct Component;

wasi::cli::command::export!(Component);

impl wasi::exports::cli::run::Guest for Component {
    async fn run() -> Result<(), ()> {
        let (mut tx, mut rx) = mpsc::channel(1);
        let (_, stdout_rx) = wit_stream::new();
        wasi::cli::stdout::set_stdout(stdout_rx);
        join!(
            async {
                _ = rx.next().await;
            },
            async {
                _ = tx.send(()).await;
            }
        );
        Ok(())
    }
}

Generally, it appears that there are 2 things happening here:

1. when the component's "futures unordered" set is empty, the component is unable to await async functions not provided by the runtime
2. when the component's "futures unordered" set is non-empty, calling Waker::wake in the guest does not awake the task and so the component ends up in a deadlock state