How to await a promise in Rust without costly type conversions?

[alshdavid]

I am trying to await the result of a promise in Rust where I'd like the value returned to be a JsUnknown to avoid casting into Rust types unless the caller needs to.

I have tried the naive implementation of using a channel to wait for a response from the callback

fn promise_await(env: Env, promise: JsObject) -> napi::Result<JsUnknown> {
  let (tx, mut rx) = tokio::sync::mpsc::channel::<JsUnknown>(1);

  let fn_then_callback = env.create_function_from_closure("callback", move |ctx| {
    tx.try_send( ctx.get(0)?).unwrap();
    Ok(ctx.env.get_undefined())
  })?;

  promise
    .get_named_property_unchecked::<JsFunction>("then")?
    .call(Some(promise), &[&fn_then_callback])?;

  // "rx.blocking_recv()" blocks the thread and pauses JS execution 
  // preventing the "then" callback from running causing a deadlock
  Ok(rx.blocking_recv().unwrap())
}

#[napi]
fn start(env: Env, promise: JsUnknown) -> napi::Result<JsUndefined> {
  let resolved = await_promise(env, promise);

  // Assume it's a string
  let value = unsafe { resolved.cast::<JsString>() };
  println!("Got: {}", v.into_utf8()?.into_owned()?);

  env.get_undefined()
}

However this doesn't work because the receiver blocks the Js thread which prevents further Js execution thereby preventing the promise callback from running.

Any tips for this?

[Brooooooklyn]

https://github.com/napi-rs/napi-rs/blob/main/examples/napi/src/promise.rs#L4