Calling block_on from within wasm.

[simonwarchol]

I'm trying to call block_on and capture the result of my async function in wasm but getting runtime errors. Does anyone have suggestions?

let result = tokio::runtime::Builder::new_current_thread()
.build()
.unwrap()
.block_on(async_fn())

[Darksonn]

Tokio's WASM support is very limited. Trying to use something unsupported results in a runtime error.

Without further information, I cannot help you.

[9SMTM6]

In what way is does it seem incorrect? That would be helpful, if it works, but documenting things the wrong way only makes things worse:-).

Could be that it is very limited. My small experiment was adding an async task looking on a channel receive to a localSet, and await that from a wasm_bindgen_futures::spawn_local. Then I can successfully trigger the code in that loop from elsewhere in my application. And that worked fine.

[9SMTM6]

I guess I'm just gonna be vague that it seems to work in some contexts. I dont have time to test all setups, but I think what I know should already be helpful. I'm now gonna run this in my little application, if I notice anything else I'll be sure to update it.

[Darksonn]

Basically, if you end up in a situation where all tasks are idle, you get that error. If you don't end up in such a situation, you don't. Using LocalSet has no effect on this.

[9SMTM6]

Using LocalSet has no effect on this.

With that you mean that an LocalSet will panic just the same as a runtime, or that a localset, as its no runtime, is not influenced by this?

The way I use the LocalSet I have no (tokio) runtime at all - it seems (I'm certainly not starting one myself or indirectoy via a macro). I was using the LocalSet just as a scheduler of sorts, I guess.

I wasn't sure if LocalSet had any dependencies on the tokio runtime, but so far I did not stumble upon a failure. And it works well enough for me, spawn_local was the only thing I would've been able to use for these tasks anyways, as they're not send nor sync.

Btw, the PR is #6835.

[Darksonn]

It sounds like you are using the LocalSet as a FuturesUnordered.