Async task using `Mut<Resource>`

[odd hull]

I have a resource ResMut<ClientConnection> and I would like to start an async task on it, like so:

fn connect(mut netclient: ResMut<ClientConnection>,) {
    info!("calling connect on netclient");
    let task_handle = IoTaskPool::get().spawn(Compat::new(async { netclient.connect().await }));

    // do other stuff with the task handle
}

The issue is that I can't pass the &mut Res into the task; I get an error like

407 | fn connect(mut netclient: ResMut<ClientConnection>, mut state: ResMut<ConnectingState>) {
    |            -------------
    |            |
    |            `netclient` is a reference that is only valid in the function body
    |            has type `bevy::prelude::ResMut<'1, connection::client::ClientConnection>`
408 |     info!("calling connect on netclient");
409 |     let connection_task = IoTaskPool::get().spawn(Compat::new(async { netclient.connect().await }));
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |                                                   |
    |                                                   `netclient` escapes the function body here
    |                                                   argument requires that `'1` must outlive `'static`
    |
    = note: requirement occurs because of a mutable reference to `bevy::prelude::ResMut<'_, connection::client::ClientConnection>`
    = note: mutable references are invariant over their type parameter
    = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

I can't use async move either because I don't own the resource.

I guess a solution would be to remove the resource from the world so that I own it, launch the task, and then maybe re-add the resource to the world after the task ends, but that isn't very ergonomic.

Is there a good way to do this?

[hexed forge]

Alternatively you can put some internal state behind an Arc and clone the resource into the task like AssetServer

[untold surge]

You can take a page out of bevy_defer's book and spam channels for 2 way communication

you can try some actor pattern shenanigans on the async side

[odd hull]

You mean I put the data in my resource behind an Arc? Yes maybe that would work. I'd hope to avoid that because this is only for this one async task.
Also I would need Arc<Mutex<>> no? since i need to update the value from inside the task. How come the AssetServer avoids the Mutex?

[odd hull]

I'm not sure I understand the approach; do you have more information?

[untold surge]

It's hard to explain like this but in general a good async pattern is not to pass state around but to communicate

this video might be good inspiration (although it's 3 hours long 😅 )
https://www.youtube.com/watch?v=o2ob8zkeq2s

[odd hull]

Yeah maybe I can make it work like this:

• the function is sync instead of async
• inside the function I create a task which I detach
• then I listen for the result of the task in a separate system via a signal

[untold surge]

yeah that can work

[odd hull]

Would something like your AsyncResource in bevy_defer be helpful for my problem?

[untold surge]

I'm not sure, but be skeptical since bevy defer is not designed for networking and may block the main thread

but if you push the resource to thread locals you can get immediate access

[odd hull]

@hexed forge I wonder if you have thoughts about this; (needing a mutex)