Passing join handle to select! in a loop

[sandeepmukh]

I have some async code (a state machine running an algorithm) running in parallel to processing those messages and then terminating once the state machine returns its final state. Is there a way to do this?

Code outline:

let join_handle = tokio::spawn(async move { match state_machine.execute().await  });
loop {
    tokio::select! {
        result = join_handle => { // this is the trouble line!
                  // process result
                  break;
          },
           event = handle_incoming_messages() => { // handle messages },
        }
 }

Error:
use of moved value: join_handle

value moved here, in previous iteration of looprustc(E0382)
main.rs(169, 9): move occurs because join_handle has type tokio::task::JoinHandle<_>, which does not implement the Copy trait
main.rs(178, 22): value moved here, in previous iteration of loop

[Darksonn]

You can do this by passing a mutable reference to the JoinHandle.

tokio::select! {
    result = &mut join_handle => {
        // process result
        break;
    },
    event = handle_incoming_messages() => {
        // handle messages
    },
}

This ensures that the JoinHandle is not consumed. Note that it isn't even consumed when the JoinHandle completes. Any use of the JoinHandle after completion will result in a panic.

[sandeepmukh]

Thank you! Do you know how I can check when the join handle completes? I'm trying to use now_or_never but it always seems to return None.

[Darksonn]

You can do it with now_or_never. You can open a new question if you're running into trouble with it.