Rust + Tokio: Process First Task to Complete

Question

In Tokio, how do you wait for the first task in a list of tasks to complete? The size of the list is unknown.

More specifically:

1. You have a list of tasks.
2. Wait for the first that completes.
3. Run some code, including possibly adding more tasks to the list.
4. Repeat until the list is empty.

The context is that I'm trying to write a function that will run at most N tasks at a time.

Answer 1

If using tokio , the general template is this:

use futures::stream::FuturesUnordered;
use tokio::task::JoinHandle;

let futs = data
    .into_iter()
    .map(|datum| {
        tokio::spawn(async move {
            compute_value(datum)
        })
    })
    // The _ is inferred to be the return type of `compute_value`
    .collect::<FuturesUnordered<JoinHandle<_>>>();

while let Some(result) = futs.next().await {
    let value = result?;  // a potential stream error
    // use value
}