How do I get the futures back from select_all?

Question

I need to download a lot of files, but I want to process only 3 files at the same time. I'm receiving the next file URL from a Tokio MPSC channel. I need to cover the case when I'm already downloading 2 files and waiting for a new one.

How can I wait for one of 3 futures and extend a previous select_all?

A very simplified example: playground

use futures::{
    future::{select, select_all, Either},
    FutureExt,
};
use tokio::time::{delay_for, Duration};

async fn exec(val: usize) {
    println!("Started {}", val);
    delay_for(Duration::from_millis(100)).await;
    println!("Executed {}", val);
}

async fn get_next() -> usize {
    use rand::Rng;
    delay_for(Duration::from_millis(100)).await;
    rand::thread_rng().gen_range(0, 10)
}

#[tokio::main]
async fn main() {
    let mut tasks = Vec::new();
    for _ in 0..10 {
        if tasks.len() == 0 {
            let next = get_next().await;
            tasks.push(tokio::spawn(exec(next)));
            continue;
        }

        let all = select_all(tasks.drain(..));
        let next = get_next().boxed();

        match select(all, next).await {
            Either::Left(((_, _, left), _)) => tasks = left,
            Either::Right((a, _other)) => {
                // How can I get tasks futures from 'other'?
                // So on the next iteration, I'll be able to do select with an additional task.
                tasks.push(tokio::spawn(exec(a)));
            }
        };
    }
}