Parent future is too big in memory

[Timo Carlin-Burns]

If a parent future spawns a child future with TaskGroup::spawn, the memory used by the child is included in the memory used by the parent.

Example

use std::mem::size_of_val;

use task_group::TaskGroup;

fn main() {
    assert_eq!(size_of_val(&big_future()), 4098);
    assert_eq!(size_of_val(&empty_future()), 1);
    assert_eq!(
        size_of_val(&spawns_big_future_using_tokio()),
        size_of_val(&spawns_empty_future_using_tokio())
    );

    assert_eq!(
        size_of_val(&spawns_big_future_using_task_group()),
        size_of_val(&spawns_empty_future_using_task_group())
    );
    // ^ Fails
    // thread 'main' panicked at 'assertion failed: `(left == right)`
    //   left: `4368`,
    //   right: `272`', src/main.rs:13:5
}

async fn spawns_big_future_using_task_group() {
    let (task_group, task_manager) = TaskGroup::new();
    task_group.spawn("big future", big_future()).await.unwrap();
    drop(task_group);
    task_manager.await.unwrap();
}

async fn spawns_empty_future_using_task_group() {
    let (task_group, task_manager) = TaskGroup::new();
    task_group
        .spawn("empty future", empty_future())
        .await
        .unwrap();
    drop(task_group);
    task_manager.await.unwrap();
}

async fn spawns_big_future_using_tokio() {
    tokio::spawn(big_future()).await.unwrap().unwrap();
}

async fn spawns_empty_future_using_tokio() {
    tokio::spawn(empty_future()).await.unwrap().unwrap();
}

async fn big_future() -> Result<(), ()> {
    let big_object = [0_u8; 4096];
    // Hold _big_object across an await point
    async { () }.await;
    drop(big_object);
    Ok(())
}

async fn empty_future() -> Result<(), ()> {
    Ok(())
}

I believe this is caused by TaskGroup::spawn being an async function, so all the arguments are represented in the async function's initial state. This can be fixed by switching it to a normal function that returns impl Future, and starting the async block after the task has been spawned.

PS: thanks for this awesome library!

[Pat Hickey]

Thanks for the detailed report!