SignalVecExt::flatten stops polling after first top-level item if the item is empty

[martin-kolarik]

If the first polled item of flatten is empty SignalVec, inner polling of this SignalVec ends with Pending, and therefore top-level poll loop does not continue. If top-level SignalVec contains more items in this situation, they are not polled immediately and wait until next wake up.

[Pauan]

Thanks for the report, but I'm having a hard time reproducing it. Could you give an example which fails?

[martin-kolarik]

I apologize, I will provide code later, I am a bit short of time now.

It will be something like (not tested):

let a = MutableVec<MutableVec<u32>>::new();
a.lock_mut().push_cloned(MutableVec::new());
a.lock_mut().push_cloned(MutableVec::new_with_values(vec![15]));

let s = a.signal_vec_cloned().map(|v| v.signal_vec()).flatten();

The behavior in handling Flatten then is that the first (top-level) VecDiff::Push is loaded and stored into this.inner, but then polling stops (my continue solves it in the proposed fix), so the next VecDiff::Push carrying vec![15] is delayed till next wakeup.

[martin-kolarik]

So, here is the code:

use std::future::ready;

use async_std::task::{sleep, spawn};
use futures_signals::signal_vec::{MutableVec, SignalVecExt};

#[async_std::main]
async fn main() {
    let outer: MutableVec<MutableVec<usize>> = MutableVec::new();

    let flattened = outer
        .signal_vec_cloned()
        .map(|inner| inner.signal_vec())
        .flatten();

    spawn(flattened.map(|x| println!("{x}")).for_each(|_| ready(())));

    sleep(std::time::Duration::from_secs(1)).await;

    outer.lock_mut().push_cloned(MutableVec::new());
    outer
        .lock_mut()
        .push_cloned(MutableVec::new_with_values(vec![42]));

    loop {
        sleep(std::time::Duration::from_secs(1)).await;
    }
}

The sleep afer spawn ensures in a dirty way that spawn starts polling before values are inserted.

If flatten implementation uses continue, as in #80, it prints out 42. If flatten uses false, as in current 0.3.33 code, it prints out nothing.

[Pauan]

Sorry about the delay on this, I've been quite busy. Your example was helpful, and I've fixed this.