One less thread available than expected in TaskPool Resource.

[GrantMoyer]

Even though ComputeTaskPool::thread_num() reports 4, apparently only 3 are used for work when accessing the ComputeTaskPool resource. Maybe one of the threads in the Resource is inadvertently blocked.

Affects master branch. Does not affect AsyncComputeTaskPool resource or IOTaskPool resource.

Code to reproduce:

use bevy::{
    prelude::*,
    tasks::{prelude::*, TaskPoolBuilder},
};
use std::{
    io::{stdout, Write},
    thread,
    time::{Duration, Instant},
};

fn task_system(pool: Res<ComputeTaskPool>) {
    let t0 = Instant::now();
    println!("{} threads", pool.thread_num());
    pool.scope(|s| {
        for i in 0..8 {
            s.spawn(async move {
                print!("{}", i);
                stdout().flush().unwrap();
                thread::sleep(Duration::from_secs(1));
            })
        }
    });
    let t1 = Instant::now();
    println!("\nTook {:.3}s", (t1 - t0).as_secs_f32());
}

fn main() {
    let pool = ComputeTaskPool(TaskPoolBuilder::new().num_threads(4).build());
    let t0 = Instant::now();
    println!("{} threads", pool.thread_num());
    pool.scope(|s| {
        for i in 0..8 {
            s.spawn(async move {
                print!("{}", i);
                stdout().flush().unwrap();
                thread::sleep(Duration::from_secs(1));
            })
        }
    });
    let t1 = Instant::now();
    println!("\nTook {:.3}s", (t1 - t0).as_secs_f32());

    App::build().add_startup_system(task_system.system()).run();
}

Output:

4 threads
17405326
Took 2.001s
4 threads
02631745
Took 3.001s

Expected output:

4 threads
17405326
Took 2.001s
4 threads
02631745
Took 2.001s

[GrantMoyer]

Comment from @aclysma on discord:

that's expected right now, only 3 threads are available for your work because one of those threads is blocked by parallel_executor doing a blocking wait on a channel

So it looks like this is caused by #405.

[Cupnfish]

Now run your code on my computer, the output is this, I think this bug is solved.

4 threads
02475613
Took 2.009s
6 threads
01475623
Took 2.014s

[GrantMoyer]

Fixed by #437.