Nested task_spawn with block_on is not working

[manikawnth]

Nested block_on works:

task::block_on(async {
    task::block_on(fib_async(45));
});

Nested task_spawn with await works:

let s = task::spawn( async {
    task::spawn(fib_async(45)).await;
});
    
task::block_on(s);

but nested task_spawn with block_on doesn't work:

let s = task::spawn( async {
    let s1 = task::spawn(fib_async(45));
    task::block_on(s1);
});
    
task::block_on(s);

Version:
async-std = "1.5.0"

Not sure if I'm doing something obviously wrong!

[skade]

Could you please clarify what "doesn't work"? Do you observe a panic or a lock?

[manikawnth]

Either it's a lock or an infinite loop. The terminal hangs forever.

This is the full example

use async_std::task;

fn main() {
    let t = task::spawn(async {        
        println!("parent task spawned");
        let nested_t = task::spawn(async {
            println!("nested task spawned");
        });        
        task::block_on(nested_t); //this doesn't return
        //nested_t.await;   //this yields
    });    
    task::block_on(t);    
    println!("never reaches here incase of nested block_on");
}

[dignifiedquire]

block_on inside spawn should really be detected, it doesn't make much sense asfaiu. If you want to block, you can use .await

[Licenser]

I'm not sure "doesn't make sense" applies here. Yes in the above, simplified, for example it seems nonsensical but in most applications we don't have control over every library that is used or it is possible for some code path to be synchronous for other reasons.

So if a library uses block_on anywhere in their code it becomes unusable in asynchronous code which is a bit of a bummer.

And the same applies to block_on in a block_on too.

[dignifiedquire]

Fixed in #799