CODEWITHSUNDEEP
rustasync-awaitfuture
Anunaki
Anunaki

Reputation: 881

Why is an async fn() called from Future::poll() not executed at the time of execution?

I'm calling an async fn() in Future::poll() but the .await statement and the code behind it is not executed at the time of execution.

use futures::FutureExt;
use std::future::Future;
use std::pin::Pin;
use std::sync::{Arc, Mutex};
use std::task::{Context, Poll};

#[pin_project::pin_project]
struct Person<'a> {
    name: &'a str,
    age: i32,
}

impl<'a> Future for Person<'a> {
    type Output = i32;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let mut this = self.project();
        *this.age += 10;

        let mut fut1 = fn1();
        let pinfut1 = Pin::new(&mut fut1);
        //let pinfut1 = unsafe { Pin::new_unchecked(&mut fut1) };
        pinfut1.poll(cx)
    }
}

fn fn1() -> impl Future<Output = i32> + Unpin {
    async {
        dbg!("sleep start!"); // execute here!
        async_std::task::sleep(std::time::Duration::from_secs(5)).await; //  <--- blocked here ?
        dbg!("sleep done!"); // Never execute here！
        123
    }
    .boxed()
}

fn main() {
    let p1 = Person {
        name: "jack",
        age: Default::default(),
    };
    async_std::task::block_on(async {
        let a = p1.await;
        dbg!(a); // Never execute here!
    });
    std::thread::park();
}

playground

Cargo.toml:

[package]
name = "test-poll"
version = "0.1.0"
authors = ["xx <[email protected]>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
async-std="1.8.0"
pin-project="0.4.6"
futures=""

Upvotes: 0

Views: 818

Answers (1)

Shepmaster
Shepmaster

Reputation: 432059

Every time your Person future is polled, you create a brand new fn1 future:

let mut fut1 = fn1();

That future waits for 5 seconds, then wakes the executor, which polls Person, which then creates a brand new fn1 future, which waits for 5 seconds...

See How to implement a Future or Stream that polls an async fn? for instructions on how to do this correctly (although I agree that you probably don't want to do this in the first place).

Upvotes: 3

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