Race condition in task communication causes deadlocks

[carllerche]

Repro:

extern crate time;

use std::io::IoResult;
use std::io::timer::Timer;
use std::sync::Future;

fn repro() -> IoResult<()> {
    let (tx, rx): (SyncSender<&'static str>, Receiver<&'static str>) = sync_channel(3);

    let mut f = Future::spawn(proc() {
        let mut timer = try!(Timer::new());
        let tick = timer.periodic(10000);

        loop {
            println!("select; {}", time::precise_time_ns());
            select! {
                _ = tick.recv() => {
                    fail!("nope, received tick first; {}", time::precise_time_ns());
                },
                msg = rx.recv() => {
                    println!("Received msg; {}", msg);

                    if msg == "hi2u" {
                        break;
                    }
                }
            }
        }

        Ok(())
    });

    tx.try_send("zomg").unwrap();
    tx.try_send("hi2u").unwrap();

    f.get()
}

pub fn main() {
    let mut i = 0u;
    loop {
        i += 1;
        println!("starting iter {}; {}", i, time::precise_time_ns());
        repro().unwrap();
    }
}

[carllerche]

It seems that the select! is not necessarily related:

extern crate time;

use std::io::IoResult;
use std::io::timer::Timer;
use std::sync::Future;

fn repro() -> IoResult<()> {
    let (tx, rx): (SyncSender<&'static str>, Receiver<&'static str>) = sync_channel(3);

    let mut f = Future::spawn(proc() {
        rx.recv();
        rx.recv();

        Ok(())
    });

    tx.try_send("zomg").unwrap();
    tx.try_send("hi2u").unwrap();

    f.get()
}

pub fn main() {
    let mut i = 0u;
    loop {
        i += 1;
        println!("starting iter {}; {}", i, time::precise_time_ns());
        repro().unwrap();
    }
}

[carllerche]

Removing Future deadlocks as well:

extern crate time;

use std::io::IoResult;
use std::io::timer::Timer;
use std::sync::Future;

fn repro() -> IoResult<()> {
    let (tx1, rx1): (SyncSender<&'static str>, Receiver<&'static str>) = sync_channel(3);
    let (tx2, rx2): (SyncSender<&'static str>, Receiver<&'static str>) = sync_channel(3);

    spawn(proc() {
        rx1.recv();
        tx2.try_send("pong").unwrap();
    });

    tx1.try_send("ping").unwrap();
    rx2.recv();

    Ok(())
}

pub fn main() {
    let mut i = 0u;
    loop {
        i += 1;
        println!("starting iter {}; {}", i, time::precise_time_ns());
        repro().unwrap();
    }
}

[carllerche]

It seems that SyncSender#try_send is the culprit.