Process::signal(0) returns Ok() when child process was killed externally

[japaric]

Scripts to reproduce

// loopback.rs
use std::io::stdio::stdin;

fn main() {
    for line in stdin().lines() {
        match line {
            Ok(line) => print!("{}", line),
            Err(e)   => fail!("whoops: {}", e),
        }
    }
}

// heartbeat.rs
use std::io::process::Process;
use std::io::timer::Timer;

fn main() {
    let mut timer = Timer::new().unwrap();
    let periodic = timer.periodic(1000);

    let mut child = Process::new("./loopback", []).unwrap();

    loop {
        match child.signal(0) {
            Err(_) => fail!("child process is dead"),
            Ok(_)  => println!("*heartbeat*"),
        }

        periodic.recv();
    }
}

Steps to reproduce:

$ ./heartbeat
$ killall loopback
# heartbeat never ends

Version:

rustc 0.10-pre (e6468a8 2014-03-24 10:01:57 -0700)
host: x86_64-unknown-linux-gnu

Probably related to issue #13123