How to handle I/O of a subprocess asynchronously?

Question

I have a subprocess, which may or may not write something to it's stdout in a specific amount of time, e.g. 3 seconds.

If a new line in the subprocess stdout starts with the correct thing, I want to return the line. Optimally I would like to realize something like this:

use std::io::{BufRead, BufReader};
use std::thread;
use std::time::Duration;

pub fn wait_for_or_exit(
    reader: &BufReader<&mut std::process::ChildStdout>,
    wait_time: u64,
    cmd: &str,
) -> Option<String> {
    let signal: Arc<AtomicBool> = Arc::new(AtomicBool::new(false));
    let signal_clone = signal.clone();
    let child = thread::spawn(move || {
        thread::sleep(Duration::from_millis(wait_time));
        signal_clone.store(true, Ordering::Relaxed);
    });
    let mut line = String::new();
    while !signal.load(Ordering::Relaxed) {
        //Sleep a really small amount of time not to block cpu
        thread::sleep(Duration::from_millis(10));
        //This line is obviously invalid!
        if reader.has_input() {
            line.clear();
            reader.read_line(&mut line).unwrap();
            if line.starts_with(cmd) {
                return Some(line);
            }
        }
    }
    None
}

The only line not working here is reader.has_input().

Obviously, if the subprocess answers much faster than the wait_time for a repeated amount of times, there will be a lot of sleeping threads, but I can take care of that with channels.

Answer 1

There are two approaches.

1. You can spin up a separate thread, and then use some mechanism (probably a channel) to signal success or failure to your waiting thread.
2. You can use async IO as you mentioned, such as the futures and tokio lib.

I'll demo both. I prefer the futures/Tokio approach, but if you're not familiar with the futures model, then option one might be better.

The Rust stdlib has a Channels API, and this channel actually features a recv_timeout which can help us out quite a bit.

use std::thread;
use std::time::Duration;
use std::sync::mpsc;

// this spins up a separate thread in which to wait for stuff to read
// from the BufReader<ChildStdout> 
// If we successfully read, we send the string over the Channel.
// Back in the original thread, we wait for an answer over the channel
// or timeout in wait_time secs. 
pub fn wait_for_or_exit(
    reader: &BufReader<&mut std::process::ChildStdout>,
    wait_time: u64,
    cmd: &str,
) -> Option<String> {
    let (sender, receiver) = mpsc::channel();

    thread::spawn(move || {
        let line = reader.read_line();
        sender.send(line);
    });

    match receiver.recv_timeout(Duration::from_secs(wait_time)) {
        Ok(line) => if line.starts_with(cmd) 
           { Some(line) } else 
           { None },
        Err(mpsc::RecvTimeoutError::Timeout) => None,
        Err(mpsc::RecvTimeoutError::Disconnected) => None  

    }
}

Option two assumes that you're building a future's based app. In order to accomplish what you want using Async IO is a file descriptor that will let us set NON_BLOCKING. Luckily we don't have to do that ourselves. The Futures and Tokio APIs handle this nicely. The trade-off, is that you have to compose your code out of non-blocking futures.

The code below was taken almost entirely from Tokio Process with a Futures timeout that comes from the Tokio API.

extern crate futures;
extern crate tokio;
extern crate tokio_process;

use std::process::Command;
use std::time::{Duration};

use futures::Future;
use tokio_process::CommandExt;
use tokio::prelude::*;

const TIMEOUT_SECS: u64 = 3;

fn main() {
    // Like above, but use `output_async` which returns a future instead of
    // immediately returning the `Child`.
    let output = Command::new("echo").arg("hello").arg("world")
                        .output_async();

    let future = output.map_err(|e| panic!("failed to collect output: {}", e))
        .map(|output| {
            assert!(output.status.success());
            assert_eq!(output.stdout, b"hello world\n");
            println!("received output: {}",     String::from_utf8(output.stdout).unwrap());
        })
        .timeout(Duration::from_secs(TIMEOUT_SECS)) // here is where we say we only want to wait TIMETOUT seconds
        .map_err(|_e| { println!("Timed out waiting for data"); });

    tokio::run(future);
}