Implementing Asynchronous Timeout in Rust

In concurrent programming, it's crucial to prevent operations from blocking indefinitely. This challenge focuses on implementing robust timeout mechanisms for asynchronous tasks in Rust. You will create a function that executes a given asynchronous operation but cancels it if it exceeds a specified duration, returning a specific result to indicate the timeout. This is vital for responsive applications, network requests, and any operation where unbounded waiting can be detrimental.

Problem Description

Your task is to create a Rust function named with_timeout that takes an asynchronous operation (a Future) and a duration. This function should execute the Future. If the Future completes within the given duration, with_timeout should return the result of the Future. If the Future does not complete within the duration, with_timeout should cancel the Future and return a specific TimeoutError value.

Key Requirements:

• The function must accept a generic Future and a std::time::Duration.
• It must return a Result type. On success, it should contain the Output of the Future. On timeout, it should contain a custom TimeoutError.
• The function should leverage Rust's asynchronous capabilities, likely using tokio or a similar runtime.
• The timeout mechanism should actively cancel the underlying Future when the duration expires.

Expected Behavior:

• If the provided Future resolves before the Duration elapses, the Result returned by with_timeout should be Ok(future_output).
• If the Duration elapses before the Future resolves, the Result returned by with_timeout should be Err(TimeoutError).

Important Edge Cases:

• Zero Duration: A duration of 0 should ideally cause an immediate timeout if the Future is not already completed.
• Immediate Completion: If the Future completes almost instantaneously, it should not be prematurely timed out.

Examples

Example 1: Successful Completion

use tokio::time::{sleep, Duration};
use std::io; // Using a common error type for demonstration

async fn quick_task() -> Result<String, io::Error> {
    sleep(Duration::from_millis(50)).await;
    Ok("Task completed successfully".to_string())
}

#[tokio::main]
async fn main() {
    let timeout_duration = Duration::from_secs(1);
    let result = with_timeout(quick_task(), timeout_duration).await;
    // Expected output: Ok(Ok("Task completed successfully"))
    // Explanation: The quick_task() completes well within the 1-second timeout.
    // The inner Result is preserved.
}

Example 2: Timeout Occurs

use tokio::time::{sleep, Duration};
use std::io;

async fn slow_task() -> Result<String, io::Error> {
    sleep(Duration::from_secs(5)).await;
    Ok("This should not be reached".to_string())
}

#[tokio::main]
async fn main() {
    let timeout_duration = Duration::from_secs(1);
    let result = with_timeout(slow_task(), timeout_duration).await;
    // Expected output: Err(TimeoutError)
    // Explanation: The slow_task() takes 5 seconds, exceeding the 1-second timeout.
    // The timeout error should be returned instead of the task's result.
}

Example 3: Timeout with a Different Return Type

use tokio::time::{sleep, Duration};
use std::error::Error;

// A simple error type for our task
#[derive(Debug)]
struct MyTaskError;

impl std::fmt::Display for MyTaskError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "My task failed")
    }
}

impl Error for MyTaskError {}

async fn task_that_might_fail() -> Result<(), MyTaskError> {
    sleep(Duration::from_millis(700)).await;
    Err(MyTaskError)
}

#[tokio::main]
async fn main() {
    let timeout_duration = Duration::from_secs(1);
    let result = with_timeout(task_that_might_fail(), timeout_duration).await;
    // Expected output: Ok(Err(MyTaskError))
    // Explanation: The task completes within the timeout, even though it returns an error.
    // The `with_timeout` function should preserve the task's original Result.
}

Constraints

• You must use the tokio runtime for managing asynchronous tasks and timers.
• The Future type passed to with_timeout can be any type that implements Future and returns a Result<T, E>.
• The timeout mechanism must be cancellation-aware. Simply polling a future after a timeout without cancelling its underlying operations is not sufficient.
• The TimeoutError type should be a distinct, identifiable error.

Notes

• Consider using tokio::time::timeout as a starting point or inspiration. Your goal is to understand how it works and potentially build a similar construct, perhaps with custom error handling or specific features.
• Think about how to combine the Future with a timer.
• The function signature should be generic enough to handle various Future types and their Ok/Err variants.
• The with_timeout function itself should return a Result to clearly indicate success or timeout. The Ok variant of this outer Result should contain the original Result returned by the Future.