Understanding Executors

An executor is the runtime that drives Futures to completion.

Core Responsibilities

1. Polling Futures - Calls poll() on tasks
2. Task Scheduling - Decides which task runs next
3. Waker Management - Tracks which tasks are ready
4. I/O Integration - Connects to OS async I/O (epoll, kqueue, IOCP)

Why Rust Needs External Runtimes

Unlike Go or JavaScript, Rust doesn't include a runtime. This is intentional:

• Zero-cost abstraction - Pay only for what you use
• Flexibility - Choose the right runtime for your use case
• No_std support - Embedded systems can use async

Popular Runtimes

Executor Loop (Simplified)

rust
loop {
    // Wait for any task to become ready
    let ready_tasks = wait_for_wakeups();
    
    for task in ready_tasks {
        match task.poll() {
            Poll::Ready(result) => complete_task(task, result),
            Poll::Pending => continue, // Will be woken later
        }
    }
}

// Minimal executor concept (educational)
use std::future::Future;
use std::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
use std::pin::Pin;

fn block_on<F: Future>(mut future: F) -> F::Output {
    let waker = dummy_waker();
    let mut cx = Context::from_waker(&waker);
    let mut future = unsafe { Pin::new_unchecked(&mut future) };
    
    loop {
        match future.as_mut().poll(&mut cx) {
            Poll::Ready(output) => return output,
            Poll::Pending => std::thread::yield_now(),
        }
    }
}