# Futures in Rust

We'll create our own `Futures` together with a fake reactor and a simple
executor which allows you to edit, run an play around with the code right here
in your browser.

I'll walk you through the example, but if you want to check it out closer, you
can always [clone the repository][example_repo] and play around with the code yourself.

There are several branches explained in the readme, but two are
relevant for this chapter. The `main` branch is the example we go through here,
and the `basic_example_commented` branch is this example with extensive
comments.

> If you want to follow along as we go through, initialize a new cargo project
> by creating a new folder and run `cargo init` inside it. Everything we write
> here will be in `main.rs`

## Implementing our own Futures

Let's start off by getting all our imports right away so you can follow along

```rust, noplaypen, ignore
use std::{
    future::Future, pin::Pin, sync::{mpsc::{channel, Sender}, Arc, Mutex},
    task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
    thread::{self, JoinHandle}, time::{Duration, Instant}
};
```

## The Executor

The executors responsibility is to take one or more futures and run them to completion.

The first thing an `executor` does when it gets a `Future` is polling it.

**When polled one of three things can happen:**

- The future returns `Ready` and we schedule whatever chained operations to run
- The future hasn't been polled before so we pass it a `Waker` and suspend it
- The futures has been polled before but is not ready and returns `Pending`

Rust provides a way for the Reactor and Executor to communicate through the `Waker`. The reactor stores this `Waker` and calls `Waker::wake()` on it once
a `Future` has resolved and should be polled again.

**Our Executor will look like this:**

```rust, noplaypen, ignore
// Our executor takes any object which implements the `Future` trait
fn block_on<F: Future>(mut future: F) -> F::Output {
    // the first thing we do is to construct a `Waker` which we'll pass on to
    // the `reactor` so it can wake us up when an event is ready. 
    let mywaker = Arc::new(MyWaker{ thread: thread::current() }); 
    let waker = waker_into_waker(Arc::into_raw(mywaker));
    // The context struct is just a wrapper for a `Waker` object. Maybe in the
    // future this will do more, but right now it's just a wrapper.
    let mut cx = Context::from_waker(&waker);

    // We poll in a loop, but it's not a busy loop. It will only run when
    // an event occurs, or a thread has a "spurious wakeup" (an unexpected wakeup
    // that can happen for no good reason).
    let val = loop {
        // So, since we run this on one thread and run one future to completion
        // we can pin the `Future` to the stack. This is unsafe, but saves an
        // allocation. We could `Box::pin` it too if we wanted. This is however
        // safe since we don't move the `Future` here.
        let pinned = unsafe { Pin::new_unchecked(&mut future) };
        match Future::poll(pinned, &mut cx) {
            // when the Future is ready we're finished
            Poll::Ready(val) => break val,
            // If we get a `pending` future we just go to sleep...
            Poll::Pending => thread::park(),
        };
    };
    val
}
```

Inn all the examples here I've chose to comment the code extensively. I find it
easier to follow that way than dividing if up into many paragraphs.

We'll see more about the `Waker` in the next paragraph, but just look at it like
a _trait object_ similar to the one we constructed  in the first chapter.

> `Context` is just a wrapper around the `Waker`. At the time of writing this
book it's nothing more. In the future it might be possible that the `Context`
object will do more than just wrapping a `Future` so having this extra 
abstraction gives some flexibility.

You'll notice how we use `Pin` here to pin the future when we poll it.

Now that you've read so much about `Generators` and `Pin` already this should
be rather easy to understand. `Future` is a state machine, every `await` point
is a `yield` point. We could borrow data across `await` points and we meet the
exact same challenges as we do when borrowing across `yield` points.

As we explained in the [chapter about generators](./3_generators_pin.md), we use
`Pin` and the guarantees that give us to allow `Futures` to have self
references.

## The `Future` implementation

In Rust we call an interruptible task a `Future`. Futures has a well defined interface, which means they can be used across the entire ecosystem. We can chain
these `Futures` so that once a "leaf future" is ready we'll perform a set of
operations.

These chained operations can spawn new leaf futures themselves.

**Our Future implementation looks like this:**

```rust, noplaypen, ignore
// This is the definition of our `Waker`. We use a regular thread-handle here.
// It works but it's not a good solution. It's easy to fix though, I'll explain
// after this code snippet.
#[derive(Clone)]
struct MyWaker {
    thread: thread::Thread,
}

// This is the definition of our `Future`. It keeps all the information we
// need. This one holds a reference to our `reactor`, that's just to make
// this example as easy as possible. It doesn't need to hold a reference to
// the whole reactor, but it needs to be able to register itself with the
// reactor.
#[derive(Clone)]
pub struct Task {
    id: usize,
    reactor: Arc<Mutex<Reactor>>,
    data: u64,
    is_registered: bool,
}

// These are function definitions we'll use for our waker. Remember the
// "Trait Objects" chapter earlier.
fn mywaker_wake(s: &MyWaker) {
    let waker_ptr: *const MyWaker = s;
    let waker_arc = unsafe {Arc::from_raw(waker_ptr)};
    waker_arc.thread.unpark();
}

// Since we use an `Arc` cloning is just increasing the refcount on the smart
// pointer.
fn mywaker_clone(s: &MyWaker) -> RawWaker {
    let arc = unsafe { Arc::from_raw(s).clone() };
    std::mem::forget(arc.clone()); // increase ref count
    RawWaker::new(Arc::into_raw(arc) as *const (), &VTABLE)
}

// This is actually a "helper funtcion" to create a `Waker` vtable. In contrast
// to when we created a `Trait Object` from scratch we don't need to concern
// ourselves with the actual layout of the `vtable` and only provide a fixed
// set of functions
const VTABLE: RawWakerVTable = unsafe {
    RawWakerVTable::new(
        |s| mywaker_clone(&*(s as *const MyWaker)),     // clone
        |s| mywaker_wake(&*(s as *const MyWaker)),      // wake
        |s| mywaker_wake(*(s as *const &MyWaker)),      // wake by ref
        |s| drop(Arc::from_raw(s as *const MyWaker)),   // decrease refcount
    )
};

// Instead of implementing this on the `MyWaker` oject in `impl Mywaker...` we
// just use this pattern instead since it saves us some lines of code.
fn waker_into_waker(s: *const MyWaker) -> Waker {
    let raw_waker = RawWaker::new(s as *const (), &VTABLE);
    unsafe { Waker::from_raw(raw_waker) }
}

impl Task {
    fn new(reactor: Arc<Mutex<Reactor>>, data: u64, id: usize) -> Self {
        Task {
            id,
            reactor,
            data,
            is_registered: false,
        }
    }
}

// This is our `Future` implementation
impl Future for Task {
    // The output for our kind of `leaf future` is just an `usize`. For other
    // futures this could be something more interesting like a byte array.
    type Output = usize;
    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let mut r = self.reactor.lock().unwrap();
        // we check with the `Reactor` if this future is in its "readylist"
        // i.e. if it's `Ready`
        if r.is_ready(self.id) {
            // if it is, we return the data. In this case it's just the ID of
            // the task since this is just a very simple example.
            Poll::Ready(self.id)
        } else if self.is_registered {
            // If the future is registered alredy, we just return `Pending`
            Poll::Pending
        } else {
            // If we get here, it must be the first time this `Future` is polled
            // so we register a task with our `reactor`
            r.register(self.data, cx.waker().clone(), self.id);
            // oh, we have to drop the lock on our `Mutex` here because we can't
            // have a shared and exclusive borrow at the same time
            drop(r);
            self.is_registered = true;
            Poll::Pending
        }
    }
}
```

This is mostly pretty straight forward. The confusing part is the strange way
we need to construct the `Waker`, but since we've already created our own
_trait objects_ from raw parts, this looks pretty familiar. Actually, it's
even a bit easier.

We use an `Arc` here to pass out a ref-counted borrow of our `MyWaker`. This
is pretty normal, and makes this easy and safe to work with. Cloning a `Waker`
is just increasing the refcount in this case.

Dropping a `Waker` is as easy as decreasing the refcount. Now, in special
cases we could choose to not use an `Arc`. So this low-level method is there
to allow such cases. 

Indeed, if we only used `Arc` there is no reason for us to go through all the 
trouble of creating our own `vtable` and a `RawWaker`. We could just implement
a normal trait.

Fortunately, in the future this will probably be possible in the standard
library as well. For now, [this trait lives in the nursery][arc_wake], but my
guess is that this will be a part of the standard library after som maturing.

We choose to pass in a reference to the whole `Reactor` here. This isn't normal.
The reactor will often be a global resource which let's us register interests
without passing around a reference.

### Why using thread park/unpark is a bad idea for a library

It could deadlock easily since anyone could get a handle to the `executor thread`
and call park/unpark on it.

If one of our `Futures` holds a handle to our thread, or any unrelated code
calls `unpark` on our thread, the following could happen:

1. A future could call `unpark` on the executor thread from a different thread
2. Our `executor` thinks that data is ready and wakes up and polls the future
3. The future is not ready yet when polled, but at that exact same time the 
`Reactor` gets an event and calls `wake()` which also unparks our thread.
4. This could happen before we go to sleep again since these processes
run in parallel.
5. Our reactor has called `wake` but our thread is still sleeping since it was
awake already at that point.
6. We're deadlocked and our program stops working

> There is also the case that our thread could have what's called a
`spurious wakeup` ([which can happen unexpectedly][spurious_wakeup]), which
could cause the same deadlock if we're unlucky.

There are many better solutions, here are some:

  - Use [std::sync::CondVar][condvar]
  - Use [crossbeam::sync::Parker][crossbeam_parker]

## The Reactor

This is the home stretch, and not strictly `Future` related, but we need one
to have an example to run.

Since concurrency mostly makes sense when interacting with the outside world (or
at least some peripheral), we need something to actually abstract over this
interaction in an asynchronous way. 

This is the `Reactors` job. Most often you'll see reactors in Rust use a library called [Mio][mio], which provides non 
blocking APIs and event notification for several platforms.

The reactor will typically give you something like a `TcpStream` (or any other resource) which you'll use to create an I/O request. What you get in return
is a `Future`. 

>If the `Reactor` is registered as a global resource (which
>is pretty normal), our `Task` in would instead be a special `TcpStream` which
>registers interest with the global `Reactor` and no reference is needed.

We can call this kind of `Future` a "leaf Future", since it's some operation 
we'll actually wait on and that we can chain operations on which are performed
once the leaf future is ready. 

**Our Reactor will look like this:**

```rust, noplaypen, ignore
// This is a "fake" reactor. It does no real I/O, but that also makes our
// code possible to run in the book and in the playground
struct Reactor {
    // we need some way of registering a Task with the reactor. Normally this
    // would be an "interest" in an I/O event
    dispatcher: Sender<Event>,
    handle: Option<JoinHandle<()>>,
    // This is a list of tasks that are ready, which means they should be polled
    // for data.
    readylist: Arc<Mutex<Vec<usize>>>,
}

// We just have two kind of events. An event called `Timeout`
// and a `Close` event to close down our reactor.
#[derive(Debug)]
enum Event {
    Close,
    Timeout(Waker, u64, usize),
}

impl Reactor {
    fn new() -> Self {
        // The way we register new events with our reactor is using a regular
        // channel
        let (tx, rx) = channel::<Event>();
        let readylist = Arc::new(Mutex::new(vec![]));
        let rl_clone = readylist.clone();

        // This `Vec` will hold handles to all threads we spawn so we can
        // join them later on and finish our programm in a good manner
        let mut handles = vec![];
        // This will be the "Reactor thread"
        let handle = thread::spawn(move || {
            for event in rx {
                let rl_clone = rl_clone.clone();
                match event {
                    // If we get a close event we break out of the loop we're in
                    Event::Close => break,
                    Event::Timeout(waker, duration, id) => {

                        // When we get an event we simply spawn a new thread
                        // which will simulate some I/O resource...
                        let event_handle = thread::spawn(move || {
                            //... by sleeping for the number of seconds
                            // we provided when creating the `Task`.
                            thread::sleep(Duration::from_secs(duration));
                            // When it's done sleeping we put the ID of this task
                            // on the "readylist"
                            rl_clone.lock().map(|mut rl| rl.push(id)).unwrap();
                            // Then we call `wake` which will wake up our
                            // executor and start polling the futures
                            waker.wake();
                        });

                        handles.push(event_handle);
                    }
                }
            }

            // When we exit the Reactor we first join all the handles on
            // the child threads we've spawned so we catch any panics and
            // release any resources.
            for handle in handles {
                handle.join().unwrap();
            }
        });

        Reactor {
            readylist,
            dispatcher: tx,
            handle: Some(handle),
        }
    }

    fn register(&mut self, duration: u64, waker: Waker, data: usize) {
        // registering an event is as simple as sending an `Event` through
        // the channel.
        self.dispatcher
            .send(Event::Timeout(waker, duration, data))
            .unwrap();
    }

    fn close(&mut self) {
        self.dispatcher.send(Event::Close).unwrap();
    }

    // We need a way to check if any event's are ready. This will simply
    // look through the "readylist" for an event macthing the ID we want to
    // check for.
    fn is_ready(&self, id_to_check: usize) -> bool {
        self.readylist
            .lock()
            .map(|rl| rl.iter().any(|id| *id == id_to_check))
            .unwrap()
    }
}

// When our `Reactor` is dropped we join the reactor thread with the thread
// owning our `Reactor` so we catch any panics and release all resources.
// It's not needed for this to work, but it really is a best practice to join
// all threads you spawn.
impl Drop for Reactor {
    fn drop(&mut self) {
        self.handle.take().map(|h| h.join().unwrap()).unwrap();
    }
}
```

It's a lot of code though, but essentially we just spawn off a new thread
and make it sleep for some time which we specify when we create a `Task`.

Now, let's test our code and see if it works. This code is actually runnable
if you press the "play" button. Since we're sleeping for a couple of seconds
here, just give it some time to run.

In the last chapter we have the [whole 200 lines in an editable window](./8_finished_example.md). You can
also copy that or edit it right in this book.

```rust, edition2018
# use std::{
#     future::Future, pin::Pin, sync::{mpsc::{channel, Sender}, Arc, Mutex},
#     task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
#     thread::{self, JoinHandle}, time::{Duration, Instant}
# };
# 
fn main() {
    // This is just to make it easier for us to see when our Future was resolved
    let start = Instant::now();

    // Many runtimes create a glocal `reactor` we pass it as an argument
    let reactor = Reactor::new();
    // Since we'll share this between threads we wrap it in a 
    // atmically-refcounted- mutex.
    let reactor = Arc::new(Mutex::new(reactor));
    
    // We create two tasks:
    // - first parameter is the `reactor`
    // - the second is a timeout in seconds
    // - the third is an `id` to identify the task
    let future1 = Task::new(reactor.clone(), 1, 1);
    let future2 = Task::new(reactor.clone(), 2, 2);

    // an `async` block works the same way as an `async fn` in that it compiles
    // our code into a state machine, `yielding` at every `await` point.
    let fut1 = async {
        let val = future1.await;
        let dur = (Instant::now() - start).as_secs_f32();
        println!("Future got {} at time: {:.2}.", val, dur);
    };

    let fut2 = async {
        let val = future2.await;
        let dur = (Instant::now() - start).as_secs_f32();
        println!("Future got {} at time: {:.2}.", val, dur);
    };

    // Our executor can only run one and one future, this is pretty normal
    // though. You have a set of operations containing many futures that
    // ends up as a single future that drives them all to completion.
    let mainfut = async {
        fut1.await;
        fut2.await;
    };

    // This executor will block the main thread until the futures is resolved
    block_on(mainfut);
    // When we're done, we want to shut down our reactor thread so our program
    // ends nicely.
    reactor.lock().map(|mut r| r.close()).unwrap();
}

# // ============================= EXECUTOR ====================================
# fn block_on<F: Future>(mut future: F) -> F::Output {
#     let mywaker = Arc::new(MyWaker{ thread: thread::current() }); 
#     let waker = waker_into_waker(Arc::into_raw(mywaker));
#     let mut cx = Context::from_waker(&waker);
#     let val = loop {
#         let pinned = unsafe { Pin::new_unchecked(&mut future) };
#         match Future::poll(pinned, &mut cx) {
#             Poll::Ready(val) => break val,
#             Poll::Pending => thread::park(),
#         };
#     };
#     val
# }
# 
# fn spawn<F: Future>(future: F) -> Pin<Box<F>> {
#     let mywaker = Arc::new(MyWaker{ thread: thread::current() }); 
#     let waker = waker_into_waker(Arc::into_raw(mywaker)); 
#     let mut cx = Context::from_waker(&waker); 
#     let mut boxed = Box::pin(future);
#     let _ = Future::poll(boxed.as_mut(), &mut cx); 
#     boxed
# }
# 
# // ====================== FUTURE IMPLEMENTATION ==============================
# #[derive(Clone)]
# struct MyWaker {
#     thread: thread::Thread,
# }
# 
# #[derive(Clone)]
# pub struct Task {
#     id: usize,
#     reactor: Arc<Mutex<Reactor>>,
#     data: u64,
#     is_registered: bool,
# }
# 
# fn mywaker_wake(s: &MyWaker) {
#     let waker_ptr: *const MyWaker = s;
#     let waker_arc = unsafe {Arc::from_raw(waker_ptr)};
#     waker_arc.thread.unpark();
# }
# 
# fn mywaker_clone(s: &MyWaker) -> RawWaker {
#     let arc = unsafe { Arc::from_raw(s).clone() };
#     std::mem::forget(arc.clone()); // increase ref count
#     RawWaker::new(Arc::into_raw(arc) as *const (), &VTABLE)
# }
# 
# const VTABLE: RawWakerVTable = unsafe {
#     RawWakerVTable::new(
#         |s| mywaker_clone(&*(s as *const MyWaker)),     // clone
#         |s| mywaker_wake(&*(s as *const MyWaker)),      // wake
#         |s| mywaker_wake(*(s as *const &MyWaker)),      // wake by ref
#         |s| drop(Arc::from_raw(s as *const MyWaker)),   // decrease refcount
#     )
# };
# 
# fn waker_into_waker(s: *const MyWaker) -> Waker {
#     let raw_waker = RawWaker::new(s as *const (), &VTABLE);
#     unsafe { Waker::from_raw(raw_waker) }
# }
# 
# impl Task {
#     fn new(reactor: Arc<Mutex<Reactor>>, data: u64, id: usize) -> Self {
#         Task {
#             id,
#             reactor,
#             data,
#             is_registered: false,
#         }
#     }
# }
# 
# impl Future for Task {
#     type Output = usize;
#     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
#         let mut r = self.reactor.lock().unwrap();
#         if r.is_ready(self.id) {
#             Poll::Ready(self.id)
#         } else if self.is_registered {
#             Poll::Pending
#         } else {
#             r.register(self.data, cx.waker().clone(), self.id);
#             drop(r);
#             self.is_registered = true;
#             Poll::Pending
#         }
#     }
# }
# 
# // =============================== REACTOR ===================================
# struct Reactor {
#     dispatcher: Sender<Event>,
#     handle: Option<JoinHandle<()>>,
#     readylist: Arc<Mutex<Vec<usize>>>,
# }
# #[derive(Debug)]
# enum Event {
#     Close,
#     Timeout(Waker, u64, usize),
# }
# 
# impl Reactor {
#     fn new() -> Self {
#         let (tx, rx) = channel::<Event>();
#         let readylist = Arc::new(Mutex::new(vec![]));
#         let rl_clone = readylist.clone();
#         let mut handles = vec![];
#         let handle = thread::spawn(move || {
#             // This simulates some I/O resource
#             for event in rx {
#                 println!("REACTOR: {:?}", event);
#                 let rl_clone = rl_clone.clone();
#                 match event {
#                     Event::Close => break,
#                     Event::Timeout(waker, duration, id) => {
#                         let event_handle = thread::spawn(move || {
#                             thread::sleep(Duration::from_secs(duration));
#                             rl_clone.lock().map(|mut rl| rl.push(id)).unwrap();
#                             waker.wake();
#                         });
# 
#                         handles.push(event_handle);
#                     }
#                 }
#             }
# 
#             for handle in handles {
#                 handle.join().unwrap();
#             }
#         });
# 
#         Reactor {
#             readylist,
#             dispatcher: tx,
#             handle: Some(handle),
#         }
#     }
# 
#     fn register(&mut self, duration: u64, waker: Waker, data: usize) {
#         self.dispatcher
#             .send(Event::Timeout(waker, duration, data))
#             .unwrap();
#     }
# 
#     fn close(&mut self) {
#         self.dispatcher.send(Event::Close).unwrap();
#     }
# 
#     fn is_ready(&self, id_to_check: usize) -> bool {
#         self.readylist
#             .lock()
#             .map(|rl| rl.iter().any(|id| *id == id_to_check))
#             .unwrap()
#     }
# }
# 
# impl Drop for Reactor {
#     fn drop(&mut self) {
#         self.handle.take().map(|h| h.join().unwrap()).unwrap();
#     }
# }
```

I added a debug printout of the events the reactor registered interest for so we can observe
two things:

1. How the `Waker` object looks just like the _trait object_ we talked about in an earlier chapter
2. In what order the events register interest with the reactor

The last point is relevant when we move on the the last paragraph.

## Async/Await and concurrent Futures

This is the first time we actually see the `async/await` syntax so let's
finish this book by explaining them briefly.

Hopefully, the `await` syntax looks pretty familiar. It has a lot in common
with `yield` and indeed, it works in much the same way.

The `async` keyword can be used on functions as in `async fn(...)` or on a
block as in `async { ... }`. Both will turn your function, or block, into a
`Future`.

These `Futures` are rather simple. Imagine our generator from a few chapters
back. Every `await` point is like a `yield` point.

Instead of `yielding` a value we pass in, it yields the `Future` we're awaiting.
In turn this `Future` is polled. 

Now, as is the case in our code, our `mainfut` contains two non-leaf futures
which it awaits, and all that happens is that these state machines are polled
as well until some "leaf future" in the end is finally polled and either
returns `Ready` or `Pending`.

The way our example is right now, it's not much better than regular synchronous
code. For us to actually await multiple futures at the same time we somehow need
to `spawn` them so they're polled once, but does not cause our thread to sleep
and wait for them one after one.

Our example as it stands now returns this:

```ignore
Future got 1 at time: 1.00.
Future got 2 at time: 3.00.
```

If these `Futures` were executed asynchronously we would expect to see:

```ignore
Future got 1 at time: 1.00.
Future got 2 at time: 2.00.
```

To accomplish this we can create the simplest possible `spawn` function I could
come up with:

```rust, ignore, noplaypen
fn spawn<F: Future>(future: F) -> Pin<Box<F>> {
    // We start off the same way as we did before
    let mywaker = Arc::new(MyWaker{ thread: thread::current() });
    let waker = waker_into_waker(Arc::into_raw(mywaker));
    let mut cx = Context::from_waker(&waker);
    
    // But we need to Box this Future. We can't pin it to this stack frame
    // since we'll return before the `Future` is resolved so it must be pinned
    // to the heap.
    let mut boxed = Box::pin(future);
    // Now we poll and just discard the result. This way, we register a `Waker`
    // with our `Reactor` and kick of whatever operation we're expecting.
    let _ = Future::poll(boxed.as_mut(), &mut cx);
    
    // We still need this `Future` since we'll await it later so we return it...
    boxed
}
```

Now if we change our code in `main` to look like this instead.

```rust, edition2018
# use std::{
#     future::Future, pin::Pin, sync::{mpsc::{channel, Sender}, Arc, Mutex},
#     task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
#     thread::{self, JoinHandle}, time::{Duration, Instant}
# };
fn main() {
    let start = Instant::now();
    let reactor = Reactor::new();
    let reactor = Arc::new(Mutex::new(reactor));
    let future1 = Task::new(reactor.clone(), 1, 1);
    let future2 = Task::new(reactor.clone(), 2, 2);

    let fut1 = async {
        let val = future1.await;
        let dur = (Instant::now() - start).as_secs_f32();
        println!("Future got {} at time: {:.2}.", val, dur);
    };

    let fut2 = async {
        let val = future2.await;
        let dur = (Instant::now() - start).as_secs_f32();
        println!("Future got {} at time: {:.2}.", val, dur);
    };

    // You'll notice everything stays the same until this point
    let mainfut = async {
        // Here we "kick off" our first `Future`
        let handle1 = spawn(fut1);
        // And the second one
        let handle2 = spawn(fut2);

        // Now, they're already started, and when they get polled in our
        // executor now they will just return `Pending`, or if we somehow used
        // so much time that they're already resolved, they will return `Ready`.
        handle1.await;
        handle2.await;
    };

    block_on(mainfut);
    reactor.lock().map(|mut r| r.close()).unwrap();
}
# // ============================= EXECUTOR ====================================
# fn block_on<F: Future>(mut future: F) -> F::Output {
#     let mywaker = Arc::new(MyWaker{ thread: thread::current() }); 
#     let waker = waker_into_waker(Arc::into_raw(mywaker));
#     let mut cx = Context::from_waker(&waker);
#     let val = loop {
#         let pinned = unsafe { Pin::new_unchecked(&mut future) };
#         match Future::poll(pinned, &mut cx) {
#             Poll::Ready(val) => break val,
#             Poll::Pending => thread::park(),
#         };
#     };
#     val
# }
# 
# fn spawn<F: Future>(future: F) -> Pin<Box<F>> {
#     let mywaker = Arc::new(MyWaker{ thread: thread::current() }); 
#     let waker = waker_into_waker(Arc::into_raw(mywaker)); 
#     let mut cx = Context::from_waker(&waker); 
#     let mut boxed = Box::pin(future);
#     let _ = Future::poll(boxed.as_mut(), &mut cx); 
#     boxed
# }
# 
# // ====================== FUTURE IMPLEMENTATION ==============================
# #[derive(Clone)]
# struct MyWaker {
#     thread: thread::Thread,
# }
# 
# #[derive(Clone)]
# pub struct Task {
#     id: usize,
#     reactor: Arc<Mutex<Reactor>>,
#     data: u64,
#     is_registered: bool,
# }
# 
# fn mywaker_wake(s: &MyWaker) {
#     let waker_ptr: *const MyWaker = s;
#     let waker_arc = unsafe {Arc::from_raw(waker_ptr)};
#     waker_arc.thread.unpark();
# }
# 
# fn mywaker_clone(s: &MyWaker) -> RawWaker {
#     let arc = unsafe { Arc::from_raw(s).clone() };
#     std::mem::forget(arc.clone()); // increase ref count
#     RawWaker::new(Arc::into_raw(arc) as *const (), &VTABLE)
# }
# 
# const VTABLE: RawWakerVTable = unsafe {
#     RawWakerVTable::new(
#         |s| mywaker_clone(&*(s as *const MyWaker)),     // clone
#         |s| mywaker_wake(&*(s as *const MyWaker)),      // wake
#         |s| mywaker_wake(*(s as *const &MyWaker)),      // wake by ref
#         |s| drop(Arc::from_raw(s as *const MyWaker)),   // decrease refcount
#     )
# };
# 
# fn waker_into_waker(s: *const MyWaker) -> Waker {
#     let raw_waker = RawWaker::new(s as *const (), &VTABLE);
#     unsafe { Waker::from_raw(raw_waker) }
# }
# 
# impl Task {
#     fn new(reactor: Arc<Mutex<Reactor>>, data: u64, id: usize) -> Self {
#         Task {
#             id,
#             reactor,
#             data,
#             is_registered: false,
#         }
#     }
# }
# 
# impl Future for Task {
#     type Output = usize;
#     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
#         let mut r = self.reactor.lock().unwrap();
#         if r.is_ready(self.id) {
#             Poll::Ready(self.id)
#         } else if self.is_registered {
#             Poll::Pending
#         } else {
#             r.register(self.data, cx.waker().clone(), self.id);
#             drop(r);
#             self.is_registered = true;
#             Poll::Pending
#         }
#     }
# }
# 
# // =============================== REACTOR ===================================
# struct Reactor {
#     dispatcher: Sender<Event>,
#     handle: Option<JoinHandle<()>>,
#     readylist: Arc<Mutex<Vec<usize>>>,
# }
# #[derive(Debug)]
# enum Event {
#     Close,
#     Timeout(Waker, u64, usize),
# }
# 
# impl Reactor {
#     fn new() -> Self {
#         let (tx, rx) = channel::<Event>();
#         let readylist = Arc::new(Mutex::new(vec![]));
#         let rl_clone = readylist.clone();
#         let mut handles = vec![];
#         let handle = thread::spawn(move || {
#             // This simulates some I/O resource
#             for event in rx {
#                 println!("REACTOR: {:?}", event);
#                 let rl_clone = rl_clone.clone();
#                 match event {
#                     Event::Close => break,
#                     Event::Timeout(waker, duration, id) => {
#                         let event_handle = thread::spawn(move || {
#                             thread::sleep(Duration::from_secs(duration));
#                             rl_clone.lock().map(|mut rl| rl.push(id)).unwrap();
#                             waker.wake();
#                         });
# 
#                         handles.push(event_handle);
#                     }
#                 }
#             }
# 
#             for handle in handles {
#                 handle.join().unwrap();
#             }
#         });
# 
#         Reactor {
#             readylist,
#             dispatcher: tx,
#             handle: Some(handle),
#         }
#     }
# 
#     fn register(&mut self, duration: u64, waker: Waker, data: usize) {
#         self.dispatcher
#             .send(Event::Timeout(waker, duration, data))
#             .unwrap();
#     }
# 
#     fn close(&mut self) {
#         self.dispatcher.send(Event::Close).unwrap();
#     }
# 
#     fn is_ready(&self, id_to_check: usize) -> bool {
#         self.readylist
#             .lock()
#             .map(|rl| rl.iter().any(|id| *id == id_to_check))
#             .unwrap()
#     }
# }
# 
# impl Drop for Reactor {
#     fn drop(&mut self) {
#         self.handle.take().map(|h| h.join().unwrap()).unwrap();
#     }
# }
```


Now, if we try to run our example again


If you add this code to our example and run it, you'll see:

```ignore
Future got 1 at time: 1.00.
Future got 2 at time: 2.00.
```
Exactly as we expected.

Now this `spawn` method is not very sophisticated but it explains the concept.
I've [challenged you to create a better version](./conclusion.md#building-a-better-exectuor) and pointed you at a better resource
in the next chapter under [reader exercises](./conclusion.md#reader-exercises).

That's actually it for now. There are probably much more to learn, but I think it
will be easier once the fundamental concepts are there and that further
exploration will get a lot easier. 

Don't forget the exercises in the last chapter 😊. Have fun until the next time! 

[mio]: https://github.com/tokio-rs/mio
[arc_wake]: https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.13/futures/task/trait.ArcWake.html
[example_repo]: https://github.com/cfsamson/examples-futures
[playground_example]:https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=ca43dba55c6e3838c5494de45875677f
[spurious_wakeup]: https://cfsamson.github.io/book-exploring-async-basics/9_3_http_module.html#bonus-section
[condvar]: https://doc.rust-lang.org/stable/std/sync/struct.Condvar.html
[crossbeam_parker]: https://docs.rs/crossbeam/0.7.3/crossbeam/sync/struct.Parker.html