# Implementing Futures - Futures Explained in 200 Lines of Rust
This book aims to explain Futures in Rust using an example driven approach.

# 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 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

    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:

    // 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);

        // 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 shadow `future` so it can't be accessed again and will
        // not move until it's dropped.
        let mut future = unsafe { Pin::new_unchecked(&mut future) };

        // 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 {

            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, 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:

    // 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) };
        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, 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.

> 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), which could cause the same deadlock if we're unlucky.

There are several better solutions, here are some:

- Use std::sync::CondVar
- Use crossbeam::sync::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, 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 which we can chain operations on which are performed once the leaf future is ready.

The reactor we create here will also create leaf-futures, accept a waker and call it once the task is finished.

The task we're implementing is the simplest I could find. It's a timer that only spawns a thread and puts it to sleep for a number of seconds we specify when acquiring the leaf-future.

To be able to run the code here in the browser there is not much real I/O we can do so just pretend that this is actually represents some useful I/O operation for the sake of this example.

Our Reactor will look like this:

    // 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. You can also copy that or edit it right in this book.

    # 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
    # }
    #
    # // ====================== 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

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, so when we poll a future the first time we run the code up until the first `await` point where it yields a new Future we poll and so on until we reach a leaf-future.

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 until some "leaf future" in the end 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 the executor starts running them concurrently.

Our example as it stands now returns this:

    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:

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

Now, this is the point where I'll refer you to some better resources for implementing just that. You should have a pretty good understanding of the concept of Futures by now.

The next step should be getting to know how more advanced runtimes work and how they implement different ways of running Futures to completion.

If I were you I would read this next, and try to implement it for our example..

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 😊.