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@@ -9,65 +9,40 @@ can always clone the repository and play around with the code yourself. There
are two branches. The `basic_example` is this code, and the `basic_example_commented`
is this example with extensive comments.
> If you want to follow along as we go through, initalize 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 with why we wrote this book, by implementing our own `Futures`.
Let's start off by getting all our imports right away so you can follow along
```rust, edition2018
```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}
};
```
fn main() {
// This is just to make it easier for us to see when our Future was resolved
let start = Instant::now();
## The Executor
// 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(), 2, 1);
let future2 = Task::new(reactor.clone(), 1, 2);
The executors task is to take one or more futures and run them to completion.
// 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);
};
The first thing an `executor` does when it gets a `Future` is polling it.
let fut2 = async {
let val = future2.await;
let dur = (Instant::now() - start).as_secs_f32();
println!("Future got {} at time: {:.2}.", val, dur);
};
**When polled one of three things can happen:**
// 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;
};
- 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`
// 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();
}
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.
//// ============================ EXECUTOR ====================================
**Our Executor will look like this:**
```rust, noplaypen
// 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
@@ -96,25 +71,34 @@ fn block_on<F: Future>(mut future: F) -> F::Output {
};
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.
// ====================== FUTURE IMPLEMENTATION ==============================
We'll see more about the `Waker` in the next paragraph, but just look at it like
a _trait object_ like 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 in the future.
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 that chapter, we use `Pin` and the guarantees that give us to
allow `Futures` to have self references.
## The `Future` implementation
```rust, noplaypen
// This is the definition of our `Waker`. We use a regular thread-handle here.
// It works but it's not a good solution. If one of our `Futures` holds a handle
// to our thread and takes it with it to a different thread the followinc could
// happen:
// 1. Our future calls `unpark` 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 but one nanosecond later the `Reactor` gets
// an event and calles `wake()` which also unparks our thread.
// 4. This could all 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 alredy at that point.
// 6. We're deadlocked and our program stops working
// There are many better soloutions, here are some:
// - Use `std::sync::CondVar`
// - Use [crossbeam::sync::Parker](https://docs.rs/crossbeam/0.7.3/crossbeam/sync/struct.Parker.html)
// 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,
@@ -183,11 +167,12 @@ impl Task {
// This is our `Future` implementation
impl Future for Task {
// The output for this kind of `leaf future` is just an `usize`. For other
// futures this could be something more interesting like a byte stream.
// futures this could be something more interesting like a bytearray.
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.
@@ -207,9 +192,80 @@ impl Future for Task {
}
}
}
```
// =============================== REACTOR ===================================
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 as easy as increasing the refcount.
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 mye
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 is not 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 and takes it with it to a different thread the followinc 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 but one nanosecond later the `Reactor` gets
an event and calles `wake()` which also unparks our thread.
4. This could all 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 alredy at that point.
6. We're deadlocked and our program stops working
There are many better soloutions, here are some:
- Use `std::sync::CondVar`
- Use [crossbeam::sync::Parker](https://docs.rs/crossbeam/0.7.3/crossbeam/sync/struct.Parker.html)
## 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`. Or 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`.
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
// 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 {
@@ -318,29 +374,37 @@ impl Drop for Reactor {
}
```
## Our finished code
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`.
Here is the whole example. You can edit it right here in your browser and
run it yourself. Have fun!
```rust,edition2018,editable
use std::{
future::Future, pin::Pin, sync::{mpsc::{channel, Sender}, Arc, Mutex},
task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
thread::{self, JoinHandle}, time::{Duration, Instant}
};
Now, let's test our code and see if it works:
```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(), 2, 1);
let future2 = Task::new(reactor.clone(), 1, 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();
@@ -353,170 +417,274 @@ fn main() {
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,
Simple(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 {
let rl_clone = rl_clone.clone();
match event {
Event::Close => break,
Event::Simple(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::Simple(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();
}
}
#//// ============================ EXECUTOR ====================================
#
#// 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
#}
#
#// ====================== FUTURE IMPLEMENTATION ==============================
#
#// This is the definition of our `Waker`. We use a regular thread-handle here.
#// It works but it's not a good solution. If one of our `Futures` holds a handle
#// to our thread and takes it with it to a different thread the followinc could
#// happen:
#// 1. Our future calls `unpark` 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 but one nanosecond later the `Reactor` gets
#// an event and calles `wake()` which also unparks our thread.
#// 4. This could all 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 alredy at that point.
#// 6. We're deadlocked and our program stops working
#// There are many better soloutions, here are some:
#// - Use `std::sync::CondVar`
#// - Use [crossbeam::sync::Parker](https://docs.rs/crossbeam/0.7.3/crossbeam/sync/#struct.Parker.html)
##[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 from the book.
#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 this kind of `leaf future` is just an `usize`. For other
# // futures this could be something more interesting like a byte stream.
# 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"
# if r.is_ready(self.id) {
# // if it is, we return the data. In this case it's just the ID of
# // the task.
# 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
# }
# }
#}
#
# // =============================== REACTOR ===================================
# // 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. A timeout event, a "timeout" event called
# // `Simple` and a `Close` event to close down our reactor.
# #[derive(Debug)]
# enum Event {
# Close,
# Simple(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 || {
# // This simulates some I/O resource
# 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::Simple(waker, duration, id) => {
#
# // When we get an event we simply spawn a new thread...
# let event_handle = thread::spawn(move || {
# //... which will just sleep 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 all 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::Simple(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();
# }
# }
```
[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
[playground_example]:https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=ca43dba55c6e3838c5494de45875677f
[mdbook_issue]: https://github.com/rust-lang/mdBook/issues/1134