git.bitcoin.ninja Git - rust-lightning/blob - lightning/src/util/wakers.rs

   1 // This file is Copyright its original authors, visible in version control
   2 // history.
   3 //
   4 // This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
   5 // or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
   7 // You may not use this file except in accordance with one or both of these
   8 // licenses.
   9
  10 //! Utilities which allow users to block on some future notification from LDK. These are
  11 //! specifically used by [`ChannelManager`] to allow waiting until the [`ChannelManager`] needs to
  12 //! be re-persisted.
  13 //!
  14 //! [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
  15
  16 use alloc::sync::Arc;
  17 use core::mem;
  18 use crate::sync::Mutex;
  19
  20 use crate::prelude::*;
  21
  22 #[cfg(feature = "std")]
  23 use crate::sync::Condvar;
  24 #[cfg(feature = "std")]
  25 use std::time::Duration;
  26
  27 use core::future::Future as StdFuture;
  28 use core::task::{Context, Poll};
  29 use core::pin::Pin;
  30
  31
  32 /// Used to signal to one of many waiters that the condition they're waiting on has happened.
  33 pub(crate) struct Notifier {
  34         notify_pending: Mutex<(bool, Option<Arc<Mutex<FutureState>>>)>,
  35 }
  36
  37 impl Notifier {
  38         pub(crate) fn new() -> Self {
  39                 Self {
  40                         notify_pending: Mutex::new((false, None)),
  41                 }
  42         }
  43
  44         /// Wake waiters, tracking that wake needs to occur even if there are currently no waiters.
  45         pub(crate) fn notify(&self) {
  46                 let mut lock = self.notify_pending.lock().unwrap();
  47                 if let Some(future_state) = &lock.1 {
  48                         if complete_future(future_state) {
  49                                 lock.1 = None;
  50                                 return;
  51                         }
  52                 }
  53                 lock.0 = true;
  54         }
  55
  56         /// Gets a [`Future`] that will get woken up with any waiters
  57         pub(crate) fn get_future(&self) -> Future {
  58                 let mut lock = self.notify_pending.lock().unwrap();
  59                 if let Some(existing_state) = &lock.1 {
  60                         if existing_state.lock().unwrap().callbacks_made {
  61                                 // If the existing `FutureState` has completed and actually made callbacks,
  62                                 // consider the notification flag to have been cleared and reset the future state.
  63                                 lock.1.take();
  64                                 lock.0 = false;
  65                         }
  66                 }
  67                 if let Some(existing_state) = &lock.1 {
  68                         Future { state: Arc::clone(&existing_state) }
  69                 } else {
  70                         let state = Arc::new(Mutex::new(FutureState {
  71                                 callbacks: Vec::new(),
  72                                 callbacks_with_state: Vec::new(),
  73                                 complete: lock.0,
  74                                 callbacks_made: false,
  75                         }));
  76                         lock.1 = Some(Arc::clone(&state));
  77                         Future { state }
  78                 }
  79         }
  80
  81         #[cfg(any(test, feature = "_test_utils"))]
  82         pub fn notify_pending(&self) -> bool {
  83                 self.notify_pending.lock().unwrap().0
  84         }
  85 }
  86
  87 macro_rules! define_callback { ($($bounds: path),*) => {
  88 /// A callback which is called when a [`Future`] completes.
  89 ///
  90 /// Note that this MUST NOT call back into LDK directly, it must instead schedule actions to be
  91 /// taken later. Rust users should use the [`std::future::Future`] implementation for [`Future`]
  92 /// instead.
  93 ///
  94 /// Note that the [`std::future::Future`] implementation may only work for runtimes which schedule
  95 /// futures when they receive a wake, rather than immediately executing them.
  96 pub trait FutureCallback : $($bounds +)* {
  97         /// The method which is called.
  98         fn call(&self);
  99 }
 100
 101 impl<F: Fn() $(+ $bounds)*> FutureCallback for F {
 102         fn call(&self) { (self)(); }
 103 }
 104 } }
 105
 106 #[cfg(feature = "std")]
 107 define_callback!(Send);
 108 #[cfg(not(feature = "std"))]
 109 define_callback!();
 110
 111 pub(crate) struct FutureState {
 112         // When we're tracking whether a callback counts as having woken the user's code, we check the
 113         // first bool - set to false if we're just calling a Waker, and true if we're calling an actual
 114         // user-provided function.
 115         callbacks: Vec<(bool, Box<dyn FutureCallback>)>,
 116         callbacks_with_state: Vec<(bool, Box<dyn Fn(&Arc<Mutex<FutureState>>) -> () + Send>)>,
 117         complete: bool,
 118         callbacks_made: bool,
 119 }
 120
 121 fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
 122         let mut state_lock = this.lock().unwrap();
 123         let state = &mut *state_lock;
 124         for (counts_as_call, callback) in state.callbacks.drain(..) {
 125                 callback.call();
 126                 state.callbacks_made |= counts_as_call;
 127         }
 128         for (counts_as_call, callback) in state.callbacks_with_state.drain(..) {
 129                 (callback)(this);
 130                 state.callbacks_made |= counts_as_call;
 131         }
 132         state.complete = true;
 133         state.callbacks_made
 134 }
 135
 136 /// A simple future which can complete once, and calls some callback(s) when it does so.
 137 ///
 138 /// Clones can be made and all futures cloned from the same source will complete at the same time.
 139 #[derive(Clone)]
 140 pub struct Future {
 141         state: Arc<Mutex<FutureState>>,
 142 }
 143
 144 impl Future {
 145         /// Registers a callback to be called upon completion of this future. If the future has already
 146         /// completed, the callback will be called immediately.
 147         ///
 148         /// This is not exported to bindings users, use the bindings-only `register_callback_fn` instead
 149         pub fn register_callback(&self, callback: Box<dyn FutureCallback>) {
 150                 let mut state = self.state.lock().unwrap();
 151                 if state.complete {
 152                         state.callbacks_made = true;
 153                         mem::drop(state);
 154                         callback.call();
 155                 } else {
 156                         state.callbacks.push((true, callback));
 157                 }
 158         }
 159
 160         // C bindings don't (currently) know how to map `Box<dyn Trait>`, and while it could add the
 161         // following wrapper, doing it in the bindings is currently much more work than simply doing it
 162         // here.
 163         /// Registers a callback to be called upon completion of this future. If the future has already
 164         /// completed, the callback will be called immediately.
 165         #[cfg(c_bindings)]
 166         pub fn register_callback_fn<F: 'static + FutureCallback>(&self, callback: F) {
 167                 self.register_callback(Box::new(callback));
 168         }
 169
 170         /// Waits until this [`Future`] completes.
 171         #[cfg(feature = "std")]
 172         pub fn wait(self) {
 173                 Sleeper::from_single_future(self).wait();
 174         }
 175
 176         /// Waits until this [`Future`] completes or the given amount of time has elapsed.
 177         ///
 178         /// Returns true if the [`Future`] completed, false if the time elapsed.
 179         #[cfg(feature = "std")]
 180         pub fn wait_timeout(self, max_wait: Duration) -> bool {
 181                 Sleeper::from_single_future(self).wait_timeout(max_wait)
 182         }
 183
 184         #[cfg(test)]
 185         pub fn poll_is_complete(&self) -> bool {
 186                 let mut state = self.state.lock().unwrap();
 187                 if state.complete {
 188                         state.callbacks_made = true;
 189                         true
 190                 } else { false }
 191         }
 192 }
 193
 194 use core::task::Waker;
 195 struct StdWaker(pub Waker);
 196 impl FutureCallback for StdWaker {
 197         fn call(&self) { self.0.wake_by_ref() }
 198 }
 199
 200 /// This is not exported to bindings users as Rust Futures aren't usable in language bindings.
 201 impl<'a> StdFuture for Future {
 202         type Output = ();
 203
 204         fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 205                 let mut state = self.state.lock().unwrap();
 206                 if state.complete {
 207                         state.callbacks_made = true;
 208                         Poll::Ready(())
 209                 } else {
 210                         let waker = cx.waker().clone();
 211                         state.callbacks.push((false, Box::new(StdWaker(waker))));
 212                         Poll::Pending
 213                 }
 214         }
 215 }
 216
 217 /// A struct which can be used to select across many [`Future`]s at once without relying on a full
 218 /// async context.
 219 #[cfg(feature = "std")]
 220 pub struct Sleeper {
 221         notifiers: Vec<Arc<Mutex<FutureState>>>,
 222 }
 223
 224 #[cfg(feature = "std")]
 225 impl Sleeper {
 226         /// Constructs a new sleeper from one future, allowing blocking on it.
 227         pub fn from_single_future(future: Future) -> Self {
 228                 Self { notifiers: vec![future.state] }
 229         }
 230         /// Constructs a new sleeper from two futures, allowing blocking on both at once.
 231         // Note that this is the common case - a ChannelManager and ChainMonitor.
 232         pub fn from_two_futures(fut_a: Future, fut_b: Future) -> Self {
 233                 Self { notifiers: vec![fut_a.state, fut_b.state] }
 234         }
 235         /// Constructs a new sleeper on many futures, allowing blocking on all at once.
 236         pub fn new(futures: Vec<Future>) -> Self {
 237                 Self { notifiers: futures.into_iter().map(|f| f.state).collect() }
 238         }
 239         /// Prepares to go into a wait loop body, creating a condition variable which we can block on
 240         /// and an `Arc<Mutex<Option<_>>>` which gets set to the waking `Future`'s state prior to the
 241         /// condition variable being woken.
 242         fn setup_wait(&self) -> (Arc<Condvar>, Arc<Mutex<Option<Arc<Mutex<FutureState>>>>>) {
 243                 let cv = Arc::new(Condvar::new());
 244                 let notified_fut_mtx = Arc::new(Mutex::new(None));
 245                 {
 246                         for notifier_mtx in self.notifiers.iter() {
 247                                 let cv_ref = Arc::clone(&cv);
 248                                 let notified_fut_ref = Arc::clone(&notified_fut_mtx);
 249                                 let mut notifier = notifier_mtx.lock().unwrap();
 250                                 if notifier.complete {
 251                                         *notified_fut_mtx.lock().unwrap() = Some(Arc::clone(&notifier_mtx));
 252                                         break;
 253                                 }
 254                                 notifier.callbacks_with_state.push((false, Box::new(move |notifier_ref| {
 255                                         *notified_fut_ref.lock().unwrap() = Some(Arc::clone(notifier_ref));
 256                                         cv_ref.notify_all();
 257                                 })));
 258                         }
 259                 }
 260                 (cv, notified_fut_mtx)
 261         }
 262
 263         /// Wait until one of the [`Future`]s registered with this [`Sleeper`] has completed.
 264         pub fn wait(&self) {
 265                 let (cv, notified_fut_mtx) = self.setup_wait();
 266                 let notified_fut = cv.wait_while(notified_fut_mtx.lock().unwrap(), |fut_opt| fut_opt.is_none())
 267                         .unwrap().take().expect("CV wait shouldn't have returned until the notifying future was set");
 268                 notified_fut.lock().unwrap().callbacks_made = true;
 269         }
 270
 271         /// Wait until one of the [`Future`]s registered with this [`Sleeper`] has completed or the
 272         /// given amount of time has elapsed. Returns true if a [`Future`] completed, false if the time
 273         /// elapsed.
 274         pub fn wait_timeout(&self, max_wait: Duration) -> bool {
 275                 let (cv, notified_fut_mtx) = self.setup_wait();
 276                 let notified_fut =
 277                         match cv.wait_timeout_while(notified_fut_mtx.lock().unwrap(), max_wait, |fut_opt| fut_opt.is_none()) {
 278                                 Ok((_, e)) if e.timed_out() => return false,
 279                                 Ok((mut notified_fut, _)) =>
 280                                         notified_fut.take().expect("CV wait shouldn't have returned until the notifying future was set"),
 281                                 Err(_) => panic!("Previous panic while a lock was held led to a lock panic"),
 282                         };
 283                 notified_fut.lock().unwrap().callbacks_made = true;
 284                 true
 285         }
 286 }
 287
 288 #[cfg(test)]
 289 mod tests {
 290         use super::*;
 291         use core::sync::atomic::{AtomicBool, Ordering};
 292         use core::future::Future as FutureTrait;
 293         use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 294
 295         #[test]
 296         fn notifier_pre_notified_future() {
 297                 // Previously, if we generated a future after a `Notifier` had been notified, the future
 298                 // would never complete. This tests this behavior, ensuring the future instead completes
 299                 // immediately.
 300                 let notifier = Notifier::new();
 301                 notifier.notify();
 302
 303                 let callback = Arc::new(AtomicBool::new(false));
 304                 let callback_ref = Arc::clone(&callback);
 305                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 306                 assert!(callback.load(Ordering::SeqCst));
 307         }
 308
 309         #[test]
 310         fn notifier_future_completes_wake() {
 311                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 312                 // been notified, we'd never mark the notifier as not-awaiting-notify. This caused the
 313                 // `lightning-background-processor` to persist in a tight loop.
 314                 let notifier = Notifier::new();
 315
 316                 // First check the simple case, ensuring if we get notified a new future isn't woken until
 317                 // a second `notify`.
 318                 let callback = Arc::new(AtomicBool::new(false));
 319                 let callback_ref = Arc::clone(&callback);
 320                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 321                 assert!(!callback.load(Ordering::SeqCst));
 322
 323                 notifier.notify();
 324                 assert!(callback.load(Ordering::SeqCst));
 325
 326                 let callback = Arc::new(AtomicBool::new(false));
 327                 let callback_ref = Arc::clone(&callback);
 328                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 329                 assert!(!callback.load(Ordering::SeqCst));
 330
 331                 notifier.notify();
 332                 assert!(callback.load(Ordering::SeqCst));
 333
 334                 // Then check the case where the future is fetched before the notification, but a callback
 335                 // is only registered after the `notify`, ensuring that it is still sufficient to ensure we
 336                 // don't get an instant-wake when we get a new future.
 337                 let future = notifier.get_future();
 338                 notifier.notify();
 339
 340                 let callback = Arc::new(AtomicBool::new(false));
 341                 let callback_ref = Arc::clone(&callback);
 342                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 343                 assert!(callback.load(Ordering::SeqCst));
 344
 345                 let callback = Arc::new(AtomicBool::new(false));
 346                 let callback_ref = Arc::clone(&callback);
 347                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 348                 assert!(!callback.load(Ordering::SeqCst));
 349         }
 350
 351         #[test]
 352         fn new_future_wipes_notify_bit() {
 353                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 354                 // been notified, we'd never mark the notifier as not-awaiting-notify if a `Future` is
 355                 // fetched after the notify bit has been set.
 356                 let notifier = Notifier::new();
 357                 notifier.notify();
 358
 359                 let callback = Arc::new(AtomicBool::new(false));
 360                 let callback_ref = Arc::clone(&callback);
 361                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 362                 assert!(callback.load(Ordering::SeqCst));
 363
 364                 let callback = Arc::new(AtomicBool::new(false));
 365                 let callback_ref = Arc::clone(&callback);
 366                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 367                 assert!(!callback.load(Ordering::SeqCst));
 368
 369                 notifier.notify();
 370                 assert!(callback.load(Ordering::SeqCst));
 371         }
 372
 373         #[cfg(feature = "std")]
 374         #[test]
 375         fn test_wait_timeout() {
 376                 use crate::sync::Arc;
 377                 use std::thread;
 378
 379                 let persistence_notifier = Arc::new(Notifier::new());
 380                 let thread_notifier = Arc::clone(&persistence_notifier);
 381
 382                 let exit_thread = Arc::new(AtomicBool::new(false));
 383                 let exit_thread_clone = exit_thread.clone();
 384                 thread::spawn(move || {
 385                         loop {
 386                                 thread_notifier.notify();
 387                                 if exit_thread_clone.load(Ordering::SeqCst) {
 388                                         break
 389                                 }
 390                         }
 391                 });
 392
 393                 // Check that we can block indefinitely until updates are available.
 394                 let _ = persistence_notifier.get_future().wait();
 395
 396                 // Check that the Notifier will return after the given duration if updates are
 397                 // available.
 398                 loop {
 399                         if persistence_notifier.get_future().wait_timeout(Duration::from_millis(100)) {
 400                                 break
 401                         }
 402                 }
 403
 404                 exit_thread.store(true, Ordering::SeqCst);
 405
 406                 // Check that the Notifier will return after the given duration even if no updates
 407                 // are available.
 408                 loop {
 409                         if !persistence_notifier.get_future().wait_timeout(Duration::from_millis(100)) {
 410                                 break
 411                         }
 412                 }
 413         }
 414
 415         #[cfg(feature = "std")]
 416         #[test]
 417         fn test_state_drops() {
 418                 // Previously, there was a leak if a `Notifier` was `drop`ed without ever being notified
 419                 // but after having been slept-on. This tests for that leak.
 420                 use crate::sync::Arc;
 421                 use std::thread;
 422
 423                 let notifier_a = Arc::new(Notifier::new());
 424                 let notifier_b = Arc::new(Notifier::new());
 425
 426                 let thread_notifier_a = Arc::clone(&notifier_a);
 427
 428                 let future_a = notifier_a.get_future();
 429                 let future_state_a = Arc::downgrade(&future_a.state);
 430
 431                 let future_b = notifier_b.get_future();
 432                 let future_state_b = Arc::downgrade(&future_b.state);
 433
 434                 let join_handle = thread::spawn(move || {
 435                         // Let the other thread get to the wait point, then notify it.
 436                         std::thread::sleep(Duration::from_millis(50));
 437                         thread_notifier_a.notify();
 438                 });
 439
 440                 // Wait on the other thread to finish its sleep, note that the leak only happened if we
 441                 // actually have to sleep here, not if we immediately return.
 442                 Sleeper::from_two_futures(future_a, future_b).wait();
 443
 444                 join_handle.join().unwrap();
 445
 446                 // then drop the notifiers and make sure the future states are gone.
 447                 mem::drop(notifier_a);
 448                 mem::drop(notifier_b);
 449
 450                 assert!(future_state_a.upgrade().is_none() && future_state_b.upgrade().is_none());
 451         }
 452
 453         #[test]
 454         fn test_future_callbacks() {
 455                 let future = Future {
 456                         state: Arc::new(Mutex::new(FutureState {
 457                                 callbacks: Vec::new(),
 458                                 callbacks_with_state: Vec::new(),
 459                                 complete: false,
 460                                 callbacks_made: false,
 461                         }))
 462                 };
 463                 let callback = Arc::new(AtomicBool::new(false));
 464                 let callback_ref = Arc::clone(&callback);
 465                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 466
 467                 assert!(!callback.load(Ordering::SeqCst));
 468                 complete_future(&future.state);
 469                 assert!(callback.load(Ordering::SeqCst));
 470                 complete_future(&future.state);
 471         }
 472
 473         #[test]
 474         fn test_pre_completed_future_callbacks() {
 475                 let future = Future {
 476                         state: Arc::new(Mutex::new(FutureState {
 477                                 callbacks: Vec::new(),
 478                                 callbacks_with_state: Vec::new(),
 479                                 complete: false,
 480                                 callbacks_made: false,
 481                         }))
 482                 };
 483                 complete_future(&future.state);
 484
 485                 let callback = Arc::new(AtomicBool::new(false));
 486                 let callback_ref = Arc::clone(&callback);
 487                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 488
 489                 assert!(callback.load(Ordering::SeqCst));
 490                 assert!(future.state.lock().unwrap().callbacks.is_empty());
 491         }
 492
 493         // Rather annoyingly, there's no safe way in Rust std to construct a Waker despite it being
 494         // totally possible to construct from a trait implementation (though somewhat less effecient
 495         // compared to a raw VTable). Instead, we have to write out a lot of boilerplate to build a
 496         // waker, which we do here with a trivial Arc<AtomicBool> data element to track woke-ness.
 497         const WAKER_V_TABLE: RawWakerVTable = RawWakerVTable::new(waker_clone, wake, wake_by_ref, drop);
 498         unsafe fn wake_by_ref(ptr: *const ()) { let p = ptr as *const Arc<AtomicBool>; assert!(!(*p).fetch_or(true, Ordering::SeqCst)); }
 499         unsafe fn drop(ptr: *const ()) { let p = ptr as *mut Arc<AtomicBool>; let _freed = Box::from_raw(p); }
 500         unsafe fn wake(ptr: *const ()) { wake_by_ref(ptr); drop(ptr); }
 501         unsafe fn waker_clone(ptr: *const ()) -> RawWaker {
 502                 let p = ptr as *const Arc<AtomicBool>;
 503                 RawWaker::new(Box::into_raw(Box::new(Arc::clone(&*p))) as *const (), &WAKER_V_TABLE)
 504         }
 505
 506         fn create_waker() -> (Arc<AtomicBool>, Waker) {
 507                 let a = Arc::new(AtomicBool::new(false));
 508                 let waker = unsafe { Waker::from_raw(waker_clone((&a as *const Arc<AtomicBool>) as *const ())) };
 509                 (a, waker)
 510         }
 511
 512         #[test]
 513         fn test_future() {
 514                 let mut future = Future {
 515                         state: Arc::new(Mutex::new(FutureState {
 516                                 callbacks: Vec::new(),
 517                                 callbacks_with_state: Vec::new(),
 518                                 complete: false,
 519                                 callbacks_made: false,
 520                         }))
 521                 };
 522                 let mut second_future = Future { state: Arc::clone(&future.state) };
 523
 524                 let (woken, waker) = create_waker();
 525                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 526                 assert!(!woken.load(Ordering::SeqCst));
 527
 528                 let (second_woken, second_waker) = create_waker();
 529                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Pending);
 530                 assert!(!second_woken.load(Ordering::SeqCst));
 531
 532                 complete_future(&future.state);
 533                 assert!(woken.load(Ordering::SeqCst));
 534                 assert!(second_woken.load(Ordering::SeqCst));
 535                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 536                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Ready(()));
 537         }
 538
 539         #[test]
 540         #[cfg(feature = "std")]
 541         fn test_dropped_future_doesnt_count() {
 542                 // Tests that if a Future gets drop'd before it is poll()ed `Ready` it doesn't count as
 543                 // having been woken, leaving the notify-required flag set.
 544                 let notifier = Notifier::new();
 545                 notifier.notify();
 546
 547                 // If we get a future and don't touch it we're definitely still notify-required.
 548                 notifier.get_future();
 549                 assert!(notifier.get_future().wait_timeout(Duration::from_millis(1)));
 550                 assert!(!notifier.get_future().wait_timeout(Duration::from_millis(1)));
 551
 552                 // Even if we poll'd once but didn't observe a `Ready`, we should be notify-required.
 553                 let mut future = notifier.get_future();
 554                 let (woken, waker) = create_waker();
 555                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 556
 557                 notifier.notify();
 558                 assert!(woken.load(Ordering::SeqCst));
 559                 assert!(notifier.get_future().wait_timeout(Duration::from_millis(1)));
 560
 561                 // However, once we do poll `Ready` it should wipe the notify-required flag.
 562                 let mut future = notifier.get_future();
 563                 let (woken, waker) = create_waker();
 564                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 565
 566                 notifier.notify();
 567                 assert!(woken.load(Ordering::SeqCst));
 568                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 569                 assert!(!notifier.get_future().wait_timeout(Duration::from_millis(1)));
 570         }
 571
 572         #[test]
 573         fn test_poll_post_notify_completes() {
 574                 // Tests that if we have a future state that has completed, and we haven't yet requested a
 575                 // new future, if we get a notify prior to requesting that second future it is generated
 576                 // pre-completed.
 577                 let notifier = Notifier::new();
 578
 579                 notifier.notify();
 580                 let mut future = notifier.get_future();
 581                 let (woken, waker) = create_waker();
 582                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 583                 assert!(!woken.load(Ordering::SeqCst));
 584
 585                 notifier.notify();
 586                 let mut future = notifier.get_future();
 587                 let (woken, waker) = create_waker();
 588                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 589                 assert!(!woken.load(Ordering::SeqCst));
 590
 591                 let mut future = notifier.get_future();
 592                 let (woken, waker) = create_waker();
 593                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 594                 assert!(!woken.load(Ordering::SeqCst));
 595
 596                 notifier.notify();
 597                 assert!(woken.load(Ordering::SeqCst));
 598                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 599         }
 600
 601         #[test]
 602         fn test_poll_post_notify_completes_initial_notified() {
 603                 // Identical to the previous test, but the first future completes via a wake rather than an
 604                 // immediate `Poll::Ready`.
 605                 let notifier = Notifier::new();
 606
 607                 let mut future = notifier.get_future();
 608                 let (woken, waker) = create_waker();
 609                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 610
 611                 notifier.notify();
 612                 assert!(woken.load(Ordering::SeqCst));
 613                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 614
 615                 notifier.notify();
 616                 let mut future = notifier.get_future();
 617                 let (woken, waker) = create_waker();
 618                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 619                 assert!(!woken.load(Ordering::SeqCst));
 620
 621                 let mut future = notifier.get_future();
 622                 let (woken, waker) = create_waker();
 623                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 624                 assert!(!woken.load(Ordering::SeqCst));
 625
 626                 notifier.notify();
 627                 assert!(woken.load(Ordering::SeqCst));
 628                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 629         }
 630
 631         #[test]
 632         #[cfg(feature = "std")]
 633         fn test_multi_future_sleep() {
 634                 // Tests the `Sleeper` with multiple futures.
 635                 let notifier_a = Notifier::new();
 636                 let notifier_b = Notifier::new();
 637
 638                 // Set both notifiers as woken without sleeping yet.
 639                 notifier_a.notify();
 640                 notifier_b.notify();
 641                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 642
 643                 // One future has woken us up, but the other should still have a pending notification.
 644                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 645
 646                 // However once we've slept twice, we should no longer have any pending notifications
 647                 assert!(!Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future())
 648                         .wait_timeout(Duration::from_millis(10)));
 649
 650                 // Test ordering somewhat more.
 651                 notifier_a.notify();
 652                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 653         }
 654
 655         #[test]
 656         #[cfg(feature = "std")]
 657         fn sleeper_with_pending_callbacks() {
 658                 // This is similar to the above `test_multi_future_sleep` test, but in addition registers
 659                 // "normal" callbacks which will cause the futures to assume notification has occurred,
 660                 // rather than waiting for a woken sleeper.
 661                 let notifier_a = Notifier::new();
 662                 let notifier_b = Notifier::new();
 663
 664                 // Set both notifiers as woken without sleeping yet.
 665                 notifier_a.notify();
 666                 notifier_b.notify();
 667
 668                 // After sleeping one future (not guaranteed which one, however) will have its notification
 669                 // bit cleared.
 670                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 671
 672                 // By registering a callback on the futures for both notifiers, one will complete
 673                 // immediately, but one will remain tied to the notifier, and will complete once the
 674                 // notifier is next woken, which will be considered the completion of the notification.
 675                 let callback_a = Arc::new(AtomicBool::new(false));
 676                 let callback_b = Arc::new(AtomicBool::new(false));
 677                 let callback_a_ref = Arc::clone(&callback_a);
 678                 let callback_b_ref = Arc::clone(&callback_b);
 679                 notifier_a.get_future().register_callback(Box::new(move || assert!(!callback_a_ref.fetch_or(true, Ordering::SeqCst))));
 680                 notifier_b.get_future().register_callback(Box::new(move || assert!(!callback_b_ref.fetch_or(true, Ordering::SeqCst))));
 681                 assert!(callback_a.load(Ordering::SeqCst) ^ callback_b.load(Ordering::SeqCst));
 682
 683                 // If we now notify both notifiers again, the other callback will fire, completing the
 684                 // notification, and we'll be back to one pending notification.
 685                 notifier_a.notify();
 686                 notifier_b.notify();
 687
 688                 assert!(callback_a.load(Ordering::SeqCst) && callback_b.load(Ordering::SeqCst));
 689                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 690                 assert!(!Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future())
 691                         .wait_timeout(Duration::from_millis(10)));
 692         }
 693 }