_ 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                                 std_future_callbacks: Vec::new(),
  73                                 callbacks_with_state: Vec::new(),
  74                                 complete: lock.0,
  75                                 callbacks_made: false,
  76                         }));
  77                         lock.1 = Some(Arc::clone(&state));
  78                         Future { state }
  79                 }
  80         }
  81
  82         #[cfg(any(test, feature = "_test_utils"))]
  83         pub fn notify_pending(&self) -> bool {
  84                 self.notify_pending.lock().unwrap().0
  85         }
  86 }
  87
  88 macro_rules! define_callback { ($($bounds: path),*) => {
  89 /// A callback which is called when a [`Future`] completes.
  90 ///
  91 /// Note that this MUST NOT call back into LDK directly, it must instead schedule actions to be
  92 /// taken later. Rust users should use the [`std::future::Future`] implementation for [`Future`]
  93 /// instead.
  94 ///
  95 /// Note that the [`std::future::Future`] implementation may only work for runtimes which schedule
  96 /// futures when they receive a wake, rather than immediately executing them.
  97 pub trait FutureCallback : $($bounds +)* {
  98         /// The method which is called.
  99         fn call(&self);
 100 }
 101
 102 impl<F: Fn() $(+ $bounds)*> FutureCallback for F {
 103         fn call(&self) { (self)(); }
 104 }
 105 } }
 106
 107 #[cfg(feature = "std")]
 108 define_callback!(Send);
 109 #[cfg(not(feature = "std"))]
 110 define_callback!();
 111
 112 pub(crate) struct FutureState {
 113         // `callbacks` count as having woken the users' code (as they go direct to the user), but
 114         // `std_future_callbacks` and `callbacks_with_state` do not (as the first just wakes a future,
 115         // we only count it after another `poll()` and the second wakes a `Sleeper` which handles
 116         // setting `callbacks_made` itself).
 117         callbacks: Vec<Box<dyn FutureCallback>>,
 118         std_future_callbacks: Vec<StdWaker>,
 119         callbacks_with_state: Vec<Box<dyn Fn(&Arc<Mutex<FutureState>>) -> () + Send>>,
 120         complete: bool,
 121         callbacks_made: bool,
 122 }
 123
 124 fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
 125         let mut state_lock = this.lock().unwrap();
 126         let state = &mut *state_lock;
 127         for callback in state.callbacks.drain(..) {
 128                 callback.call();
 129                 state.callbacks_made = true;
 130         }
 131         for waker in state.std_future_callbacks.drain(..) {
 132                 waker.0.wake_by_ref();
 133         }
 134         for callback in state.callbacks_with_state.drain(..) {
 135                 (callback)(this);
 136         }
 137         state.complete = true;
 138         state.callbacks_made
 139 }
 140
 141 /// A simple future which can complete once, and calls some callback(s) when it does so.
 142 ///
 143 /// Clones can be made and all futures cloned from the same source will complete at the same time.
 144 #[derive(Clone)]
 145 pub struct Future {
 146         state: Arc<Mutex<FutureState>>,
 147 }
 148
 149 impl Future {
 150         /// Registers a callback to be called upon completion of this future. If the future has already
 151         /// completed, the callback will be called immediately.
 152         ///
 153         /// This is not exported to bindings users, use the bindings-only `register_callback_fn` instead
 154         pub fn register_callback(&self, callback: Box<dyn FutureCallback>) {
 155                 let mut state = self.state.lock().unwrap();
 156                 if state.complete {
 157                         state.callbacks_made = true;
 158                         mem::drop(state);
 159                         callback.call();
 160                 } else {
 161                         state.callbacks.push(callback);
 162                 }
 163         }
 164
 165         // C bindings don't (currently) know how to map `Box<dyn Trait>`, and while it could add the
 166         // following wrapper, doing it in the bindings is currently much more work than simply doing it
 167         // here.
 168         /// Registers a callback to be called upon completion of this future. If the future has already
 169         /// completed, the callback will be called immediately.
 170         #[cfg(c_bindings)]
 171         pub fn register_callback_fn<F: 'static + FutureCallback>(&self, callback: F) {
 172                 self.register_callback(Box::new(callback));
 173         }
 174
 175         /// Waits until this [`Future`] completes.
 176         #[cfg(feature = "std")]
 177         pub fn wait(self) {
 178                 Sleeper::from_single_future(self).wait();
 179         }
 180
 181         /// Waits until this [`Future`] completes or the given amount of time has elapsed.
 182         ///
 183         /// Returns true if the [`Future`] completed, false if the time elapsed.
 184         #[cfg(feature = "std")]
 185         pub fn wait_timeout(self, max_wait: Duration) -> bool {
 186                 Sleeper::from_single_future(self).wait_timeout(max_wait)
 187         }
 188
 189         #[cfg(test)]
 190         pub fn poll_is_complete(&self) -> bool {
 191                 let mut state = self.state.lock().unwrap();
 192                 if state.complete {
 193                         state.callbacks_made = true;
 194                         true
 195                 } else { false }
 196         }
 197 }
 198
 199 use core::task::Waker;
 200 struct StdWaker(pub Waker);
 201
 202 /// This is not exported to bindings users as Rust Futures aren't usable in language bindings.
 203 impl<'a> StdFuture for Future {
 204         type Output = ();
 205
 206         fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 207                 let mut state = self.state.lock().unwrap();
 208                 if state.complete {
 209                         state.callbacks_made = true;
 210                         Poll::Ready(())
 211                 } else {
 212                         let waker = cx.waker().clone();
 213                         state.std_future_callbacks.push(StdWaker(waker));
 214                         Poll::Pending
 215                 }
 216         }
 217 }
 218
 219 /// A struct which can be used to select across many [`Future`]s at once without relying on a full
 220 /// async context.
 221 #[cfg(feature = "std")]
 222 pub struct Sleeper {
 223         notifiers: Vec<Arc<Mutex<FutureState>>>,
 224 }
 225
 226 #[cfg(feature = "std")]
 227 impl Sleeper {
 228         /// Constructs a new sleeper from one future, allowing blocking on it.
 229         pub fn from_single_future(future: Future) -> Self {
 230                 Self { notifiers: vec![future.state] }
 231         }
 232         /// Constructs a new sleeper from two futures, allowing blocking on both at once.
 233         // Note that this is the common case - a ChannelManager and ChainMonitor.
 234         pub fn from_two_futures(fut_a: Future, fut_b: Future) -> Self {
 235                 Self { notifiers: vec![fut_a.state, fut_b.state] }
 236         }
 237         /// Constructs a new sleeper on many futures, allowing blocking on all at once.
 238         pub fn new(futures: Vec<Future>) -> Self {
 239                 Self { notifiers: futures.into_iter().map(|f| f.state).collect() }
 240         }
 241         /// Prepares to go into a wait loop body, creating a condition variable which we can block on
 242         /// and an `Arc<Mutex<Option<_>>>` which gets set to the waking `Future`'s state prior to the
 243         /// condition variable being woken.
 244         fn setup_wait(&self) -> (Arc<Condvar>, Arc<Mutex<Option<Arc<Mutex<FutureState>>>>>) {
 245                 let cv = Arc::new(Condvar::new());
 246                 let notified_fut_mtx = Arc::new(Mutex::new(None));
 247                 {
 248                         for notifier_mtx in self.notifiers.iter() {
 249                                 let cv_ref = Arc::clone(&cv);
 250                                 let notified_fut_ref = Arc::clone(&notified_fut_mtx);
 251                                 let mut notifier = notifier_mtx.lock().unwrap();
 252                                 if notifier.complete {
 253                                         *notified_fut_mtx.lock().unwrap() = Some(Arc::clone(&notifier_mtx));
 254                                         break;
 255                                 }
 256                                 notifier.callbacks_with_state.push(Box::new(move |notifier_ref| {
 257                                         *notified_fut_ref.lock().unwrap() = Some(Arc::clone(notifier_ref));
 258                                         cv_ref.notify_all();
 259                                 }));
 260                         }
 261                 }
 262                 (cv, notified_fut_mtx)
 263         }
 264
 265         /// Wait until one of the [`Future`]s registered with this [`Sleeper`] has completed.
 266         pub fn wait(&self) {
 267                 let (cv, notified_fut_mtx) = self.setup_wait();
 268                 let notified_fut = cv.wait_while(notified_fut_mtx.lock().unwrap(), |fut_opt| fut_opt.is_none())
 269                         .unwrap().take().expect("CV wait shouldn't have returned until the notifying future was set");
 270                 notified_fut.lock().unwrap().callbacks_made = true;
 271         }
 272
 273         /// Wait until one of the [`Future`]s registered with this [`Sleeper`] has completed or the
 274         /// given amount of time has elapsed. Returns true if a [`Future`] completed, false if the time
 275         /// elapsed.
 276         pub fn wait_timeout(&self, max_wait: Duration) -> bool {
 277                 let (cv, notified_fut_mtx) = self.setup_wait();
 278                 let notified_fut =
 279                         match cv.wait_timeout_while(notified_fut_mtx.lock().unwrap(), max_wait, |fut_opt| fut_opt.is_none()) {
 280                                 Ok((_, e)) if e.timed_out() => return false,
 281                                 Ok((mut notified_fut, _)) =>
 282                                         notified_fut.take().expect("CV wait shouldn't have returned until the notifying future was set"),
 283                                 Err(_) => panic!("Previous panic while a lock was held led to a lock panic"),
 284                         };
 285                 notified_fut.lock().unwrap().callbacks_made = true;
 286                 true
 287         }
 288 }
 289
 290 #[cfg(test)]
 291 mod tests {
 292         use super::*;
 293         use core::sync::atomic::{AtomicBool, Ordering};
 294         use core::future::Future as FutureTrait;
 295         use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 296
 297         #[test]
 298         fn notifier_pre_notified_future() {
 299                 // Previously, if we generated a future after a `Notifier` had been notified, the future
 300                 // would never complete. This tests this behavior, ensuring the future instead completes
 301                 // immediately.
 302                 let notifier = Notifier::new();
 303                 notifier.notify();
 304
 305                 let callback = Arc::new(AtomicBool::new(false));
 306                 let callback_ref = Arc::clone(&callback);
 307                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 308                 assert!(callback.load(Ordering::SeqCst));
 309         }
 310
 311         #[test]
 312         fn notifier_future_completes_wake() {
 313                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 314                 // been notified, we'd never mark the notifier as not-awaiting-notify. This caused the
 315                 // `lightning-background-processor` to persist in a tight loop.
 316                 let notifier = Notifier::new();
 317
 318                 // First check the simple case, ensuring if we get notified a new future isn't woken until
 319                 // a second `notify`.
 320                 let callback = Arc::new(AtomicBool::new(false));
 321                 let callback_ref = Arc::clone(&callback);
 322                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 323                 assert!(!callback.load(Ordering::SeqCst));
 324
 325                 notifier.notify();
 326                 assert!(callback.load(Ordering::SeqCst));
 327
 328                 let callback = Arc::new(AtomicBool::new(false));
 329                 let callback_ref = Arc::clone(&callback);
 330                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 331                 assert!(!callback.load(Ordering::SeqCst));
 332
 333                 notifier.notify();
 334                 assert!(callback.load(Ordering::SeqCst));
 335
 336                 // Then check the case where the future is fetched before the notification, but a callback
 337                 // is only registered after the `notify`, ensuring that it is still sufficient to ensure we
 338                 // don't get an instant-wake when we get a new future.
 339                 let future = notifier.get_future();
 340                 notifier.notify();
 341
 342                 let callback = Arc::new(AtomicBool::new(false));
 343                 let callback_ref = Arc::clone(&callback);
 344                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 345                 assert!(callback.load(Ordering::SeqCst));
 346
 347                 let callback = Arc::new(AtomicBool::new(false));
 348                 let callback_ref = Arc::clone(&callback);
 349                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 350                 assert!(!callback.load(Ordering::SeqCst));
 351         }
 352
 353         #[test]
 354         fn new_future_wipes_notify_bit() {
 355                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 356                 // been notified, we'd never mark the notifier as not-awaiting-notify if a `Future` is
 357                 // fetched after the notify bit has been set.
 358                 let notifier = Notifier::new();
 359                 notifier.notify();
 360
 361                 let callback = Arc::new(AtomicBool::new(false));
 362                 let callback_ref = Arc::clone(&callback);
 363                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 364                 assert!(callback.load(Ordering::SeqCst));
 365
 366                 let callback = Arc::new(AtomicBool::new(false));
 367                 let callback_ref = Arc::clone(&callback);
 368                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 369                 assert!(!callback.load(Ordering::SeqCst));
 370
 371                 notifier.notify();
 372                 assert!(callback.load(Ordering::SeqCst));
 373         }
 374
 375         #[cfg(feature = "std")]
 376         #[test]
 377         fn test_wait_timeout() {
 378                 use crate::sync::Arc;
 379                 use std::thread;
 380
 381                 let persistence_notifier = Arc::new(Notifier::new());
 382                 let thread_notifier = Arc::clone(&persistence_notifier);
 383
 384                 let exit_thread = Arc::new(AtomicBool::new(false));
 385                 let exit_thread_clone = exit_thread.clone();
 386                 thread::spawn(move || {
 387                         loop {
 388                                 thread_notifier.notify();
 389                                 if exit_thread_clone.load(Ordering::SeqCst) {
 390                                         break
 391                                 }
 392                         }
 393                 });
 394
 395                 // Check that we can block indefinitely until updates are available.
 396                 let _ = persistence_notifier.get_future().wait();
 397
 398                 // Check that the Notifier will return after the given duration if updates are
 399                 // available.
 400                 loop {
 401                         if persistence_notifier.get_future().wait_timeout(Duration::from_millis(100)) {
 402                                 break
 403                         }
 404                 }
 405
 406                 exit_thread.store(true, Ordering::SeqCst);
 407
 408                 // Check that the Notifier will return after the given duration even if no updates
 409                 // are available.
 410                 loop {
 411                         if !persistence_notifier.get_future().wait_timeout(Duration::from_millis(100)) {
 412                                 break
 413                         }
 414                 }
 415         }
 416
 417         #[cfg(feature = "std")]
 418         #[test]
 419         fn test_state_drops() {
 420                 // Previously, there was a leak if a `Notifier` was `drop`ed without ever being notified
 421                 // but after having been slept-on. This tests for that leak.
 422                 use crate::sync::Arc;
 423                 use std::thread;
 424
 425                 let notifier_a = Arc::new(Notifier::new());
 426                 let notifier_b = Arc::new(Notifier::new());
 427
 428                 let thread_notifier_a = Arc::clone(&notifier_a);
 429
 430                 let future_a = notifier_a.get_future();
 431                 let future_state_a = Arc::downgrade(&future_a.state);
 432
 433                 let future_b = notifier_b.get_future();
 434                 let future_state_b = Arc::downgrade(&future_b.state);
 435
 436                 let join_handle = thread::spawn(move || {
 437                         // Let the other thread get to the wait point, then notify it.
 438                         std::thread::sleep(Duration::from_millis(50));
 439                         thread_notifier_a.notify();
 440                 });
 441
 442                 // Wait on the other thread to finish its sleep, note that the leak only happened if we
 443                 // actually have to sleep here, not if we immediately return.
 444                 Sleeper::from_two_futures(future_a, future_b).wait();
 445
 446                 join_handle.join().unwrap();
 447
 448                 // then drop the notifiers and make sure the future states are gone.
 449                 mem::drop(notifier_a);
 450                 mem::drop(notifier_b);
 451
 452                 assert!(future_state_a.upgrade().is_none() && future_state_b.upgrade().is_none());
 453         }
 454
 455         #[test]
 456         fn test_future_callbacks() {
 457                 let future = Future {
 458                         state: Arc::new(Mutex::new(FutureState {
 459                                 callbacks: Vec::new(),
 460                                 std_future_callbacks: Vec::new(),
 461                                 callbacks_with_state: Vec::new(),
 462                                 complete: false,
 463                                 callbacks_made: false,
 464                         }))
 465                 };
 466                 let callback = Arc::new(AtomicBool::new(false));
 467                 let callback_ref = Arc::clone(&callback);
 468                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 469
 470                 assert!(!callback.load(Ordering::SeqCst));
 471                 complete_future(&future.state);
 472                 assert!(callback.load(Ordering::SeqCst));
 473                 complete_future(&future.state);
 474         }
 475
 476         #[test]
 477         fn test_pre_completed_future_callbacks() {
 478                 let future = Future {
 479                         state: Arc::new(Mutex::new(FutureState {
 480                                 callbacks: Vec::new(),
 481                                 callbacks_with_state: Vec::new(),
 482                                 complete: false,
 483                                 callbacks_made: false,
 484                         }))
 485                 };
 486                 complete_future(&future.state);
 487
 488                 let callback = Arc::new(AtomicBool::new(false));
 489                 let callback_ref = Arc::clone(&callback);
 490                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 491
 492                 assert!(callback.load(Ordering::SeqCst));
 493                 assert!(future.state.lock().unwrap().callbacks.is_empty());
 494         }
 495
 496         // Rather annoyingly, there's no safe way in Rust std to construct a Waker despite it being
 497         // totally possible to construct from a trait implementation (though somewhat less efficient
 498         // compared to a raw VTable). Instead, we have to write out a lot of boilerplate to build a
 499         // waker, which we do here with a trivial Arc<AtomicBool> data element to track woke-ness.
 500         const WAKER_V_TABLE: RawWakerVTable = RawWakerVTable::new(waker_clone, wake, wake_by_ref, drop);
 501         unsafe fn wake_by_ref(ptr: *const ()) { let p = ptr as *const Arc<AtomicBool>; assert!(!(*p).fetch_or(true, Ordering::SeqCst)); }
 502         unsafe fn drop(ptr: *const ()) { let p = ptr as *mut Arc<AtomicBool>; let _freed = Box::from_raw(p); }
 503         unsafe fn wake(ptr: *const ()) { wake_by_ref(ptr); drop(ptr); }
 504         unsafe fn waker_clone(ptr: *const ()) -> RawWaker {
 505                 let p = ptr as *const Arc<AtomicBool>;
 506                 RawWaker::new(Box::into_raw(Box::new(Arc::clone(&*p))) as *const (), &WAKER_V_TABLE)
 507         }
 508
 509         fn create_waker() -> (Arc<AtomicBool>, Waker) {
 510                 let a = Arc::new(AtomicBool::new(false));
 511                 let waker = unsafe { Waker::from_raw(waker_clone((&a as *const Arc<AtomicBool>) as *const ())) };
 512                 (a, waker)
 513         }
 514
 515         #[test]
 516         fn test_future() {
 517                 let mut future = Future {
 518                         state: Arc::new(Mutex::new(FutureState {
 519                                 callbacks: Vec::new(),
 520                                 std_future_callbacks: Vec::new(),
 521                                 callbacks_with_state: Vec::new(),
 522                                 complete: false,
 523                                 callbacks_made: false,
 524                         }))
 525                 };
 526                 let mut second_future = Future { state: Arc::clone(&future.state) };
 527
 528                 let (woken, waker) = create_waker();
 529                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 530                 assert!(!woken.load(Ordering::SeqCst));
 531
 532                 let (second_woken, second_waker) = create_waker();
 533                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Pending);
 534                 assert!(!second_woken.load(Ordering::SeqCst));
 535
 536                 complete_future(&future.state);
 537                 assert!(woken.load(Ordering::SeqCst));
 538                 assert!(second_woken.load(Ordering::SeqCst));
 539                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 540                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Ready(()));
 541         }
 542
 543         #[test]
 544         #[cfg(feature = "std")]
 545         fn test_dropped_future_doesnt_count() {
 546                 // Tests that if a Future gets drop'd before it is poll()ed `Ready` it doesn't count as
 547                 // having been woken, leaving the notify-required flag set.
 548                 let notifier = Notifier::new();
 549                 notifier.notify();
 550
 551                 // If we get a future and don't touch it we're definitely still notify-required.
 552                 notifier.get_future();
 553                 assert!(notifier.get_future().wait_timeout(Duration::from_millis(1)));
 554                 assert!(!notifier.get_future().wait_timeout(Duration::from_millis(1)));
 555
 556                 // Even if we poll'd once but didn't observe a `Ready`, we should be notify-required.
 557                 let mut future = notifier.get_future();
 558                 let (woken, waker) = create_waker();
 559                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 560
 561                 notifier.notify();
 562                 assert!(woken.load(Ordering::SeqCst));
 563                 assert!(notifier.get_future().wait_timeout(Duration::from_millis(1)));
 564
 565                 // However, once we do poll `Ready` it should wipe the notify-required flag.
 566                 let mut future = notifier.get_future();
 567                 let (woken, waker) = create_waker();
 568                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 569
 570                 notifier.notify();
 571                 assert!(woken.load(Ordering::SeqCst));
 572                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 573                 assert!(!notifier.get_future().wait_timeout(Duration::from_millis(1)));
 574         }
 575
 576         #[test]
 577         fn test_poll_post_notify_completes() {
 578                 // Tests that if we have a future state that has completed, and we haven't yet requested a
 579                 // new future, if we get a notify prior to requesting that second future it is generated
 580                 // pre-completed.
 581                 let notifier = Notifier::new();
 582
 583                 notifier.notify();
 584                 let mut future = notifier.get_future();
 585                 let (woken, waker) = create_waker();
 586                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 587                 assert!(!woken.load(Ordering::SeqCst));
 588
 589                 notifier.notify();
 590                 let mut future = notifier.get_future();
 591                 let (woken, waker) = create_waker();
 592                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 593                 assert!(!woken.load(Ordering::SeqCst));
 594
 595                 let mut future = notifier.get_future();
 596                 let (woken, waker) = create_waker();
 597                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 598                 assert!(!woken.load(Ordering::SeqCst));
 599
 600                 notifier.notify();
 601                 assert!(woken.load(Ordering::SeqCst));
 602                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 603         }
 604
 605         #[test]
 606         fn test_poll_post_notify_completes_initial_notified() {
 607                 // Identical to the previous test, but the first future completes via a wake rather than an
 608                 // immediate `Poll::Ready`.
 609                 let notifier = Notifier::new();
 610
 611                 let mut future = notifier.get_future();
 612                 let (woken, waker) = create_waker();
 613                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 614
 615                 notifier.notify();
 616                 assert!(woken.load(Ordering::SeqCst));
 617                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 618
 619                 notifier.notify();
 620                 let mut future = notifier.get_future();
 621                 let (woken, waker) = create_waker();
 622                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 623                 assert!(!woken.load(Ordering::SeqCst));
 624
 625                 let mut future = notifier.get_future();
 626                 let (woken, waker) = create_waker();
 627                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 628                 assert!(!woken.load(Ordering::SeqCst));
 629
 630                 notifier.notify();
 631                 assert!(woken.load(Ordering::SeqCst));
 632                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 633         }
 634
 635         #[test]
 636         #[cfg(feature = "std")]
 637         fn test_multi_future_sleep() {
 638                 // Tests the `Sleeper` with multiple futures.
 639                 let notifier_a = Notifier::new();
 640                 let notifier_b = Notifier::new();
 641
 642                 // Set both notifiers as woken without sleeping yet.
 643                 notifier_a.notify();
 644                 notifier_b.notify();
 645                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 646
 647                 // One future has woken us up, but the other should still have a pending notification.
 648                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 649
 650                 // However once we've slept twice, we should no longer have any pending notifications
 651                 assert!(!Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future())
 652                         .wait_timeout(Duration::from_millis(10)));
 653
 654                 // Test ordering somewhat more.
 655                 notifier_a.notify();
 656                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 657         }
 658
 659         #[test]
 660         #[cfg(feature = "std")]
 661         fn sleeper_with_pending_callbacks() {
 662                 // This is similar to the above `test_multi_future_sleep` test, but in addition registers
 663                 // "normal" callbacks which will cause the futures to assume notification has occurred,
 664                 // rather than waiting for a woken sleeper.
 665                 let notifier_a = Notifier::new();
 666                 let notifier_b = Notifier::new();
 667
 668                 // Set both notifiers as woken without sleeping yet.
 669                 notifier_a.notify();
 670                 notifier_b.notify();
 671
 672                 // After sleeping one future (not guaranteed which one, however) will have its notification
 673                 // bit cleared.
 674                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 675
 676                 // By registering a callback on the futures for both notifiers, one will complete
 677                 // immediately, but one will remain tied to the notifier, and will complete once the
 678                 // notifier is next woken, which will be considered the completion of the notification.
 679                 let callback_a = Arc::new(AtomicBool::new(false));
 680                 let callback_b = Arc::new(AtomicBool::new(false));
 681                 let callback_a_ref = Arc::clone(&callback_a);
 682                 let callback_b_ref = Arc::clone(&callback_b);
 683                 notifier_a.get_future().register_callback(Box::new(move || assert!(!callback_a_ref.fetch_or(true, Ordering::SeqCst))));
 684                 notifier_b.get_future().register_callback(Box::new(move || assert!(!callback_b_ref.fetch_or(true, Ordering::SeqCst))));
 685                 assert!(callback_a.load(Ordering::SeqCst) ^ callback_b.load(Ordering::SeqCst));
 686
 687                 // If we now notify both notifiers again, the other callback will fire, completing the
 688                 // notification, and we'll be back to one pending notification.
 689                 notifier_a.notify();
 690                 notifier_b.notify();
 691
 692                 assert!(callback_a.load(Ordering::SeqCst) && callback_b.load(Ordering::SeqCst));
 693                 Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
 694                 assert!(!Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future())
 695                         .wait_timeout(Duration::from_millis(10)));
 696         }
 697 }