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                 let mut self_idx = 0;
  60                 if let Some(existing_state) = &lock.1 {
  61                         let mut locked = existing_state.lock().unwrap();
  62                         if locked.callbacks_made {
  63                                 // If the existing `FutureState` has completed and actually made callbacks,
  64                                 // consider the notification flag to have been cleared and reset the future state.
  65                                 mem::drop(locked);
  66                                 lock.1.take();
  67                                 lock.0 = false;
  68                         } else {
  69                                 self_idx = locked.next_idx;
  70                                 locked.next_idx += 1;
  71                         }
  72                 }
  73                 if let Some(existing_state) = &lock.1 {
  74                         Future { state: Arc::clone(&existing_state), self_idx }
  75                 } else {
  76                         let state = Arc::new(Mutex::new(FutureState {
  77                                 callbacks: Vec::new(),
  78                                 std_future_callbacks: Vec::new(),
  79                                 callbacks_with_state: Vec::new(),
  80                                 complete: lock.0,
  81                                 callbacks_made: false,
  82                                 next_idx: 1,
  83                         }));
  84                         lock.1 = Some(Arc::clone(&state));
  85                         Future { state, self_idx: 0 }
  86                 }
  87         }
  88
  89         #[cfg(any(test, feature = "_test_utils"))]
  90         pub fn notify_pending(&self) -> bool {
  91                 self.notify_pending.lock().unwrap().0
  92         }
  93 }
  94
  95 macro_rules! define_callback { ($($bounds: path),*) => {
  96 /// A callback which is called when a [`Future`] completes.
  97 ///
  98 /// Note that this MUST NOT call back into LDK directly, it must instead schedule actions to be
  99 /// taken later. Rust users should use the [`std::future::Future`] implementation for [`Future`]
 100 /// instead.
 101 ///
 102 /// Note that the [`std::future::Future`] implementation may only work for runtimes which schedule
 103 /// futures when they receive a wake, rather than immediately executing them.
 104 pub trait FutureCallback : $($bounds +)* {
 105         /// The method which is called.
 106         fn call(&self);
 107 }
 108
 109 impl<F: Fn() $(+ $bounds)*> FutureCallback for F {
 110         fn call(&self) { (self)(); }
 111 }
 112 } }
 113
 114 #[cfg(feature = "std")]
 115 define_callback!(Send);
 116 #[cfg(not(feature = "std"))]
 117 define_callback!();
 118
 119 pub(crate) struct FutureState {
 120         // `callbacks` count as having woken the users' code (as they go direct to the user), but
 121         // `std_future_callbacks` and `callbacks_with_state` do not (as the first just wakes a future,
 122         // we only count it after another `poll()` and the second wakes a `Sleeper` which handles
 123         // setting `callbacks_made` itself).
 124         callbacks: Vec<Box<dyn FutureCallback>>,
 125         std_future_callbacks: Vec<(usize, StdWaker)>,
 126         callbacks_with_state: Vec<Box<dyn Fn(&Arc<Mutex<FutureState>>) -> () + Send>>,
 127         complete: bool,
 128         callbacks_made: bool,
 129         next_idx: usize,
 130 }
 131
 132 fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
 133         let mut state_lock = this.lock().unwrap();
 134         let state = &mut *state_lock;
 135         for callback in state.callbacks.drain(..) {
 136                 callback.call();
 137                 state.callbacks_made = true;
 138         }
 139         for (_, waker) in state.std_future_callbacks.drain(..) {
 140                 waker.0.wake_by_ref();
 141         }
 142         for callback in state.callbacks_with_state.drain(..) {
 143                 (callback)(this);
 144         }
 145         state.complete = true;
 146         state.callbacks_made
 147 }
 148
 149 /// A simple future which can complete once, and calls some callback(s) when it does so.
 150 pub struct Future {
 151         state: Arc<Mutex<FutureState>>,
 152         self_idx: usize,
 153 }
 154
 155 impl Future {
 156         /// Registers a callback to be called upon completion of this future. If the future has already
 157         /// completed, the callback will be called immediately.
 158         ///
 159         /// This is not exported to bindings users, use the bindings-only `register_callback_fn` instead
 160         pub fn register_callback(&self, callback: Box<dyn FutureCallback>) {
 161                 let mut state = self.state.lock().unwrap();
 162                 if state.complete {
 163                         state.callbacks_made = true;
 164                         mem::drop(state);
 165                         callback.call();
 166                 } else {
 167                         state.callbacks.push(callback);
 168                 }
 169         }
 170
 171         // C bindings don't (currently) know how to map `Box<dyn Trait>`, and while it could add the
 172         // following wrapper, doing it in the bindings is currently much more work than simply doing it
 173         // here.
 174         /// Registers a callback to be called upon completion of this future. If the future has already
 175         /// completed, the callback will be called immediately.
 176         #[cfg(c_bindings)]
 177         pub fn register_callback_fn<F: 'static + FutureCallback>(&self, callback: F) {
 178                 self.register_callback(Box::new(callback));
 179         }
 180
 181         /// Waits until this [`Future`] completes.
 182         #[cfg(feature = "std")]
 183         pub fn wait(&self) {
 184                 Sleeper::from_single_future(&self).wait();
 185         }
 186
 187         /// Waits until this [`Future`] completes or the given amount of time has elapsed.
 188         ///
 189         /// Returns true if the [`Future`] completed, false if the time elapsed.
 190         #[cfg(feature = "std")]
 191         pub fn wait_timeout(&self, max_wait: Duration) -> bool {
 192                 Sleeper::from_single_future(&self).wait_timeout(max_wait)
 193         }
 194
 195         #[cfg(test)]
 196         pub fn poll_is_complete(&self) -> bool {
 197                 let mut state = self.state.lock().unwrap();
 198                 if state.complete {
 199                         state.callbacks_made = true;
 200                         true
 201                 } else { false }
 202         }
 203 }
 204
 205 impl Drop for Future {
 206         fn drop(&mut self) {
 207                 self.state.lock().unwrap().std_future_callbacks.retain(|(idx, _)| *idx != self.self_idx);
 208         }
 209 }
 210
 211 use core::task::Waker;
 212 struct StdWaker(pub Waker);
 213
 214 /// This is not exported to bindings users as Rust Futures aren't usable in language bindings.
 215 impl<'a> StdFuture for Future {
 216         type Output = ();
 217
 218         fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 219                 let mut state = self.state.lock().unwrap();
 220                 if state.complete {
 221                         state.callbacks_made = true;
 222                         Poll::Ready(())
 223                 } else {
 224                         let waker = cx.waker().clone();
 225                         state.std_future_callbacks.retain(|(idx, _)| *idx != self.self_idx);
 226                         state.std_future_callbacks.push((self.self_idx, StdWaker(waker)));
 227                         Poll::Pending
 228                 }
 229         }
 230 }
 231
 232 /// A struct which can be used to select across many [`Future`]s at once without relying on a full
 233 /// async context.
 234 #[cfg(feature = "std")]
 235 pub struct Sleeper {
 236         notifiers: Vec<Arc<Mutex<FutureState>>>,
 237 }
 238
 239 #[cfg(feature = "std")]
 240 impl Sleeper {
 241         /// Constructs a new sleeper from one future, allowing blocking on it.
 242         pub fn from_single_future(future: &Future) -> Self {
 243                 Self { notifiers: vec![Arc::clone(&future.state)] }
 244         }
 245         /// Constructs a new sleeper from two futures, allowing blocking on both at once.
 246         // Note that this is the common case - a ChannelManager and ChainMonitor.
 247         pub fn from_two_futures(fut_a: &Future, fut_b: &Future) -> Self {
 248                 Self { notifiers: vec![Arc::clone(&fut_a.state), Arc::clone(&fut_b.state)] }
 249         }
 250         /// Constructs a new sleeper on many futures, allowing blocking on all at once.
 251         pub fn new(futures: Vec<Future>) -> Self {
 252                 Self { notifiers: futures.into_iter().map(|f| Arc::clone(&f.state)).collect() }
 253         }
 254         /// Prepares to go into a wait loop body, creating a condition variable which we can block on
 255         /// and an `Arc<Mutex<Option<_>>>` which gets set to the waking `Future`'s state prior to the
 256         /// condition variable being woken.
 257         fn setup_wait(&self) -> (Arc<Condvar>, Arc<Mutex<Option<Arc<Mutex<FutureState>>>>>) {
 258                 let cv = Arc::new(Condvar::new());
 259                 let notified_fut_mtx = Arc::new(Mutex::new(None));
 260                 {
 261                         for notifier_mtx in self.notifiers.iter() {
 262                                 let cv_ref = Arc::clone(&cv);
 263                                 let notified_fut_ref = Arc::clone(&notified_fut_mtx);
 264                                 let mut notifier = notifier_mtx.lock().unwrap();
 265                                 if notifier.complete {
 266                                         *notified_fut_mtx.lock().unwrap() = Some(Arc::clone(&notifier_mtx));
 267                                         break;
 268                                 }
 269                                 notifier.callbacks_with_state.push(Box::new(move |notifier_ref| {
 270                                         *notified_fut_ref.lock().unwrap() = Some(Arc::clone(notifier_ref));
 271                                         cv_ref.notify_all();
 272                                 }));
 273                         }
 274                 }
 275                 (cv, notified_fut_mtx)
 276         }
 277
 278         /// Wait until one of the [`Future`]s registered with this [`Sleeper`] has completed.
 279         pub fn wait(&self) {
 280                 let (cv, notified_fut_mtx) = self.setup_wait();
 281                 let notified_fut = cv.wait_while(notified_fut_mtx.lock().unwrap(), |fut_opt| fut_opt.is_none())
 282                         .unwrap().take().expect("CV wait shouldn't have returned until the notifying future was set");
 283                 notified_fut.lock().unwrap().callbacks_made = true;
 284         }
 285
 286         /// Wait until one of the [`Future`]s registered with this [`Sleeper`] has completed or the
 287         /// given amount of time has elapsed. Returns true if a [`Future`] completed, false if the time
 288         /// elapsed.
 289         pub fn wait_timeout(&self, max_wait: Duration) -> bool {
 290                 let (cv, notified_fut_mtx) = self.setup_wait();
 291                 let notified_fut =
 292                         match cv.wait_timeout_while(notified_fut_mtx.lock().unwrap(), max_wait, |fut_opt| fut_opt.is_none()) {
 293                                 Ok((_, e)) if e.timed_out() => return false,
 294                                 Ok((mut notified_fut, _)) =>
 295                                         notified_fut.take().expect("CV wait shouldn't have returned until the notifying future was set"),
 296                                 Err(_) => panic!("Previous panic while a lock was held led to a lock panic"),
 297                         };
 298                 notified_fut.lock().unwrap().callbacks_made = true;
 299                 true
 300         }
 301 }
 302
 303 #[cfg(test)]
 304 mod tests {
 305         use super::*;
 306         use core::sync::atomic::{AtomicBool, Ordering};
 307         use core::future::Future as FutureTrait;
 308         use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 309
 310         #[test]
 311         fn notifier_pre_notified_future() {
 312                 // Previously, if we generated a future after a `Notifier` had been notified, the future
 313                 // would never complete. This tests this behavior, ensuring the future instead completes
 314                 // immediately.
 315                 let notifier = Notifier::new();
 316                 notifier.notify();
 317
 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
 324         #[test]
 325         fn notifier_future_completes_wake() {
 326                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 327                 // been notified, we'd never mark the notifier as not-awaiting-notify. This caused the
 328                 // `lightning-background-processor` to persist in a tight loop.
 329                 let notifier = Notifier::new();
 330
 331                 // First check the simple case, ensuring if we get notified a new future isn't woken until
 332                 // a second `notify`.
 333                 let callback = Arc::new(AtomicBool::new(false));
 334                 let callback_ref = Arc::clone(&callback);
 335                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 336                 assert!(!callback.load(Ordering::SeqCst));
 337
 338                 notifier.notify();
 339                 assert!(callback.load(Ordering::SeqCst));
 340
 341                 let callback = Arc::new(AtomicBool::new(false));
 342                 let callback_ref = Arc::clone(&callback);
 343                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 344                 assert!(!callback.load(Ordering::SeqCst));
 345
 346                 notifier.notify();
 347                 assert!(callback.load(Ordering::SeqCst));
 348
 349                 // Then check the case where the future is fetched before the notification, but a callback
 350                 // is only registered after the `notify`, ensuring that it is still sufficient to ensure we
 351                 // don't get an instant-wake when we get a new future.
 352                 let future = notifier.get_future();
 353                 notifier.notify();
 354
 355                 let callback = Arc::new(AtomicBool::new(false));
 356                 let callback_ref = Arc::clone(&callback);
 357                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 358                 assert!(callback.load(Ordering::SeqCst));
 359
 360                 let callback = Arc::new(AtomicBool::new(false));
 361                 let callback_ref = Arc::clone(&callback);
 362                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 363                 assert!(!callback.load(Ordering::SeqCst));
 364         }
 365
 366         #[test]
 367         fn new_future_wipes_notify_bit() {
 368                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 369                 // been notified, we'd never mark the notifier as not-awaiting-notify if a `Future` is
 370                 // fetched after the notify bit has been set.
 371                 let notifier = Notifier::new();
 372                 notifier.notify();
 373
 374                 let callback = Arc::new(AtomicBool::new(false));
 375                 let callback_ref = Arc::clone(&callback);
 376                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 377                 assert!(callback.load(Ordering::SeqCst));
 378
 379                 let callback = Arc::new(AtomicBool::new(false));
 380                 let callback_ref = Arc::clone(&callback);
 381                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 382                 assert!(!callback.load(Ordering::SeqCst));
 383
 384                 notifier.notify();
 385                 assert!(callback.load(Ordering::SeqCst));
 386         }
 387
 388         #[cfg(feature = "std")]
 389         #[test]
 390         fn test_wait_timeout() {
 391                 use crate::sync::Arc;
 392                 use std::thread;
 393
 394                 let persistence_notifier = Arc::new(Notifier::new());
 395                 let thread_notifier = Arc::clone(&persistence_notifier);
 396
 397                 let exit_thread = Arc::new(AtomicBool::new(false));
 398                 let exit_thread_clone = exit_thread.clone();
 399                 thread::spawn(move || {
 400                         loop {
 401                                 thread_notifier.notify();
 402                                 if exit_thread_clone.load(Ordering::SeqCst) {
 403                                         break
 404                                 }
 405                         }
 406                 });
 407
 408                 // Check that we can block indefinitely until updates are available.
 409                 let _ = persistence_notifier.get_future().wait();
 410
 411                 // Check that the Notifier will return after the given duration if updates are
 412                 // available.
 413                 loop {
 414                         if persistence_notifier.get_future().wait_timeout(Duration::from_millis(100)) {
 415                                 break
 416                         }
 417                 }
 418
 419                 exit_thread.store(true, Ordering::SeqCst);
 420
 421                 // Check that the Notifier will return after the given duration even if no updates
 422                 // are available.
 423                 loop {
 424                         if !persistence_notifier.get_future().wait_timeout(Duration::from_millis(100)) {
 425                                 break
 426                         }
 427                 }
 428         }
 429
 430         #[cfg(feature = "std")]
 431         #[test]
 432         fn test_state_drops() {
 433                 // Previously, there was a leak if a `Notifier` was `drop`ed without ever being notified
 434                 // but after having been slept-on. This tests for that leak.
 435                 use crate::sync::Arc;
 436                 use std::thread;
 437
 438                 let notifier_a = Arc::new(Notifier::new());
 439                 let notifier_b = Arc::new(Notifier::new());
 440
 441                 let thread_notifier_a = Arc::clone(&notifier_a);
 442
 443                 let future_a = notifier_a.get_future();
 444                 let future_state_a = Arc::downgrade(&future_a.state);
 445
 446                 let future_b = notifier_b.get_future();
 447                 let future_state_b = Arc::downgrade(&future_b.state);
 448
 449                 let join_handle = thread::spawn(move || {
 450                         // Let the other thread get to the wait point, then notify it.
 451                         std::thread::sleep(Duration::from_millis(50));
 452                         thread_notifier_a.notify();
 453                 });
 454
 455                 // Wait on the other thread to finish its sleep, note that the leak only happened if we
 456                 // actually have to sleep here, not if we immediately return.
 457                 Sleeper::from_two_futures(&future_a, &future_b).wait();
 458
 459                 join_handle.join().unwrap();
 460
 461                 // then drop the notifiers and make sure the future states are gone.
 462                 mem::drop(notifier_a);
 463                 mem::drop(notifier_b);
 464                 mem::drop(future_a);
 465                 mem::drop(future_b);
 466
 467                 assert!(future_state_a.upgrade().is_none() && future_state_b.upgrade().is_none());
 468         }
 469
 470         #[test]
 471         fn test_future_callbacks() {
 472                 let future = Future {
 473                         state: Arc::new(Mutex::new(FutureState {
 474                                 callbacks: Vec::new(),
 475                                 std_future_callbacks: Vec::new(),
 476                                 callbacks_with_state: Vec::new(),
 477                                 complete: false,
 478                                 callbacks_made: false,
 479                                 next_idx: 1,
 480                         })),
 481                         self_idx: 0,
 482                 };
 483                 let callback = Arc::new(AtomicBool::new(false));
 484                 let callback_ref = Arc::clone(&callback);
 485                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 486
 487                 assert!(!callback.load(Ordering::SeqCst));
 488                 complete_future(&future.state);
 489                 assert!(callback.load(Ordering::SeqCst));
 490                 complete_future(&future.state);
 491         }
 492
 493         #[test]
 494         fn test_pre_completed_future_callbacks() {
 495                 let future = Future {
 496                         state: Arc::new(Mutex::new(FutureState {
 497                                 callbacks: Vec::new(),
 498                                 std_future_callbacks: Vec::new(),
 499                                 callbacks_with_state: Vec::new(),
 500                                 complete: false,
 501                                 callbacks_made: false,
 502                                 next_idx: 1,
 503                         })),
 504                         self_idx: 0,
 505                 };
 506                 complete_future(&future.state);
 507
 508                 let callback = Arc::new(AtomicBool::new(false));
 509                 let callback_ref = Arc::clone(&callback);
 510                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 511
 512                 assert!(callback.load(Ordering::SeqCst));
 513                 assert!(future.state.lock().unwrap().callbacks.is_empty());
 514         }
 515
 516         // Rather annoyingly, there's no safe way in Rust std to construct a Waker despite it being
 517         // totally possible to construct from a trait implementation (though somewhat less efficient
 518         // compared to a raw VTable). Instead, we have to write out a lot of boilerplate to build a
 519         // waker, which we do here with a trivial Arc<AtomicBool> data element to track woke-ness.
 520         const WAKER_V_TABLE: RawWakerVTable = RawWakerVTable::new(waker_clone, wake, wake_by_ref, drop);
 521         unsafe fn wake_by_ref(ptr: *const ()) { let p = ptr as *const Arc<AtomicBool>; assert!(!(*p).fetch_or(true, Ordering::SeqCst)); }
 522         unsafe fn drop(ptr: *const ()) { let p = ptr as *mut Arc<AtomicBool>; let _freed = Box::from_raw(p); }
 523         unsafe fn wake(ptr: *const ()) { wake_by_ref(ptr); drop(ptr); }
 524         unsafe fn waker_clone(ptr: *const ()) -> RawWaker {
 525                 let p = ptr as *const Arc<AtomicBool>;
 526                 RawWaker::new(Box::into_raw(Box::new(Arc::clone(&*p))) as *const (), &WAKER_V_TABLE)
 527         }
 528
 529         fn create_waker() -> (Arc<AtomicBool>, Waker) {
 530                 let a = Arc::new(AtomicBool::new(false));
 531                 let waker = unsafe { Waker::from_raw(waker_clone((&a as *const Arc<AtomicBool>) as *const ())) };
 532                 (a, waker)
 533         }
 534
 535         #[test]
 536         fn test_future() {
 537                 let mut future = Future {
 538                         state: Arc::new(Mutex::new(FutureState {
 539                                 callbacks: Vec::new(),
 540                                 std_future_callbacks: Vec::new(),
 541                                 callbacks_with_state: Vec::new(),
 542                                 complete: false,
 543                                 callbacks_made: false,
 544                                 next_idx: 2,
 545                         })),
 546                         self_idx: 0,
 547                 };
 548                 let mut second_future = Future { state: Arc::clone(&future.state), self_idx: 1 };
 549
 550                 let (woken, waker) = create_waker();
 551                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 552                 assert!(!woken.load(Ordering::SeqCst));
 553
 554                 let (second_woken, second_waker) = create_waker();
 555                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Pending);
 556                 assert!(!second_woken.load(Ordering::SeqCst));
 557
 558                 complete_future(&future.state);
 559                 assert!(woken.load(Ordering::SeqCst));
 560                 assert!(second_woken.load(Ordering::SeqCst));
 561                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 562                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Ready(()));
 563         }
 564
 565         #[test]
 566         #[cfg(feature = "std")]
 567         fn test_dropped_future_doesnt_count() {
 568                 // Tests that if a Future gets drop'd before it is poll()ed `Ready` it doesn't count as
 569                 // having been woken, leaving the notify-required flag set.
 570                 let notifier = Notifier::new();
 571                 notifier.notify();
 572
 573                 // If we get a future and don't touch it we're definitely still notify-required.
 574                 notifier.get_future();
 575                 assert!(notifier.get_future().wait_timeout(Duration::from_millis(1)));
 576                 assert!(!notifier.get_future().wait_timeout(Duration::from_millis(1)));
 577
 578                 // Even if we poll'd once but didn't observe a `Ready`, we should be notify-required.
 579                 let mut future = notifier.get_future();
 580                 let (woken, waker) = create_waker();
 581                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 582
 583                 notifier.notify();
 584                 assert!(woken.load(Ordering::SeqCst));
 585                 assert!(notifier.get_future().wait_timeout(Duration::from_millis(1)));
 586
 587                 // However, once we do poll `Ready` it should wipe the notify-required flag.
 588                 let mut future = notifier.get_future();
 589                 let (woken, waker) = create_waker();
 590                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 591
 592                 notifier.notify();
 593                 assert!(woken.load(Ordering::SeqCst));
 594                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 595                 assert!(!notifier.get_future().wait_timeout(Duration::from_millis(1)));
 596         }
 597
 598         #[test]
 599         fn test_poll_post_notify_completes() {
 600                 // Tests that if we have a future state that has completed, and we haven't yet requested a
 601                 // new future, if we get a notify prior to requesting that second future it is generated
 602                 // pre-completed.
 603                 let notifier = Notifier::new();
 604
 605                 notifier.notify();
 606                 let mut future = notifier.get_future();
 607                 let (woken, waker) = create_waker();
 608                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 609                 assert!(!woken.load(Ordering::SeqCst));
 610
 611                 notifier.notify();
 612                 let mut future = notifier.get_future();
 613                 let (woken, waker) = create_waker();
 614                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 615                 assert!(!woken.load(Ordering::SeqCst));
 616
 617                 let mut future = notifier.get_future();
 618                 let (woken, waker) = create_waker();
 619                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 620                 assert!(!woken.load(Ordering::SeqCst));
 621
 622                 notifier.notify();
 623                 assert!(woken.load(Ordering::SeqCst));
 624                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 625         }
 626
 627         #[test]
 628         fn test_poll_post_notify_completes_initial_notified() {
 629                 // Identical to the previous test, but the first future completes via a wake rather than an
 630                 // immediate `Poll::Ready`.
 631                 let notifier = Notifier::new();
 632
 633                 let mut future = notifier.get_future();
 634                 let (woken, waker) = create_waker();
 635                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 636
 637                 notifier.notify();
 638                 assert!(woken.load(Ordering::SeqCst));
 639                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 640
 641                 notifier.notify();
 642                 let mut future = notifier.get_future();
 643                 let (woken, waker) = create_waker();
 644                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 645                 assert!(!woken.load(Ordering::SeqCst));
 646
 647                 let mut future = notifier.get_future();
 648                 let (woken, waker) = create_waker();
 649                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 650                 assert!(!woken.load(Ordering::SeqCst));
 651
 652                 notifier.notify();
 653                 assert!(woken.load(Ordering::SeqCst));
 654                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 655         }
 656
 657         #[test]
 658         #[cfg(feature = "std")]
 659         fn test_multi_future_sleep() {
 660                 // Tests the `Sleeper` with multiple futures.
 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                 Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 668
 669                 // One future has woken us up, but the other should still have a pending notification.
 670                 Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 671
 672                 // However once we've slept twice, we should no longer have any pending notifications
 673                 assert!(!Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future())
 674                         .wait_timeout(Duration::from_millis(10)));
 675
 676                 // Test ordering somewhat more.
 677                 notifier_a.notify();
 678                 Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 679         }
 680
 681         #[test]
 682         #[cfg(feature = "std")]
 683         fn sleeper_with_pending_callbacks() {
 684                 // This is similar to the above `test_multi_future_sleep` test, but in addition registers
 685                 // "normal" callbacks which will cause the futures to assume notification has occurred,
 686                 // rather than waiting for a woken sleeper.
 687                 let notifier_a = Notifier::new();
 688                 let notifier_b = Notifier::new();
 689
 690                 // Set both notifiers as woken without sleeping yet.
 691                 notifier_a.notify();
 692                 notifier_b.notify();
 693
 694                 // After sleeping one future (not guaranteed which one, however) will have its notification
 695                 // bit cleared.
 696                 Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 697
 698                 // By registering a callback on the futures for both notifiers, one will complete
 699                 // immediately, but one will remain tied to the notifier, and will complete once the
 700                 // notifier is next woken, which will be considered the completion of the notification.
 701                 let callback_a = Arc::new(AtomicBool::new(false));
 702                 let callback_b = Arc::new(AtomicBool::new(false));
 703                 let callback_a_ref = Arc::clone(&callback_a);
 704                 let callback_b_ref = Arc::clone(&callback_b);
 705                 notifier_a.get_future().register_callback(Box::new(move || assert!(!callback_a_ref.fetch_or(true, Ordering::SeqCst))));
 706                 notifier_b.get_future().register_callback(Box::new(move || assert!(!callback_b_ref.fetch_or(true, Ordering::SeqCst))));
 707                 assert!(callback_a.load(Ordering::SeqCst) ^ callback_b.load(Ordering::SeqCst));
 708
 709                 // If we now notify both notifiers again, the other callback will fire, completing the
 710                 // notification, and we'll be back to one pending notification.
 711                 notifier_a.notify();
 712                 notifier_b.notify();
 713
 714                 assert!(callback_a.load(Ordering::SeqCst) && callback_b.load(Ordering::SeqCst));
 715                 Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 716                 assert!(!Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future())
 717                         .wait_timeout(Duration::from_millis(10)));
 718         }
 719
 720         #[test]
 721         #[cfg(feature = "std")]
 722         fn multi_poll_stores_single_waker() {
 723                 // When a `Future` is `poll()`ed multiple times, only the last `Waker` should be called,
 724                 // but previously we'd store all `Waker`s until they're all woken at once. This tests a few
 725                 // cases to ensure `Future`s avoid storing an endless set of `Waker`s.
 726                 let notifier = Notifier::new();
 727                 let future_state = Arc::clone(&notifier.get_future().state);
 728                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 0);
 729
 730                 // Test that simply polling a future twice doesn't result in two pending `Waker`s.
 731                 let mut future_a = notifier.get_future();
 732                 assert_eq!(Pin::new(&mut future_a).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
 733                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
 734                 assert_eq!(Pin::new(&mut future_a).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
 735                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
 736
 737                 // If we poll a second future, however, that will store a second `Waker`.
 738                 let mut future_b = notifier.get_future();
 739                 assert_eq!(Pin::new(&mut future_b).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
 740                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 2);
 741
 742                 // but when we drop the `Future`s, the pending Wakers will also be dropped.
 743                 mem::drop(future_a);
 744                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
 745                 mem::drop(future_b);
 746                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 0);
 747
 748                 // Further, after polling a future twice, if the notifier is woken all Wakers are dropped.
 749                 let mut future_a = notifier.get_future();
 750                 assert_eq!(Pin::new(&mut future_a).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
 751                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
 752                 assert_eq!(Pin::new(&mut future_a).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
 753                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
 754                 notifier.notify();
 755                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 0);
 756                 assert_eq!(Pin::new(&mut future_a).poll(&mut Context::from_waker(&create_waker().1)), Poll::Ready(()));
 757                 assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 0);
 758         }
 759 }