_ 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::{Condvar, Mutex, MutexGuard};
  19
  20 use crate::prelude::*;
  21
  22 #[cfg(any(test, feature = "std"))]
  23 use std::time::{Duration, Instant};
  24
  25 use core::future::Future as StdFuture;
  26 use core::task::{Context, Poll};
  27 use core::pin::Pin;
  28
  29
  30 /// Used to signal to one of many waiters that the condition they're waiting on has happened.
  31 pub(crate) struct Notifier {
  32         notify_pending: Mutex<(bool, Option<Arc<Mutex<FutureState>>>)>,
  33         condvar: Condvar,
  34 }
  35
  36 macro_rules! check_woken {
  37         ($guard: expr, $retval: expr) => { {
  38                 if $guard.0 {
  39                         $guard.0 = false;
  40                         if $guard.1.as_ref().map(|l| l.lock().unwrap().complete).unwrap_or(false) {
  41                                 // If we're about to return as woken, and the future state is marked complete, wipe
  42                                 // the future state and let the next future wait until we get a new notify.
  43                                 $guard.1.take();
  44                         }
  45                         return $retval;
  46                 }
  47         } }
  48 }
  49
  50 impl Notifier {
  51         pub(crate) fn new() -> Self {
  52                 Self {
  53                         notify_pending: Mutex::new((false, None)),
  54                         condvar: Condvar::new(),
  55                 }
  56         }
  57
  58         fn propagate_future_state_to_notify_flag(&self) -> MutexGuard<(bool, Option<Arc<Mutex<FutureState>>>)> {
  59                 let mut lock = self.notify_pending.lock().unwrap();
  60                 if let Some(existing_state) = &lock.1 {
  61                         if existing_state.lock().unwrap().callbacks_made {
  62                                 // If the existing `FutureState` has completed and actually made callbacks,
  63                                 // consider the notification flag to have been cleared and reset the future state.
  64                                 lock.1.take();
  65                                 lock.0 = false;
  66                         }
  67                 }
  68                 lock
  69         }
  70
  71         pub(crate) fn wait(&self) {
  72                 loop {
  73                         let mut guard = self.propagate_future_state_to_notify_flag();
  74                         check_woken!(guard, ());
  75                         guard = self.condvar.wait(guard).unwrap();
  76                         check_woken!(guard, ());
  77                 }
  78         }
  79
  80         #[cfg(any(test, feature = "std"))]
  81         pub(crate) fn wait_timeout(&self, max_wait: Duration) -> bool {
  82                 let current_time = Instant::now();
  83                 loop {
  84                         let mut guard = self.propagate_future_state_to_notify_flag();
  85                         check_woken!(guard, true);
  86                         guard = self.condvar.wait_timeout(guard, max_wait).unwrap().0;
  87                         check_woken!(guard, true);
  88                         // Due to spurious wakeups that can happen on `wait_timeout`, here we need to check if the
  89                         // desired wait time has actually passed, and if not then restart the loop with a reduced wait
  90                         // time. Note that this logic can be highly simplified through the use of
  91                         // `Condvar::wait_while` and `Condvar::wait_timeout_while`, if and when our MSRV is raised to
  92                         // 1.42.0.
  93                         let elapsed = current_time.elapsed();
  94                         if elapsed >= max_wait {
  95                                 return false;
  96                         }
  97                         match max_wait.checked_sub(elapsed) {
  98                                 None => return false,
  99                                 Some(_) => continue
 100                         }
 101                 }
 102         }
 103
 104         /// Wake waiters, tracking that wake needs to occur even if there are currently no waiters.
 105         pub(crate) fn notify(&self) {
 106                 let mut lock = self.notify_pending.lock().unwrap();
 107                 if let Some(future_state) = &lock.1 {
 108                         future_state.lock().unwrap().complete();
 109                 }
 110                 lock.0 = true;
 111                 mem::drop(lock);
 112                 self.condvar.notify_all();
 113         }
 114
 115         /// Gets a [`Future`] that will get woken up with any waiters
 116         pub(crate) fn get_future(&self) -> Future {
 117                 let mut lock = self.propagate_future_state_to_notify_flag();
 118                 if let Some(existing_state) = &lock.1 {
 119                         Future { state: Arc::clone(&existing_state) }
 120                 } else {
 121                         let state = Arc::new(Mutex::new(FutureState {
 122                                 callbacks: Vec::new(),
 123                                 complete: lock.0,
 124                                 callbacks_made: false,
 125                         }));
 126                         lock.1 = Some(Arc::clone(&state));
 127                         Future { state }
 128                 }
 129         }
 130
 131         #[cfg(any(test, feature = "_test_utils"))]
 132         pub fn notify_pending(&self) -> bool {
 133                 self.notify_pending.lock().unwrap().0
 134         }
 135 }
 136
 137 /// A callback which is called when a [`Future`] completes.
 138 ///
 139 /// Note that this MUST NOT call back into LDK directly, it must instead schedule actions to be
 140 /// taken later. Rust users should use the [`std::future::Future`] implementation for [`Future`]
 141 /// instead.
 142 ///
 143 /// Note that the [`std::future::Future`] implementation may only work for runtimes which schedule
 144 /// futures when they receive a wake, rather than immediately executing them.
 145 pub trait FutureCallback : Send {
 146         /// The method which is called.
 147         fn call(&self);
 148 }
 149
 150 impl<F: Fn() + Send> FutureCallback for F {
 151         fn call(&self) { (self)(); }
 152 }
 153
 154 pub(crate) struct FutureState {
 155         callbacks: Vec<Box<dyn FutureCallback>>,
 156         complete: bool,
 157         callbacks_made: bool,
 158 }
 159
 160 impl FutureState {
 161         fn complete(&mut self) {
 162                 for callback in self.callbacks.drain(..) {
 163                         callback.call();
 164                         self.callbacks_made = true;
 165                 }
 166                 self.complete = true;
 167         }
 168 }
 169
 170 /// A simple future which can complete once, and calls some callback(s) when it does so.
 171 pub struct Future {
 172         state: Arc<Mutex<FutureState>>,
 173 }
 174
 175 impl Future {
 176         /// Registers a callback to be called upon completion of this future. If the future has already
 177         /// completed, the callback will be called immediately.
 178         ///
 179         /// (C-not exported) use the bindings-only `register_callback_fn` instead
 180         pub fn register_callback(&self, callback: Box<dyn FutureCallback>) {
 181                 let mut state = self.state.lock().unwrap();
 182                 if state.complete {
 183                         state.callbacks_made = true;
 184                         mem::drop(state);
 185                         callback.call();
 186                 } else {
 187                         state.callbacks.push(callback);
 188                 }
 189         }
 190
 191         // C bindings don't (currently) know how to map `Box<dyn Trait>`, and while it could add the
 192         // following wrapper, doing it in the bindings is currently much more work than simply doing it
 193         // here.
 194         /// Registers a callback to be called upon completion of this future. If the future has already
 195         /// completed, the callback will be called immediately.
 196         #[cfg(c_bindings)]
 197         pub fn register_callback_fn<F: 'static + FutureCallback>(&self, callback: F) {
 198                 self.register_callback(Box::new(callback));
 199         }
 200 }
 201
 202 use core::task::Waker;
 203 struct StdWaker(pub Waker);
 204 impl FutureCallback for StdWaker {
 205         fn call(&self) { self.0.wake_by_ref() }
 206 }
 207
 208 /// (C-not exported) as Rust Futures aren't usable in language bindings.
 209 impl<'a> StdFuture for Future {
 210         type Output = ();
 211
 212         fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 213                 let mut state = self.state.lock().unwrap();
 214                 if state.complete {
 215                         Poll::Ready(())
 216                 } else {
 217                         let waker = cx.waker().clone();
 218                         state.callbacks.push(Box::new(StdWaker(waker)));
 219                         Poll::Pending
 220                 }
 221         }
 222 }
 223
 224 #[cfg(test)]
 225 mod tests {
 226         use super::*;
 227         use core::sync::atomic::{AtomicBool, Ordering};
 228         use core::future::Future as FutureTrait;
 229         use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 230
 231         #[test]
 232         fn notifier_pre_notified_future() {
 233                 // Previously, if we generated a future after a `Notifier` had been notified, the future
 234                 // would never complete. This tests this behavior, ensuring the future instead completes
 235                 // immediately.
 236                 let notifier = Notifier::new();
 237                 notifier.notify();
 238
 239                 let callback = Arc::new(AtomicBool::new(false));
 240                 let callback_ref = Arc::clone(&callback);
 241                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 242                 assert!(callback.load(Ordering::SeqCst));
 243         }
 244
 245         #[test]
 246         fn notifier_future_completes_wake() {
 247                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 248                 // been notified, we'd never mark the notifier as not-awaiting-notify. This caused the
 249                 // `lightning-background-processor` to persist in a tight loop.
 250                 let notifier = Notifier::new();
 251
 252                 // First check the simple case, ensuring if we get notified a new future isn't woken until
 253                 // a second `notify`.
 254                 let callback = Arc::new(AtomicBool::new(false));
 255                 let callback_ref = Arc::clone(&callback);
 256                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 257                 assert!(!callback.load(Ordering::SeqCst));
 258
 259                 notifier.notify();
 260                 assert!(callback.load(Ordering::SeqCst));
 261
 262                 let callback = Arc::new(AtomicBool::new(false));
 263                 let callback_ref = Arc::clone(&callback);
 264                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 265                 assert!(!callback.load(Ordering::SeqCst));
 266
 267                 notifier.notify();
 268                 assert!(callback.load(Ordering::SeqCst));
 269
 270                 // Then check the case where the future is fetched before the notification, but a callback
 271                 // is only registered after the `notify`, ensuring that it is still sufficient to ensure we
 272                 // don't get an instant-wake when we get a new future.
 273                 let future = notifier.get_future();
 274                 notifier.notify();
 275
 276                 let callback = Arc::new(AtomicBool::new(false));
 277                 let callback_ref = Arc::clone(&callback);
 278                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 279                 assert!(callback.load(Ordering::SeqCst));
 280
 281                 let callback = Arc::new(AtomicBool::new(false));
 282                 let callback_ref = Arc::clone(&callback);
 283                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 284                 assert!(!callback.load(Ordering::SeqCst));
 285         }
 286
 287         #[test]
 288         fn new_future_wipes_notify_bit() {
 289                 // Previously, if we were only using the `Future` interface to learn when a `Notifier` has
 290                 // been notified, we'd never mark the notifier as not-awaiting-notify if a `Future` is
 291                 // fetched after the notify bit has been set.
 292                 let notifier = Notifier::new();
 293                 notifier.notify();
 294
 295                 let callback = Arc::new(AtomicBool::new(false));
 296                 let callback_ref = Arc::clone(&callback);
 297                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 298                 assert!(callback.load(Ordering::SeqCst));
 299
 300                 let callback = Arc::new(AtomicBool::new(false));
 301                 let callback_ref = Arc::clone(&callback);
 302                 notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 303                 assert!(!callback.load(Ordering::SeqCst));
 304
 305                 notifier.notify();
 306                 assert!(callback.load(Ordering::SeqCst));
 307         }
 308
 309         #[cfg(feature = "std")]
 310         #[test]
 311         fn test_wait_timeout() {
 312                 use crate::sync::Arc;
 313                 use std::thread;
 314
 315                 let persistence_notifier = Arc::new(Notifier::new());
 316                 let thread_notifier = Arc::clone(&persistence_notifier);
 317
 318                 let exit_thread = Arc::new(AtomicBool::new(false));
 319                 let exit_thread_clone = exit_thread.clone();
 320                 thread::spawn(move || {
 321                         loop {
 322                                 let mut lock = thread_notifier.notify_pending.lock().unwrap();
 323                                 lock.0 = true;
 324                                 thread_notifier.condvar.notify_all();
 325
 326                                 if exit_thread_clone.load(Ordering::SeqCst) {
 327                                         break
 328                                 }
 329                         }
 330                 });
 331
 332                 // Check that we can block indefinitely until updates are available.
 333                 let _ = persistence_notifier.wait();
 334
 335                 // Check that the Notifier will return after the given duration if updates are
 336                 // available.
 337                 loop {
 338                         if persistence_notifier.wait_timeout(Duration::from_millis(100)) {
 339                                 break
 340                         }
 341                 }
 342
 343                 exit_thread.store(true, Ordering::SeqCst);
 344
 345                 // Check that the Notifier will return after the given duration even if no updates
 346                 // are available.
 347                 loop {
 348                         if !persistence_notifier.wait_timeout(Duration::from_millis(100)) {
 349                                 break
 350                         }
 351                 }
 352         }
 353
 354         #[test]
 355         fn test_future_callbacks() {
 356                 let future = Future {
 357                         state: Arc::new(Mutex::new(FutureState {
 358                                 callbacks: Vec::new(),
 359                                 complete: false,
 360                                 callbacks_made: false,
 361                         }))
 362                 };
 363                 let callback = Arc::new(AtomicBool::new(false));
 364                 let callback_ref = Arc::clone(&callback);
 365                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 366
 367                 assert!(!callback.load(Ordering::SeqCst));
 368                 future.state.lock().unwrap().complete();
 369                 assert!(callback.load(Ordering::SeqCst));
 370                 future.state.lock().unwrap().complete();
 371         }
 372
 373         #[test]
 374         fn test_pre_completed_future_callbacks() {
 375                 let future = Future {
 376                         state: Arc::new(Mutex::new(FutureState {
 377                                 callbacks: Vec::new(),
 378                                 complete: false,
 379                                 callbacks_made: false,
 380                         }))
 381                 };
 382                 future.state.lock().unwrap().complete();
 383
 384                 let callback = Arc::new(AtomicBool::new(false));
 385                 let callback_ref = Arc::clone(&callback);
 386                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 387
 388                 assert!(callback.load(Ordering::SeqCst));
 389                 assert!(future.state.lock().unwrap().callbacks.is_empty());
 390         }
 391
 392         // Rather annoyingly, there's no safe way in Rust std to construct a Waker despite it being
 393         // totally possible to construct from a trait implementation (though somewhat less effecient
 394         // compared to a raw VTable). Instead, we have to write out a lot of boilerplate to build a
 395         // waker, which we do here with a trivial Arc<AtomicBool> data element to track woke-ness.
 396         const WAKER_V_TABLE: RawWakerVTable = RawWakerVTable::new(waker_clone, wake, wake_by_ref, drop);
 397         unsafe fn wake_by_ref(ptr: *const ()) { let p = ptr as *const Arc<AtomicBool>; assert!(!(*p).fetch_or(true, Ordering::SeqCst)); }
 398         unsafe fn drop(ptr: *const ()) { let p = ptr as *mut Arc<AtomicBool>; let _freed = Box::from_raw(p); }
 399         unsafe fn wake(ptr: *const ()) { wake_by_ref(ptr); drop(ptr); }
 400         unsafe fn waker_clone(ptr: *const ()) -> RawWaker {
 401                 let p = ptr as *const Arc<AtomicBool>;
 402                 RawWaker::new(Box::into_raw(Box::new(Arc::clone(&*p))) as *const (), &WAKER_V_TABLE)
 403         }
 404
 405         fn create_waker() -> (Arc<AtomicBool>, Waker) {
 406                 let a = Arc::new(AtomicBool::new(false));
 407                 let waker = unsafe { Waker::from_raw(waker_clone((&a as *const Arc<AtomicBool>) as *const ())) };
 408                 (a, waker)
 409         }
 410
 411         #[test]
 412         fn test_future() {
 413                 let mut future = Future {
 414                         state: Arc::new(Mutex::new(FutureState {
 415                                 callbacks: Vec::new(),
 416                                 complete: false,
 417                                 callbacks_made: false,
 418                         }))
 419                 };
 420                 let mut second_future = Future { state: Arc::clone(&future.state) };
 421
 422                 let (woken, waker) = create_waker();
 423                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 424                 assert!(!woken.load(Ordering::SeqCst));
 425
 426                 let (second_woken, second_waker) = create_waker();
 427                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Pending);
 428                 assert!(!second_woken.load(Ordering::SeqCst));
 429
 430                 future.state.lock().unwrap().complete();
 431                 assert!(woken.load(Ordering::SeqCst));
 432                 assert!(second_woken.load(Ordering::SeqCst));
 433                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 434                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Ready(()));
 435         }
 436 }