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