_ 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 core::time::Duration;
  19 use sync::{Condvar, Mutex};
  20
  21 use prelude::*;
  22
  23 #[cfg(any(test, feature = "std"))]
  24 use std::time::Instant;
  25
  26 use core::future::Future as StdFuture;
  27 use core::task::{Context, Poll};
  28 use core::pin::Pin;
  29
  30
  31 /// Used to signal to one of many waiters that the condition they're waiting on has happened.
  32 pub(crate) struct Notifier {
  33         notify_pending: Mutex<(bool, Option<Arc<Mutex<FutureState>>>)>,
  34         condvar: Condvar,
  35 }
  36
  37 impl Notifier {
  38         pub(crate) fn new() -> Self {
  39                 Self {
  40                         notify_pending: Mutex::new((false, None)),
  41                         condvar: Condvar::new(),
  42                 }
  43         }
  44
  45         pub(crate) fn wait(&self) {
  46                 loop {
  47                         let mut guard = self.notify_pending.lock().unwrap();
  48                         if guard.0 {
  49                                 guard.0 = false;
  50                                 return;
  51                         }
  52                         guard = self.condvar.wait(guard).unwrap();
  53                         let result = guard.0;
  54                         if result {
  55                                 guard.0 = false;
  56                                 return
  57                         }
  58                 }
  59         }
  60
  61         #[cfg(any(test, feature = "std"))]
  62         pub(crate) fn wait_timeout(&self, max_wait: Duration) -> bool {
  63                 let current_time = Instant::now();
  64                 loop {
  65                         let mut guard = self.notify_pending.lock().unwrap();
  66                         if guard.0 {
  67                                 guard.0 = false;
  68                                 return true;
  69                         }
  70                         guard = self.condvar.wait_timeout(guard, max_wait).unwrap().0;
  71                         // Due to spurious wakeups that can happen on `wait_timeout`, here we need to check if the
  72                         // desired wait time has actually passed, and if not then restart the loop with a reduced wait
  73                         // time. Note that this logic can be highly simplified through the use of
  74                         // `Condvar::wait_while` and `Condvar::wait_timeout_while`, if and when our MSRV is raised to
  75                         // 1.42.0.
  76                         let elapsed = current_time.elapsed();
  77                         let result = guard.0;
  78                         if result || elapsed >= max_wait {
  79                                 guard.0 = false;
  80                                 return result;
  81                         }
  82                         match max_wait.checked_sub(elapsed) {
  83                                 None => return result,
  84                                 Some(_) => continue
  85                         }
  86                 }
  87         }
  88
  89         /// Wake waiters, tracking that wake needs to occur even if there are currently no waiters.
  90         pub(crate) fn notify(&self) {
  91                 let mut lock = self.notify_pending.lock().unwrap();
  92                 lock.0 = true;
  93                 if let Some(future_state) = lock.1.take() {
  94                         future_state.lock().unwrap().complete();
  95                 }
  96                 mem::drop(lock);
  97                 self.condvar.notify_all();
  98         }
  99
 100         /// Gets a [`Future`] that will get woken up with any waiters
 101         pub(crate) fn get_future(&self) -> Future {
 102                 let mut lock = self.notify_pending.lock().unwrap();
 103                 if lock.0 {
 104                         Future {
 105                                 state: Arc::new(Mutex::new(FutureState {
 106                                         callbacks: Vec::new(),
 107                                         complete: false,
 108                                 }))
 109                         }
 110                 } else if let Some(existing_state) = &lock.1 {
 111                         Future { state: Arc::clone(&existing_state) }
 112                 } else {
 113                         let state = Arc::new(Mutex::new(FutureState {
 114                                 callbacks: Vec::new(),
 115                                 complete: false,
 116                         }));
 117                         lock.1 = Some(Arc::clone(&state));
 118                         Future { state }
 119                 }
 120         }
 121
 122         #[cfg(any(test, feature = "_test_utils"))]
 123         pub fn notify_pending(&self) -> bool {
 124                 self.notify_pending.lock().unwrap().0
 125         }
 126 }
 127
 128 /// A callback which is called when a [`Future`] completes.
 129 ///
 130 /// Note that this MUST NOT call back into LDK directly, it must instead schedule actions to be
 131 /// taken later. Rust users should use the [`std::future::Future`] implementation for [`Future`]
 132 /// instead.
 133 ///
 134 /// Note that the [`std::future::Future`] implementation may only work for runtimes which schedule
 135 /// futures when they receive a wake, rather than immediately executing them.
 136 pub trait FutureCallback : Send {
 137         /// The method which is called.
 138         fn call(&self);
 139 }
 140
 141 impl<F: Fn() + Send> FutureCallback for F {
 142         fn call(&self) { (self)(); }
 143 }
 144
 145 pub(crate) struct FutureState {
 146         callbacks: Vec<Box<dyn FutureCallback>>,
 147         complete: bool,
 148 }
 149
 150 impl FutureState {
 151         fn complete(&mut self) {
 152                 for callback in self.callbacks.drain(..) {
 153                         callback.call();
 154                 }
 155                 self.complete = true;
 156         }
 157 }
 158
 159 /// A simple future which can complete once, and calls some callback(s) when it does so.
 160 pub struct Future {
 161         state: Arc<Mutex<FutureState>>,
 162 }
 163
 164 impl Future {
 165         /// Registers a callback to be called upon completion of this future. If the future has already
 166         /// completed, the callback will be called immediately.
 167         pub fn register_callback(&self, callback: Box<dyn FutureCallback>) {
 168                 let mut state = self.state.lock().unwrap();
 169                 if state.complete {
 170                         mem::drop(state);
 171                         callback.call();
 172                 } else {
 173                         state.callbacks.push(callback);
 174                 }
 175         }
 176 }
 177
 178 mod std_future {
 179         use core::task::Waker;
 180         pub struct StdWaker(pub Waker);
 181         impl super::FutureCallback for StdWaker {
 182                 fn call(&self) { self.0.wake_by_ref() }
 183         }
 184 }
 185
 186 /// (C-not exported) as Rust Futures aren't usable in language bindings.
 187 impl<'a> StdFuture for Future {
 188         type Output = ();
 189
 190         fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 191                 let mut state = self.state.lock().unwrap();
 192                 if state.complete {
 193                         Poll::Ready(())
 194                 } else {
 195                         let waker = cx.waker().clone();
 196                         state.callbacks.push(Box::new(std_future::StdWaker(waker)));
 197                         Poll::Pending
 198                 }
 199         }
 200 }
 201
 202 #[cfg(test)]
 203 mod tests {
 204         use super::*;
 205         use core::sync::atomic::{AtomicBool, Ordering};
 206         use core::future::Future as FutureTrait;
 207         use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
 208
 209         #[cfg(feature = "std")]
 210         #[test]
 211         fn test_wait_timeout() {
 212                 use sync::Arc;
 213                 use std::thread;
 214
 215                 let persistence_notifier = Arc::new(Notifier::new());
 216                 let thread_notifier = Arc::clone(&persistence_notifier);
 217
 218                 let exit_thread = Arc::new(AtomicBool::new(false));
 219                 let exit_thread_clone = exit_thread.clone();
 220                 thread::spawn(move || {
 221                         loop {
 222                                 let mut lock = thread_notifier.notify_pending.lock().unwrap();
 223                                 lock.0 = true;
 224                                 thread_notifier.condvar.notify_all();
 225
 226                                 if exit_thread_clone.load(Ordering::SeqCst) {
 227                                         break
 228                                 }
 229                         }
 230                 });
 231
 232                 // Check that we can block indefinitely until updates are available.
 233                 let _ = persistence_notifier.wait();
 234
 235                 // Check that the Notifier will return after the given duration if updates are
 236                 // available.
 237                 loop {
 238                         if persistence_notifier.wait_timeout(Duration::from_millis(100)) {
 239                                 break
 240                         }
 241                 }
 242
 243                 exit_thread.store(true, Ordering::SeqCst);
 244
 245                 // Check that the Notifier will return after the given duration even if no updates
 246                 // are available.
 247                 loop {
 248                         if !persistence_notifier.wait_timeout(Duration::from_millis(100)) {
 249                                 break
 250                         }
 251                 }
 252         }
 253
 254         #[test]
 255         fn test_future_callbacks() {
 256                 let future = Future {
 257                         state: Arc::new(Mutex::new(FutureState {
 258                                 callbacks: Vec::new(),
 259                                 complete: false,
 260                         }))
 261                 };
 262                 let callback = Arc::new(AtomicBool::new(false));
 263                 let callback_ref = Arc::clone(&callback);
 264                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 265
 266                 assert!(!callback.load(Ordering::SeqCst));
 267                 future.state.lock().unwrap().complete();
 268                 assert!(callback.load(Ordering::SeqCst));
 269                 future.state.lock().unwrap().complete();
 270         }
 271
 272         #[test]
 273         fn test_pre_completed_future_callbacks() {
 274                 let future = Future {
 275                         state: Arc::new(Mutex::new(FutureState {
 276                                 callbacks: Vec::new(),
 277                                 complete: false,
 278                         }))
 279                 };
 280                 future.state.lock().unwrap().complete();
 281
 282                 let callback = Arc::new(AtomicBool::new(false));
 283                 let callback_ref = Arc::clone(&callback);
 284                 future.register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
 285
 286                 assert!(callback.load(Ordering::SeqCst));
 287                 assert!(future.state.lock().unwrap().callbacks.is_empty());
 288         }
 289
 290         // Rather annoyingly, there's no safe way in Rust std to construct a Waker despite it being
 291         // totally possible to construct from a trait implementation (though somewhat less effecient
 292         // compared to a raw VTable). Instead, we have to write out a lot of boilerplate to build a
 293         // waker, which we do here with a trivial Arc<AtomicBool> data element to track woke-ness.
 294         const WAKER_V_TABLE: RawWakerVTable = RawWakerVTable::new(waker_clone, wake, wake_by_ref, drop);
 295         unsafe fn wake_by_ref(ptr: *const ()) { let p = ptr as *const Arc<AtomicBool>; assert!(!(*p).fetch_or(true, Ordering::SeqCst)); }
 296         unsafe fn drop(ptr: *const ()) { let p = ptr as *mut Arc<AtomicBool>; let _freed = Box::from_raw(p); }
 297         unsafe fn wake(ptr: *const ()) { wake_by_ref(ptr); drop(ptr); }
 298         unsafe fn waker_clone(ptr: *const ()) -> RawWaker {
 299                 let p = ptr as *const Arc<AtomicBool>;
 300                 RawWaker::new(Box::into_raw(Box::new(Arc::clone(&*p))) as *const (), &WAKER_V_TABLE)
 301         }
 302
 303         fn create_waker() -> (Arc<AtomicBool>, Waker) {
 304                 let a = Arc::new(AtomicBool::new(false));
 305                 let waker = unsafe { Waker::from_raw(waker_clone((&a as *const Arc<AtomicBool>) as *const ())) };
 306                 (a, waker)
 307         }
 308
 309         #[test]
 310         fn test_future() {
 311                 let mut future = Future {
 312                         state: Arc::new(Mutex::new(FutureState {
 313                                 callbacks: Vec::new(),
 314                                 complete: false,
 315                         }))
 316                 };
 317                 let mut second_future = Future { state: Arc::clone(&future.state) };
 318
 319                 let (woken, waker) = create_waker();
 320                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
 321                 assert!(!woken.load(Ordering::SeqCst));
 322
 323                 let (second_woken, second_waker) = create_waker();
 324                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Pending);
 325                 assert!(!second_woken.load(Ordering::SeqCst));
 326
 327                 future.state.lock().unwrap().complete();
 328                 assert!(woken.load(Ordering::SeqCst));
 329                 assert!(second_woken.load(Ordering::SeqCst));
 330                 assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 331                 assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Ready(()));
 332         }
 333 }