Merge pull request #1851 from TheBlueMatt/2022-11-fix-broken-futures-----again

Unset the needs-notify bit in a Notifier when a Future is fetched

Author: TheBlueMatt

lightning/src/util/wakers.rs

@@ -15,7 +15,7 @@
 
 use alloc::sync::Arc;
 use core::mem;
-use crate::sync::{Condvar, Mutex};
+use crate::sync::{Condvar, Mutex, MutexGuard};
 
 use crate::prelude::*;
 
@@ -33,6 +33,20 @@ pub(crate) struct Notifier {
 	condvar: Condvar,
 }
 
+macro_rules! check_woken {
+	($guard: expr, $retval: expr) => { {
+		if $guard.0 {
+			$guard.0 = false;
+			if $guard.1.as_ref().map(|l| l.lock().unwrap().complete).unwrap_or(false) {
+				// If we're about to return as woken, and the future state is marked complete, wipe
+				// the future state and let the next future wait until we get a new notify.
+				$guard.1.take();
+			}
+			return $retval;
+		}
+	} }
+}
+
 impl Notifier {
 	pub(crate) fn new() -> Self {
 		Self {
@@ -41,45 +55,47 @@ impl Notifier {
 		}
 	}
 
+	fn propagate_future_state_to_notify_flag(&self) -> MutexGuard<(bool, Option<Arc<Mutex<FutureState>>>)> {
+		let mut lock = self.notify_pending.lock().unwrap();
+		if let Some(existing_state) = &lock.1 {
+			if existing_state.lock().unwrap().callbacks_made {
+				// If the existing `FutureState` has completed and actually made callbacks,
+				// consider the notification flag to have been cleared and reset the future state.
+				lock.1.take();
+				lock.0 = false;
+			}
+		}
+		lock
+	}
+
 	pub(crate) fn wait(&self) {
 		loop {
-			let mut guard = self.notify_pending.lock().unwrap();
-			if guard.0 {
-				guard.0 = false;
-				return;
-			}
+			let mut guard = self.propagate_future_state_to_notify_flag();
+			check_woken!(guard, ());
 			guard = self.condvar.wait(guard).unwrap();
-			let result = guard.0;
-			if result {
-				guard.0 = false;
-				return
-			}
+			check_woken!(guard, ());
 		}
 	}
 
 	#[cfg(any(test, feature = "std"))]
 	pub(crate) fn wait_timeout(&self, max_wait: Duration) -> bool {
 		let current_time = Instant::now();
 		loop {
-			let mut guard = self.notify_pending.lock().unwrap();
-			if guard.0 {
-				guard.0 = false;
-				return true;
-			}
+			let mut guard = self.propagate_future_state_to_notify_flag();
+			check_woken!(guard, true);
 			guard = self.condvar.wait_timeout(guard, max_wait).unwrap().0;
+			check_woken!(guard, true);
 			// Due to spurious wakeups that can happen on `wait_timeout`, here we need to check if the
 			// desired wait time has actually passed, and if not then restart the loop with a reduced wait
 			// time. Note that this logic can be highly simplified through the use of
 			// `Condvar::wait_while` and `Condvar::wait_timeout_while`, if and when our MSRV is raised to
 			// 1.42.0.
 			let elapsed = current_time.elapsed();
-			let result = guard.0;
-			if result || elapsed >= max_wait {
-				guard.0 = false;
-				return result;
+			if elapsed >= max_wait {
+				return false;
 			}
 			match max_wait.checked_sub(elapsed) {
-				None => return result,
+				None => return false,
 				Some(_) => continue
 			}
 		}
@@ -88,17 +104,8 @@ impl Notifier {
 	/// Wake waiters, tracking that wake needs to occur even if there are currently no waiters.
 	pub(crate) fn notify(&self) {
 		let mut lock = self.notify_pending.lock().unwrap();
-		let mut future_probably_generated_calls = false;
-		if let Some(future_state) = lock.1.take() {
-			future_probably_generated_calls |= future_state.lock().unwrap().complete();
-			future_probably_generated_calls |= Arc::strong_count(&future_state) > 1;
-		}
-		if future_probably_generated_calls {
-			// If a future made some callbacks or has not yet been drop'd (i.e. the state has more
-			// than the one reference we hold), assume the user was notified and skip setting the
-			// notification-required flag. This will not cause the `wait` functions above to return
-			// and avoid any future `Future`s starting in a completed state.
-			return;
+		if let Some(future_state) = &lock.1 {
+			future_state.lock().unwrap().complete();
 		}
 		lock.0 = true;
 		mem::drop(lock);
@@ -107,20 +114,14 @@ impl Notifier {
 
 	/// Gets a [`Future`] that will get woken up with any waiters
 	pub(crate) fn get_future(&self) -> Future {
-		let mut lock = self.notify_pending.lock().unwrap();
-		if lock.0 {
-			Future {
-				state: Arc::new(Mutex::new(FutureState {
-					callbacks: Vec::new(),
-					complete: true,
-				}))
-			}
-		} else if let Some(existing_state) = &lock.1 {
+		let mut lock = self.propagate_future_state_to_notify_flag();
+		if let Some(existing_state) = &lock.1 {
 			Future { state: Arc::clone(&existing_state) }
 		} else {
 			let state = Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
-				complete: false,
+				complete: lock.0,
+				callbacks_made: false,
 			}));
 			lock.1 = Some(Arc::clone(&state));
 			Future { state }
@@ -151,19 +152,21 @@ impl<F: Fn() + Send> FutureCallback for F {
 }
 
 pub(crate) struct FutureState {
-	callbacks: Vec<Box<dyn FutureCallback>>,
+	// When we're tracking whether a callback counts as having woken the user's code, we check the
+	// first bool - set to false if we're just calling a Waker, and true if we're calling an actual
+	// user-provided function.
+	callbacks: Vec<(bool, Box<dyn FutureCallback>)>,
 	complete: bool,
+	callbacks_made: bool,
 }
 
 impl FutureState {
-	fn complete(&mut self) -> bool {
-		let mut made_calls = false;
-		for callback in self.callbacks.drain(..) {
+	fn complete(&mut self) {
+		for (counts_as_call, callback) in self.callbacks.drain(..) {
 			callback.call();
-			made_calls = true;
+			self.callbacks_made |= counts_as_call;
 		}
 		self.complete = true;
-		made_calls
 	}
 }
 
@@ -180,10 +183,11 @@ impl Future {
 	pub fn register_callback(&self, callback: Box<dyn FutureCallback>) {
 		let mut state = self.state.lock().unwrap();
 		if state.complete {
+			state.callbacks_made = true;
 			mem::drop(state);
 			callback.call();
 		} else {
-			state.callbacks.push(callback);
+			state.callbacks.push((true, callback));
 		}
 	}
 
@@ -198,12 +202,10 @@ impl Future {
 	}
 }
 
-mod std_future {
-	use core::task::Waker;
-	pub struct StdWaker(pub Waker);
-	impl super::FutureCallback for StdWaker {
-		fn call(&self) { self.0.wake_by_ref() }
-	}
+use core::task::Waker;
+struct StdWaker(pub Waker);
+impl FutureCallback for StdWaker {
+	fn call(&self) { self.0.wake_by_ref() }
 }
 
 /// (C-not exported) as Rust Futures aren't usable in language bindings.
@@ -213,10 +215,11 @@ impl<'a> StdFuture for Future {
 	fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
 		let mut state = self.state.lock().unwrap();
 		if state.complete {
+			state.callbacks_made = true;
 			Poll::Ready(())
 		} else {
 			let waker = cx.waker().clone();
-			state.callbacks.push(Box::new(std_future::StdWaker(waker)));
+			state.callbacks.push((false, Box::new(StdWaker(waker))));
 			Poll::Pending
 		}
 	}
@@ -285,6 +288,28 @@ mod tests {
 		assert!(!callback.load(Ordering::SeqCst));
 	}
 
+	#[test]
+	fn new_future_wipes_notify_bit() {
+		// Previously, if we were only using the `Future` interface to learn when a `Notifier` has
+		// been notified, we'd never mark the notifier as not-awaiting-notify if a `Future` is
+		// fetched after the notify bit has been set.
+		let notifier = Notifier::new();
+		notifier.notify();
+
+		let callback = Arc::new(AtomicBool::new(false));
+		let callback_ref = Arc::clone(&callback);
+		notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
+		assert!(callback.load(Ordering::SeqCst));
+
+		let callback = Arc::new(AtomicBool::new(false));
+		let callback_ref = Arc::clone(&callback);
+		notifier.get_future().register_callback(Box::new(move || assert!(!callback_ref.fetch_or(true, Ordering::SeqCst))));
+		assert!(!callback.load(Ordering::SeqCst));
+
+		notifier.notify();
+		assert!(callback.load(Ordering::SeqCst));
+	}
+
 	#[cfg(feature = "std")]
 	#[test]
 	fn test_wait_timeout() {
@@ -336,6 +361,7 @@ mod tests {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
 				complete: false,
+				callbacks_made: false,
 			}))
 		};
 		let callback = Arc::new(AtomicBool::new(false));
@@ -354,6 +380,7 @@ mod tests {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
 				complete: false,
+				callbacks_made: false,
 			}))
 		};
 		future.state.lock().unwrap().complete();
@@ -391,6 +418,7 @@ mod tests {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
 				complete: false,
+				callbacks_made: false,
 			}))
 		};
 		let mut second_future = Future { state: Arc::clone(&future.state) };
@@ -409,4 +437,36 @@ mod tests {
 		assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
 		assert_eq!(Pin::new(&mut second_future).poll(&mut Context::from_waker(&second_waker)), Poll::Ready(()));
 	}
+
+	#[test]
+	fn test_dropped_future_doesnt_count() {
+		// Tests that if a Future gets drop'd before it is poll()ed `Ready` it doesn't count as
+		// having been woken, leaving the notify-required flag set.
+		let notifier = Notifier::new();
+		notifier.notify();
+
+		// If we get a future and don't touch it we're definitely still notify-required.
+		notifier.get_future();
+		assert!(notifier.wait_timeout(Duration::from_millis(1)));
+		assert!(!notifier.wait_timeout(Duration::from_millis(1)));
+
+		// Even if we poll'd once but didn't observe a `Ready`, we should be notify-required.
+		let mut future = notifier.get_future();
+		let (woken, waker) = create_waker();
+		assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
+
+		notifier.notify();
+		assert!(woken.load(Ordering::SeqCst));
+		assert!(notifier.wait_timeout(Duration::from_millis(1)));
+
+		// However, once we do poll `Ready` it should wipe the notify-required flag.
+		let mut future = notifier.get_future();
+		let (woken, waker) = create_waker();
+		assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);
+
+		notifier.notify();
+		assert!(woken.load(Ordering::SeqCst));
+		assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Ready(()));
+		assert!(!notifier.wait_timeout(Duration::from_millis(1)));
+	}
 }