Never store more than one StdWaker per live Future

When an `std::future::Future` is `poll()`ed, we're only supposed to
use the latest `Waker` provided. However, we currently push an
`StdWaker` onto our callback list every time `poll` is called,
waking every `Waker` but also using more and more memory until the
`Future` itself is woken.

Here we fix this by removing any `StdWaker`s stored for a given
`Future` when it is `drop`ped or prior to pushing a new `StdWaker`
onto the list when `poll`ed.

Sadly, the introduction of a `Drop` impl for `Future` means we
can't trivially destructure the struct any longer, causing a few
methods to need to take `Future`s by reference rather than
ownership and `clone` a few `Arc`s.

Fixes lightningdevkit#2874

lightning-background-processor/src/lib.rs

@@ -854,8 +854,8 @@ impl BackgroundProcessor {
 				peer_manager.onion_message_handler().process_pending_events(&event_handler),
 				gossip_sync, logger, scorer, stop_thread.load(Ordering::Acquire),
 				{ Sleeper::from_two_futures(
-					channel_manager.get_event_or_persistence_needed_future(),
-					chain_monitor.get_update_future()
+					&channel_manager.get_event_or_persistence_needed_future(),
+					&chain_monitor.get_update_future()
 				).wait_timeout(Duration::from_millis(100)); },
 				|_| Instant::now(), |time: &Instant, dur| time.elapsed().as_secs() > dur, false,
 				|| {

lightning/src/util/wakers.rs

@@ -180,16 +180,16 @@ impl Future {
 
 	/// Waits until this [`Future`] completes.
 	#[cfg(feature = "std")]
-	pub fn wait(self) {
-		Sleeper::from_single_future(self).wait();
+	pub fn wait(&self) {
+		Sleeper::from_single_future(&self).wait();
 	}
 
 	/// Waits until this [`Future`] completes or the given amount of time has elapsed.
 	///
 	/// Returns true if the [`Future`] completed, false if the time elapsed.
 	#[cfg(feature = "std")]
-	pub fn wait_timeout(self, max_wait: Duration) -> bool {
-		Sleeper::from_single_future(self).wait_timeout(max_wait)
+	pub fn wait_timeout(&self, max_wait: Duration) -> bool {
+		Sleeper::from_single_future(&self).wait_timeout(max_wait)
 	}
 
 	#[cfg(test)]
@@ -202,6 +202,12 @@ impl Future {
 	}
 }
 
+impl Drop for Future {
+	fn drop(&mut self) {
+		self.state.lock().unwrap().std_future_callbacks.retain(|(idx, _)| *idx != self.self_idx);
+	}
+}
+
 use core::task::Waker;
 struct StdWaker(pub Waker);
 
@@ -216,6 +222,7 @@ impl<'a> StdFuture for Future {
 			Poll::Ready(())
 		} else {
 			let waker = cx.waker().clone();
+			state.std_future_callbacks.retain(|(idx, _)| *idx != self.self_idx);
 			state.std_future_callbacks.push((self.self_idx, StdWaker(waker)));
 			Poll::Pending
 		}
@@ -232,17 +239,17 @@ pub struct Sleeper {
 #[cfg(feature = "std")]
 impl Sleeper {
 	/// Constructs a new sleeper from one future, allowing blocking on it.
-	pub fn from_single_future(future: Future) -> Self {
-		Self { notifiers: vec![future.state] }
+	pub fn from_single_future(future: &Future) -> Self {
+		Self { notifiers: vec![Arc::clone(&future.state)] }
 	}
 	/// Constructs a new sleeper from two futures, allowing blocking on both at once.
 	// Note that this is the common case - a ChannelManager and ChainMonitor.
-	pub fn from_two_futures(fut_a: Future, fut_b: Future) -> Self {
-		Self { notifiers: vec![fut_a.state, fut_b.state] }
+	pub fn from_two_futures(fut_a: &Future, fut_b: &Future) -> Self {
+		Self { notifiers: vec![Arc::clone(&fut_a.state), Arc::clone(&fut_b.state)] }
 	}
 	/// Constructs a new sleeper on many futures, allowing blocking on all at once.
 	pub fn new(futures: Vec<Future>) -> Self {
-		Self { notifiers: futures.into_iter().map(|f| f.state).collect() }
+		Self { notifiers: futures.into_iter().map(|f| Arc::clone(&f.state)).collect() }
 	}
 	/// Prepares to go into a wait loop body, creating a condition variable which we can block on
 	/// and an `Arc<Mutex<Option<_>>>` which gets set to the waking `Future`'s state prior to the
@@ -447,13 +454,15 @@ mod tests {
 
 		// Wait on the other thread to finish its sleep, note that the leak only happened if we
 		// actually have to sleep here, not if we immediately return.
-		Sleeper::from_two_futures(future_a, future_b).wait();
+		Sleeper::from_two_futures(&future_a, &future_b).wait();
 
 		join_handle.join().unwrap();
 
 		// then drop the notifiers and make sure the future states are gone.
 		mem::drop(notifier_a);
 		mem::drop(notifier_b);
+		mem::drop(future_a);
+		mem::drop(future_b);
 
 		assert!(future_state_a.upgrade().is_none() && future_state_b.upgrade().is_none());
 	}
@@ -655,18 +664,18 @@ mod tests {
 		// Set both notifiers as woken without sleeping yet.
 		notifier_a.notify();
 		notifier_b.notify();
-		Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
+		Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 
 		// One future has woken us up, but the other should still have a pending notification.
-		Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
+		Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 
 		// However once we've slept twice, we should no longer have any pending notifications
-		assert!(!Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future())
+		assert!(!Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future())
 			.wait_timeout(Duration::from_millis(10)));
 
 		// Test ordering somewhat more.
 		notifier_a.notify();
-		Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
+		Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 	}
 
 	#[test]
@@ -684,7 +693,7 @@ mod tests {
 
 		// After sleeping one future (not guaranteed which one, however) will have its notification
 		// bit cleared.
-		Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
+		Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
 
 		// By registering a callback on the futures for both notifiers, one will complete
 		// immediately, but one will remain tied to the notifier, and will complete once the
@@ -703,8 +712,37 @@ mod tests {
 		notifier_b.notify();
 
 		assert!(callback_a.load(Ordering::SeqCst) && callback_b.load(Ordering::SeqCst));
-		Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future()).wait();
-		assert!(!Sleeper::from_two_futures(notifier_a.get_future(), notifier_b.get_future())
+		Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future()).wait();
+		assert!(!Sleeper::from_two_futures(&notifier_a.get_future(), &notifier_b.get_future())
 			.wait_timeout(Duration::from_millis(10)));
 	}
+
+	#[test]
+	#[cfg(feature = "std")]
+	fn multi_poll_stores_single_waker() {
+		// When a `Future` is `poll()`ed multiple times, only the last `Waker` should be called,
+		// but previously we'd store all `Waker`s until they're all woken at once. This tests a few
+		// cases to ensure `Future`s avoid storing an endless set of `Waker`s.
+		let notifier = Notifier::new();
+		let future_state = Arc::clone(&notifier.get_future().state);
+		assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 0);
+
+		// Test that simply polling a future twice doesn't result in two pending `Waker`s.
+		let mut future_a = notifier.get_future();
+		assert_eq!(Pin::new(&mut future_a).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
+		assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
+		assert_eq!(Pin::new(&mut future_a).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
+		assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
+
+		// If we poll a second future, however, that will store a second `Waker`.
+		let mut future_b = notifier.get_future();
+		assert_eq!(Pin::new(&mut future_b).poll(&mut Context::from_waker(&create_waker().1)), Poll::Pending);
+		assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 2);
+
+		// but when we drop the `Future`s, the pending Wakers will also be dropped.
+		mem::drop(future_a);
+		assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 1);
+		mem::drop(future_b);
+		assert_eq!(future_state.lock().unwrap().std_future_callbacks.len(), 0);
+	}
 }