Fix a leak in FutureState when a Notifier is dropped un-woken

If a `Notifier` has an internal `FutureState` which gathers some
sleeper callbacks, but is never actaully woken, those callbacks
will leak due to a circular `Arc` reference when the `Notifier` is
`drop`'d.

Because `Notifier`s are rarely `drop`'d in production this isn't a
huge deal, but shows up materially in bindings tests as they spawn
many nodes over the course of a short test.

Fixes #2232

Author: TheBlueMatt

lightning/src/util/wakers.rs

@@ -69,6 +69,7 @@ impl Notifier {
 		} else {
 			let state = Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
+				callbacks_with_state: Vec::new(),
 				complete: lock.0,
 				callbacks_made: false,
 			}));
@@ -112,6 +113,7 @@ pub(crate) struct FutureState {
 	// 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>)>,
+	callbacks_with_state: Vec<(bool, Box<dyn Fn(&Arc<Mutex<FutureState>>) -> () + Send>)>,
 	complete: bool,
 	callbacks_made: bool,
 }
@@ -123,6 +125,10 @@ fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
 		callback.call();
 		state.callbacks_made |= counts_as_call;
 	}
+	for (counts_as_call, callback) in state.callbacks_with_state.drain(..) {
+		(callback)(this);
+		state.callbacks_made |= counts_as_call;
+	}
 	state.complete = true;
 	state.callbacks_made
 }
@@ -240,14 +246,13 @@ impl Sleeper {
 			for notifier_mtx in self.notifiers.iter() {
 				let cv_ref = Arc::clone(&cv);
 				let notified_fut_ref = Arc::clone(&notified_fut_mtx);
-				let notifier_ref = Arc::clone(&notifier_mtx);
 				let mut notifier = notifier_mtx.lock().unwrap();
 				if notifier.complete {
-					*notified_fut_mtx.lock().unwrap() = Some(notifier_ref);
+					*notified_fut_mtx.lock().unwrap() = Some(Arc::clone(&notifier_mtx));
 					break;
 				}
-				notifier.callbacks.push((false, Box::new(move || {
-					*notified_fut_ref.lock().unwrap() = Some(Arc::clone(&notifier_ref));
+				notifier.callbacks_with_state.push((false, Box::new(move |notifier_ref| {
+					*notified_fut_ref.lock().unwrap() = Some(Arc::clone(notifier_ref));
 					cv_ref.notify_all();
 				})));
 			}
@@ -407,11 +412,50 @@ mod tests {
 		}
 	}
 
+	#[cfg(feature = "std")]
+	#[test]
+	fn test_state_drops() {
+		// Previously, there was a leak if a `Notifier` was `drop`ed without ever being notified
+		// but after having been slept-on. This tests for that leak.
+		use crate::sync::Arc;
+		use std::thread;
+
+		let notifier_a = Arc::new(Notifier::new());
+		let notifier_b = Arc::new(Notifier::new());
+
+		let thread_notifier_a = Arc::clone(&notifier_a);
+
+		let future_a = notifier_a.get_future();
+		let future_state_a = Arc::downgrade(&future_a.state);
+
+		let future_b = notifier_b.get_future();
+		let future_state_b = Arc::downgrade(&future_b.state);
+
+		let join_handle = thread::spawn(move || {
+			// Let the other thread get to the wait point, then notify it.
+			std::thread::sleep(Duration::from_millis(50));
+			thread_notifier_a.notify();
+		});
+
+		// 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();
+
+		join_handle.join().unwrap();
+
+		// then drop the notifiers and make sure the future states are gone.
+		mem::drop(notifier_a);
+		mem::drop(notifier_b);
+
+		assert!(future_state_a.upgrade().is_none() && future_state_b.upgrade().is_none());
+	}
+
 	#[test]
 	fn test_future_callbacks() {
 		let future = Future {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
+				callbacks_with_state: Vec::new(),
 				complete: false,
 				callbacks_made: false,
 			}))
@@ -431,6 +475,7 @@ mod tests {
 		let future = Future {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
+				callbacks_with_state: Vec::new(),
 				complete: false,
 				callbacks_made: false,
 			}))
@@ -469,6 +514,7 @@ mod tests {
 		let mut future = Future {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
+				callbacks_with_state: Vec::new(),
 				complete: false,
 				callbacks_made: false,
 			}))