From 2c987209f9fdeeebd7f2f9f13fd6fee97542ce1b Mon Sep 17 00:00:00 2001
From: Matt Corallo <git@bluematt.me>
Date: Tue, 13 Feb 2024 21:58:46 +0000
Subject: [PATCH] Split lists of `Waker` and directly-registered `Future`
 callbacks

In the next commit we'll fix a memory leak due to keeping too many
`std::task::Waker` callbacks in `FutureState` from redundant `poll`
calls, but first we need to split handling of `StdWaker`-based
future wake callbacks from normal ones, which we do here.
---
 lightning/src/util/wakers.rs | 36 ++++++++++++++++++++----------------
 1 file changed, 20 insertions(+), 16 deletions(-)

diff --git a/lightning/src/util/wakers.rs b/lightning/src/util/wakers.rs
index 14e6bbe6..d22932ac 100644
--- a/lightning/src/util/wakers.rs
+++ b/lightning/src/util/wakers.rs
@@ -69,6 +69,7 @@ impl Notifier {
 		} else {
 			let state = Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
+				std_future_callbacks: Vec::new(),
 				callbacks_with_state: Vec::new(),
 				complete: lock.0,
 				callbacks_made: false,
@@ -109,11 +110,13 @@ define_callback!(Send);
 define_callback!();
 
 pub(crate) struct FutureState {
-	// 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>)>,
-	callbacks_with_state: Vec<(bool, Box<dyn Fn(&Arc<Mutex<FutureState>>) -> () + Send>)>,
+	// `callbacks` count as having woken the users' code (as they go direct to the user), but
+	// `std_future_callbacks` and `callbacks_with_state` do not (as the first just wakes a future,
+	// we only count it after another `poll()` and the second wakes a `Sleeper` which handles
+	// setting `callbacks_made` itself).
+	callbacks: Vec<Box<dyn FutureCallback>>,
+	std_future_callbacks: Vec<StdWaker>,
+	callbacks_with_state: Vec<Box<dyn Fn(&Arc<Mutex<FutureState>>) -> () + Send>>,
 	complete: bool,
 	callbacks_made: bool,
 }
@@ -121,13 +124,15 @@ pub(crate) struct FutureState {
 fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
 	let mut state_lock = this.lock().unwrap();
 	let state = &mut *state_lock;
-	for (counts_as_call, callback) in state.callbacks.drain(..) {
+	for callback in state.callbacks.drain(..) {
 		callback.call();
-		state.callbacks_made |= counts_as_call;
+		state.callbacks_made = true;
 	}
-	for (counts_as_call, callback) in state.callbacks_with_state.drain(..) {
+	for waker in state.std_future_callbacks.drain(..) {
+		waker.0.wake_by_ref();
+	}
+	for callback in state.callbacks_with_state.drain(..) {
 		(callback)(this);
-		state.callbacks_made |= counts_as_call;
 	}
 	state.complete = true;
 	state.callbacks_made
@@ -153,7 +158,7 @@ impl Future {
 			mem::drop(state);
 			callback.call();
 		} else {
-			state.callbacks.push((true, callback));
+			state.callbacks.push(callback);
 		}
 	}
 
@@ -193,9 +198,6 @@ impl Future {
 
 use core::task::Waker;
 struct StdWaker(pub Waker);
-impl FutureCallback for StdWaker {
-	fn call(&self) { self.0.wake_by_ref() }
-}
 
 /// This is not exported to bindings users as Rust Futures aren't usable in language bindings.
 impl<'a> StdFuture for Future {
@@ -208,7 +210,7 @@ impl<'a> StdFuture for Future {
 			Poll::Ready(())
 		} else {
 			let waker = cx.waker().clone();
-			state.callbacks.push((false, Box::new(StdWaker(waker))));
+			state.std_future_callbacks.push(StdWaker(waker));
 			Poll::Pending
 		}
 	}
@@ -251,10 +253,10 @@ impl Sleeper {
 					*notified_fut_mtx.lock().unwrap() = Some(Arc::clone(&notifier_mtx));
 					break;
 				}
-				notifier.callbacks_with_state.push((false, Box::new(move |notifier_ref| {
+				notifier.callbacks_with_state.push(Box::new(move |notifier_ref| {
 					*notified_fut_ref.lock().unwrap() = Some(Arc::clone(notifier_ref));
 					cv_ref.notify_all();
-				})));
+				}));
 			}
 		}
 		(cv, notified_fut_mtx)
@@ -455,6 +457,7 @@ mod tests {
 		let future = Future {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
+				std_future_callbacks: Vec::new(),
 				callbacks_with_state: Vec::new(),
 				complete: false,
 				callbacks_made: false,
@@ -514,6 +517,7 @@ mod tests {
 		let mut future = Future {
 			state: Arc::new(Mutex::new(FutureState {
 				callbacks: Vec::new(),
+				std_future_callbacks: Vec::new(),
 				callbacks_with_state: Vec::new(),
 				complete: false,
 				callbacks_made: false,
-- 
2.30.2