Give Futures for a FutureState an idx and track StdWaker idxn

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 take a step towards fixing this by giving each `Future` a
unique index and storing which `Future` an `StdWaker` came from in
the callback list. This sets us up to deduplicate `StdWaker`s by
`Future`s in the next commit.

Author: TheBlueMatt

lightning/src/util/wakers.rs

@@ -56,26 +56,33 @@ 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();
+		let mut self_idx = 0;
 		if let Some(existing_state) = &lock.1 {
-			if existing_state.lock().unwrap().callbacks_made {
+			let mut locked = existing_state.lock().unwrap();
+			if locked.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.
+				mem::drop(locked);
 				lock.1.take();
 				lock.0 = false;
+			} else {
+				self_idx = locked.next_idx;
+				locked.next_idx += 1;
 			}
 		}
 		if let Some(existing_state) = &lock.1 {
-			Future { state: Arc::clone(&existing_state) }
+			Future { state: Arc::clone(&existing_state), self_idx }
 		} 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,
+				next_idx: 1,
 			}));
 			lock.1 = Some(Arc::clone(&state));
-			Future { state }
+			Future { state, self_idx: 0 }
 		}
 	}
 
@@ -115,10 +122,11 @@ pub(crate) struct FutureState {
 	// 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>,
+	std_future_callbacks: Vec<(usize, StdWaker)>,
 	callbacks_with_state: Vec<Box<dyn Fn(&Arc<Mutex<FutureState>>) -> () + Send>>,
 	complete: bool,
 	callbacks_made: bool,
+	next_idx: usize,
 }
 
 fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
@@ -128,7 +136,7 @@ fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
 		callback.call();
 		state.callbacks_made = true;
 	}
-	for waker in state.std_future_callbacks.drain(..) {
+	for (_, waker) in state.std_future_callbacks.drain(..) {
 		waker.0.wake_by_ref();
 	}
 	for callback in state.callbacks_with_state.drain(..) {
@@ -139,11 +147,9 @@ fn complete_future(this: &Arc<Mutex<FutureState>>) -> bool {
 }
 
 /// A simple future which can complete once, and calls some callback(s) when it does so.
-///
-/// Clones can be made and all futures cloned from the same source will complete at the same time.
-#[derive(Clone)]
 pub struct Future {
 	state: Arc<Mutex<FutureState>>,
+	self_idx: usize,
 }
 
 impl Future {
@@ -210,7 +216,7 @@ impl<'a> StdFuture for Future {
 			Poll::Ready(())
 		} else {
 			let waker = cx.waker().clone();
-			state.std_future_callbacks.push(StdWaker(waker));
+			state.std_future_callbacks.push((self.self_idx, StdWaker(waker)));
 			Poll::Pending
 		}
 	}
@@ -461,7 +467,9 @@ mod tests {
 				callbacks_with_state: Vec::new(),
 				complete: false,
 				callbacks_made: false,
-			}))
+				next_idx: 1,
+			})),
+			self_idx: 0,
 		};
 		let callback = Arc::new(AtomicBool::new(false));
 		let callback_ref = Arc::clone(&callback);
@@ -478,10 +486,13 @@ 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,
-			}))
+				next_idx: 1,
+			})),
+			self_idx: 0,
 		};
 		complete_future(&future.state);
 
@@ -521,9 +532,11 @@ mod tests {
 				callbacks_with_state: Vec::new(),
 				complete: false,
 				callbacks_made: false,
-			}))
+				next_idx: 2,
+			})),
+			self_idx: 0,
 		};
-		let mut second_future = Future { state: Arc::clone(&future.state) };
+		let mut second_future = Future { state: Arc::clone(&future.state), self_idx: 1 };
 
 		let (woken, waker) = create_waker();
 		assert_eq!(Pin::new(&mut future).poll(&mut Context::from_waker(&waker)), Poll::Pending);