Wake the background processor if an async monitor update completes

If the `ChainMonitor` gets an async monitor update completion, this
means the `ChannelManager` needs to be polled for event processing.
Here we wake it using the new multi-`Future`-await `Sleeper`, or
the existing `select` block in the async BP.

Fixes #2052.

lightning-background-processor/src/lib.rs

@@ -38,6 +38,8 @@ use lightning::routing::router::Router;
 use lightning::routing::scoring::{Score, WriteableScore};
 use lightning::util::logger::Logger;
 use lightning::util::persist::Persister;
+#[cfg(feature = "std")]
+use lightning::util::wakers::Sleeper;
 use lightning_rapid_gossip_sync::RapidGossipSync;
 
 use core::ops::Deref;
@@ -388,15 +390,20 @@ pub(crate) mod futures_util {
 	use core::task::{Poll, Waker, RawWaker, RawWakerVTable};
 	use core::pin::Pin;
 	use core::marker::Unpin;
-	pub(crate) struct Selector<A: Future<Output=()> + Unpin, B: Future<Output=bool> + Unpin> {
+	pub(crate) struct Selector<
+		A: Future<Output=()> + Unpin, B: Future<Output=()> + Unpin, C: Future<Output=bool> + Unpin
+	> {
 		pub a: A,
 		pub b: B,
+		pub c: C,
 	}
 	pub(crate) enum SelectorOutput {
-		A, B(bool),
+		A, B, C(bool),
 	}
 
-	impl<A: Future<Output=()> + Unpin, B: Future<Output=bool> + Unpin> Future for Selector<A, B> {
+	impl<
+		A: Future<Output=()> + Unpin, B: Future<Output=()> + Unpin, C: Future<Output=bool> + Unpin
+	> Future for Selector<A, B, C> {
 		type Output = SelectorOutput;
 		fn poll(mut self: Pin<&mut Self>, ctx: &mut core::task::Context<'_>) -> Poll<SelectorOutput> {
 			match Pin::new(&mut self.a).poll(ctx) {
@@ -404,7 +411,11 @@ pub(crate) mod futures_util {
 				Poll::Pending => {},
 			}
 			match Pin::new(&mut self.b).poll(ctx) {
-				Poll::Ready(res) => { return Poll::Ready(SelectorOutput::B(res)); },
+				Poll::Ready(()) => { return Poll::Ready(SelectorOutput::B); },
+				Poll::Pending => {},
+			}
+			match Pin::new(&mut self.c).poll(ctx) {
+				Poll::Ready(res) => { return Poll::Ready(SelectorOutput::C(res)); },
 				Poll::Pending => {},
 			}
 			Poll::Pending
@@ -514,11 +525,13 @@ where
 		gossip_sync, peer_manager, logger, scorer, should_break, {
 			let fut = Selector {
 				a: channel_manager.get_persistable_update_future(),
-				b: sleeper(Duration::from_millis(100)),
+				b: chain_monitor.get_update_future(),
+				c: sleeper(Duration::from_millis(100)),
 			};
 			match fut.await {
 				SelectorOutput::A => true,
-				SelectorOutput::B(exit) => {
+				SelectorOutput::B => false,
+				SelectorOutput::C(exit) => {
 					should_break = exit;
 					false
 				}
@@ -643,7 +656,10 @@ impl BackgroundProcessor {
 			define_run_body!(persister, chain_monitor, chain_monitor.process_pending_events(&event_handler),
 				channel_manager, channel_manager.process_pending_events(&event_handler),
 				gossip_sync, peer_manager, logger, scorer, stop_thread.load(Ordering::Acquire),
-				channel_manager.get_persistable_update_future().wait_timeout(Duration::from_millis(100)),
+				Sleeper::from_two_futures(
+					channel_manager.get_persistable_update_future(),
+					chain_monitor.get_update_future()
+				).wait_timeout(Duration::from_millis(100)),
 				|_| Instant::now(), |time: &Instant, dur| time.elapsed().as_secs() > dur)
 		});
 		Self { stop_thread: stop_thread_clone, thread_handle: Some(handle) }

lightning/src/chain/chainmonitor.rs

@@ -37,6 +37,7 @@ use crate::events::{Event, EventHandler};
 use crate::util::atomic_counter::AtomicCounter;
 use crate::util::logger::Logger;
 use crate::util::errors::APIError;
+use crate::util::wakers::{Future, Notifier};
 use crate::ln::channelmanager::ChannelDetails;
 
 use crate::prelude::*;
@@ -240,6 +241,8 @@ pub struct ChainMonitor<ChannelSigner: WriteableEcdsaChannelSigner, C: Deref, T:
 	pending_monitor_events: Mutex<Vec<(OutPoint, Vec<MonitorEvent>, Option<PublicKey>)>>,
 	/// The best block height seen, used as a proxy for the passage of time.
 	highest_chain_height: AtomicUsize,
+
+	event_notifier: Notifier,
 }
 
 impl<ChannelSigner: WriteableEcdsaChannelSigner, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref> ChainMonitor<ChannelSigner, C, T, F, L, P>
@@ -300,6 +303,7 @@ where C::Target: chain::Filter,
 					ChannelMonitorUpdateStatus::PermanentFailure => {
 						monitor_state.channel_perm_failed.store(true, Ordering::Release);
 						self.pending_monitor_events.lock().unwrap().push((*funding_outpoint, vec![MonitorEvent::UpdateFailed(*funding_outpoint)], monitor.get_counterparty_node_id()));
+						self.event_notifier.notify();
 					},
 					ChannelMonitorUpdateStatus::InProgress => {
 						log_debug!(self.logger, "Channel Monitor sync for channel {} in progress, holding events until completion!", log_funding_info!(monitor));
@@ -345,6 +349,7 @@ where C::Target: chain::Filter,
 			persister,
 			pending_monitor_events: Mutex::new(Vec::new()),
 			highest_chain_height: AtomicUsize::new(0),
+			event_notifier: Notifier::new(),
 		}
 	}
 
@@ -472,6 +477,7 @@ where C::Target: chain::Filter,
 				}
 			},
 		}
+		self.event_notifier.notify();
 		Ok(())
 	}
 
@@ -486,6 +492,7 @@ where C::Target: chain::Filter,
 			funding_txo,
 			monitor_update_id,
 		}], counterparty_node_id));
+		self.event_notifier.notify();
 	}
 
 	#[cfg(any(test, fuzzing, feature = "_test_utils"))]
@@ -514,6 +521,18 @@ where C::Target: chain::Filter,
 			handler(event).await;
 		}
 	}
+
+	/// Gets a [`Future`] that completes when an event is available either via
+	/// [`chain::Watch::release_pending_monitor_events`] or
+	/// [`EventsProvider::process_pending_events`].
+	///
+	/// Note that callbacks registered on the [`Future`] MUST NOT call back into this
+	/// [`ChainMonitor`] and should instead register actions to be taken later.
+	///
+	/// [`EventsProvider::process_pending_events`]: crate::events::EventsProvider::process_pending_events
+	pub fn get_update_future(&self) -> Future {
+		self.event_notifier.get_future()
+	}
 }
 
 impl<ChannelSigner: WriteableEcdsaChannelSigner, C: Deref, T: Deref, F: Deref, L: Deref, P: Deref>