Introduce next_event_async allowing to poll event queue

bindings/kotlin/ldk-node-android/lib/build.gradle.kts

@@ -43,6 +43,7 @@ android {
 dependencies {
     implementation("net.java.dev.jna:jna:5.12.0@aar")
     implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk7")
+    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4")
     implementation("androidx.appcompat:appcompat:1.4.0")
     implementation("androidx.core:core-ktx:1.7.0")
     api("org.slf4j:slf4j-api:1.7.30")

bindings/kotlin/ldk-node-jvm/lib/build.gradle.kts

@@ -46,6 +46,7 @@ dependencies {
 
     // Use the Kotlin JDK 8 standard library.
     implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
+    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4")
 
     implementation("net.java.dev.jna:jna:5.12.0")
 }

bindings/ldk_node.udl

@@ -43,6 +43,8 @@ interface LDKNode {
 	void stop();
 	Event? next_event();
 	Event wait_next_event();
+	[Async]
+	Event next_event_async();
 	void event_handled();
 	PublicKey node_id();
 	sequence<SocketAddress>? listening_addresses();

src/event.rs

@@ -26,6 +26,8 @@ use lightning::util::ser::{Readable, ReadableArgs, Writeable, Writer};
 use bitcoin::blockdata::locktime::absolute::LockTime;
 use bitcoin::secp256k1::PublicKey;
 use bitcoin::OutPoint;
+use core::future::Future;
+use core::task::{Poll, Waker};
 use rand::{thread_rng, Rng};
 use std::collections::VecDeque;
 use std::ops::Deref;
@@ -125,7 +127,8 @@ pub struct EventQueue<K: KVStore + Sync + Send, L: Deref>
 where
 	L::Target: Logger,
 {
-	queue: Mutex<VecDeque<Event>>,
+	queue: Arc<Mutex<VecDeque<Event>>>,
+	waker: Arc<Mutex<Option<Waker>>>,
 	notifier: Condvar,
 	kv_store: Arc<K>,
 	logger: L,
@@ -136,9 +139,10 @@ where
 	L::Target: Logger,
 {
 	pub(crate) fn new(kv_store: Arc<K>, logger: L) -> Self {
-		let queue: Mutex<VecDeque<Event>> = Mutex::new(VecDeque::new());
+		let queue = Arc::new(Mutex::new(VecDeque::new()));
+		let waker = Arc::new(Mutex::new(None));
 		let notifier = Condvar::new();
-		Self { queue, notifier, kv_store, logger }
+		Self { queue, waker, notifier, kv_store, logger }
 	}
 
 	pub(crate) fn add_event(&self, event: Event) -> Result<(), Error> {
@@ -149,6 +153,10 @@ where
 		}
 
 		self.notifier.notify_one();
+
+		if let Some(waker) = self.waker.lock().unwrap().take() {
+			waker.wake();
+		}
 		Ok(())
 	}
 
@@ -157,6 +165,10 @@ where
 		locked_queue.front().map(|e| e.clone())
 	}
 
+	pub(crate) async fn next_event_async(&self) -> Event {
+		EventFuture { event_queue: Arc::clone(&self.queue), waker: Arc::clone(&self.waker) }.await
+	}
+
 	pub(crate) fn wait_next_event(&self) -> Event {
 		let locked_queue =
 			self.notifier.wait_while(self.queue.lock().unwrap(), |queue| queue.is_empty()).unwrap();
@@ -170,6 +182,10 @@ where
 			self.persist_queue(&locked_queue)?;
 		}
 		self.notifier.notify_one();
+
+		if let Some(waker) = self.waker.lock().unwrap().take() {
+			waker.wake();
+		}
 		Ok(())
 	}
 
@@ -207,9 +223,10 @@ where
 	) -> Result<Self, lightning::ln::msgs::DecodeError> {
 		let (kv_store, logger) = args;
 		let read_queue: EventQueueDeserWrapper = Readable::read(reader)?;
-		let queue: Mutex<VecDeque<Event>> = Mutex::new(read_queue.0);
+		let queue = Arc::new(Mutex::new(read_queue.0));
+		let waker = Arc::new(Mutex::new(None));
 		let notifier = Condvar::new();
-		Ok(Self { queue, notifier, kv_store, logger })
+		Ok(Self { queue, waker, notifier, kv_store, logger })
 	}
 }
 
@@ -240,6 +257,26 @@ impl Writeable for EventQueueSerWrapper<'_> {
 	}
 }
 
+struct EventFuture {
+	event_queue: Arc<Mutex<VecDeque<Event>>>,
+	waker: Arc<Mutex<Option<Waker>>>,
+}
+
+impl Future for EventFuture {
+	type Output = Event;
+
+	fn poll(
+		self: core::pin::Pin<&mut Self>, cx: &mut core::task::Context<'_>,
+	) -> core::task::Poll<Self::Output> {
+		if let Some(event) = self.event_queue.lock().unwrap().front() {
+			Poll::Ready(event.clone())
+		} else {
+			*self.waker.lock().unwrap() = Some(cx.waker().clone());
+			Poll::Pending
+		}
+	}
+}
+
 pub(crate) struct EventHandler<K: KVStore + Sync + Send, L: Deref>
 where
 	L::Target: Logger,
@@ -796,12 +833,14 @@ where
 mod tests {
 	use super::*;
 	use lightning::util::test_utils::{TestLogger, TestStore};
+	use std::sync::atomic::{AtomicU16, Ordering};
+	use std::time::Duration;
 
-	#[test]
-	fn event_queue_persistence() {
+	#[tokio::test]
+	async fn event_queue_persistence() {
 		let store = Arc::new(TestStore::new(false));
 		let logger = Arc::new(TestLogger::new());
-		let event_queue = EventQueue::new(Arc::clone(&store), Arc::clone(&logger));
+		let event_queue = Arc::new(EventQueue::new(Arc::clone(&store), Arc::clone(&logger)));
 		assert_eq!(event_queue.next_event(), None);
 
 		let expected_event = Event::ChannelReady {
@@ -814,6 +853,7 @@ mod tests {
 		// Check we get the expected event and that it is returned until we mark it handled.
 		for _ in 0..5 {
 			assert_eq!(event_queue.wait_next_event(), expected_event);
+			assert_eq!(event_queue.next_event_async().await, expected_event);
 			assert_eq!(event_queue.next_event(), Some(expected_event.clone()));
 		}
 
@@ -832,4 +872,96 @@ mod tests {
 		event_queue.event_handled().unwrap();
 		assert_eq!(event_queue.next_event(), None);
 	}
+
+	#[tokio::test]
+	async fn event_queue_concurrency() {
+		let store = Arc::new(TestStore::new(false));
+		let logger = Arc::new(TestLogger::new());
+		let event_queue = Arc::new(EventQueue::new(Arc::clone(&store), Arc::clone(&logger)));
+		assert_eq!(event_queue.next_event(), None);
+
+		let expected_event = Event::ChannelReady {
+			channel_id: ChannelId([23u8; 32]),
+			user_channel_id: UserChannelId(2323),
+			counterparty_node_id: None,
+		};
+
+		// Check `next_event_async` won't return if the queue is empty and always rather timeout.
+		tokio::select! {
+			_ = tokio::time::sleep(Duration::from_secs(1)) => {
+				// Timeout
+			}
+			_ = event_queue.next_event_async() => {
+				panic!();
+			}
+		}
+
+		assert_eq!(event_queue.next_event(), None);
+		// Check we get the expected number of events when polling/enqueuing concurrently.
+		let enqueued_events = AtomicU16::new(0);
+		let received_events = AtomicU16::new(0);
+		let mut delayed_enqueue = false;
+
+		for _ in 0..25 {
+			event_queue.add_event(expected_event.clone()).unwrap();
+			enqueued_events.fetch_add(1, Ordering::SeqCst);
+		}
+
+		loop {
+			tokio::select! {
+				_ = tokio::time::sleep(Duration::from_millis(10)), if !delayed_enqueue => {
+					event_queue.add_event(expected_event.clone()).unwrap();
+					enqueued_events.fetch_add(1, Ordering::SeqCst);
+					delayed_enqueue = true;
+				}
+				e = event_queue.next_event_async() => {
+					assert_eq!(e, expected_event);
+					event_queue.event_handled().unwrap();
+					received_events.fetch_add(1, Ordering::SeqCst);
+
+					event_queue.add_event(expected_event.clone()).unwrap();
+					enqueued_events.fetch_add(1, Ordering::SeqCst);
+				}
+				e = event_queue.next_event_async() => {
+					assert_eq!(e, expected_event);
+					event_queue.event_handled().unwrap();
+					received_events.fetch_add(1, Ordering::SeqCst);
+				}
+			}
+
+			if delayed_enqueue
+				&& received_events.load(Ordering::SeqCst) == enqueued_events.load(Ordering::SeqCst)
+			{
+				break;
+			}
+		}
+		assert_eq!(event_queue.next_event(), None);
+
+		// Check we operate correctly, even when mixing and matching blocking and async API calls.
+		let (tx, mut rx) = tokio::sync::watch::channel(());
+		let thread_queue = Arc::clone(&event_queue);
+		let thread_event = expected_event.clone();
+		std::thread::spawn(move || {
+			let e = thread_queue.wait_next_event();
+			assert_eq!(e, thread_event);
+			thread_queue.event_handled().unwrap();
+			tx.send(()).unwrap();
+		});
+
+		let thread_queue = Arc::clone(&event_queue);
+		let thread_event = expected_event.clone();
+		std::thread::spawn(move || {
+			// Sleep a bit before we enqueue the events everybody is waiting for.
+			std::thread::sleep(Duration::from_millis(20));
+			thread_queue.add_event(thread_event.clone()).unwrap();
+			thread_queue.add_event(thread_event.clone()).unwrap();
+		});
+
+		let e = event_queue.next_event_async().await;
+		assert_eq!(e, expected_event.clone());
+		event_queue.event_handled().unwrap();
+
+		rx.changed().await.unwrap();
+		assert_eq!(event_queue.next_event(), None);
+	}
 }

src/lib.rs

@@ -811,6 +811,15 @@ impl<K: KVStore + Sync + Send + 'static> Node<K> {
 		self.event_queue.next_event()
 	}
 
+	/// Returns the next event in the event queue.
+	///
+	/// Will asynchronously poll the event queue until the next event is ready.
+	///
+	/// **Note:** this will always return the same event until handling is confirmed via [`Node::event_handled`].
+	pub async fn next_event_async(&self) -> Event {
+		self.event_queue.next_event_async().await
+	}
+
 	/// Returns the next event in the event queue.
 	///
 	/// Will block the current thread until the next event is available.