BABE slot and epoch event notifications (#6563)

* BabeWorker -> BabeSlotWorker

* SlotWorker::notify_slot: similar to claim_slot, but called no matter authoring

* Wrap the future with a new struct BabeWorker

* Add type definition slot_notification_sinks

* Function slot_notification_streams for the receiver side

* Get a handle of slot_notification_sinks in BabeSlotWorker

* Implement notify_slot

* Switch to use bounded mpsc

* Do not drop the sink when channel is full

Only skip sending the message and emit a warning, because it is recoverable.

* Fix future type bounds

* Add must_use and sink type alias

substrate/Cargo.lock

@@ -6389,6 +6389,7 @@ dependencies = [
  "pdqselect",
  "rand 0.7.3",
  "rand_chacha 0.2.2",
+ "retain_mut",
  "sc-block-builder",
  "sc-client-api",
  "sc-consensus-epochs",
@@ -6415,6 +6416,7 @@ dependencies = [
  "sp-keyring",
  "sp-runtime",
  "sp-timestamp",
+ "sp-utils",
  "sp-version",
  "substrate-prometheus-endpoint",
  "substrate-test-runtime-client",

substrate/client/consensus/babe/Cargo.toml

@@ -37,6 +37,7 @@ sp-consensus-vrf = { version = "0.8.0-rc5", path = "../../../primitives/consensu
 sc-consensus-uncles = { version = "0.8.0-rc5", path = "../uncles" }
 sc-consensus-slots = { version = "0.8.0-rc5", path = "../slots" }
 sp-runtime = { version = "2.0.0-rc5", path = "../../../primitives/runtime" }
+sp-utils = { version = "2.0.0-rc5", path = "../../../primitives/utils" }
 fork-tree = { version = "2.0.0-rc5", path = "../../../utils/fork-tree" }
 prometheus-endpoint = { package = "substrate-prometheus-endpoint", path = "../../../utils/prometheus", version = "0.8.0-rc5"}
 futures = "0.3.4"
@@ -48,6 +49,7 @@ rand = "0.7.2"
 merlin = "2.0"
 pdqselect = "0.1.0"
 derive_more = "0.99.2"
+retain_mut = "0.1.1"
 
 [dev-dependencies]
 sp-keyring = { version = "2.0.0-rc5", path = "../../../primitives/keyring" }

substrate/client/consensus/babe/src/lib.rs

@@ -106,6 +106,8 @@ use sc_client_api::{
 	BlockchainEvents, ProvideUncles,
 };
 use sp_block_builder::BlockBuilder as BlockBuilderApi;
+use futures::channel::mpsc::{channel, Sender, Receiver};
+use retain_mut::RetainMut;
 
 use futures::prelude::*;
 use log::{debug, info, log, trace, warn};
@@ -370,7 +372,7 @@ pub fn start_babe<B, C, SC, E, I, SO, CAW, Error>(BabeParams {
 	babe_link,
 	can_author_with,
 }: BabeParams<B, C, E, I, SO, SC, CAW>) -> Result<
-	impl futures::Future<Output=()>,
+	BabeWorker<B>,
 	sp_consensus::Error,
 > where
 	B: BlockT,
@@ -378,16 +380,18 @@ pub fn start_babe<B, C, SC, E, I, SO, CAW, Error>(BabeParams {
 		+ HeaderBackend<B> + HeaderMetadata<B, Error = ClientError> + Send + Sync + 'static,
 	C::Api: BabeApi<B>,
 	SC: SelectChain<B> + 'static,
-	E: Environment<B, Error = Error> + Send + Sync,
+	E: Environment<B, Error = Error> + Send + Sync + 'static,
 	E::Proposer: Proposer<B, Error = Error, Transaction = sp_api::TransactionFor<C, B>>,
 	I: BlockImport<B, Error = ConsensusError, Transaction = sp_api::TransactionFor<C, B>> + Send
 		+ Sync + 'static,
 	Error: std::error::Error + Send + From<ConsensusError> + From<I::Error> + 'static,
-	SO: SyncOracle + Send + Sync + Clone,
-	CAW: CanAuthorWith<B> + Send,
+	SO: SyncOracle + Send + Sync + Clone + 'static,
+	CAW: CanAuthorWith<B> + Send + 'static,
 {
 	let config = babe_link.config;
-	let worker = BabeWorker {
+	let slot_notification_sinks = Arc::new(Mutex::new(Vec::new()));
+
+	let worker = BabeSlotWorker {
 		client: client.clone(),
 		block_import: Arc::new(Mutex::new(block_import)),
 		env,
@@ -395,6 +399,7 @@ pub fn start_babe<B, C, SC, E, I, SO, CAW, Error>(BabeParams {
 		force_authoring,
 		keystore,
 		epoch_changes: babe_link.epoch_changes.clone(),
+		slot_notification_sinks: slot_notification_sinks.clone(),
 		config: config.clone(),
 	};
 
@@ -406,7 +411,7 @@ pub fn start_babe<B, C, SC, E, I, SO, CAW, Error>(BabeParams {
 	)?;
 
 	info!(target: "babe", "👶 Starting BABE Authorship worker");
-	Ok(sc_consensus_slots::start_slot_worker(
+	let inner = sc_consensus_slots::start_slot_worker(
 		config.0,
 		select_chain,
 		worker,
@@ -414,10 +419,49 @@ pub fn start_babe<B, C, SC, E, I, SO, CAW, Error>(BabeParams {
 		inherent_data_providers,
 		babe_link.time_source,
 		can_author_with,
-	))
+	);
+	Ok(BabeWorker {
+		inner: Box::pin(inner),
+		slot_notification_sinks,
+	})
 }
 
-struct BabeWorker<B: BlockT, C, E, I, SO> {
+/// Worker for Babe which implements `Future<Output=()>`. This must be polled.
+#[must_use]
+pub struct BabeWorker<B: BlockT> {
+	inner: Pin<Box<dyn futures::Future<Output=()> + Send + 'static>>,
+	slot_notification_sinks: Arc<Mutex<Vec<Sender<(u64, ViableEpochDescriptor<B::Hash, NumberFor<B>, Epoch>)>>>>,
+}
+
+impl<B: BlockT> BabeWorker<B> {
+	/// Return an event stream of notifications for when new slot happens, and the corresponding
+	/// epoch descriptor.
+	pub fn slot_notification_stream(
+		&self
+	) -> Receiver<(u64, ViableEpochDescriptor<B::Hash, NumberFor<B>, Epoch>)> {
+		const CHANNEL_BUFFER_SIZE: usize = 1024;
+
+		let (sink, stream) = channel(CHANNEL_BUFFER_SIZE);
+		self.slot_notification_sinks.lock().push(sink);
+		stream
+	}
+}
+
+impl<B: BlockT> futures::Future for BabeWorker<B> {
+	type Output = ();
+
+	fn poll(
+		mut self: Pin<&mut Self>,
+		cx: &mut futures::task::Context
+	) -> futures::task::Poll<Self::Output> {
+		self.inner.as_mut().poll(cx)
+	}
+}
+
+/// Slot notification sinks.
+type SlotNotificationSinks<B> = Arc<Mutex<Vec<Sender<(u64, ViableEpochDescriptor<<B as BlockT>::Hash, NumberFor<B>, Epoch>)>>>>;
+
+struct BabeSlotWorker<B: BlockT, C, E, I, SO> {
 	client: Arc<C>,
 	block_import: Arc<Mutex<I>>,
 	env: E,
@@ -425,10 +469,11 @@ struct BabeWorker<B: BlockT, C, E, I, SO> {
 	force_authoring: bool,
 	keystore: KeyStorePtr,
 	epoch_changes: SharedEpochChanges<B, Epoch>,
+	slot_notification_sinks: SlotNotificationSinks<B>,
 	config: Config,
 }
 
-impl<B, C, E, I, Error, SO> sc_consensus_slots::SimpleSlotWorker<B> for BabeWorker<B, C, E, I, SO> where
+impl<B, C, E, I, Error, SO> sc_consensus_slots::SimpleSlotWorker<B> for BabeSlotWorker<B, C, E, I, SO> where
 	B: BlockT,
 	C: ProvideRuntimeApi<B> +
 		ProvideCache<B> +
@@ -502,6 +547,28 @@ impl<B, C, E, I, Error, SO> sc_consensus_slots::SimpleSlotWorker<B> for BabeWork
 		s
 	}
 
+	fn notify_slot(
+		&self,
+		_parent_header: &B::Header,
+		slot_number: SlotNumber,
+		epoch_descriptor: &ViableEpochDescriptor<B::Hash, NumberFor<B>, Epoch>,
+	) {
+		self.slot_notification_sinks.lock()
+			.retain_mut(|sink| {
+				match sink.try_send((slot_number, epoch_descriptor.clone())) {
+					Ok(()) => true,
+					Err(e) => {
+						if e.is_full() {
+							warn!(target: "babe", "Trying to notify a slot but the channel is full");
+							true
+						} else {
+							false
+						}
+					},
+				}
+			});
+	}
+
 	fn pre_digest_data(
 		&self,
 		_slot_number: u64,
@@ -599,7 +666,7 @@ impl<B, C, E, I, Error, SO> sc_consensus_slots::SimpleSlotWorker<B> for BabeWork
 	}
 }
 
-impl<B, C, E, I, Error, SO> SlotWorker<B> for BabeWorker<B, C, E, I, SO> where
+impl<B, C, E, I, Error, SO> SlotWorker<B> for BabeSlotWorker<B, C, E, I, SO> where
 	B: BlockT,
 	C: ProvideRuntimeApi<B> +
 		ProvideCache<B> +

substrate/client/consensus/slots/src/lib.rs

@@ -104,6 +104,15 @@ pub trait SimpleSlotWorker<B: BlockT> {
 		epoch_data: &Self::EpochData,
 	) -> Option<Self::Claim>;
 
+	/// Notifies the given slot. Similar to `claim_slot`, but will be called no matter whether we
+	/// need to author blocks or not.
+	fn notify_slot(
+		&self,
+		_header: &B::Header,
+		_slot_number: u64,
+		_epoch_data: &Self::EpochData,
+	) { }
+
 	/// Return the pre digest data to include in a block authored with the given claim.
 	fn pre_digest_data(
 		&self,
@@ -191,6 +200,8 @@ pub trait SimpleSlotWorker<B: BlockT> {
 			}
 		};
 
+		self.notify_slot(&chain_head, slot_number, &epoch_data);
+
 		let authorities_len = self.authorities_len(&epoch_data);
 
 		if !self.force_authoring() &&