grandpa: always create and send justification if there are any subscribers (#6935)

* grandpa: use bytes type for justification rpc notification

* grandpa: always create justification if there are rpc subscribers

* grandpa: wording

* grandpa: replace notify_justification macro with function

* grandpa: prefer Option<&T> over &Option<T>

substrate/client/finality-grandpa/rpc/Cargo.toml

@@ -8,9 +8,10 @@ edition = "2018"
 license = "GPL-3.0-or-later WITH Classpath-exception-2.0"
 
 [dependencies]
-sc-rpc = { version = "2.0.0-rc6", path = "../../rpc" }
-sp-runtime = { version = "2.0.0-rc6", path = "../../../primitives/runtime" }
 sc-finality-grandpa = { version = "0.8.0-rc6", path = "../" }
+sc-rpc = { version = "2.0.0-rc6", path = "../../rpc" }
+sp-core = { version = "2.0.0-rc6", path = "../../../primitives/core" }
+sp-runtime = { version = "2.0.0-rc6", path = "../../../primitives/runtime" }
 finality-grandpa = { version = "0.12.3", features = ["derive-codec"] }
 jsonrpc-core = "14.2.0"
 jsonrpc-core-client = "14.2.0"

substrate/client/finality-grandpa/rpc/src/lib.rs

@@ -406,7 +406,7 @@ mod tests {
 
 		// Notify with a header and justification
 		let justification = create_justification();
-		let _ = justification_sender.notify(justification.clone()).unwrap();
+		justification_sender.notify(|| Ok(justification.clone())).unwrap();
 
 		// Inspect what we received
 		let recv = receiver.take(1).wait().flatten().collect::<Vec<_>>();
@@ -418,7 +418,7 @@ mod tests {
 
 		let recv_sub_id: String =
 			serde_json::from_value(json_map["subscription"].take()).unwrap();
-		let recv_justification: Vec<u8> =
+		let recv_justification: sp_core::Bytes =
 			serde_json::from_value(json_map["result"].take()).unwrap();
 		let recv_justification: GrandpaJustification<Block> =
 			Decode::decode(&mut &recv_justification[..]).unwrap();

substrate/client/finality-grandpa/rpc/src/notification.rs

@@ -23,10 +23,10 @@ use sc_finality_grandpa::GrandpaJustification;
 
 /// An encoded justification proving that the given header has been finalized
 #[derive(Clone, Serialize, Deserialize)]
-pub struct JustificationNotification(Vec<u8>);
+pub struct JustificationNotification(sp_core::Bytes);
 
 impl<Block: BlockT> From<GrandpaJustification<Block>> for JustificationNotification {
 	fn from(notification: GrandpaJustification<Block>) -> Self {
-		JustificationNotification(notification.encode())
+		JustificationNotification(notification.encode().into())
 	}
 }

substrate/client/finality-grandpa/src/environment.rs

@@ -645,7 +645,8 @@ pub(crate) fn ancestry<Block: BlockT, Client>(
 	client: &Arc<Client>,
 	base: Block::Hash,
 	block: Block::Hash,
-) -> Result<Vec<Block::Hash>, GrandpaError> where
+) -> Result<Vec<Block::Hash>, GrandpaError>
+where
 	Client: HeaderMetadata<Block, Error = sp_blockchain::Error>,
 {
 	if base == block { return Err(GrandpaError::NotDescendent) }
@@ -671,15 +672,14 @@ pub(crate) fn ancestry<Block: BlockT, Client>(
 	Ok(tree_route.retracted().iter().skip(1).map(|e| e.hash).collect())
 }
 
-impl<B, Block: BlockT, C, N, SC, VR>
-	voter::Environment<Block::Hash, NumberFor<Block>>
-for Environment<B, Block, C, N, SC, VR>
+impl<B, Block: BlockT, C, N, SC, VR> voter::Environment<Block::Hash, NumberFor<Block>>
+	for Environment<B, Block, C, N, SC, VR>
 where
 	Block: 'static,
 	B: Backend<Block>,
 	C: crate::ClientForGrandpa<Block, B> + 'static,
 	C::Api: GrandpaApi<Block, Error = sp_blockchain::Error>,
- 	N: NetworkT<Block> + 'static + Send + Sync,
+	N: NetworkT<Block> + 'static + Send + Sync,
 	SC: SelectChain<Block> + 'static,
 	VR: VotingRule<Block, C>,
 	NumberFor<Block>: BlockNumberOps,
@@ -1023,7 +1023,7 @@ where
 			number,
 			(round, commit).into(),
 			false,
-			&self.justification_sender,
+			self.justification_sender.as_ref(),
 		)
 	}
 
@@ -1088,9 +1088,10 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 	number: NumberFor<Block>,
 	justification_or_commit: JustificationOrCommit<Block>,
 	initial_sync: bool,
-	justification_sender: &Option<GrandpaJustificationSender<Block>>,
-) -> Result<(), CommandOrError<Block::Hash, NumberFor<Block>>> where
-	Block:  BlockT,
+	justification_sender: Option<&GrandpaJustificationSender<Block>>,
+) -> Result<(), CommandOrError<Block::Hash, NumberFor<Block>>>
+where
+	Block: BlockT,
 	BE: Backend<Block>,
 	Client: crate::ClientForGrandpa<Block, BE>,
 {
@@ -1154,6 +1155,18 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 			}
 		}
 
+		// send a justification notification if a sender exists and in case of error log it.
+		fn notify_justification<Block: BlockT>(
+			justification_sender: Option<&GrandpaJustificationSender<Block>>,
+			justification: impl FnOnce() -> Result<GrandpaJustification<Block>, Error>,
+		) {
+			if let Some(sender) = justification_sender {
+				if let Err(err) = sender.notify(justification) {
+					warn!(target: "afg", "Error creating justification for subscriber: {:?}", err);
+				}
+			}
+		}
+
 		// NOTE: this code assumes that honest voters will never vote past a
 		// transition block, thus we don't have to worry about the case where
 		// we have a transition with `effective_block = N`, but we finalize
@@ -1161,7 +1174,10 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 		// justifications for transition blocks which will be requested by
 		// syncing clients.
 		let justification = match justification_or_commit {
-			JustificationOrCommit::Justification(justification) => Some(justification),
+			JustificationOrCommit::Justification(justification) => {
+				notify_justification(justification_sender, || Ok(justification.clone()));
+				Some(justification.encode())
+			},
 			JustificationOrCommit::Commit((round_number, commit)) => {
 				let mut justification_required =
 					// justification is always required when block that enacts new authorities
@@ -1181,29 +1197,31 @@ pub(crate) fn finalize_block<BE, Block, Client>(
 					}
 				}
 
-				if justification_required {
-					let justification = GrandpaJustification::from_commit(
-						&client,
-						round_number,
-						commit,
-					)?;
+				// NOTE: the code below is a bit more verbose because we
+				// really want to avoid creating a justification if it isn't
+				// needed (e.g. if there's no subscribers), and also to avoid
+				// creating it twice. depending on the vote tree for the round,
+				// creating a justification might require multiple fetches of
+				// headers from the database.
+				let justification = || GrandpaJustification::from_commit(
+					&client,
+					round_number,
+					commit,
+				);
 
-					Some(justification)
+				if justification_required {
+					let justification = justification()?;
+					notify_justification(justification_sender, || Ok(justification.clone()));
+
+					Some(justification.encode())
 				} else {
+					notify_justification(justification_sender, justification);
+
 					None
 				}
 			},
 		};
 
-		// Notify any registered listeners in case we have a justification
-		if let Some(sender) = justification_sender {
-			if let Some(ref justification) = justification {
-				let _ = sender.notify(justification.clone());
-			}
-		}
-
-		let justification = justification.map(|j| j.encode());
-
 		debug!(target: "afg", "Finalizing blocks up to ({:?}, {})", number, hash);
 
 		// ideally some handle to a synchronization oracle would be used

substrate/client/finality-grandpa/src/import.rs

@@ -619,7 +619,6 @@ where
 	Client: crate::ClientForGrandpa<Block, BE>,
 	NumberFor<Block>: finality_grandpa::BlockNumberOps,
 {
-
 	/// Import a block justification and finalize the block.
 	///
 	/// If `enacts_change` is set to true, then finalizing this block *must*
@@ -653,7 +652,7 @@ where
 			number,
 			justification.into(),
 			initial_sync,
-			&Some(self.justification_sender.clone()),
+			Some(&self.justification_sender),
 		);
 
 		match result {

substrate/client/finality-grandpa/src/notification.rs

@@ -20,9 +20,10 @@ use std::sync::Arc;
 use parking_lot::Mutex;
 
 use sp_runtime::traits::Block as BlockT;
-use sp_utils::mpsc::{tracing_unbounded, TracingUnboundedSender, TracingUnboundedReceiver};
+use sp_utils::mpsc::{tracing_unbounded, TracingUnboundedReceiver, TracingUnboundedSender};
 
 use crate::justification::GrandpaJustification;
+use crate::Error;
 
 // Stream of justifications returned when subscribing.
 type JustificationStream<Block> = TracingUnboundedReceiver<GrandpaJustification<Block>>;
@@ -54,10 +55,22 @@ impl<Block: BlockT> GrandpaJustificationSender<Block> {
 
 	/// Send out a notification to all subscribers that a new justification
 	/// is available for a block.
-	pub fn notify(&self, notification: GrandpaJustification<Block>) -> Result<(), ()> {
-		self.subscribers.lock().retain(|n| {
-			!n.is_closed() && n.unbounded_send(notification.clone()).is_ok()
-		});
+	pub fn notify(
+		&self,
+		justification: impl FnOnce() -> Result<GrandpaJustification<Block>, Error>,
+	) -> Result<(), Error> {
+		let mut subscribers = self.subscribers.lock();
+
+		// do an initial prune on closed subscriptions
+		subscribers.retain(|n| !n.is_closed());
+
+		// if there's no subscribers we avoid creating
+		// the justification which is a costly operation
+		if !subscribers.is_empty() {
+			let justification = justification()?;
+			subscribers.retain(|n| n.unbounded_send(justification.clone()).is_ok());
+		}
+
 		Ok(())
 	}
 }

substrate/client/finality-grandpa/src/observer.rs

@@ -74,11 +74,10 @@ fn grandpa_observer<BE, Block: BlockT, Client, S, F>(
 	last_finalized_number: NumberFor<Block>,
 	commits: S,
 	note_round: F,
-) -> impl Future<Output=Result<(), CommandOrError<Block::Hash, NumberFor<Block>>>> where
+) -> impl Future<Output = Result<(), CommandOrError<Block::Hash, NumberFor<Block>>>>
+where
 	NumberFor<Block>: BlockNumberOps,
-	S: Stream<
-		Item = Result<CommunicationIn<Block>, CommandOrError<Block::Hash, NumberFor<Block>>>,
-	>,
+	S: Stream<Item = Result<CommunicationIn<Block>, CommandOrError<Block::Hash, NumberFor<Block>>>>,
 	F: Fn(u64),
 	BE: Backend<Block>,
 	Client: crate::ClientForGrandpa<Block, BE>,
@@ -130,7 +129,7 @@ fn grandpa_observer<BE, Block: BlockT, Client, S, F>(
 				finalized_number,
 				(round, commit).into(),
 				false,
-				&justification_sender,
+				justification_sender.as_ref(),
 			) {
 				Ok(_) => {},
 				Err(e) => return future::err(e),