Register listeners in statement distribution

node/network/statement-distribution/src/lib.rs

@@ -36,7 +36,7 @@ use polkadot_node_network_protocol::{
 };
 
 use futures::prelude::*;
-use futures::channel::oneshot;
+use futures::channel::{mpsc, oneshot};
 use indexmap::IndexSet;
 
 use std::collections::{HashMap, HashSet};
@@ -476,6 +476,21 @@ fn check_statement_signature(
 		.and_then(|v| statement.check_signature(&signing_context, v))
 }
 
+/// Informs all registered listeners about a newly received statement.
+///
+/// Removes all closed listeners.
+async fn inform_statement_listeners(
+	statement: SignedFullStatement,
+	listeners: &mut Vec<mpsc::Sender<SignedFullStatement>>,
+) {
+	// Ignore the errors since these will be removed later.
+	for listener in listeners.iter_mut() {
+		let _ = listener.send(statement.clone()).await;
+	}
+	// Remove any closed listeners.
+	listeners.retain(|tx| !tx.is_closed());
+}
+
 /// Places the statement in storage if it is new, and then
 /// circulates the statement to all peers who have not seen it yet, and
 /// sends all statements dependent on that statement to peers who could previously not receive
@@ -821,6 +836,7 @@ async fn run(
 	let mut peers: HashMap<PeerId, PeerData> = HashMap::new();
 	let mut our_view = View::default();
 	let mut active_heads: HashMap<Hash, ActiveHeadData> = HashMap::new();
+	let mut statement_listeners: Vec<mpsc::Sender<SignedFullStatement>> = Vec::new();
 
 	loop {
 		let message = ctx.recv().await?;
@@ -874,14 +890,19 @@ async fn run(
 			}
 			FromOverseer::Signal(OverseerSignal::Conclude) => break,
 			FromOverseer::Communication { msg } => match msg {
-				StatementDistributionMessage::Share(relay_parent, statement) =>
+				StatementDistributionMessage::Share(relay_parent, statement) => {
+					inform_statement_listeners(
+						statement.clone(),
+						&mut statement_listeners,
+					).await;
 					circulate_statement_and_dependents(
 						&mut peers,
 						&mut active_heads,
 						&mut ctx,
 						relay_parent,
 						statement,
-					).await?,
+					).await?;
+				}
 				StatementDistributionMessage::NetworkBridgeUpdateV1(event) =>
 					handle_network_update(
 						&mut peers,
@@ -890,6 +911,9 @@ async fn run(
 						&mut our_view,
 						event,
 					).await?,
+				StatementDistributionMessage::RegisterStatementListener(tx) => {
+					statement_listeners.push(tx);
+				}
 			}
 		}
 	}

node/subsystem/src/messages.rs

@@ -415,6 +415,8 @@ pub enum StatementDistributionMessage {
 	Share(Hash, SignedFullStatement),
 	/// Event from the network bridge.
 	NetworkBridgeUpdateV1(NetworkBridgeEvent<protocol_v1::StatementDistributionMessage>),
+	/// Register a listener for shared statements.
+	RegisterStatementListener(mpsc::Sender<SignedFullStatement>),
 }
 
 impl StatementDistributionMessage {
@@ -423,6 +425,7 @@ impl StatementDistributionMessage {
 		match self {
 			Self::Share(hash, _) => Some(*hash),
 			Self::NetworkBridgeUpdateV1(_) => None,
+			Self::RegisterStatementListener(_) => None,
 		}
 	}
 }

roadmap/implementers-guide/src/types/overseer-protocol.md

@@ -362,6 +362,8 @@ enum StatementDistributionMessage {
 	/// The statement distribution subsystem assumes that the statement should be correctly
 	/// signed.
 	Share(Hash, SignedFullStatement),
+	/// Register a listener to be notified on any new statements.
+	RegisterStatementListener(ResponseChannel<SignedFullStatement>),
 }
 ```