subsystem for issuing background connection requests (#2538)

* initial subsystem for issuing connection requests

* finish the initial impl

* integrate with the overseer

* rename to gossip-support

* fix renamings leftover

* remove run_inner

* fix compilation

* random subset of sqrt

polkadot/Cargo.lock

@@ -5273,6 +5273,20 @@ dependencies = [
  "thiserror",
 ]
 
+[[package]]
+name = "polkadot-gossip-support"
+version = "0.1.0"
+dependencies = [
+ "futures 0.3.12",
+ "polkadot-node-network-protocol",
+ "polkadot-node-subsystem",
+ "polkadot-node-subsystem-util",
+ "polkadot-primitives",
+ "rand 0.8.3",
+ "tracing",
+ "tracing-futures",
+]
+
 [[package]]
 name = "polkadot-network-bridge"
 version = "0.1.0"
@@ -5994,6 +6008,7 @@ dependencies = [
  "polkadot-availability-distribution",
  "polkadot-availability-recovery",
  "polkadot-collator-protocol",
+ "polkadot-gossip-support",
  "polkadot-network-bridge",
  "polkadot-node-collation-generation",
  "polkadot-node-core-approval-voting",

polkadot/Cargo.toml

@@ -64,6 +64,7 @@ members = [
 	"node/network/availability-distribution",
 	"node/network/availability-recovery",
 	"node/network/collator-protocol",
+	"node/network/gossip-support",
 	"node/overseer",
 	"node/primitives",
 	"node/service",

polkadot/node/network/bridge/src/lib.rs

@@ -1547,6 +1547,7 @@ mod tests {
 				AllMessages::CollationGeneration(_) => unreachable!("Not interested in network events"),
 				AllMessages::ApprovalVoting(_) => unreachable!("Not interested in network events"),
 				AllMessages::ApprovalDistribution(_) => { cnt += 1; }
+				AllMessages::GossipSupport(_) => unreachable!("Not interested in network events"),
 				// Add variants here as needed, `{ cnt += 1; }` for those that need to be
 				// notified, `unreachable!()` for those that should not.
 			}

polkadot/node/network/gossip-support/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "polkadot-gossip-support"
+version = "0.1.0"
+authors = ["Parity Technologies <admin@parity.io>"]
+edition = "2018"
+
+[dependencies]
+polkadot-node-network-protocol = { path = "../protocol" }
+polkadot-node-subsystem = { path = "../../subsystem" }
+polkadot-node-subsystem-util = { path = "../../subsystem-util" }
+polkadot-primitives = { path = "../../../primitives" }
+
+futures = "0.3.8"
+tracing = "0.1.22"
+tracing-futures = "0.2.4"
+rand = "0.8.3"

polkadot/node/network/gossip-support/src/lib.rs

@@ -0,0 +1,170 @@
+// Copyright 2021 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Polkadot is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! This subsystem is responsible for keeping track of session changes
+//! and issuing a connection request to the validators relevant to
+//! the gossiping subsystems on every new session.
+
+use futures::FutureExt as _;
+use rand::seq::SliceRandom as _;
+use polkadot_node_subsystem::{
+	messages::{
+		GossipSupportMessage,
+	},
+	ActiveLeavesUpdate, FromOverseer, OverseerSignal,
+	Subsystem, SpawnedSubsystem, SubsystemContext,
+};
+use polkadot_node_subsystem_util::{
+	validator_discovery::{ConnectionRequest, self},
+	self as util,
+};
+use polkadot_primitives::v1::{
+	Hash, ValidatorId, SessionIndex,
+};
+use polkadot_node_network_protocol::peer_set::PeerSet;
+
+const LOG_TARGET: &str = "gossip_support";
+
+/// The Gossip Support subsystem.
+pub struct GossipSupport {}
+
+#[derive(Default)]
+struct State {
+	last_session_index: Option<SessionIndex>,
+	/// when we overwrite this, it automatically drops the previous request
+	last_connection_request: Option<ConnectionRequest>,
+}
+
+impl GossipSupport {
+	/// Create a new instance of the [`GossipSupport`] subsystem.
+	pub fn new() -> Self {
+		Self {}
+	}
+
+	#[tracing::instrument(skip(self, ctx), fields(subsystem = LOG_TARGET))]
+	async fn run<Context>(self, mut ctx: Context)
+	where
+		Context: SubsystemContext<Message = GossipSupportMessage>,
+	{
+		let mut state = State::default();
+		loop {
+			let message = match ctx.recv().await {
+				Ok(message) => message,
+				Err(e) => {
+					tracing::debug!(
+						target: LOG_TARGET,
+						err = ?e,
+						"Failed to receive a message from Overseer, exiting"
+					);
+					return;
+				},
+			};
+			match message {
+				FromOverseer::Communication { .. } => {},
+				FromOverseer::Signal(OverseerSignal::ActiveLeaves(ActiveLeavesUpdate {
+					activated,
+					..
+				})) => {
+					tracing::trace!(target: LOG_TARGET, "active leaves signal");
+
+					let leaves = activated.into_iter().map(|(h, _)| h);
+					if let Err(e) = state.handle_active_leaves(&mut ctx, leaves).await {
+						tracing::debug!(target: LOG_TARGET, "Error {}", e);
+					}
+				}
+				FromOverseer::Signal(OverseerSignal::BlockFinalized(_hash, _number)) => {},
+				FromOverseer::Signal(OverseerSignal::Conclude) => {
+					return;
+				}
+			}
+		}
+	}
+}
+
+async fn determine_relevant_validators(
+	ctx: &mut impl SubsystemContext,
+	relay_parent: Hash,
+	_session: SessionIndex,
+) -> Result<Vec<ValidatorId>, util::Error> {
+	let validators = util::request_validators_ctx(relay_parent, ctx).await?.await??;
+	Ok(validators)
+}
+
+// chooses a random subset of sqrt(v.len()), but at least 25 elements
+fn choose_random_subset<T>(mut v: Vec<T>) -> Vec<T> {
+	let mut rng = rand::thread_rng();
+	v.shuffle(&mut rng);
+
+	let sqrt = (v.len() as f64).sqrt() as usize;
+	let len = std::cmp::max(25, sqrt);
+	v.truncate(len);
+	v
+}
+
+impl State {
+	/// 1. Determine if the current session index has changed.
+	/// 2. If it has, determine relevant validators
+	///    and issue a connection request.
+	async fn handle_active_leaves(
+		&mut self,
+		ctx: &mut impl SubsystemContext,
+		leaves: impl Iterator<Item = Hash>,
+	) -> Result<(), util::Error> {
+		for leaf in leaves {
+			let current_index = util::request_session_index_for_child_ctx(leaf, ctx).await?.await??;
+			let maybe_new_session = match self.last_session_index {
+				Some(i) if i <= current_index => None,
+				_ => Some((current_index, leaf)),
+			};
+
+			if let Some((new_session, relay_parent)) = maybe_new_session {
+				tracing::debug!(target: LOG_TARGET, "New session detected {}", new_session);
+				let validators = determine_relevant_validators(ctx, relay_parent, new_session).await?;
+				let validators = choose_random_subset(validators);
+				tracing::debug!(target: LOG_TARGET, "Issuing a connection request to {:?}", validators);
+
+				let request = validator_discovery::connect_to_validators_in_session(
+					ctx,
+					relay_parent,
+					validators,
+					PeerSet::Validation,
+					new_session,
+				).await?;
+
+				self.last_session_index = Some(new_session);
+				self.last_connection_request = Some(request);
+			}
+		}
+
+		Ok(())
+	}
+}
+
+impl<C> Subsystem<C> for GossipSupport
+where
+	C: SubsystemContext<Message = GossipSupportMessage> + Sync + Send,
+{
+	fn start(self, ctx: C) -> SpawnedSubsystem {
+		let future = self.run(ctx)
+			.map(|_| Ok(()))
+			.boxed();
+
+		SpawnedSubsystem {
+			name: "gossip-support-subsystem",
+			future,
+		}
+	}
+}

polkadot/node/overseer/src/lib.rs

@@ -86,7 +86,7 @@ use polkadot_subsystem::messages::{
 	ProvisionerMessage, PoVDistributionMessage, RuntimeApiMessage,
 	AvailabilityStoreMessage, NetworkBridgeMessage, AllMessages, CollationGenerationMessage,
 	CollatorProtocolMessage, AvailabilityRecoveryMessage, ApprovalDistributionMessage,
-	ApprovalVotingMessage,
+	ApprovalVotingMessage, GossipSupportMessage,
 };
 pub use polkadot_subsystem::{
 	Subsystem, SubsystemContext, OverseerSignal, FromOverseer, SubsystemError, SubsystemResult,
@@ -565,6 +565,9 @@ pub struct Overseer<S> {
 	/// An Approval Voting subsystem.
 	approval_voting_subsystem: OverseenSubsystem<ApprovalVotingMessage>,
 
+	/// A Gossip Support subsystem.
+	gossip_support_subsystem: OverseenSubsystem<GossipSupportMessage>,
+
 	/// Spawner to spawn tasks to.
 	s: S,
 
@@ -606,6 +609,7 @@ pub struct Overseer<S> {
 pub struct AllSubsystems<
 	CV = (), CB = (), CS = (), SD = (), AD = (), AR = (), BS = (), BD = (), P = (),
 	PoVD = (), RA = (), AS = (), NB = (), CA = (), CG = (), CP = (), ApD = (), ApV = (),
+	GS = (),
 > {
 	/// A candidate validation subsystem.
 	pub candidate_validation: CV,
@@ -643,10 +647,12 @@ pub struct AllSubsystems<
 	pub approval_distribution: ApD,
 	/// An Approval Voting subsystem.
 	pub approval_voting: ApV,
+	/// A Connection Request Issuer subsystem.
+	pub gossip_support: GS,
 }
 
-impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
-	AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
+impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS>
+	AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS>
 {
 	/// Create a new instance of [`AllSubsystems`].
 	///
@@ -679,6 +685,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 		DummySubsystem,
 		DummySubsystem,
 		DummySubsystem,
+		DummySubsystem,
 	> {
 		AllSubsystems {
 			candidate_validation: DummySubsystem,
@@ -699,6 +706,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: DummySubsystem,
 			approval_distribution: DummySubsystem,
 			approval_voting: DummySubsystem,
+			gossip_support: DummySubsystem,
 		}
 	}
 
@@ -706,7 +714,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_candidate_validation<NEW>(
 		self,
 		candidate_validation: NEW,
-	) -> AllSubsystems<NEW, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<NEW, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -726,6 +734,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -733,7 +742,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_candidate_backing<NEW>(
 		self,
 		candidate_backing: NEW,
-	) -> AllSubsystems<CV, NEW, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, NEW, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing,
@@ -753,6 +762,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -760,7 +770,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_candidate_selection<NEW>(
 		self,
 		candidate_selection: NEW,
-	) -> AllSubsystems<CV, CB, NEW, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, NEW, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -780,6 +790,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -787,7 +798,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_statement_distribution<NEW>(
 		self,
 		statement_distribution: NEW,
-	) -> AllSubsystems<CV, CB, CS, NEW, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, NEW, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -807,6 +818,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -814,7 +826,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_availability_distribution<NEW>(
 		self,
 		availability_distribution: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, NEW, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, NEW, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -834,6 +846,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -841,7 +854,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_availability_recovery<NEW>(
 		self,
 		availability_recovery: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, NEW, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, NEW, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -861,6 +874,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -868,7 +882,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_bitfield_signing<NEW>(
 		self,
 		bitfield_signing: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, NEW, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, NEW, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -888,6 +902,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -895,7 +910,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_bitfield_distribution<NEW>(
 		self,
 		bitfield_distribution: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, NEW, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, NEW, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -915,6 +930,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -922,7 +938,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_provisioner<NEW>(
 		self,
 		provisioner: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, NEW, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, NEW, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -942,6 +958,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -949,7 +966,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_pov_distribution<NEW>(
 		self,
 		pov_distribution: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, NEW, RA, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, NEW, RA, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -969,6 +986,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -976,7 +994,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_runtime_api<NEW>(
 		self,
 		runtime_api: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, NEW, AS, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, NEW, AS, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -996,6 +1014,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -1003,7 +1022,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_availability_store<NEW>(
 		self,
 		availability_store: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, NEW, NB, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, NEW, NB, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -1023,6 +1042,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -1030,7 +1050,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_network_bridge<NEW>(
 		self,
 		network_bridge: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NEW, CA, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NEW, CA, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -1050,6 +1070,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -1057,7 +1078,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_chain_api<NEW>(
 		self,
 		chain_api: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, NEW, CG, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, NEW, CG, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -1077,6 +1098,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -1084,7 +1106,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_collation_generation<NEW>(
 		self,
 		collation_generation: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, NEW, CP, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, NEW, CP, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -1104,6 +1126,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -1111,7 +1134,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_collator_protocol<NEW>(
 		self,
 		collator_protocol: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, NEW, ApD, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, NEW, ApD, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -1131,6 +1154,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol,
 			approval_distribution: self.approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -1138,7 +1162,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_approval_distribution<NEW>(
 		self,
 		approval_distribution: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, NEW, ApV> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, NEW, ApV, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -1158,6 +1182,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collator_protocol: self.collator_protocol,
 			approval_distribution,
 			approval_voting: self.approval_voting,
+			gossip_support: self.gossip_support,
 		}
 	}
 
@@ -1165,7 +1190,7 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 	pub fn replace_approval_voting<NEW>(
 		self,
 		approval_voting: NEW,
-	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, NEW> {
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, NEW, GS> {
 		AllSubsystems {
 			candidate_validation: self.candidate_validation,
 			candidate_backing: self.candidate_backing,
@@ -1184,7 +1209,36 @@ impl<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>
 			collation_generation: self.collation_generation,
 			collator_protocol: self.collator_protocol,
 			approval_distribution: self.approval_distribution,
-			approval_voting
+			approval_voting,
+			gossip_support: self.gossip_support,
+		}
+	}
+
+	/// Replace the `gossip_support` instance in `self`.
+	pub fn replace_gossip_support<NEW>(
+		self,
+		gossip_support: NEW,
+	) -> AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, NEW> {
+		AllSubsystems {
+			candidate_validation: self.candidate_validation,
+			candidate_backing: self.candidate_backing,
+			candidate_selection: self.candidate_selection,
+			statement_distribution: self.statement_distribution,
+			availability_distribution: self.availability_distribution,
+			availability_recovery: self.availability_recovery,
+			bitfield_signing: self.bitfield_signing,
+			bitfield_distribution: self.bitfield_distribution,
+			provisioner: self.provisioner,
+			pov_distribution: self.pov_distribution,
+			runtime_api: self.runtime_api,
+			availability_store: self.availability_store,
+			network_bridge: self.network_bridge,
+			chain_api: self.chain_api,
+			collation_generation: self.collation_generation,
+			collator_protocol: self.collator_protocol,
+			approval_distribution: self.approval_distribution,
+			approval_voting: self.approval_voting,
+			gossip_support,
 		}
 	}
 }
@@ -1395,9 +1449,9 @@ where
 	/// #
 	/// # }); }
 	/// ```
-	pub fn new<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>(
+	pub fn new<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS>(
 		leaves: impl IntoIterator<Item = BlockInfo>,
-		all_subsystems: AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV>,
+		all_subsystems: AllSubsystems<CV, CB, CS, SD, AD, AR, BS, BD, P, PoVD, RA, AS, NB, CA, CG, CP, ApD, ApV, GS>,
 		prometheus_registry: Option<&prometheus::Registry>,
 		mut s: S,
 	) -> SubsystemResult<(Self, OverseerHandler)>
@@ -1420,6 +1474,7 @@ where
 		CP: Subsystem<OverseerSubsystemContext<CollatorProtocolMessage>> + Send,
 		ApD: Subsystem<OverseerSubsystemContext<ApprovalDistributionMessage>> + Send,
 		ApV: Subsystem<OverseerSubsystemContext<ApprovalVotingMessage>> + Send,
+		GS: Subsystem<OverseerSubsystemContext<GossipSupportMessage>> + Send,
 	{
 		let (events_tx, events_rx) = metered::channel(CHANNEL_CAPACITY, "overseer_events");
 
@@ -1631,6 +1686,16 @@ where
 			TaskKind::Blocking,
 		)?;
 
+		let gossip_support_subsystem = spawn(
+			&mut s,
+			&mut running_subsystems,
+			metered::UnboundedMeteredSender::<_>::clone(&to_overseer_tx),
+			all_subsystems.gossip_support,
+			&metrics,
+			&mut seed,
+			TaskKind::Regular,
+		)?;
+
 		let leaves = leaves
 			.into_iter()
 			.map(|BlockInfo { hash, parent_hash: _, number }| (hash, number))
@@ -1658,6 +1723,7 @@ where
 			collator_protocol_subsystem,
 			approval_distribution_subsystem,
 			approval_voting_subsystem,
+			gossip_support_subsystem,
 			s,
 			running_subsystems,
 			to_overseer_rx: to_overseer_rx.fuse(),
@@ -1692,6 +1758,7 @@ where
 		let _ = self.collation_generation_subsystem.send_signal(OverseerSignal::Conclude).await;
 		let _ = self.approval_distribution_subsystem.send_signal(OverseerSignal::Conclude).await;
 		let _ = self.approval_voting_subsystem.send_signal(OverseerSignal::Conclude).await;
+		let _ = self.gossip_support_subsystem.send_signal(OverseerSignal::Conclude).await;
 
 		let mut stop_delay = Delay::new(Duration::from_secs(STOP_DELAY)).fuse();
 
@@ -1861,7 +1928,8 @@ where
 		self.collator_protocol_subsystem.send_signal(signal.clone()).await?;
 		self.collation_generation_subsystem.send_signal(signal.clone()).await?;
 		self.approval_distribution_subsystem.send_signal(signal.clone()).await?;
-		self.approval_voting_subsystem.send_signal(signal).await?;
+		self.approval_voting_subsystem.send_signal(signal.clone()).await?;
+		self.gossip_support_subsystem.send_signal(signal).await?;
 
 		Ok(())
 	}
@@ -1925,6 +1993,9 @@ where
 			AllMessages::ApprovalVoting(msg) => {
 				self.approval_voting_subsystem.send_message(msg).await?;
 			},
+			AllMessages::GossipSupport(msg) => {
+				self.gossip_support_subsystem.send_message(msg).await?;
+			},
 		}
 
 		Ok(())
@@ -2824,6 +2895,7 @@ mod tests {
 				chain_api: subsystem.clone(),
 				approval_distribution: subsystem.clone(),
 				approval_voting: subsystem.clone(),
+				gossip_support: subsystem.clone(),
 			};
 			let (overseer, mut handler) = Overseer::new(
 				vec![],
@@ -2843,7 +2915,7 @@ mod tests {
 			}).await;
 
 			// send a msg to each subsystem
-			// except for BitfieldSigning as the message is not instantiable
+			// except for BitfieldSigning and GossipSupport as the messages are not instantiable
 			handler.send_msg(AllMessages::CandidateValidation(test_candidate_validation_msg())).await;
 			handler.send_msg(AllMessages::CandidateBacking(test_candidate_backing_msg())).await;
 			handler.send_msg(AllMessages::CandidateSelection(test_candidate_selection_msg())).await;
@@ -2852,6 +2924,7 @@ mod tests {
 			handler.send_msg(AllMessages::StatementDistribution(test_statement_distribution_msg())).await;
 			handler.send_msg(AllMessages::AvailabilityRecovery(test_availability_recovery_msg())).await;
 			// handler.send_msg(AllMessages::BitfieldSigning(test_bitfield_signing_msg())).await;
+			// handler.send_msg(AllMessages::GossipSupport(test_bitfield_signing_msg())).await;
 			handler.send_msg(AllMessages::BitfieldDistribution(test_bitfield_distribution_msg())).await;
 			handler.send_msg(AllMessages::Provisioner(test_provisioner_msg())).await;
 			handler.send_msg(AllMessages::PoVDistribution(test_pov_distribution_msg())).await;
@@ -2867,13 +2940,12 @@ mod tests {
 
 			select! {
 				res = overseer_fut => {
-					const NUM_SUBSYSTEMS: usize = 18;
+					const NUM_SUBSYSTEMS: usize = 19;
 
 					assert_eq!(stop_signals_received.load(atomic::Ordering::SeqCst), NUM_SUBSYSTEMS);
-					// x2 because of broadcast_signal on startup
 					assert_eq!(signals_received.load(atomic::Ordering::SeqCst), NUM_SUBSYSTEMS);
-					// -2 for BitfieldSigning and Availability distribution
-					assert_eq!(msgs_received.load(atomic::Ordering::SeqCst), NUM_SUBSYSTEMS - 2);
+					// -3 for BitfieldSigning, GossipSupport and AvailabilityDistribution
+					assert_eq!(msgs_received.load(atomic::Ordering::SeqCst), NUM_SUBSYSTEMS - 3);
 
 					assert!(res.is_ok());
 				},

polkadot/node/service/Cargo.toml

@@ -83,6 +83,7 @@ polkadot-availability-bitfield-distribution = { path = "../network/bitfield-dist
 polkadot-availability-distribution = { path = "../network/availability-distribution", optional = true }
 polkadot-availability-recovery = { path = "../network/availability-recovery", optional = true }
 polkadot-collator-protocol = { path = "../network/collator-protocol", optional = true }
+polkadot-gossip-support = { path = "../network/gossip-support", optional = true }
 polkadot-network-bridge = { path = "../network/bridge", optional = true }
 polkadot-node-collation-generation = { path = "../collation-generation", optional = true }
 polkadot-node-core-av-store = { path = "../core/av-store", optional = true }
@@ -128,6 +129,7 @@ real-overseer = [
 	"polkadot-availability-distribution",
 	"polkadot-availability-recovery",
 	"polkadot-collator-protocol",
+	"polkadot-gossip-support",
 	"polkadot-network-bridge",
 	"polkadot-node-collation-generation",
 	"polkadot-node-core-backing",

polkadot/node/service/src/lib.rs

@@ -418,6 +418,7 @@ where
 	use polkadot_availability_recovery::AvailabilityRecoverySubsystem;
 	use polkadot_approval_distribution::ApprovalDistribution as ApprovalDistributionSubsystem;
 	use polkadot_node_core_approval_voting::ApprovalVotingSubsystem;
+	use polkadot_gossip_support::GossipSupport as GossipSupportSubsystem;
 
 	let all_subsystems = AllSubsystems {
 		availability_distribution: AvailabilityDistributionSubsystem::new(
@@ -497,6 +498,7 @@ where
 			approval_voting_config,
 			keystore.clone(),
 		)?,
+		gossip_support: GossipSupportSubsystem::new(),
 	};
 
 	Overseer::new(

polkadot/node/subsystem/src/messages.rs

@@ -682,6 +682,11 @@ pub enum ApprovalDistributionMessage {
 	NetworkBridgeUpdateV1(NetworkBridgeEvent<protocol_v1::ApprovalDistributionMessage>),
 }
 
+/// Message to the Gossip Support subsystem.
+#[derive(Debug)]
+pub enum GossipSupportMessage {
+}
+
 /// A message type tying together all message types that are used across Subsystems.
 #[subsystem_dispatch_gen(NetworkBridgeEvent<protocol_v1::ValidationProtocol>)]
 #[derive(Debug, derive_more::From)]
@@ -735,6 +740,9 @@ pub enum AllMessages {
 	ApprovalVoting(ApprovalVotingMessage),
 	/// Message for the Approval Distribution subsystem.
 	ApprovalDistribution(ApprovalDistributionMessage),
+	/// Message for the Gossip Support subsystem.
+	#[skip]
+	GossipSupport(GossipSupportMessage),
 }
 
 impl From<IncomingRequest<req_res_v1::AvailabilityFetchingRequest>> for AllMessages {