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/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,
+		}
+	}
+}