Extract warp sync strategy from ChainSync (#2467)

Extract `WarpSync` (and `StateSync` as part of warp sync) from
`ChainSync` as independent syncing strategy called by `SyncingEngine`.
Introduce `SyncingStrategy` enum as a proxy between `SyncingEngine` and
specific syncing strategies.

## Limitations
Gap sync is kept in `ChainSync` for now because it shares the same set
of peers as block syncing implementation in `ChainSync`. Extraction of a
common context responsible for peer management in syncing strategies
able to run in parallel is planned for a follow-up PR.

## Further improvements
A possibility of conversion of `SyncingStartegy` into a trait should be
evaluated. The main stopper for this is that different strategies need
to communicate different actions to `SyncingEngine` and respond to
different events / provide different APIs (e.g., requesting
justifications is only possible via `ChainSync` and not through
`WarpSync`; `SendWarpProofRequest` action is only relevant to
`WarpSync`, etc.)

---------

Co-authored-by: Aaro Altonen <48052676+altonen@users.noreply.github.com>

substrate/client/network/sync/src/strategy/state.rs

@@ -0,0 +1,754 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program 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.
+
+// This program 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 this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! State sync strategy.
+
+use crate::{
+	schema::v1::StateResponse,
+	strategy::state_sync::{ImportResult, StateSync, StateSyncProvider},
+	types::{BadPeer, OpaqueStateRequest, OpaqueStateResponse, SyncState, SyncStatus},
+	LOG_TARGET,
+};
+use libp2p::PeerId;
+use log::{debug, error, trace};
+use sc_client_api::ProofProvider;
+use sc_consensus::{BlockImportError, BlockImportStatus, IncomingBlock};
+use sc_network_common::sync::message::BlockAnnounce;
+use sp_consensus::BlockOrigin;
+use sp_runtime::{
+	traits::{Block as BlockT, Header, NumberFor},
+	Justifications, SaturatedConversion,
+};
+use std::{collections::HashMap, sync::Arc};
+
+mod rep {
+	use sc_network::ReputationChange as Rep;
+
+	/// Peer response data does not have requested bits.
+	pub const BAD_RESPONSE: Rep = Rep::new(-(1 << 12), "Incomplete response");
+
+	/// Reputation change for peers which send us a known bad state.
+	pub const BAD_STATE: Rep = Rep::new(-(1 << 29), "Bad state");
+}
+
+/// Action that should be performed on [`StateStrategy`]'s behalf.
+pub enum StateStrategyAction<B: BlockT> {
+	/// Send state request to peer.
+	SendStateRequest { peer_id: PeerId, request: OpaqueStateRequest },
+	/// Disconnect and report peer.
+	DropPeer(BadPeer),
+	/// Import blocks.
+	ImportBlocks { origin: BlockOrigin, blocks: Vec<IncomingBlock<B>> },
+	/// State sync has finished.
+	Finished,
+}
+
+enum PeerState {
+	Available,
+	DownloadingState,
+}
+
+impl PeerState {
+	fn is_available(&self) -> bool {
+		matches!(self, PeerState::Available)
+	}
+}
+
+struct Peer<B: BlockT> {
+	best_number: NumberFor<B>,
+	state: PeerState,
+}
+
+/// Syncing strategy that downloads and imports a recent state directly.
+pub struct StateStrategy<B: BlockT> {
+	state_sync: Box<dyn StateSyncProvider<B>>,
+	peers: HashMap<PeerId, Peer<B>>,
+	actions: Vec<StateStrategyAction<B>>,
+	succeded: bool,
+}
+
+impl<B: BlockT> StateStrategy<B> {
+	/// Create a new instance.
+	pub fn new<Client>(
+		client: Arc<Client>,
+		target_header: B::Header,
+		target_body: Option<Vec<B::Extrinsic>>,
+		target_justifications: Option<Justifications>,
+		skip_proof: bool,
+		initial_peers: impl Iterator<Item = (PeerId, NumberFor<B>)>,
+	) -> Self
+	where
+		Client: ProofProvider<B> + Send + Sync + 'static,
+	{
+		let peers = initial_peers
+			.map(|(peer_id, best_number)| {
+				(peer_id, Peer { best_number, state: PeerState::Available })
+			})
+			.collect();
+		Self {
+			state_sync: Box::new(StateSync::new(
+				client,
+				target_header,
+				target_body,
+				target_justifications,
+				skip_proof,
+			)),
+			peers,
+			actions: Vec::new(),
+			succeded: false,
+		}
+	}
+
+	// Create a new instance with a custom state sync provider.
+	// Used in tests.
+	#[cfg(test)]
+	fn new_with_provider(
+		state_sync_provider: Box<dyn StateSyncProvider<B>>,
+		initial_peers: impl Iterator<Item = (PeerId, NumberFor<B>)>,
+	) -> Self {
+		Self {
+			state_sync: state_sync_provider,
+			peers: initial_peers
+				.map(|(peer_id, best_number)| {
+					(peer_id, Peer { best_number, state: PeerState::Available })
+				})
+				.collect(),
+			actions: Vec::new(),
+			succeded: false,
+		}
+	}
+
+	/// Notify that a new peer has connected.
+	pub fn add_peer(&mut self, peer_id: PeerId, _best_hash: B::Hash, best_number: NumberFor<B>) {
+		self.peers.insert(peer_id, Peer { best_number, state: PeerState::Available });
+	}
+
+	/// Notify that a peer has disconnected.
+	pub fn remove_peer(&mut self, peer_id: &PeerId) {
+		self.peers.remove(peer_id);
+	}
+
+	/// Submit a validated block announcement.
+	///
+	/// Returns new best hash & best number of the peer if they are updated.
+	#[must_use]
+	pub fn on_validated_block_announce(
+		&mut self,
+		is_best: bool,
+		peer_id: PeerId,
+		announce: &BlockAnnounce<B::Header>,
+	) -> Option<(B::Hash, NumberFor<B>)> {
+		is_best.then_some({
+			let best_number = *announce.header.number();
+			let best_hash = announce.header.hash();
+			if let Some(ref mut peer) = self.peers.get_mut(&peer_id) {
+				peer.best_number = best_number;
+			}
+			// Let `SyncingEngine` know that we should update the peer info.
+			(best_hash, best_number)
+		})
+	}
+
+	/// Process state response.
+	pub fn on_state_response(&mut self, peer_id: PeerId, response: OpaqueStateResponse) {
+		if let Err(bad_peer) = self.on_state_response_inner(peer_id, response) {
+			self.actions.push(StateStrategyAction::DropPeer(bad_peer));
+		}
+	}
+
+	fn on_state_response_inner(
+		&mut self,
+		peer_id: PeerId,
+		response: OpaqueStateResponse,
+	) -> Result<(), BadPeer> {
+		if let Some(peer) = self.peers.get_mut(&peer_id) {
+			peer.state = PeerState::Available;
+		}
+
+		let response: Box<StateResponse> = response.0.downcast().map_err(|_error| {
+			error!(
+				target: LOG_TARGET,
+				"Failed to downcast opaque state response, this is an implementation bug."
+			);
+			debug_assert!(false);
+
+			BadPeer(peer_id, rep::BAD_RESPONSE)
+		})?;
+
+		debug!(
+			target: LOG_TARGET,
+			"Importing state data from {} with {} keys, {} proof nodes.",
+			peer_id,
+			response.entries.len(),
+			response.proof.len(),
+		);
+
+		match self.state_sync.import(*response) {
+			ImportResult::Import(hash, header, state, body, justifications) => {
+				let origin = BlockOrigin::NetworkInitialSync;
+				let block = IncomingBlock {
+					hash,
+					header: Some(header),
+					body,
+					indexed_body: None,
+					justifications,
+					origin: None,
+					allow_missing_state: true,
+					import_existing: true,
+					skip_execution: true,
+					state: Some(state),
+				};
+				debug!(target: LOG_TARGET, "State download is complete. Import is queued");
+				self.actions
+					.push(StateStrategyAction::ImportBlocks { origin, blocks: vec![block] });
+				Ok(())
+			},
+			ImportResult::Continue => Ok(()),
+			ImportResult::BadResponse => {
+				debug!(target: LOG_TARGET, "Bad state data received from {peer_id}");
+				Err(BadPeer(peer_id, rep::BAD_STATE))
+			},
+		}
+	}
+
+	/// A batch of blocks have been processed, with or without errors.
+	///
+	/// Normally this should be called when target block with state is imported.
+	pub fn on_blocks_processed(
+		&mut self,
+		imported: usize,
+		count: usize,
+		results: Vec<(Result<BlockImportStatus<NumberFor<B>>, BlockImportError>, B::Hash)>,
+	) {
+		trace!(target: LOG_TARGET, "State sync: imported {imported} of {count}.");
+
+		let results = results
+			.into_iter()
+			.filter_map(|(result, hash)| {
+				if hash == self.state_sync.target_hash() {
+					Some(result)
+				} else {
+					debug!(
+						target: LOG_TARGET,
+						"Unexpected block processed: {hash} with result {result:?}.",
+					);
+					None
+				}
+			})
+			.collect::<Vec<_>>();
+
+		if !results.is_empty() {
+			// We processed the target block
+			results.iter().filter_map(|result| result.as_ref().err()).for_each(|e| {
+				error!(
+					target: LOG_TARGET,
+					"Failed to import target block with state: {e:?}."
+				);
+			});
+			self.succeded |= results.into_iter().any(|result| result.is_ok());
+			self.actions.push(StateStrategyAction::Finished);
+		}
+	}
+
+	/// Produce state request.
+	fn state_request(&mut self) -> Option<(PeerId, OpaqueStateRequest)> {
+		if self.state_sync.is_complete() {
+			return None
+		}
+
+		if self
+			.peers
+			.values()
+			.any(|peer| matches!(peer.state, PeerState::DownloadingState))
+		{
+			// Only one state request at a time is possible.
+			return None
+		}
+
+		let peer_id =
+			self.schedule_next_peer(PeerState::DownloadingState, self.state_sync.target_number())?;
+		let request = self.state_sync.next_request();
+		trace!(
+			target: LOG_TARGET,
+			"New state request to {peer_id}: {request:?}.",
+		);
+		Some((peer_id, OpaqueStateRequest(Box::new(request))))
+	}
+
+	fn schedule_next_peer(
+		&mut self,
+		new_state: PeerState,
+		min_best_number: NumberFor<B>,
+	) -> Option<PeerId> {
+		let mut targets: Vec<_> = self.peers.values().map(|p| p.best_number).collect();
+		if targets.is_empty() {
+			return None
+		}
+		targets.sort();
+		let median = targets[targets.len() / 2];
+		let threshold = std::cmp::max(median, min_best_number);
+		// Find a random peer that is synced as much as peer majority and is above
+		// `min_best_number`.
+		for (peer_id, peer) in self.peers.iter_mut() {
+			if peer.state.is_available() && peer.best_number >= threshold {
+				peer.state = new_state;
+				return Some(*peer_id)
+			}
+		}
+		None
+	}
+
+	/// Returns the current sync status.
+	pub fn status(&self) -> SyncStatus<B> {
+		SyncStatus {
+			state: if self.state_sync.is_complete() {
+				SyncState::Idle
+			} else {
+				SyncState::Downloading { target: self.state_sync.target_number() }
+			},
+			best_seen_block: Some(self.state_sync.target_number()),
+			num_peers: self.peers.len().saturated_into(),
+			num_connected_peers: self.peers.len().saturated_into(),
+			queued_blocks: 0,
+			state_sync: Some(self.state_sync.progress()),
+			warp_sync: None,
+		}
+	}
+
+	/// Get the number of peers known to syncing.
+	pub fn num_peers(&self) -> usize {
+		self.peers.len()
+	}
+
+	/// Get actions that should be performed by the owner on [`WarpSync`]'s behalf
+	#[must_use]
+	pub fn actions(&mut self) -> impl Iterator<Item = StateStrategyAction<B>> {
+		let state_request = self
+			.state_request()
+			.into_iter()
+			.map(|(peer_id, request)| StateStrategyAction::SendStateRequest { peer_id, request });
+		self.actions.extend(state_request);
+
+		std::mem::take(&mut self.actions).into_iter()
+	}
+
+	/// Check if state sync has succeded.
+	#[must_use]
+	pub fn is_succeded(&self) -> bool {
+		self.succeded
+	}
+}
+
+#[cfg(test)]
+mod test {
+	use super::*;
+	use crate::{
+		schema::v1::{StateRequest, StateResponse},
+		strategy::state_sync::{ImportResult, StateSyncProgress, StateSyncProvider},
+	};
+	use codec::Decode;
+	use sc_block_builder::BlockBuilderBuilder;
+	use sc_client_api::KeyValueStates;
+	use sc_consensus::{ImportedAux, ImportedState};
+	use sp_runtime::traits::Zero;
+	use substrate_test_runtime_client::{
+		runtime::{Block, Hash},
+		BlockBuilderExt, DefaultTestClientBuilderExt, TestClientBuilder, TestClientBuilderExt,
+	};
+
+	mockall::mock! {
+		pub StateSync<B: BlockT> {}
+
+		impl<B: BlockT> StateSyncProvider<B> for StateSync<B> {
+			fn import(&mut self, response: StateResponse) -> ImportResult<B>;
+			fn next_request(&self) -> StateRequest;
+			fn is_complete(&self) -> bool;
+			fn target_number(&self) -> NumberFor<B>;
+			fn target_hash(&self) -> B::Hash;
+			fn progress(&self) -> StateSyncProgress;
+		}
+	}
+
+	#[test]
+	fn no_peer_is_scheduled_if_no_peers_connected() {
+		let client = Arc::new(TestClientBuilder::new().set_no_genesis().build());
+		let target_block = BlockBuilderBuilder::new(&*client)
+			.on_parent_block(client.chain_info().best_hash)
+			.with_parent_block_number(client.chain_info().best_number)
+			.build()
+			.unwrap()
+			.build()
+			.unwrap()
+			.block;
+		let target_header = target_block.header().clone();
+
+		let mut state_strategy =
+			StateStrategy::new(client, target_header, None, None, false, std::iter::empty());
+
+		assert!(state_strategy
+			.schedule_next_peer(PeerState::DownloadingState, Zero::zero())
+			.is_none());
+	}
+
+	#[test]
+	fn at_least_median_synced_peer_is_scheduled() {
+		let client = Arc::new(TestClientBuilder::new().set_no_genesis().build());
+		let target_block = BlockBuilderBuilder::new(&*client)
+			.on_parent_block(client.chain_info().best_hash)
+			.with_parent_block_number(client.chain_info().best_number)
+			.build()
+			.unwrap()
+			.build()
+			.unwrap()
+			.block;
+
+		for _ in 0..100 {
+			let peers = (1..=10)
+				.map(|best_number| (PeerId::random(), best_number))
+				.collect::<HashMap<_, _>>();
+			let initial_peers = peers.iter().map(|(p, n)| (*p, *n));
+
+			let mut state_strategy = StateStrategy::new(
+				client.clone(),
+				target_block.header().clone(),
+				None,
+				None,
+				false,
+				initial_peers,
+			);
+
+			let peer_id =
+				state_strategy.schedule_next_peer(PeerState::DownloadingState, Zero::zero());
+			assert!(*peers.get(&peer_id.unwrap()).unwrap() >= 6);
+		}
+	}
+
+	#[test]
+	fn min_best_number_peer_is_scheduled() {
+		let client = Arc::new(TestClientBuilder::new().set_no_genesis().build());
+		let target_block = BlockBuilderBuilder::new(&*client)
+			.on_parent_block(client.chain_info().best_hash)
+			.with_parent_block_number(client.chain_info().best_number)
+			.build()
+			.unwrap()
+			.build()
+			.unwrap()
+			.block;
+
+		for _ in 0..10 {
+			let peers = (1..=10)
+				.map(|best_number| (PeerId::random(), best_number))
+				.collect::<HashMap<_, _>>();
+			let initial_peers = peers.iter().map(|(p, n)| (*p, *n));
+
+			let mut state_strategy = StateStrategy::new(
+				client.clone(),
+				target_block.header().clone(),
+				None,
+				None,
+				false,
+				initial_peers,
+			);
+
+			let peer_id = state_strategy.schedule_next_peer(PeerState::DownloadingState, 10);
+			assert!(*peers.get(&peer_id.unwrap()).unwrap() == 10);
+		}
+	}
+
+	#[test]
+	fn state_request_contains_correct_hash() {
+		let client = Arc::new(TestClientBuilder::new().set_no_genesis().build());
+		let target_block = BlockBuilderBuilder::new(&*client)
+			.on_parent_block(client.chain_info().best_hash)
+			.with_parent_block_number(client.chain_info().best_number)
+			.build()
+			.unwrap()
+			.build()
+			.unwrap()
+			.block;
+
+		let initial_peers = (1..=10).map(|best_number| (PeerId::random(), best_number));
+
+		let mut state_strategy = StateStrategy::new(
+			client.clone(),
+			target_block.header().clone(),
+			None,
+			None,
+			false,
+			initial_peers,
+		);
+
+		let (_peer_id, mut opaque_request) = state_strategy.state_request().unwrap();
+		let request: &mut StateRequest = opaque_request.0.downcast_mut().unwrap();
+		let hash = Hash::decode(&mut &*request.block).unwrap();
+
+		assert_eq!(hash, target_block.header().hash());
+	}
+
+	#[test]
+	fn no_parallel_state_requests() {
+		let client = Arc::new(TestClientBuilder::new().set_no_genesis().build());
+		let target_block = BlockBuilderBuilder::new(&*client)
+			.on_parent_block(client.chain_info().best_hash)
+			.with_parent_block_number(client.chain_info().best_number)
+			.build()
+			.unwrap()
+			.build()
+			.unwrap()
+			.block;
+
+		let initial_peers = (1..=10).map(|best_number| (PeerId::random(), best_number));
+
+		let mut state_strategy = StateStrategy::new(
+			client.clone(),
+			target_block.header().clone(),
+			None,
+			None,
+			false,
+			initial_peers,
+		);
+
+		// First request is sent.
+		assert!(state_strategy.state_request().is_some());
+
+		// No parallel request is sent.
+		assert!(state_strategy.state_request().is_none());
+	}
+
+	#[test]
+	fn received_state_response_makes_peer_available_again() {
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		state_sync_provider.expect_import().return_once(|_| ImportResult::Continue);
+		let peer_id = PeerId::random();
+		let initial_peers = std::iter::once((peer_id, 10));
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), initial_peers);
+		// Manually set the peer's state.
+		state_strategy.peers.get_mut(&peer_id).unwrap().state = PeerState::DownloadingState;
+
+		let dummy_response = OpaqueStateResponse(Box::new(StateResponse::default()));
+		state_strategy.on_state_response(peer_id, dummy_response);
+
+		assert!(state_strategy.peers.get(&peer_id).unwrap().state.is_available());
+	}
+
+	#[test]
+	fn bad_state_response_drops_peer() {
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		// Provider says that state response is bad.
+		state_sync_provider.expect_import().return_once(|_| ImportResult::BadResponse);
+		let peer_id = PeerId::random();
+		let initial_peers = std::iter::once((peer_id, 10));
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), initial_peers);
+		// Manually set the peer's state.
+		state_strategy.peers.get_mut(&peer_id).unwrap().state = PeerState::DownloadingState;
+		let dummy_response = OpaqueStateResponse(Box::new(StateResponse::default()));
+		// Receiving response drops the peer.
+		assert!(matches!(
+			state_strategy.on_state_response_inner(peer_id, dummy_response),
+			Err(BadPeer(id, _rep)) if id == peer_id,
+		));
+	}
+
+	#[test]
+	fn partial_state_response_doesnt_generate_actions() {
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		// Sync provider says that the response is partial.
+		state_sync_provider.expect_import().return_once(|_| ImportResult::Continue);
+		let peer_id = PeerId::random();
+		let initial_peers = std::iter::once((peer_id, 10));
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), initial_peers);
+		// Manually set the peer's state .
+		state_strategy.peers.get_mut(&peer_id).unwrap().state = PeerState::DownloadingState;
+
+		let dummy_response = OpaqueStateResponse(Box::new(StateResponse::default()));
+		state_strategy.on_state_response(peer_id, dummy_response);
+
+		// No actions generated.
+		assert_eq!(state_strategy.actions.len(), 0)
+	}
+
+	#[test]
+	fn complete_state_response_leads_to_block_import() {
+		// Build block to use for checks.
+		let client = Arc::new(TestClientBuilder::new().set_no_genesis().build());
+		let mut block_builder = BlockBuilderBuilder::new(&*client)
+			.on_parent_block(client.chain_info().best_hash)
+			.with_parent_block_number(client.chain_info().best_number)
+			.build()
+			.unwrap();
+		block_builder.push_storage_change(vec![1, 2, 3], Some(vec![4, 5, 6])).unwrap();
+		let block = block_builder.build().unwrap().block;
+		let header = block.header().clone();
+		let hash = header.hash();
+		let body = Some(block.extrinsics().iter().cloned().collect::<Vec<_>>());
+		let state = ImportedState { block: hash, state: KeyValueStates(Vec::new()) };
+		let justifications = Some(Justifications::from((*b"FRNK", Vec::new())));
+
+		// Prepare `StateSync`
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		let import = ImportResult::Import(
+			hash,
+			header.clone(),
+			state.clone(),
+			body.clone(),
+			justifications.clone(),
+		);
+		state_sync_provider.expect_import().return_once(move |_| import);
+
+		// Reference values to check against.
+		let expected_origin = BlockOrigin::NetworkInitialSync;
+		let expected_block = IncomingBlock {
+			hash,
+			header: Some(header),
+			body,
+			indexed_body: None,
+			justifications,
+			origin: None,
+			allow_missing_state: true,
+			import_existing: true,
+			skip_execution: true,
+			state: Some(state),
+		};
+		let expected_blocks = vec![expected_block];
+
+		// Prepare `StateStrategy`.
+		let peer_id = PeerId::random();
+		let initial_peers = std::iter::once((peer_id, 10));
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), initial_peers);
+		// Manually set the peer's state .
+		state_strategy.peers.get_mut(&peer_id).unwrap().state = PeerState::DownloadingState;
+
+		// Receive response.
+		let dummy_response = OpaqueStateResponse(Box::new(StateResponse::default()));
+		state_strategy.on_state_response(peer_id, dummy_response);
+
+		assert_eq!(state_strategy.actions.len(), 1);
+		assert!(matches!(
+			&state_strategy.actions[0],
+			StateStrategyAction::ImportBlocks { origin, blocks }
+				if *origin == expected_origin && *blocks == expected_blocks,
+		));
+	}
+
+	#[test]
+	fn importing_unknown_block_doesnt_finish_strategy() {
+		let target_hash = Hash::random();
+		let unknown_hash = Hash::random();
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		state_sync_provider.expect_target_hash().return_const(target_hash);
+
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), std::iter::empty());
+
+		// Unknown block imported.
+		state_strategy.on_blocks_processed(
+			1,
+			1,
+			vec![(
+				Ok(BlockImportStatus::ImportedUnknown(1, ImportedAux::default(), None)),
+				unknown_hash,
+			)],
+		);
+
+		// No actions generated.
+		assert_eq!(state_strategy.actions.len(), 0);
+	}
+
+	#[test]
+	fn succesfully_importing_target_block_finishes_strategy() {
+		let target_hash = Hash::random();
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		state_sync_provider.expect_target_hash().return_const(target_hash);
+
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), std::iter::empty());
+
+		// Target block imported.
+		state_strategy.on_blocks_processed(
+			1,
+			1,
+			vec![(
+				Ok(BlockImportStatus::ImportedUnknown(1, ImportedAux::default(), None)),
+				target_hash,
+			)],
+		);
+
+		// Strategy finishes.
+		assert_eq!(state_strategy.actions.len(), 1);
+		assert!(matches!(&state_strategy.actions[0], StateStrategyAction::Finished));
+	}
+
+	#[test]
+	fn failure_to_import_target_block_finishes_strategy() {
+		let target_hash = Hash::random();
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		state_sync_provider.expect_target_hash().return_const(target_hash);
+
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), std::iter::empty());
+
+		// Target block import failed.
+		state_strategy.on_blocks_processed(
+			1,
+			1,
+			vec![(
+				Err(BlockImportError::VerificationFailed(None, String::from("test-error"))),
+				target_hash,
+			)],
+		);
+
+		// Strategy finishes.
+		assert_eq!(state_strategy.actions.len(), 1);
+		assert!(matches!(&state_strategy.actions[0], StateStrategyAction::Finished));
+	}
+
+	#[test]
+	fn finished_strategy_doesnt_generate_more_actions() {
+		let target_hash = Hash::random();
+		let mut state_sync_provider = MockStateSync::<Block>::new();
+		state_sync_provider.expect_target_hash().return_const(target_hash);
+		state_sync_provider.expect_is_complete().return_const(true);
+
+		// Get enough peers for possible spurious requests.
+		let initial_peers = (1..=10).map(|best_number| (PeerId::random(), best_number));
+
+		let mut state_strategy =
+			StateStrategy::new_with_provider(Box::new(state_sync_provider), initial_peers);
+
+		state_strategy.on_blocks_processed(
+			1,
+			1,
+			vec![(
+				Ok(BlockImportStatus::ImportedUnknown(1, ImportedAux::default(), None)),
+				target_hash,
+			)],
+		);
+
+		// Strategy finishes.
+		let actions = state_strategy.actions().collect::<Vec<_>>();
+		assert_eq!(actions.len(), 1);
+		assert!(matches!(&actions[0], StateStrategyAction::Finished));
+
+		// No more actions generated.
+		assert_eq!(state_strategy.actions().count(), 0);
+	}
+}