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/chain_sync/test.rs

@@ -0,0 +1,967 @@
+// 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/>.
+
+//! Tests of [`ChainSync`].
+
+use super::*;
+use futures::executor::block_on;
+use sc_block_builder::BlockBuilderBuilder;
+use sc_network_common::sync::message::{BlockAnnounce, BlockData, BlockState, FromBlock};
+use sp_blockchain::HeaderBackend;
+use substrate_test_runtime_client::{
+	runtime::{Block, Hash, Header},
+	BlockBuilderExt, ClientBlockImportExt, ClientExt, DefaultTestClientBuilderExt, TestClient,
+	TestClientBuilder, TestClientBuilderExt,
+};
+
+#[test]
+fn processes_empty_response_on_justification_request_for_unknown_block() {
+	// if we ask for a justification for a given block to a peer that doesn't know that block
+	// (different from not having a justification), the peer will reply with an empty response.
+	// internally we should process the response as the justification not being available.
+
+	let client = Arc::new(TestClientBuilder::new().build());
+	let peer_id = PeerId::random();
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 1, 64, None).unwrap();
+
+	let (a1_hash, a1_number) = {
+		let a1 = 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;
+		(a1.hash(), *a1.header.number())
+	};
+
+	// add a new peer with the same best block
+	sync.add_peer(peer_id, a1_hash, a1_number);
+
+	// and request a justification for the block
+	sync.request_justification(&a1_hash, a1_number);
+
+	// the justification request should be scheduled to that peer
+	assert!(sync
+		.justification_requests()
+		.iter()
+		.any(|(who, request)| { *who == peer_id && request.from == FromBlock::Hash(a1_hash) }));
+
+	// there are no extra pending requests
+	assert_eq!(sync.extra_justifications.pending_requests().count(), 0);
+
+	// there's one in-flight extra request to the expected peer
+	assert!(sync.extra_justifications.active_requests().any(|(who, (hash, number))| {
+		*who == peer_id && *hash == a1_hash && *number == a1_number
+	}));
+
+	// if the peer replies with an empty response (i.e. it doesn't know the block),
+	// the active request should be cleared.
+	sync.on_block_justification(peer_id, BlockResponse::<Block> { id: 0, blocks: vec![] })
+		.unwrap();
+
+	// there should be no in-flight requests
+	assert_eq!(sync.extra_justifications.active_requests().count(), 0);
+
+	// and the request should now be pending again, waiting for reschedule
+	assert!(sync
+		.extra_justifications
+		.pending_requests()
+		.any(|(hash, number)| { *hash == a1_hash && *number == a1_number }));
+}
+
+#[test]
+fn restart_doesnt_affect_peers_downloading_finality_data() {
+	let mut client = Arc::new(TestClientBuilder::new().build());
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 1, 64, None).unwrap();
+
+	let peer_id1 = PeerId::random();
+	let peer_id2 = PeerId::random();
+	let peer_id3 = PeerId::random();
+
+	let mut new_blocks = |n| {
+		for _ in 0..n {
+			let 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;
+			block_on(client.import(BlockOrigin::Own, block.clone())).unwrap();
+		}
+
+		let info = client.info();
+		(info.best_hash, info.best_number)
+	};
+
+	let (b1_hash, b1_number) = new_blocks(50);
+
+	// add 2 peers at blocks that we don't have locally
+	sync.add_peer(peer_id1, Hash::random(), 42);
+	sync.add_peer(peer_id2, Hash::random(), 10);
+
+	// we wil send block requests to these peers
+	// for these blocks we don't know about
+	assert!(sync
+		.block_requests()
+		.into_iter()
+		.all(|(p, _)| { p == peer_id1 || p == peer_id2 }));
+
+	// add a new peer at a known block
+	sync.add_peer(peer_id3, b1_hash, b1_number);
+
+	// we request a justification for a block we have locally
+	sync.request_justification(&b1_hash, b1_number);
+
+	// the justification request should be scheduled to the
+	// new peer which is at the given block
+	assert!(sync.justification_requests().iter().any(|(p, r)| {
+		*p == peer_id3 &&
+			r.fields == BlockAttributes::JUSTIFICATION &&
+			r.from == FromBlock::Hash(b1_hash)
+	}));
+
+	assert_eq!(
+		sync.peers.get(&peer_id3).unwrap().state,
+		PeerSyncState::DownloadingJustification(b1_hash),
+	);
+
+	// clear old actions
+	let _ = sync.take_actions();
+
+	// we restart the sync state
+	sync.restart();
+	let actions = sync.take_actions().collect::<Vec<_>>();
+
+	// which should make us send out block requests to the first two peers
+	assert_eq!(actions.len(), 2);
+	assert!(actions.iter().all(|action| match action {
+		ChainSyncAction::SendBlockRequest { peer_id, .. } =>
+			peer_id == &peer_id1 || peer_id == &peer_id2,
+		_ => false,
+	}));
+
+	// peer 3 should be unaffected it was downloading finality data
+	assert_eq!(
+		sync.peers.get(&peer_id3).unwrap().state,
+		PeerSyncState::DownloadingJustification(b1_hash),
+	);
+
+	// Set common block to something that we don't have (e.g. failed import)
+	sync.peers.get_mut(&peer_id3).unwrap().common_number = 100;
+	sync.restart();
+	assert_eq!(sync.peers.get(&peer_id3).unwrap().common_number, 50);
+}
+
+/// Send a block annoucnement for the given `header`.
+fn send_block_announce(header: Header, peer_id: PeerId, sync: &mut ChainSync<Block, TestClient>) {
+	let announce = BlockAnnounce {
+		header: header.clone(),
+		state: Some(BlockState::Best),
+		data: Some(Vec::new()),
+	};
+
+	let _ = sync.on_validated_block_announce(true, peer_id, &announce);
+}
+
+/// Create a block response from the given `blocks`.
+fn create_block_response(blocks: Vec<Block>) -> BlockResponse<Block> {
+	BlockResponse::<Block> {
+		id: 0,
+		blocks: blocks
+			.into_iter()
+			.map(|b| BlockData::<Block> {
+				hash: b.hash(),
+				header: Some(b.header().clone()),
+				body: Some(b.deconstruct().1),
+				indexed_body: None,
+				receipt: None,
+				message_queue: None,
+				justification: None,
+				justifications: None,
+			})
+			.collect(),
+	}
+}
+
+/// Get a block request from `sync` and check that is matches the expected request.
+fn get_block_request(
+	sync: &mut ChainSync<Block, TestClient>,
+	from: FromBlock<Hash, u64>,
+	max: u32,
+	peer: &PeerId,
+) -> BlockRequest<Block> {
+	let requests = sync.block_requests();
+
+	log::trace!(target: LOG_TARGET, "Requests: {requests:?}");
+
+	assert_eq!(1, requests.len());
+	assert_eq!(*peer, requests[0].0);
+
+	let request = requests[0].1.clone();
+
+	assert_eq!(from, request.from);
+	assert_eq!(Some(max), request.max);
+	request
+}
+
+/// Build and import a new best block.
+fn build_block(client: &mut Arc<TestClient>, at: Option<Hash>, fork: bool) -> Block {
+	let at = at.unwrap_or_else(|| client.info().best_hash);
+
+	let mut block_builder = BlockBuilderBuilder::new(&**client)
+		.on_parent_block(at)
+		.fetch_parent_block_number(&**client)
+		.unwrap()
+		.build()
+		.unwrap();
+
+	if fork {
+		block_builder.push_storage_change(vec![1, 2, 3], Some(vec![4, 5, 6])).unwrap();
+	}
+
+	let block = block_builder.build().unwrap().block;
+
+	block_on(client.import(BlockOrigin::Own, block.clone())).unwrap();
+	block
+}
+
+fn unwrap_from_block_number(from: FromBlock<Hash, u64>) -> u64 {
+	if let FromBlock::Number(from) = from {
+		from
+	} else {
+		panic!("Expected a number!");
+	}
+}
+
+/// A regression test for a behavior we have seen on a live network.
+///
+/// The scenario is that the node is doing a full resync and is connected to some node that is
+/// doing a major sync as well. This other node that is doing a major sync will finish before
+/// our node and send a block announcement message, but we don't have seen any block
+/// announcement from this node in its sync process. Meaning our common number didn't change. It
+/// is now expected that we start an ancestor search to find the common number.
+#[test]
+fn do_ancestor_search_when_common_block_to_best_qeued_gap_is_to_big() {
+	sp_tracing::try_init_simple();
+
+	let blocks = {
+		let mut client = Arc::new(TestClientBuilder::new().build());
+		(0..MAX_DOWNLOAD_AHEAD * 2)
+			.map(|_| build_block(&mut client, None, false))
+			.collect::<Vec<_>>()
+	};
+
+	let mut client = Arc::new(TestClientBuilder::new().build());
+	let info = client.info();
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 5, 64, None).unwrap();
+
+	let peer_id1 = PeerId::random();
+	let peer_id2 = PeerId::random();
+
+	let best_block = blocks.last().unwrap().clone();
+	let max_blocks_to_request = sync.max_blocks_per_request;
+	// Connect the node we will sync from
+	sync.add_peer(peer_id1, best_block.hash(), *best_block.header().number());
+	sync.add_peer(peer_id2, info.best_hash, 0);
+
+	let mut best_block_num = 0;
+	while best_block_num < MAX_DOWNLOAD_AHEAD {
+		let request = get_block_request(
+			&mut sync,
+			FromBlock::Number(max_blocks_to_request as u64 + best_block_num as u64),
+			max_blocks_to_request as u32,
+			&peer_id1,
+		);
+
+		let from = unwrap_from_block_number(request.from.clone());
+
+		let mut resp_blocks = blocks[best_block_num as usize..from as usize].to_vec();
+		resp_blocks.reverse();
+
+		let response = create_block_response(resp_blocks.clone());
+
+		// Clear old actions to not deal with them
+		let _ = sync.take_actions();
+
+		sync.on_block_data(&peer_id1, Some(request), response).unwrap();
+
+		let actions = sync.take_actions().collect::<Vec<_>>();
+		assert_eq!(actions.len(), 1);
+		assert!(matches!(
+			&actions[0],
+			ChainSyncAction::ImportBlocks{ origin: _, blocks } if blocks.len() == max_blocks_to_request as usize,
+		));
+
+		best_block_num += max_blocks_to_request as u32;
+
+		let _ = sync.on_blocks_processed(
+			max_blocks_to_request as usize,
+			max_blocks_to_request as usize,
+			resp_blocks
+				.iter()
+				.rev()
+				.map(|b| {
+					(
+						Ok(BlockImportStatus::ImportedUnknown(
+							*b.header().number(),
+							Default::default(),
+							Some(peer_id1),
+						)),
+						b.hash(),
+					)
+				})
+				.collect(),
+		);
+
+		resp_blocks
+			.into_iter()
+			.rev()
+			.for_each(|b| block_on(client.import_as_final(BlockOrigin::Own, b)).unwrap());
+	}
+
+	// "Wait" for the queue to clear
+	sync.queue_blocks.clear();
+
+	// Let peer2 announce that it finished syncing
+	send_block_announce(best_block.header().clone(), peer_id2, &mut sync);
+
+	let (peer1_req, peer2_req) =
+		sync.block_requests().into_iter().fold((None, None), |res, req| {
+			if req.0 == peer_id1 {
+				(Some(req.1), res.1)
+			} else if req.0 == peer_id2 {
+				(res.0, Some(req.1))
+			} else {
+				panic!("Unexpected req: {:?}", req)
+			}
+		});
+
+	// We should now do an ancestor search to find the correct common block.
+	let peer2_req = peer2_req.unwrap();
+	assert_eq!(Some(1), peer2_req.max);
+	assert_eq!(FromBlock::Number(best_block_num as u64), peer2_req.from);
+
+	let response = create_block_response(vec![blocks[(best_block_num - 1) as usize].clone()]);
+
+	// Clear old actions to not deal with them
+	let _ = sync.take_actions();
+
+	sync.on_block_data(&peer_id2, Some(peer2_req), response).unwrap();
+
+	let actions = sync.take_actions().collect::<Vec<_>>();
+	assert!(actions.is_empty());
+
+	let peer1_from = unwrap_from_block_number(peer1_req.unwrap().from);
+
+	// As we are on the same chain, we should directly continue with requesting blocks from
+	// peer 2 as well.
+	get_block_request(
+		&mut sync,
+		FromBlock::Number(peer1_from + max_blocks_to_request as u64),
+		max_blocks_to_request as u32,
+		&peer_id2,
+	);
+}
+
+/// A test that ensures that we can sync a huge fork.
+///
+/// The following scenario:
+/// A peer connects to us and we both have the common block 512. The last finalized is 2048.
+/// Our best block is 4096. The peer send us a block announcement with 4097 from a fork.
+///
+/// We will first do an ancestor search to find the common block. After that we start to sync
+/// the fork and finish it ;)
+#[test]
+fn can_sync_huge_fork() {
+	sp_tracing::try_init_simple();
+
+	let mut client = Arc::new(TestClientBuilder::new().build());
+	let blocks = (0..MAX_BLOCKS_TO_LOOK_BACKWARDS * 4)
+		.map(|_| build_block(&mut client, None, false))
+		.collect::<Vec<_>>();
+
+	let fork_blocks = {
+		let mut client = Arc::new(TestClientBuilder::new().build());
+		let fork_blocks = blocks[..MAX_BLOCKS_TO_LOOK_BACKWARDS as usize * 2]
+			.into_iter()
+			.inspect(|b| block_on(client.import(BlockOrigin::Own, (*b).clone())).unwrap())
+			.cloned()
+			.collect::<Vec<_>>();
+
+		fork_blocks
+			.into_iter()
+			.chain(
+				(0..MAX_BLOCKS_TO_LOOK_BACKWARDS * 2 + 1)
+					.map(|_| build_block(&mut client, None, true)),
+			)
+			.collect::<Vec<_>>()
+	};
+
+	let info = client.info();
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 5, 64, None).unwrap();
+
+	let finalized_block = blocks[MAX_BLOCKS_TO_LOOK_BACKWARDS as usize * 2 - 1].clone();
+	let just = (*b"TEST", Vec::new());
+	client.finalize_block(finalized_block.hash(), Some(just)).unwrap();
+	sync.update_chain_info(&info.best_hash, info.best_number);
+
+	let peer_id1 = PeerId::random();
+
+	let common_block = blocks[MAX_BLOCKS_TO_LOOK_BACKWARDS as usize / 2].clone();
+	// Connect the node we will sync from
+	sync.add_peer(peer_id1, common_block.hash(), *common_block.header().number());
+
+	send_block_announce(fork_blocks.last().unwrap().header().clone(), peer_id1, &mut sync);
+
+	let mut request =
+		get_block_request(&mut sync, FromBlock::Number(info.best_number), 1, &peer_id1);
+
+	// Discard old actions we are not interested in
+	let _ = sync.take_actions();
+
+	// Do the ancestor search
+	loop {
+		let block = &fork_blocks[unwrap_from_block_number(request.from.clone()) as usize - 1];
+		let response = create_block_response(vec![block.clone()]);
+
+		sync.on_block_data(&peer_id1, Some(request), response).unwrap();
+
+		let actions = sync.take_actions().collect::<Vec<_>>();
+
+		request = if actions.is_empty() {
+			// We found the ancenstor
+			break
+		} else {
+			assert_eq!(actions.len(), 1);
+			match &actions[0] {
+				ChainSyncAction::SendBlockRequest { peer_id: _, request } => request.clone(),
+				action @ _ => panic!("Unexpected action: {action:?}"),
+			}
+		};
+
+		log::trace!(target: LOG_TARGET, "Request: {request:?}");
+	}
+
+	// Now request and import the fork.
+	let mut best_block_num = *finalized_block.header().number() as u32;
+	let max_blocks_to_request = sync.max_blocks_per_request;
+	while best_block_num < *fork_blocks.last().unwrap().header().number() as u32 - 1 {
+		let request = get_block_request(
+			&mut sync,
+			FromBlock::Number(max_blocks_to_request as u64 + best_block_num as u64),
+			max_blocks_to_request as u32,
+			&peer_id1,
+		);
+
+		let from = unwrap_from_block_number(request.from.clone());
+
+		let mut resp_blocks = fork_blocks[best_block_num as usize..from as usize].to_vec();
+		resp_blocks.reverse();
+
+		let response = create_block_response(resp_blocks.clone());
+
+		sync.on_block_data(&peer_id1, Some(request), response).unwrap();
+
+		let actions = sync.take_actions().collect::<Vec<_>>();
+		assert_eq!(actions.len(), 1);
+		assert!(matches!(
+			&actions[0],
+			ChainSyncAction::ImportBlocks{ origin: _, blocks } if blocks.len() == sync.max_blocks_per_request as usize
+		));
+
+		best_block_num += sync.max_blocks_per_request as u32;
+
+		sync.on_blocks_processed(
+			max_blocks_to_request as usize,
+			max_blocks_to_request as usize,
+			resp_blocks
+				.iter()
+				.rev()
+				.map(|b| {
+					(
+						Ok(BlockImportStatus::ImportedUnknown(
+							*b.header().number(),
+							Default::default(),
+							Some(peer_id1),
+						)),
+						b.hash(),
+					)
+				})
+				.collect(),
+		);
+
+		// Discard pending actions
+		let _ = sync.take_actions();
+
+		resp_blocks
+			.into_iter()
+			.rev()
+			.for_each(|b| block_on(client.import(BlockOrigin::Own, b)).unwrap());
+	}
+
+	// Request the tip
+	get_block_request(&mut sync, FromBlock::Hash(fork_blocks.last().unwrap().hash()), 1, &peer_id1);
+}
+
+#[test]
+fn syncs_fork_without_duplicate_requests() {
+	sp_tracing::try_init_simple();
+
+	let mut client = Arc::new(TestClientBuilder::new().build());
+	let blocks = (0..MAX_BLOCKS_TO_LOOK_BACKWARDS * 4)
+		.map(|_| build_block(&mut client, None, false))
+		.collect::<Vec<_>>();
+
+	let fork_blocks = {
+		let mut client = Arc::new(TestClientBuilder::new().build());
+		let fork_blocks = blocks[..MAX_BLOCKS_TO_LOOK_BACKWARDS as usize * 2]
+			.into_iter()
+			.inspect(|b| block_on(client.import(BlockOrigin::Own, (*b).clone())).unwrap())
+			.cloned()
+			.collect::<Vec<_>>();
+
+		fork_blocks
+			.into_iter()
+			.chain(
+				(0..MAX_BLOCKS_TO_LOOK_BACKWARDS * 2 + 1)
+					.map(|_| build_block(&mut client, None, true)),
+			)
+			.collect::<Vec<_>>()
+	};
+
+	let info = client.info();
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 5, 64, None).unwrap();
+
+	let finalized_block = blocks[MAX_BLOCKS_TO_LOOK_BACKWARDS as usize * 2 - 1].clone();
+	let just = (*b"TEST", Vec::new());
+	client.finalize_block(finalized_block.hash(), Some(just)).unwrap();
+	sync.update_chain_info(&info.best_hash, info.best_number);
+
+	let peer_id1 = PeerId::random();
+
+	let common_block = blocks[MAX_BLOCKS_TO_LOOK_BACKWARDS as usize / 2].clone();
+	// Connect the node we will sync from
+	sync.add_peer(peer_id1, common_block.hash(), *common_block.header().number());
+
+	send_block_announce(fork_blocks.last().unwrap().header().clone(), peer_id1, &mut sync);
+
+	let mut request =
+		get_block_request(&mut sync, FromBlock::Number(info.best_number), 1, &peer_id1);
+
+	// Discard pending actions
+	let _ = sync.take_actions();
+
+	// Do the ancestor search
+	loop {
+		let block = &fork_blocks[unwrap_from_block_number(request.from.clone()) as usize - 1];
+		let response = create_block_response(vec![block.clone()]);
+
+		sync.on_block_data(&peer_id1, Some(request), response).unwrap();
+
+		let actions = sync.take_actions().collect::<Vec<_>>();
+
+		request = if actions.is_empty() {
+			// We found the ancenstor
+			break
+		} else {
+			assert_eq!(actions.len(), 1);
+			match &actions[0] {
+				ChainSyncAction::SendBlockRequest { peer_id: _, request } => request.clone(),
+				action @ _ => panic!("Unexpected action: {action:?}"),
+			}
+		};
+
+		log::trace!(target: LOG_TARGET, "Request: {request:?}");
+	}
+
+	// Now request and import the fork.
+	let mut best_block_num = *finalized_block.header().number() as u32;
+	let max_blocks_to_request = sync.max_blocks_per_request;
+
+	let mut request = get_block_request(
+		&mut sync,
+		FromBlock::Number(max_blocks_to_request as u64 + best_block_num as u64),
+		max_blocks_to_request as u32,
+		&peer_id1,
+	);
+	let last_block_num = *fork_blocks.last().unwrap().header().number() as u32 - 1;
+	while best_block_num < last_block_num {
+		let from = unwrap_from_block_number(request.from.clone());
+
+		let mut resp_blocks = fork_blocks[best_block_num as usize..from as usize].to_vec();
+		resp_blocks.reverse();
+
+		let response = create_block_response(resp_blocks.clone());
+
+		// Discard old actions
+		let _ = sync.take_actions();
+
+		sync.on_block_data(&peer_id1, Some(request.clone()), response).unwrap();
+
+		let actions = sync.take_actions().collect::<Vec<_>>();
+		assert_eq!(actions.len(), 1);
+		assert!(matches!(
+			&actions[0],
+			ChainSyncAction::ImportBlocks{ origin: _, blocks } if blocks.len() == max_blocks_to_request as usize
+		));
+
+		best_block_num += max_blocks_to_request as u32;
+
+		if best_block_num < last_block_num {
+			// make sure we're not getting a duplicate request in the time before the blocks are
+			// processed
+			request = get_block_request(
+				&mut sync,
+				FromBlock::Number(max_blocks_to_request as u64 + best_block_num as u64),
+				max_blocks_to_request as u32,
+				&peer_id1,
+			);
+		}
+
+		let mut notify_imported: Vec<_> = resp_blocks
+			.iter()
+			.rev()
+			.map(|b| {
+				(
+					Ok(BlockImportStatus::ImportedUnknown(
+						*b.header().number(),
+						Default::default(),
+						Some(peer_id1),
+					)),
+					b.hash(),
+				)
+			})
+			.collect();
+
+		// The import queue may send notifications in batches of varying size. So we simulate
+		// this here by splitting the batch into 2 notifications.
+		let max_blocks_to_request = sync.max_blocks_per_request;
+		let second_batch = notify_imported.split_off(notify_imported.len() / 2);
+		let _ = sync.on_blocks_processed(
+			max_blocks_to_request as usize,
+			max_blocks_to_request as usize,
+			notify_imported,
+		);
+
+		let _ = sync.on_blocks_processed(
+			max_blocks_to_request as usize,
+			max_blocks_to_request as usize,
+			second_batch,
+		);
+
+		resp_blocks
+			.into_iter()
+			.rev()
+			.for_each(|b| block_on(client.import(BlockOrigin::Own, b)).unwrap());
+	}
+
+	// Request the tip
+	get_block_request(&mut sync, FromBlock::Hash(fork_blocks.last().unwrap().hash()), 1, &peer_id1);
+}
+
+#[test]
+fn removes_target_fork_on_disconnect() {
+	sp_tracing::try_init_simple();
+	let mut client = Arc::new(TestClientBuilder::new().build());
+	let blocks = (0..3).map(|_| build_block(&mut client, None, false)).collect::<Vec<_>>();
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 1, 64, None).unwrap();
+
+	let peer_id1 = PeerId::random();
+	let common_block = blocks[1].clone();
+	// Connect the node we will sync from
+	sync.add_peer(peer_id1, common_block.hash(), *common_block.header().number());
+
+	// Create a "new" header and announce it
+	let mut header = blocks[0].header().clone();
+	header.number = 4;
+	send_block_announce(header, peer_id1, &mut sync);
+	assert!(sync.fork_targets.len() == 1);
+
+	let _ = sync.remove_peer(&peer_id1);
+	assert!(sync.fork_targets.len() == 0);
+}
+
+#[test]
+fn can_import_response_with_missing_blocks() {
+	sp_tracing::try_init_simple();
+	let mut client2 = Arc::new(TestClientBuilder::new().build());
+	let blocks = (0..4).map(|_| build_block(&mut client2, None, false)).collect::<Vec<_>>();
+
+	let empty_client = Arc::new(TestClientBuilder::new().build());
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, empty_client.clone(), 1, 64, None).unwrap();
+
+	let peer_id1 = PeerId::random();
+	let best_block = blocks[3].clone();
+	sync.add_peer(peer_id1, best_block.hash(), *best_block.header().number());
+
+	sync.peers.get_mut(&peer_id1).unwrap().state = PeerSyncState::Available;
+	sync.peers.get_mut(&peer_id1).unwrap().common_number = 0;
+
+	// Request all missing blocks and respond only with some.
+	let request = get_block_request(&mut sync, FromBlock::Hash(best_block.hash()), 4, &peer_id1);
+	let response =
+		create_block_response(vec![blocks[3].clone(), blocks[2].clone(), blocks[1].clone()]);
+	sync.on_block_data(&peer_id1, Some(request.clone()), response).unwrap();
+	assert_eq!(sync.best_queued_number, 0);
+
+	// Request should only contain the missing block.
+	let request = get_block_request(&mut sync, FromBlock::Number(1), 1, &peer_id1);
+	let response = create_block_response(vec![blocks[0].clone()]);
+	sync.on_block_data(&peer_id1, Some(request), response).unwrap();
+	assert_eq!(sync.best_queued_number, 4);
+}
+#[test]
+fn ancestor_search_repeat() {
+	let state = AncestorSearchState::<Block>::BinarySearch(1, 3);
+	assert!(handle_ancestor_search_state(&state, 2, true).is_none());
+}
+
+#[test]
+fn sync_restart_removes_block_but_not_justification_requests() {
+	let mut client = Arc::new(TestClientBuilder::new().build());
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 1, 64, None).unwrap();
+
+	let peers = vec![PeerId::random(), PeerId::random()];
+
+	let mut new_blocks = |n| {
+		for _ in 0..n {
+			let 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;
+			block_on(client.import(BlockOrigin::Own, block.clone())).unwrap();
+		}
+
+		let info = client.info();
+		(info.best_hash, info.best_number)
+	};
+
+	let (b1_hash, b1_number) = new_blocks(50);
+
+	// add new peer and request blocks from them
+	sync.add_peer(peers[0], Hash::random(), 42);
+
+	// we don't actually perform any requests, just keep track of peers waiting for a response
+	let mut pending_responses = HashSet::new();
+
+	// we wil send block requests to these peers
+	// for these blocks we don't know about
+	for (peer, _request) in sync.block_requests() {
+		// "send" request
+		pending_responses.insert(peer);
+	}
+
+	// add a new peer at a known block
+	sync.add_peer(peers[1], b1_hash, b1_number);
+
+	// we request a justification for a block we have locally
+	sync.request_justification(&b1_hash, b1_number);
+
+	// the justification request should be scheduled to the
+	// new peer which is at the given block
+	let mut requests = sync.justification_requests();
+	assert_eq!(requests.len(), 1);
+	let (peer, _request) = requests.remove(0);
+	// "send" request
+	assert!(pending_responses.insert(peer));
+
+	assert!(!std::matches!(
+		sync.peers.get(&peers[0]).unwrap().state,
+		PeerSyncState::DownloadingJustification(_),
+	));
+	assert_eq!(
+		sync.peers.get(&peers[1]).unwrap().state,
+		PeerSyncState::DownloadingJustification(b1_hash),
+	);
+	assert_eq!(pending_responses.len(), 2);
+
+	// discard old actions
+	let _ = sync.take_actions();
+
+	// restart sync
+	sync.restart();
+	let actions = sync.take_actions().collect::<Vec<_>>();
+	for action in actions.iter() {
+		match action {
+			ChainSyncAction::CancelBlockRequest { peer_id } => {
+				pending_responses.remove(&peer_id);
+			},
+			ChainSyncAction::SendBlockRequest { peer_id, .. } => {
+				// we drop obsolete response, but don't register a new request, it's checked in
+				// the `assert!` below
+				pending_responses.remove(&peer_id);
+			},
+			action @ _ => panic!("Unexpected action: {action:?}"),
+		}
+	}
+	assert!(actions.iter().any(|action| {
+		match action {
+			ChainSyncAction::SendBlockRequest { peer_id, .. } => peer_id == &peers[0],
+			_ => false,
+		}
+	}));
+
+	assert_eq!(pending_responses.len(), 1);
+	assert!(pending_responses.contains(&peers[1]));
+	assert_eq!(
+		sync.peers.get(&peers[1]).unwrap().state,
+		PeerSyncState::DownloadingJustification(b1_hash),
+	);
+	let _ = sync.remove_peer(&peers[1]);
+	pending_responses.remove(&peers[1]);
+	assert_eq!(pending_responses.len(), 0);
+}
+
+/// The test demonstrates https://github.com/paritytech/polkadot-sdk/issues/2094.
+/// TODO: convert it into desired behavior test once the issue is fixed (see inline comments).
+/// The issue: we currently rely on block numbers instead of block hash
+/// to download blocks from peers. As a result, we can end up with blocks
+/// from different forks as shown by the test.
+#[test]
+#[should_panic]
+fn request_across_forks() {
+	sp_tracing::try_init_simple();
+
+	let mut client = Arc::new(TestClientBuilder::new().build());
+	let blocks = (0..100).map(|_| build_block(&mut client, None, false)).collect::<Vec<_>>();
+
+	let fork_a_blocks = {
+		let mut client = Arc::new(TestClientBuilder::new().build());
+		let mut fork_blocks = blocks[..]
+			.into_iter()
+			.inspect(|b| {
+				assert!(matches!(client.block(*b.header.parent_hash()), Ok(Some(_))));
+				block_on(client.import(BlockOrigin::Own, (*b).clone())).unwrap()
+			})
+			.cloned()
+			.collect::<Vec<_>>();
+		for _ in 0..10 {
+			fork_blocks.push(build_block(&mut client, None, false));
+		}
+		fork_blocks
+	};
+
+	let fork_b_blocks = {
+		let mut client = Arc::new(TestClientBuilder::new().build());
+		let mut fork_blocks = blocks[..]
+			.into_iter()
+			.inspect(|b| {
+				assert!(matches!(client.block(*b.header.parent_hash()), Ok(Some(_))));
+				block_on(client.import(BlockOrigin::Own, (*b).clone())).unwrap()
+			})
+			.cloned()
+			.collect::<Vec<_>>();
+		for _ in 0..10 {
+			fork_blocks.push(build_block(&mut client, None, true));
+		}
+		fork_blocks
+	};
+
+	let mut sync = ChainSync::new(ChainSyncMode::Full, client.clone(), 5, 64, None).unwrap();
+
+	// Add the peers, all at the common ancestor 100.
+	let common_block = blocks.last().unwrap();
+	let peer_id1 = PeerId::random();
+	sync.add_peer(peer_id1, common_block.hash(), *common_block.header().number());
+	let peer_id2 = PeerId::random();
+	sync.add_peer(peer_id2, common_block.hash(), *common_block.header().number());
+
+	// Peer 1 announces 107 from fork 1, 100-107 get downloaded.
+	{
+		let block = (&fork_a_blocks[106]).clone();
+		let peer = peer_id1;
+		log::trace!(target: LOG_TARGET, "<1> {peer} announces from fork 1");
+		send_block_announce(block.header().clone(), peer, &mut sync);
+		let request = get_block_request(&mut sync, FromBlock::Hash(block.hash()), 7, &peer);
+		let mut resp_blocks = fork_a_blocks[100_usize..107_usize].to_vec();
+		resp_blocks.reverse();
+		let response = create_block_response(resp_blocks.clone());
+
+		// Drop old actions
+		let _ = sync.take_actions();
+
+		sync.on_block_data(&peer, Some(request), response).unwrap();
+		let actions = sync.take_actions().collect::<Vec<_>>();
+		assert_eq!(actions.len(), 1);
+		assert!(matches!(
+			&actions[0],
+			ChainSyncAction::ImportBlocks{ origin: _, blocks } if blocks.len() == 7_usize
+		));
+		assert_eq!(sync.best_queued_number, 107);
+		assert_eq!(sync.best_queued_hash, block.hash());
+		assert!(sync.is_known(&block.header.parent_hash()));
+	}
+
+	// Peer 2 also announces 107 from fork 1.
+	{
+		let prev_best_number = sync.best_queued_number;
+		let prev_best_hash = sync.best_queued_hash;
+		let peer = peer_id2;
+		log::trace!(target: LOG_TARGET, "<2> {peer} announces from fork 1");
+		for i in 100..107 {
+			let block = (&fork_a_blocks[i]).clone();
+			send_block_announce(block.header().clone(), peer, &mut sync);
+			assert!(sync.block_requests().is_empty());
+		}
+		assert_eq!(sync.best_queued_number, prev_best_number);
+		assert_eq!(sync.best_queued_hash, prev_best_hash);
+	}
+
+	// Peer 2 undergoes reorg, announces 108 from fork 2, gets downloaded even though we
+	// don't have the parent from fork 2.
+	{
+		let block = (&fork_b_blocks[107]).clone();
+		let peer = peer_id2;
+		log::trace!(target: LOG_TARGET, "<3> {peer} announces from fork 2");
+		send_block_announce(block.header().clone(), peer, &mut sync);
+		// TODO: when the issue is fixed, this test can be changed to test the
+		// expected behavior instead. The needed changes would be:
+		// 1. Remove the `#[should_panic]` directive
+		// 2. These should be changed to check that sync.block_requests().is_empty(), after the
+		//    block is announced.
+		let request = get_block_request(&mut sync, FromBlock::Hash(block.hash()), 1, &peer);
+		let response = create_block_response(vec![block.clone()]);
+
+		// Drop old actions we are not going to check
+		let _ = sync.take_actions();
+
+		sync.on_block_data(&peer, Some(request), response).unwrap();
+		let actions = sync.take_actions().collect::<Vec<_>>();
+		assert_eq!(actions.len(), 1);
+		assert!(matches!(
+			&actions[0],
+			ChainSyncAction::ImportBlocks{ origin: _, blocks } if blocks.len() == 1_usize
+		));
+		assert!(sync.is_known(&block.header.parent_hash()));
+	}
+}

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);
+	}
+}

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

@@ -0,0 +1,328 @@
+// 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 support.
+
+use crate::{
+	schema::v1::{StateEntry, StateRequest, StateResponse},
+	LOG_TARGET,
+};
+use codec::{Decode, Encode};
+use log::debug;
+use sc_client_api::{CompactProof, ProofProvider};
+use sc_consensus::ImportedState;
+use smallvec::SmallVec;
+use sp_core::storage::well_known_keys;
+use sp_runtime::{
+	traits::{Block as BlockT, Header, NumberFor},
+	Justifications,
+};
+use std::{collections::HashMap, fmt, sync::Arc};
+
+/// Generic state sync provider. Used for mocking in tests.
+pub trait StateSyncProvider<B: BlockT>: Send + Sync {
+	/// Validate and import a state response.
+	fn import(&mut self, response: StateResponse) -> ImportResult<B>;
+	/// Produce next state request.
+	fn next_request(&self) -> StateRequest;
+	/// Check if the state is complete.
+	fn is_complete(&self) -> bool;
+	/// Returns target block number.
+	fn target_number(&self) -> NumberFor<B>;
+	/// Returns target block hash.
+	fn target_hash(&self) -> B::Hash;
+	/// Returns state sync estimated progress.
+	fn progress(&self) -> StateSyncProgress;
+}
+
+// Reported state sync phase.
+#[derive(Clone, Eq, PartialEq, Debug)]
+pub enum StateSyncPhase {
+	// State download in progress.
+	DownloadingState,
+	// Download is complete, state is being imported.
+	ImportingState,
+}
+
+impl fmt::Display for StateSyncPhase {
+	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+		match self {
+			Self::DownloadingState => write!(f, "Downloading state"),
+			Self::ImportingState => write!(f, "Importing state"),
+		}
+	}
+}
+
+/// Reported state download progress.
+#[derive(Clone, Eq, PartialEq, Debug)]
+pub struct StateSyncProgress {
+	/// Estimated download percentage.
+	pub percentage: u32,
+	/// Total state size in bytes downloaded so far.
+	pub size: u64,
+	/// Current state sync phase.
+	pub phase: StateSyncPhase,
+}
+
+/// Import state chunk result.
+pub enum ImportResult<B: BlockT> {
+	/// State is complete and ready for import.
+	Import(B::Hash, B::Header, ImportedState<B>, Option<Vec<B::Extrinsic>>, Option<Justifications>),
+	/// Continue downloading.
+	Continue,
+	/// Bad state chunk.
+	BadResponse,
+}
+
+/// State sync state machine. Accumulates partial state data until it
+/// is ready to be imported.
+pub struct StateSync<B: BlockT, Client> {
+	target_block: B::Hash,
+	target_header: B::Header,
+	target_root: B::Hash,
+	target_body: Option<Vec<B::Extrinsic>>,
+	target_justifications: Option<Justifications>,
+	last_key: SmallVec<[Vec<u8>; 2]>,
+	state: HashMap<Vec<u8>, (Vec<(Vec<u8>, Vec<u8>)>, Vec<Vec<u8>>)>,
+	complete: bool,
+	client: Arc<Client>,
+	imported_bytes: u64,
+	skip_proof: bool,
+}
+
+impl<B, Client> StateSync<B, Client>
+where
+	B: BlockT,
+	Client: ProofProvider<B> + Send + Sync + 'static,
+{
+	///  Create a new instance.
+	pub fn new(
+		client: Arc<Client>,
+		target_header: B::Header,
+		target_body: Option<Vec<B::Extrinsic>>,
+		target_justifications: Option<Justifications>,
+		skip_proof: bool,
+	) -> Self {
+		Self {
+			client,
+			target_block: target_header.hash(),
+			target_root: *target_header.state_root(),
+			target_header,
+			target_body,
+			target_justifications,
+			last_key: SmallVec::default(),
+			state: HashMap::default(),
+			complete: false,
+			imported_bytes: 0,
+			skip_proof,
+		}
+	}
+}
+
+impl<B, Client> StateSyncProvider<B> for StateSync<B, Client>
+where
+	B: BlockT,
+	Client: ProofProvider<B> + Send + Sync + 'static,
+{
+	///  Validate and import a state response.
+	fn import(&mut self, response: StateResponse) -> ImportResult<B> {
+		if response.entries.is_empty() && response.proof.is_empty() {
+			debug!(target: LOG_TARGET, "Bad state response");
+			return ImportResult::BadResponse
+		}
+		if !self.skip_proof && response.proof.is_empty() {
+			debug!(target: LOG_TARGET, "Missing proof");
+			return ImportResult::BadResponse
+		}
+		let complete = if !self.skip_proof {
+			debug!(target: LOG_TARGET, "Importing state from {} trie nodes", response.proof.len());
+			let proof_size = response.proof.len() as u64;
+			let proof = match CompactProof::decode(&mut response.proof.as_ref()) {
+				Ok(proof) => proof,
+				Err(e) => {
+					debug!(target: LOG_TARGET, "Error decoding proof: {:?}", e);
+					return ImportResult::BadResponse
+				},
+			};
+			let (values, completed) = match self.client.verify_range_proof(
+				self.target_root,
+				proof,
+				self.last_key.as_slice(),
+			) {
+				Err(e) => {
+					debug!(
+						target: LOG_TARGET,
+						"StateResponse failed proof verification: {}",
+						e,
+					);
+					return ImportResult::BadResponse
+				},
+				Ok(values) => values,
+			};
+			debug!(target: LOG_TARGET, "Imported with {} keys", values.len());
+
+			let complete = completed == 0;
+			if !complete && !values.update_last_key(completed, &mut self.last_key) {
+				debug!(target: LOG_TARGET, "Error updating key cursor, depth: {}", completed);
+			};
+
+			for values in values.0 {
+				let key_values = if values.state_root.is_empty() {
+					// Read child trie roots.
+					values
+						.key_values
+						.into_iter()
+						.filter(|key_value| {
+							if well_known_keys::is_child_storage_key(key_value.0.as_slice()) {
+								self.state
+									.entry(key_value.1.clone())
+									.or_default()
+									.1
+									.push(key_value.0.clone());
+								false
+							} else {
+								true
+							}
+						})
+						.collect()
+				} else {
+					values.key_values
+				};
+				let entry = self.state.entry(values.state_root).or_default();
+				if entry.0.len() > 0 && entry.1.len() > 1 {
+					// Already imported child_trie with same root.
+					// Warning this will not work with parallel download.
+				} else if entry.0.is_empty() {
+					for (key, _value) in key_values.iter() {
+						self.imported_bytes += key.len() as u64;
+					}
+
+					entry.0 = key_values;
+				} else {
+					for (key, value) in key_values {
+						self.imported_bytes += key.len() as u64;
+						entry.0.push((key, value))
+					}
+				}
+			}
+			self.imported_bytes += proof_size;
+			complete
+		} else {
+			let mut complete = true;
+			// if the trie is a child trie and one of its parent trie is empty,
+			// the parent cursor stays valid.
+			// Empty parent trie content only happens when all the response content
+			// is part of a single child trie.
+			if self.last_key.len() == 2 && response.entries[0].entries.is_empty() {
+				// Do not remove the parent trie position.
+				self.last_key.pop();
+			} else {
+				self.last_key.clear();
+			}
+			for state in response.entries {
+				debug!(
+					target: LOG_TARGET,
+					"Importing state from {:?} to {:?}",
+					state.entries.last().map(|e| sp_core::hexdisplay::HexDisplay::from(&e.key)),
+					state.entries.first().map(|e| sp_core::hexdisplay::HexDisplay::from(&e.key)),
+				);
+
+				if !state.complete {
+					if let Some(e) = state.entries.last() {
+						self.last_key.push(e.key.clone());
+					}
+					complete = false;
+				}
+				let is_top = state.state_root.is_empty();
+				let entry = self.state.entry(state.state_root).or_default();
+				if entry.0.len() > 0 && entry.1.len() > 1 {
+					// Already imported child trie with same root.
+				} else {
+					let mut child_roots = Vec::new();
+					for StateEntry { key, value } in state.entries {
+						// Skip all child key root (will be recalculated on import).
+						if is_top && well_known_keys::is_child_storage_key(key.as_slice()) {
+							child_roots.push((value, key));
+						} else {
+							self.imported_bytes += key.len() as u64;
+							entry.0.push((key, value))
+						}
+					}
+					for (root, storage_key) in child_roots {
+						self.state.entry(root).or_default().1.push(storage_key);
+					}
+				}
+			}
+			complete
+		};
+		if complete {
+			self.complete = true;
+			ImportResult::Import(
+				self.target_block,
+				self.target_header.clone(),
+				ImportedState {
+					block: self.target_block,
+					state: std::mem::take(&mut self.state).into(),
+				},
+				self.target_body.clone(),
+				self.target_justifications.clone(),
+			)
+		} else {
+			ImportResult::Continue
+		}
+	}
+
+	/// Produce next state request.
+	fn next_request(&self) -> StateRequest {
+		StateRequest {
+			block: self.target_block.encode(),
+			start: self.last_key.clone().into_vec(),
+			no_proof: self.skip_proof,
+		}
+	}
+
+	/// Check if the state is complete.
+	fn is_complete(&self) -> bool {
+		self.complete
+	}
+
+	/// Returns target block number.
+	fn target_number(&self) -> NumberFor<B> {
+		*self.target_header.number()
+	}
+
+	/// Returns target block hash.
+	fn target_hash(&self) -> B::Hash {
+		self.target_block
+	}
+
+	/// Returns state sync estimated progress.
+	fn progress(&self) -> StateSyncProgress {
+		let cursor = *self.last_key.get(0).and_then(|last| last.get(0)).unwrap_or(&0u8);
+		let percent_done = cursor as u32 * 100 / 256;
+		StateSyncProgress {
+			percentage: percent_done,
+			size: self.imported_bytes,
+			phase: if self.complete {
+				StateSyncPhase::ImportingState
+			} else {
+				StateSyncPhase::DownloadingState
+			},
+		}
+	}
+}