sc_network_sync/

blocks.rs

// 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/>.

use crate::LOG_TARGET;
use log::trace;
use prometheus_endpoint::{register, Counter, PrometheusError, Registry, U64};
use sc_network_common::sync::message;
use sc_network_types::PeerId;
use sp_arithmetic::traits::Saturating;
use sp_runtime::traits::{Block as BlockT, NumberFor, One};
use std::{
	cmp,
	collections::{BTreeMap, HashMap},
	ops::Range,
};

/// Block data with origin.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct BlockData<B: BlockT> {
	/// The Block Message from the wire
	pub block: message::BlockData<B>,
	/// The peer, we received this from
	pub origin: Option<PeerId>,
}

#[derive(Debug)]
enum BlockRangeState<B: BlockT> {
	Downloading { len: NumberFor<B>, downloading: u32 },
	Complete(Vec<BlockData<B>>),
	Queued { len: NumberFor<B> },
}

impl<B: BlockT> BlockRangeState<B> {
	pub fn len(&self) -> NumberFor<B> {
		match *self {
			Self::Downloading { len, .. } => len,
			Self::Complete(ref blocks) => (blocks.len() as u32).into(),
			Self::Queued { len } => len,
		}
	}
}

#[derive(Clone)]
pub(crate) struct Metrics {
	stale_download_reservations: Counter<U64>,
}

impl Metrics {
	pub(crate) fn register(registry: &Registry) -> Result<Self, PrometheusError> {
		Ok(Self {
			stale_download_reservations: register(
				Counter::new(
					"substrate_sync_stale_download_reservations_total",
					"Number of stale block download reservations released before scheduling a new range",
				)?,
				registry,
			)?,
		})
	}
}

/// A collection of blocks being downloaded.
#[derive(Default)]
pub struct BlockCollection<B: BlockT> {
	/// Downloaded blocks.
	blocks: BTreeMap<NumberFor<B>, BlockRangeState<B>>,
	peer_requests: HashMap<PeerId, NumberFor<B>>,
	/// Block ranges downloaded and queued for import.
	/// Maps start_hash => (start_num, end_num).
	queued_blocks: HashMap<B::Hash, (NumberFor<B>, NumberFor<B>)>,
	/// Metrics shared by regular and gap sync.
	metrics: Option<Metrics>,
}

impl<B: BlockT> BlockCollection<B> {
	/// Create a new instance.
	pub fn new() -> Self {
		Self::with_metrics(None)
	}

	pub(crate) fn with_metrics(metrics: Option<Metrics>) -> Self {
		Self {
			blocks: BTreeMap::new(),
			peer_requests: HashMap::new(),
			queued_blocks: HashMap::new(),
			metrics,
		}
	}

	/// Clear everything.
	pub fn clear(&mut self) {
		self.blocks.clear();
		self.peer_requests.clear();
	}

	/// Insert a set of blocks into collection.
	pub fn insert(&mut self, start: NumberFor<B>, blocks: Vec<message::BlockData<B>>, who: PeerId) {
		if blocks.is_empty() {
			return;
		}

		match self.blocks.get(&start) {
			Some(&BlockRangeState::Downloading { .. }) => {
				trace!(target: LOG_TARGET, "Inserting block data still marked as being downloaded: {}", start);
			},
			Some(BlockRangeState::Complete(existing)) if existing.len() >= blocks.len() => {
				trace!(target: LOG_TARGET, "Ignored block data already downloaded: {}", start);
				return;
			},
			_ => (),
		}

		self.blocks.insert(
			start,
			BlockRangeState::Complete(
				blocks.into_iter().map(|b| BlockData { origin: Some(who), block: b }).collect(),
			),
		);
	}

	/// Returns a set of block hashes that require a header download. The returned set is marked as
	/// being downloaded.
	pub fn needed_blocks(
		&mut self,
		who: PeerId,
		count: u32,
		peer_best: NumberFor<B>,
		common: NumberFor<B>,
		max_parallel: u32,
		max_ahead: u32,
	) -> Option<Range<NumberFor<B>>> {
		// Cancellation, responses and disconnection should release the previous range.
		if self.peer_requests.contains_key(&who) {
			log::debug!(target: LOG_TARGET, "Releasing stale block download reservation for {who}");
			if let Some(metrics) = &self.metrics {
				metrics.stale_download_reservations.inc();
			}
			self.clear_peer_download(&who);
		}
		if peer_best <= common {
			// Bail out early
			return None;
		}
		// First block number that we need to download
		let first_different = common + <NumberFor<B>>::one();
		let count = (count as u32).into();
		let (mut range, downloading) = {
			// Iterate through the ranges in `self.blocks` looking for a range to download
			let mut downloading_iter = self.blocks.iter().peekable();
			let mut prev: Option<(&NumberFor<B>, &BlockRangeState<B>)> = None;
			loop {
				let next = downloading_iter.next();
				break match (prev, next) {
					// If we are already downloading this range, request it from `max_parallel`
					// peers (`max_parallel = 5` by default).
					// Do not request already downloading range from peers with common number above
					// the range start.
					(Some((start, &BlockRangeState::Downloading { ref len, downloading })), _)
						if downloading < max_parallel && *start >= first_different =>
					{
						(*start..*start + *len, downloading)
					},
					// If there is a gap between ranges requested, download this gap unless the peer
					// has common number above the gap start
					(Some((start, r)), Some((next_start, _)))
						if *start + r.len() < *next_start &&
							*start + r.len() >= first_different =>
					{
						(*start + r.len()..cmp::min(*next_start, *start + r.len() + count), 0)
					},
					// Download `count` blocks after the last range requested unless the peer
					// has common number above this new range
					(Some((start, r)), None) if *start + r.len() >= first_different => {
						(*start + r.len()..*start + r.len() + count, 0)
					},
					// If there are no ranges currently requested, download `count` blocks after
					// `common` number
					(None, None) => (first_different..first_different + count, 0),
					// If the first range starts above `common + 1`, download the gap at the start
					(None, Some((start, _))) if *start > first_different => {
						(first_different..cmp::min(first_different + count, *start), 0)
					},
					// Move on to the next range pair
					_ => {
						prev = next;
						continue;
					},
				};
			}
		};
		// crop to peers best
		if range.start > peer_best {
			trace!(target: LOG_TARGET, "Out of range for peer {} ({} vs {})", who, range.start, peer_best);
			return None;
		}

		range.end = cmp::min(peer_best.saturating_add(One::one()), range.end);

		if self
			.blocks
			.iter()
			.next()
			.map_or(false, |(n, _)| range.start > *n + max_ahead.into())
		{
			trace!(target: LOG_TARGET, "Too far ahead for peer {} ({})", who, range.start);
			return None;
		}

		if range.end <= range.start {
			debug_assert!(
				false,
				"Empty range {:?}, count={}, peer_best={}, common={}, blocks={:?}",
				range, count, peer_best, common, self.blocks
			);
			trace!(
				target: LOG_TARGET,
				"Empty range for peer {who}: {range:?}, count={count}, peer_best={peer_best}, common={common}",
			);
			return None;
		}

		self.peer_requests.insert(who, range.start);
		self.blocks.insert(
			range.start,
			BlockRangeState::Downloading {
				len: range.end - range.start,
				downloading: downloading + 1,
			},
		);

		Some(range)
	}

	/// Get a valid chain of blocks ordered in descending order and ready for importing into
	/// the blockchain.
	/// `from` is the maximum block number for the start of the range that we are interested in.
	/// The function will return empty Vec if the first block ready is higher than `from`.
	/// For each returned block hash `clear_queued` must be called at some later stage.
	pub fn ready_blocks(&mut self, from: NumberFor<B>) -> Vec<BlockData<B>> {
		let mut ready = Vec::new();

		let mut prev = from;
		for (&start, range_data) in &mut self.blocks {
			if start > prev {
				break;
			}
			let len = match range_data {
				BlockRangeState::Complete(blocks) => {
					let len = (blocks.len() as u32).into();
					prev = start + len;
					if let Some(BlockData { block, .. }) = blocks.first() {
						self.queued_blocks
							.insert(block.hash, (start, start + (blocks.len() as u32).into()));
					}
					// Remove all elements from `blocks` and add them to `ready`
					ready.append(blocks);
					len
				},
				BlockRangeState::Queued { .. } => continue,
				_ => break,
			};
			*range_data = BlockRangeState::Queued { len };
		}
		trace!(target: LOG_TARGET, "{} blocks ready for import", ready.len());
		ready
	}

	pub fn clear_queued(&mut self, hash: &B::Hash) {
		if let Some((from, to)) = self.queued_blocks.remove(hash) {
			let mut block_num = from;
			while block_num < to {
				self.blocks.remove(&block_num);
				block_num += One::one();
			}
			trace!(target: LOG_TARGET, "Cleared blocks from {:?} to {:?}", from, to);
		}
	}

	pub fn clear_peer_download(&mut self, who: &PeerId) {
		if let Some(start) = self.peer_requests.remove(who) {
			let remove = match self.blocks.get_mut(&start) {
				Some(&mut BlockRangeState::Downloading { ref mut downloading, .. })
					if *downloading > 1 =>
				{
					*downloading -= 1;
					false
				},
				Some(&mut BlockRangeState::Downloading { .. }) => true,
				_ => false,
			};
			if remove {
				self.blocks.remove(&start);
			}
		}
	}
}

#[cfg(test)]
mod test {
	use super::{BlockCollection, BlockData, BlockRangeState};
	use sc_network_common::sync::message;
	use sc_network_types::PeerId;
	use sp_core::H256;
	use sp_runtime::testing::{Block as RawBlock, MockCallU64, TestXt};

	type Block = RawBlock<TestXt<MockCallU64, ()>>;

	fn is_empty(bc: &BlockCollection<Block>) -> bool {
		bc.blocks.is_empty() && bc.peer_requests.is_empty()
	}

	fn generate_blocks(n: usize) -> Vec<message::BlockData<Block>> {
		(0..n)
			.map(|_| message::generic::BlockData {
				hash: H256::random(),
				header: None,
				body: None,
				message_queue: None,
				receipt: None,
				justification: None,
				justifications: None,
			})
			.collect()
	}

	#[test]
	fn create_clear() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));
		bc.insert(1, generate_blocks(100), PeerId::random());
		assert!(!is_empty(&bc));
		bc.clear();
		assert!(is_empty(&bc));
	}

	#[test]
	fn insert_blocks() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));
		let peer0 = PeerId::random();
		let peer1 = PeerId::random();
		let peer2 = PeerId::random();

		let blocks = generate_blocks(150);
		assert_eq!(bc.needed_blocks(peer0, 40, 150, 0, 1, 200), Some(1..41));
		assert_eq!(bc.needed_blocks(peer1, 40, 150, 0, 1, 200), Some(41..81));
		assert_eq!(bc.needed_blocks(peer2, 40, 150, 0, 1, 200), Some(81..121));

		bc.clear_peer_download(&peer1);
		bc.insert(41, blocks[41..81].to_vec(), peer1);
		assert_eq!(bc.ready_blocks(1), vec![]);
		assert_eq!(bc.needed_blocks(peer1, 40, 150, 0, 1, 200), Some(121..151));
		bc.clear_peer_download(&peer0);
		bc.insert(1, blocks[1..11].to_vec(), peer0);

		assert_eq!(bc.needed_blocks(peer0, 40, 150, 0, 1, 200), Some(11..41));
		assert_eq!(
			bc.ready_blocks(1),
			blocks[1..11]
				.iter()
				.map(|b| BlockData { block: b.clone(), origin: Some(peer0) })
				.collect::<Vec<_>>()
		);

		bc.clear_peer_download(&peer0);
		bc.insert(11, blocks[11..41].to_vec(), peer0);

		let ready = bc.ready_blocks(12);
		assert_eq!(
			ready[..30],
			blocks[11..41]
				.iter()
				.map(|b| BlockData { block: b.clone(), origin: Some(peer0) })
				.collect::<Vec<_>>()[..]
		);
		assert_eq!(
			ready[30..],
			blocks[41..81]
				.iter()
				.map(|b| BlockData { block: b.clone(), origin: Some(peer1) })
				.collect::<Vec<_>>()[..]
		);

		bc.clear_peer_download(&peer2);
		assert_eq!(bc.needed_blocks(peer2, 40, 150, 80, 1, 200), Some(81..121));
		bc.clear_peer_download(&peer2);
		bc.insert(81, blocks[81..121].to_vec(), peer2);
		bc.clear_peer_download(&peer1);
		bc.insert(121, blocks[121..150].to_vec(), peer1);

		assert_eq!(bc.ready_blocks(80), vec![]);
		let ready = bc.ready_blocks(81);
		assert_eq!(
			ready[..40],
			blocks[81..121]
				.iter()
				.map(|b| BlockData { block: b.clone(), origin: Some(peer2) })
				.collect::<Vec<_>>()[..]
		);
		assert_eq!(
			ready[40..],
			blocks[121..150]
				.iter()
				.map(|b| BlockData { block: b.clone(), origin: Some(peer1) })
				.collect::<Vec<_>>()[..]
		);
	}

	#[test]
	fn large_gap() {
		let mut bc: BlockCollection<Block> = BlockCollection::new();
		bc.blocks.insert(100, BlockRangeState::Downloading { len: 128, downloading: 1 });
		let blocks = generate_blocks(10)
			.into_iter()
			.map(|b| BlockData { block: b, origin: None })
			.collect();
		bc.blocks.insert(114305, BlockRangeState::Complete(blocks));

		// Each range is requested by a distinct peer: sync issues at most one in-flight range
		// per peer, and a peer only asks for another range once its previous one is released.
		let peer0 = PeerId::random();
		let peer1 = PeerId::random();
		let peer2 = PeerId::random();
		assert_eq!(bc.needed_blocks(peer0, 128, 10000, 0, 1, 200), Some(1..100));
		assert_eq!(bc.needed_blocks(peer1, 128, 10000, 0, 1, 200), None); // too far ahead
		assert_eq!(
			bc.needed_blocks(peer2, 128, 10000, 0, 1, 200000),
			Some(100 + 128..100 + 128 + 128)
		);
	}

	#[test]
	fn no_duplicate_requests_on_fork() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));
		let peer = PeerId::random();

		let blocks = generate_blocks(10);

		// count = 5, peer_best = 50, common = 39, max_parallel = 0, max_ahead = 200
		assert_eq!(bc.needed_blocks(peer, 5, 50, 39, 0, 200), Some(40..45));

		// got a response on the request for `40..45`
		bc.clear_peer_download(&peer);
		bc.insert(40, blocks[..5].to_vec(), peer);

		// our "node" started on a fork, with its current best = 47, which is > common
		let ready = bc.ready_blocks(48);
		assert_eq!(
			ready,
			blocks[..5]
				.iter()
				.map(|b| BlockData { block: b.clone(), origin: Some(peer) })
				.collect::<Vec<_>>()
		);

		assert_eq!(bc.needed_blocks(peer, 5, 50, 39, 0, 200), Some(45..50));
	}

	#[test]
	fn clear_queued_subsequent_ranges() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));
		// Two consecutive ranges are requested, one per peer: sync issues at most one in-flight
		// range per peer, so distinct peers stand in for the two requests.
		let peer1 = PeerId::random();
		let peer2 = PeerId::random();

		let blocks = generate_blocks(10);

		// Request 2 ranges
		assert_eq!(bc.needed_blocks(peer1, 5, 50, 39, 0, 200), Some(40..45));
		assert_eq!(bc.needed_blocks(peer2, 5, 50, 39, 0, 200), Some(45..50));

		// got a response covering `40..50`
		bc.clear_peer_download(&peer1);
		bc.clear_peer_download(&peer2);
		bc.insert(40, blocks.to_vec(), peer1);

		// request any blocks starting from 1000 or lower.
		let ready = bc.ready_blocks(1000);
		assert_eq!(
			ready,
			blocks
				.iter()
				.map(|b| BlockData { block: b.clone(), origin: Some(peer1) })
				.collect::<Vec<_>>()
		);

		bc.clear_queued(&blocks[0].hash);
		assert!(bc.blocks.is_empty());
		assert!(bc.queued_blocks.is_empty());
	}

	#[test]
	fn downloaded_range_is_requested_from_max_parallel_peers() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));

		let count = 5;
		// identical ranges requested from 2 peers
		let max_parallel = 2;
		let max_ahead = 200;

		let peer1 = PeerId::random();
		let peer2 = PeerId::random();
		let peer3 = PeerId::random();

		// common for all peers
		let best = 100;
		let common = 10;

		assert_eq!(
			bc.needed_blocks(peer1, count, best, common, max_parallel, max_ahead),
			Some(11..16)
		);
		assert_eq!(
			bc.needed_blocks(peer2, count, best, common, max_parallel, max_ahead),
			Some(11..16)
		);
		assert_eq!(
			bc.needed_blocks(peer3, count, best, common, max_parallel, max_ahead),
			Some(16..21)
		);
	}
	#[test]
	fn downloaded_range_not_requested_from_peers_with_higher_common_number() {
		// A peer connects with a common number falling behind our best number
		// (either a fork or lagging behind).
		// We request a range from this peer starting at its common number + 1.
		// Even though we have less than `max_parallel` downloads, we do not request
		// this range from peers with a common number above the start of this range.

		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));

		let count = 5;
		let max_parallel = 2;
		let max_ahead = 200;

		let peer1 = PeerId::random();
		let peer1_best = 20;
		let peer1_common = 10;

		// `peer2` has first different above the start of the range downloaded from `peer1`
		let peer2 = PeerId::random();
		let peer2_best = 20;
		let peer2_common = 11; // first_different = 12

		assert_eq!(
			bc.needed_blocks(peer1, count, peer1_best, peer1_common, max_parallel, max_ahead),
			Some(11..16),
		);
		assert_eq!(
			bc.needed_blocks(peer2, count, peer2_best, peer2_common, max_parallel, max_ahead),
			Some(16..21),
		);
	}

	#[test]
	fn requesting_a_new_range_releases_a_peers_stale_range() {
		// A peer must release its previous in-flight range before being recorded against a new
		// one. If a stale entry lingers (e.g. an obsolete response was dropped without notifying
		// sync), requesting again must not orphan the old `Downloading` marker — an orphan would
		// pin the collection's lowest block and stall gap sync behind `max_ahead`.
		let metrics = super::Metrics::register(&prometheus_endpoint::Registry::new()).unwrap();
		let mut bc = BlockCollection::with_metrics(Some(metrics.clone()));
		assert!(is_empty(&bc));

		let count = 128;
		let best = 10_000;
		let max_parallel = 1;
		let max_ahead = 2048;

		let peer = PeerId::random();

		// The first request is recorded as in-flight for the peer.
		let first = bc.needed_blocks(peer, count, best, 0, max_parallel, max_ahead).unwrap();
		assert_eq!(bc.peer_requests.get(&peer), Some(&first.start));
		assert!(matches!(bc.blocks.get(&first.start), Some(BlockRangeState::Downloading { .. }),));
		assert_eq!(metrics.stale_download_reservations.get(), 0);

		// The same peer is asked for a new range while its previous one is still tracked. The
		// stale range is released, so the peer is tracked only for the new range and exactly
		// one `Downloading` marker remains — no orphan is left pinning the collection.
		let second = bc.needed_blocks(peer, count, best, 200, max_parallel, max_ahead).unwrap();
		assert_eq!(metrics.stale_download_reservations.get(), 1);
		assert_ne!(first.start, second.start);
		assert_eq!(bc.peer_requests.get(&peer), Some(&second.start));

		let downloading = bc
			.blocks
			.iter()
			.filter(|(_, state)| matches!(state, BlockRangeState::Downloading { .. }))
			.map(|(n, _)| *n)
			.collect::<Vec<_>>();
		assert_eq!(
			downloading,
			vec![second.start],
			"stale range must be released, leaving no orphaned Downloading marker",
		);

		// Normal cancellation and a collection reset must not count as stale recovery or reset
		// the cumulative counter.
		bc.clear_peer_download(&peer);
		assert!(bc.needed_blocks(peer, count, best, 400, max_parallel, max_ahead).is_some());
		bc.clear();
		assert!(bc.needed_blocks(peer, count, best, 600, max_parallel, max_ahead).is_some());
		assert_eq!(metrics.stale_download_reservations.get(), 1);
	}

	#[test]
	fn gap_above_common_number_requested() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));

		let count = 5;
		let best = 30;
		// We need at least 3 ranges requested to have a gap, so to minimize the number of peers
		// set `max_parallel = 1`
		let max_parallel = 1;
		let max_ahead = 200;

		let peer1 = PeerId::random();
		let peer2 = PeerId::random();
		let peer3 = PeerId::random();

		let common = 10;
		assert_eq!(
			bc.needed_blocks(peer1, count, best, common, max_parallel, max_ahead),
			Some(11..16),
		);
		assert_eq!(
			bc.needed_blocks(peer2, count, best, common, max_parallel, max_ahead),
			Some(16..21),
		);
		assert_eq!(
			bc.needed_blocks(peer3, count, best, common, max_parallel, max_ahead),
			Some(21..26),
		);

		// For some reason there is now a gap at 16..21. We just disconnect `peer2`, but it might
		// also happen that 16..21 received first and got imported if our best is actually >= 15.
		bc.clear_peer_download(&peer2);

		// Some peer connects with common number below the gap. The gap is requested from it.
		assert_eq!(
			bc.needed_blocks(peer2, count, best, common, max_parallel, max_ahead),
			Some(16..21),
		);
	}

	#[test]
	fn gap_below_common_number_not_requested() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));

		let count = 5;
		let best = 30;
		// We need at least 3 ranges requested to have a gap, so to minimize the number of peers
		// set `max_parallel = 1`
		let max_parallel = 1;
		let max_ahead = 200;

		let peer1 = PeerId::random();
		let peer2 = PeerId::random();
		let peer3 = PeerId::random();

		let common = 10;
		assert_eq!(
			bc.needed_blocks(peer1, count, best, common, max_parallel, max_ahead),
			Some(11..16),
		);
		assert_eq!(
			bc.needed_blocks(peer2, count, best, common, max_parallel, max_ahead),
			Some(16..21),
		);
		assert_eq!(
			bc.needed_blocks(peer3, count, best, common, max_parallel, max_ahead),
			Some(21..26),
		);

		// For some reason there is now a gap at 16..21. We just disconnect `peer2`, but it might
		// also happen that 16..21 received first and got imported if our best is actually >= 15.
		bc.clear_peer_download(&peer2);

		// Some peer connects with common number above the gap. The gap is not requested from it.
		let common = 23;
		assert_eq!(
			bc.needed_blocks(peer2, count, best, common, max_parallel, max_ahead),
			Some(26..31), // not 16..21
		);
	}

	#[test]
	fn range_at_the_end_above_common_number_requested() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));

		let count = 5;
		let best = 30;
		let max_parallel = 1;
		let max_ahead = 200;

		let peer1 = PeerId::random();
		let peer2 = PeerId::random();

		let common = 10;
		assert_eq!(
			bc.needed_blocks(peer1, count, best, common, max_parallel, max_ahead),
			Some(11..16),
		);
		assert_eq!(
			bc.needed_blocks(peer2, count, best, common, max_parallel, max_ahead),
			Some(16..21),
		);
	}

	#[test]
	fn range_at_the_end_below_common_number_not_requested() {
		let mut bc = BlockCollection::new();
		assert!(is_empty(&bc));

		let count = 5;
		let best = 30;
		let max_parallel = 1;
		let max_ahead = 200;

		let peer1 = PeerId::random();
		let peer2 = PeerId::random();

		let common = 10;
		assert_eq!(
			bc.needed_blocks(peer1, count, best, common, max_parallel, max_ahead),
			Some(11..16),
		);

		let common = 20;
		assert_eq!(
			bc.needed_blocks(peer2, count, best, common, max_parallel, max_ahead),
			Some(21..26), // not 16..21
		);
	}
}