consensus_core/

authority_service.rs

// Copyright (c) Mysten Labs, Inc.
// SPDX-License-Identifier: Apache-2.0

use std::{
    cmp::Reverse,
    collections::{BTreeMap, BTreeSet, BinaryHeap},
    pin::Pin,
    sync::Arc,
    time::Duration,
};

use async_trait::async_trait;
use bytes::Bytes;
use consensus_config::AuthorityIndex;
use consensus_types::block::{BlockRef, Round};
use futures::{Stream, StreamExt, ready, stream, task};
use mysten_metrics::spawn_monitored_task;
use parking_lot::RwLock;
use rand::seq::SliceRandom as _;
use sui_macros::fail_point_async;
use tap::TapFallible;
use tokio::sync::broadcast;
use tokio_util::sync::ReusableBoxFuture;
use tracing::{debug, info, warn};

use crate::{
    CommitIndex,
    block::{BlockAPI as _, ExtendedBlock, GENESIS_ROUND, SignedBlock, VerifiedBlock},
    block_verifier::BlockVerifier,
    commit::{CommitAPI as _, CommitRange, TrustedCommit},
    commit_vote_monitor::CommitVoteMonitor,
    context::Context,
    core_thread::CoreThreadDispatcher,
    dag_state::DagState,
    error::{ConsensusError, ConsensusResult},
    network::{
        BlockStream, ExtendedSerializedBlock, NodeId, ObserverBlockStream, ObserverNetworkService,
        PeerId, ValidatorNetworkService,
    },
    round_tracker::RoundTracker,
    stake_aggregator::{QuorumThreshold, StakeAggregator},
    storage::Store,
    synchronizer::SynchronizerHandle,
    transaction_certifier::TransactionCertifier,
};

pub(crate) const COMMIT_LAG_MULTIPLIER: u32 = 5;

/// Authority's network service implementation, agnostic to the actual networking stack used.
pub(crate) struct AuthorityService<C: CoreThreadDispatcher> {
    context: Arc<Context>,
    commit_vote_monitor: Arc<CommitVoteMonitor>,
    block_verifier: Arc<dyn BlockVerifier>,
    synchronizer: Arc<SynchronizerHandle>,
    core_dispatcher: Arc<C>,
    rx_block_broadcast: broadcast::Receiver<ExtendedBlock>,
    subscription_counter: Arc<SubscriptionCounter>,
    transaction_certifier: TransactionCertifier,
    dag_state: Arc<RwLock<DagState>>,
    store: Arc<dyn Store>,
    round_tracker: Arc<RwLock<RoundTracker>>,
}

impl<C: CoreThreadDispatcher> AuthorityService<C> {
    pub(crate) fn new(
        context: Arc<Context>,
        block_verifier: Arc<dyn BlockVerifier>,
        commit_vote_monitor: Arc<CommitVoteMonitor>,
        round_tracker: Arc<RwLock<RoundTracker>>,
        synchronizer: Arc<SynchronizerHandle>,
        core_dispatcher: Arc<C>,
        rx_block_broadcast: broadcast::Receiver<ExtendedBlock>,
        transaction_certifier: TransactionCertifier,
        dag_state: Arc<RwLock<DagState>>,
        store: Arc<dyn Store>,
    ) -> Self {
        let subscription_counter = Arc::new(SubscriptionCounter::new(context.clone()));
        Self {
            context,
            block_verifier,
            commit_vote_monitor,
            synchronizer,
            core_dispatcher,
            rx_block_broadcast,
            subscription_counter,
            transaction_certifier,
            dag_state,
            store,
            round_tracker,
        }
    }

    // Parses and validates serialized excluded ancestors.
    fn parse_excluded_ancestors(
        &self,
        peer: AuthorityIndex,
        block: &VerifiedBlock,
        mut excluded_ancestors: Vec<Vec<u8>>,
    ) -> ConsensusResult<Vec<BlockRef>> {
        let peer_hostname = &self.context.committee.authority(peer).hostname;

        let excluded_ancestors_limit = self.context.committee.size() * 2;
        if excluded_ancestors.len() > excluded_ancestors_limit {
            debug!(
                "Dropping {} excluded ancestor(s) from {} {} due to size limit",
                excluded_ancestors.len() - excluded_ancestors_limit,
                peer,
                peer_hostname,
            );
            excluded_ancestors.truncate(excluded_ancestors_limit);
        }

        let excluded_ancestors = excluded_ancestors
            .into_iter()
            .map(|serialized| {
                let block_ref: BlockRef =
                    bcs::from_bytes(&serialized).map_err(ConsensusError::MalformedBlock)?;
                if !self.context.committee.is_valid_index(block_ref.author) {
                    return Err(ConsensusError::InvalidAuthorityIndex {
                        index: block_ref.author,
                        max: self.context.committee.size(),
                    });
                }
                if block_ref.round >= block.round() {
                    return Err(ConsensusError::InvalidAncestorRound {
                        ancestor: block_ref.round,
                        block: block.round(),
                    });
                }
                Ok(block_ref)
            })
            .collect::<ConsensusResult<Vec<BlockRef>>>()?;

        for excluded_ancestor in &excluded_ancestors {
            let excluded_ancestor_hostname = &self
                .context
                .committee
                .authority(excluded_ancestor.author)
                .hostname;
            self.context
                .metrics
                .node_metrics
                .network_excluded_ancestors_count_by_authority
                .with_label_values(&[excluded_ancestor_hostname])
                .inc();
        }
        self.context
            .metrics
            .node_metrics
            .network_received_excluded_ancestors_from_authority
            .with_label_values(&[peer_hostname])
            .inc_by(excluded_ancestors.len() as u64);

        Ok(excluded_ancestors)
    }
}

#[async_trait]
impl<C: CoreThreadDispatcher> ValidatorNetworkService for AuthorityService<C> {
    async fn handle_send_block(
        &self,
        peer: AuthorityIndex,
        serialized_block: ExtendedSerializedBlock,
    ) -> ConsensusResult<()> {
        fail_point_async!("consensus-rpc-response");

        let peer_hostname = &self.context.committee.authority(peer).hostname;

        // TODO: dedup block verifications, here and with fetched blocks.
        let signed_block: SignedBlock =
            bcs::from_bytes(&serialized_block.block).map_err(ConsensusError::MalformedBlock)?;

        // Reject blocks not produced by the peer.
        if peer != signed_block.author() {
            self.context
                .metrics
                .node_metrics
                .invalid_blocks
                .with_label_values(&[
                    peer_hostname.as_str(),
                    "handle_send_block",
                    "UnexpectedAuthority",
                ])
                .inc();
            let e = ConsensusError::UnexpectedAuthority(signed_block.author(), peer);
            info!("Block with wrong authority from {}: {}", peer, e);
            return Err(e);
        }

        // Reject blocks failing parsing and validations.
        let (verified_block, reject_txn_votes) = self
            .block_verifier
            .verify_and_vote(signed_block, serialized_block.block)
            .tap_err(|e| {
                self.context
                    .metrics
                    .node_metrics
                    .invalid_blocks
                    .with_label_values(&[peer_hostname.as_str(), "handle_send_block", e.name()])
                    .inc();
                info!("Invalid block from {}: {}", peer, e);
            })?;
        let excluded_ancestors = self
            .parse_excluded_ancestors(peer, &verified_block, serialized_block.excluded_ancestors)
            .tap_err(|e| {
                debug!("Failed to parse excluded ancestors from {peer} {peer_hostname}: {e}");
                self.context
                    .metrics
                    .node_metrics
                    .invalid_blocks
                    .with_label_values(&[peer_hostname.as_str(), "handle_send_block", e.name()])
                    .inc();
            })?;

        let block_ref = verified_block.reference();
        debug!("Received block {} via send block.", block_ref);

        self.context
            .metrics
            .node_metrics
            .verified_blocks
            .with_label_values(&[peer_hostname])
            .inc();

        let now = self.context.clock.timestamp_utc_ms();
        let forward_time_drift =
            Duration::from_millis(verified_block.timestamp_ms().saturating_sub(now));

        self.context
            .metrics
            .node_metrics
            .block_timestamp_drift_ms
            .with_label_values(&[peer_hostname.as_str(), "handle_send_block"])
            .inc_by(forward_time_drift.as_millis() as u64);

        // Observe the block for the commit votes. When local commit is lagging too much,
        // commit sync loop will trigger fetching.
        self.commit_vote_monitor.observe_block(&verified_block);

        // Update own received rounds and peer accepted rounds from this verified block.
        self.round_tracker
            .write()
            .update_from_verified_block(&ExtendedBlock {
                block: verified_block.clone(),
                excluded_ancestors: excluded_ancestors.clone(),
            });

        // Reject blocks when local commit index is lagging too far from quorum commit index,
        // to avoid the memory overhead from suspended blocks.
        //
        // IMPORTANT: this must be done after observing votes from the block, otherwise
        // observed quorum commit will no longer progress.
        //
        // Since the main issue with too many suspended blocks is memory usage not CPU,
        // it is ok to reject after block verifications instead of before.
        let last_commit_index = self.dag_state.read().last_commit_index();
        let quorum_commit_index = self.commit_vote_monitor.quorum_commit_index();
        // The threshold to ignore block should be larger than commit_sync_batch_size,
        // to avoid excessive block rejections and synchronizations.
        if last_commit_index
            + self.context.parameters.commit_sync_batch_size * COMMIT_LAG_MULTIPLIER
            < quorum_commit_index
        {
            self.context
                .metrics
                .node_metrics
                .rejected_blocks
                .with_label_values(&["commit_lagging"])
                .inc();
            debug!(
                "Block {:?} is rejected because last commit index is lagging quorum commit index too much ({} < {})",
                block_ref, last_commit_index, quorum_commit_index,
            );
            return Err(ConsensusError::BlockRejected {
                block_ref,
                reason: format!(
                    "Last commit index is lagging quorum commit index too much ({} < {})",
                    last_commit_index, quorum_commit_index,
                ),
            });
        }

        // The block is verified and current, so record own votes on the block
        // before sending the block to Core.
        if self.context.protocol_config.transaction_voting_enabled() {
            self.transaction_certifier
                .add_voted_blocks(vec![(verified_block.clone(), reject_txn_votes)]);
        }

        // Send the block to Core to try accepting it into the DAG.
        let missing_ancestors = self
            .core_dispatcher
            .add_blocks(vec![verified_block.clone()])
            .await
            .map_err(|_| ConsensusError::Shutdown)?;

        // Schedule fetching missing ancestors from this peer in the background.
        if !missing_ancestors.is_empty() {
            self.context
                .metrics
                .node_metrics
                .handler_received_block_missing_ancestors
                .with_label_values(&[peer_hostname])
                .inc_by(missing_ancestors.len() as u64);
            let synchronizer = self.synchronizer.clone();
            spawn_monitored_task!(async move {
                // This does not wait for the fetch request to complete.
                // It only waits for synchronizer to queue the request to a peer.
                // When this fails, it usually means the queue is full.
                // The fetch will retry from other peers via live and periodic syncs.
                if let Err(err) = synchronizer.fetch_blocks(missing_ancestors, peer).await {
                    debug!("Failed to fetch missing ancestors via synchronizer: {err}");
                }
            });
        }

        // Schedule fetching missing soft links from this peer in the background.
        let missing_excluded_ancestors = self
            .core_dispatcher
            .check_block_refs(excluded_ancestors)
            .await
            .map_err(|_| ConsensusError::Shutdown)?;
        if !missing_excluded_ancestors.is_empty() {
            self.context
                .metrics
                .node_metrics
                .network_excluded_ancestors_sent_to_fetch
                .with_label_values(&[peer_hostname])
                .inc_by(missing_excluded_ancestors.len() as u64);

            let synchronizer = self.synchronizer.clone();
            spawn_monitored_task!(async move {
                if let Err(err) = synchronizer
                    .fetch_blocks(missing_excluded_ancestors, peer)
                    .await
                {
                    debug!("Failed to fetch excluded ancestors via synchronizer: {err}");
                }
            });
        }

        Ok(())
    }

    async fn handle_subscribe_blocks(
        &self,
        peer: AuthorityIndex,
        last_received: Round,
    ) -> ConsensusResult<BlockStream> {
        fail_point_async!("consensus-rpc-response");

        // Find past proposed blocks as the initial blocks to send to the peer.
        //
        // If there are cached blocks in the range which the peer requested, send all of them.
        // The size is bounded by the local GC round and DagState cache size.
        //
        // Otherwise if there is no cached block in the range which the peer requested,
        // and this node has proposed blocks before, at least one block should be sent to the peer
        // to help with liveness.
        let past_proposed_blocks = {
            let dag_state = self.dag_state.read();

            let mut proposed_blocks =
                dag_state.get_cached_blocks(self.context.own_index, last_received + 1);
            if proposed_blocks.is_empty() {
                let last_proposed_block = dag_state.get_last_proposed_block();
                proposed_blocks = if last_proposed_block.round() > GENESIS_ROUND {
                    vec![last_proposed_block]
                } else {
                    vec![]
                };
            }
            stream::iter(
                proposed_blocks
                    .into_iter()
                    .map(|block| ExtendedSerializedBlock {
                        block: block.serialized().clone(),
                        excluded_ancestors: vec![],
                    }),
            )
        };

        let broadcasted_blocks = BroadcastedBlockStream::new(
            PeerId::Validator(peer),
            self.rx_block_broadcast.resubscribe(),
            self.subscription_counter.clone(),
        );

        // Return a stream of blocks that first yields missed blocks as requested, then new blocks.
        Ok(Box::pin(past_proposed_blocks.chain(
            broadcasted_blocks.map(ExtendedSerializedBlock::from),
        )))
    }

    // Handles 3 types of requests:
    // 1. Live sync:
    //    - Both missing block refs and highest accepted rounds are specified.
    //    - fetch_missing_ancestors is true.
    //    - response returns max_blocks_per_sync blocks.
    // 2. Periodic sync:
    //    - Highest accepted rounds must be specified.
    //    - Missing block refs are optional.
    //    - fetch_missing_ancestors is false (default).
    //    - response returns max_blocks_per_fetch blocks.
    // 3. Commit sync:
    //    - Missing block refs are specified.
    //    - Highest accepted rounds are empty.
    //    - fetch_missing_ancestors is false (default).
    //    - response returns max_blocks_per_fetch blocks.
    async fn handle_fetch_blocks(
        &self,
        _peer: AuthorityIndex,
        mut block_refs: Vec<BlockRef>,
        fetch_after_rounds: Vec<Round>,
        fetch_missing_ancestors: bool,
    ) -> ConsensusResult<Vec<Bytes>> {
        fail_point_async!("consensus-rpc-response");

        if block_refs.is_empty() && (fetch_missing_ancestors || fetch_after_rounds.is_empty()) {
            return Err(ConsensusError::InvalidFetchBlocksRequest("When no block refs are provided, fetch_after_rounds must be provided and fetch_missing_ancestors must be false".to_string()));
        }
        if !fetch_after_rounds.is_empty()
            && fetch_after_rounds.len() != self.context.committee.size()
        {
            return Err(ConsensusError::InvalidSizeOfHighestAcceptedRounds(
                fetch_after_rounds.len(),
                self.context.committee.size(),
            ));
        }

        // Finds the suitable limit of # of blocks to return.
        let max_response_num_blocks = if !fetch_after_rounds.is_empty() && !block_refs.is_empty() {
            self.context.parameters.max_blocks_per_sync
        } else {
            self.context.parameters.max_blocks_per_fetch
        };
        if block_refs.len() > max_response_num_blocks {
            block_refs.truncate(max_response_num_blocks);
        }

        // Validate the requested block refs.
        for block in &block_refs {
            if !self.context.committee.is_valid_index(block.author) {
                return Err(ConsensusError::InvalidAuthorityIndex {
                    index: block.author,
                    max: self.context.committee.size(),
                });
            }
            if block.round == GENESIS_ROUND {
                return Err(ConsensusError::UnexpectedGenesisBlockRequested);
            }
        }

        // Get the requested blocks first.
        let mut blocks = self
            .dag_state
            .read()
            .get_blocks(&block_refs)
            .into_iter()
            .flatten()
            .collect::<Vec<_>>();

        // When fetch_missing_ancestors is true, fetch missing ancestors of the requested blocks.
        // Otherwise, fetch additional blocks depth-first from the requested block authorities.
        if blocks.len() < max_response_num_blocks && !fetch_after_rounds.is_empty() {
            if fetch_missing_ancestors {
                // Get unique missing ancestor blocks of the requested blocks (validated to be non-empty).
                // fetch_after_rounds will only be used to filter out already accepted blocks.
                let missing_ancestors = blocks
                    .iter()
                    .flat_map(|block| block.ancestors().to_vec())
                    .filter(|block_ref| fetch_after_rounds[block_ref.author] < block_ref.round)
                    .collect::<BTreeSet<_>>()
                    .into_iter()
                    .collect::<Vec<_>>();

                // If there are too many missing ancestors, randomly select a subset to avoid
                // fetching duplicated blocks across peers.
                let selected_num_blocks = max_response_num_blocks
                    .saturating_sub(blocks.len())
                    .min(missing_ancestors.len());
                if selected_num_blocks > 0 {
                    let selected_ancestor_refs = missing_ancestors
                        .choose_multiple(&mut mysten_common::random::get_rng(), selected_num_blocks)
                        .copied()
                        .collect::<Vec<_>>();
                    let ancestor_blocks = self
                        .dag_state
                        .read()
                        .get_blocks(&selected_ancestor_refs)
                        .into_iter()
                        .flatten();
                    blocks.extend(ancestor_blocks);
                }
            } else {
                // Get additional blocks from authorities with missing block.
                // Compute the fetch round per requested authority, or all authorities.
                let mut limit_rounds = BTreeMap::<AuthorityIndex, Round>::new();
                if block_refs.is_empty() {
                    let dag_state = self.dag_state.read();
                    for (index, _authority) in self.context.committee.authorities() {
                        let last_block = dag_state.get_last_block_for_authority(index);
                        limit_rounds.insert(index, last_block.round());
                    }
                } else {
                    for block_ref in &block_refs {
                        let entry = limit_rounds
                            .entry(block_ref.author)
                            .or_insert(block_ref.round);
                        *entry = (*entry).min(block_ref.round);
                    }
                }

                // Use a min-heap to fetch blocks across authorities in ascending round order.
                // Each entry is (fetch_start_round, authority, limit_round).
                let mut heap = BinaryHeap::new();
                for (authority, limit_round) in &limit_rounds {
                    let fetch_start = fetch_after_rounds[*authority] + 1;
                    if fetch_start < *limit_round {
                        heap.push(Reverse((fetch_start, *authority, *limit_round)));
                    }
                }

                while let Some(Reverse((fetch_start, authority, limit_round))) = heap.pop() {
                    let fetched = self.store.scan_blocks_by_author_in_range(
                        authority,
                        fetch_start,
                        limit_round,
                        1,
                    )?;
                    if let Some(block) = fetched.into_iter().next() {
                        let next_start = block.round() + 1;
                        blocks.push(block);
                        if blocks.len() >= max_response_num_blocks {
                            blocks.truncate(max_response_num_blocks);
                            break;
                        }
                        if next_start < limit_round {
                            heap.push(Reverse((next_start, authority, limit_round)));
                        }
                    }
                }
            }
        }

        // Return the serialized blocks
        let bytes = blocks
            .into_iter()
            .map(|block| block.serialized().clone())
            .collect::<Vec<_>>();
        Ok(bytes)
    }

    async fn handle_fetch_commits(
        &self,
        _peer: AuthorityIndex,
        commit_range: CommitRange,
    ) -> ConsensusResult<(Vec<TrustedCommit>, Vec<VerifiedBlock>)> {
        fail_point_async!("consensus-rpc-response");

        // Compute an inclusive end index and bound the maximum number of commits scanned.
        let inclusive_end = commit_range.end().min(
            commit_range.start() + self.context.parameters.commit_sync_batch_size as CommitIndex
                - 1,
        );
        let mut commits = self
            .store
            .scan_commits((commit_range.start()..=inclusive_end).into())?;
        let mut certifier_block_refs = vec![];
        'commit: while let Some(c) = commits.last() {
            let index = c.index();
            let votes = self.store.read_commit_votes(index)?;
            let mut stake_aggregator = StakeAggregator::<QuorumThreshold>::new();
            for v in &votes {
                stake_aggregator.add(v.author, &self.context.committee);
            }
            if stake_aggregator.reached_threshold(&self.context.committee) {
                certifier_block_refs = votes;
                break 'commit;
            } else {
                debug!(
                    "Commit {} votes did not reach quorum to certify, {} < {}, skipping",
                    index,
                    stake_aggregator.stake(),
                    stake_aggregator.threshold(&self.context.committee)
                );
                self.context
                    .metrics
                    .node_metrics
                    .commit_sync_fetch_commits_handler_uncertified_skipped
                    .inc();
                commits.pop();
            }
        }
        let certifier_blocks = self
            .store
            .read_blocks(&certifier_block_refs)?
            .into_iter()
            .flatten()
            .collect();
        Ok((commits, certifier_blocks))
    }

    async fn handle_fetch_latest_blocks(
        &self,
        peer: AuthorityIndex,
        authorities: Vec<AuthorityIndex>,
    ) -> ConsensusResult<Vec<Bytes>> {
        fail_point_async!("consensus-rpc-response");

        if authorities.len() > self.context.committee.size() {
            return Err(ConsensusError::TooManyAuthoritiesProvided(peer));
        }

        // Ensure that those are valid authorities
        for authority in &authorities {
            if !self.context.committee.is_valid_index(*authority) {
                return Err(ConsensusError::InvalidAuthorityIndex {
                    index: *authority,
                    max: self.context.committee.size(),
                });
            }
        }

        // Read from the dag state to find the latest blocks.
        // TODO: at the moment we don't look into the block manager for suspended blocks. Ideally we
        // want in the future if we think we would like to tackle the majority of cases.
        let mut blocks = vec![];
        let dag_state = self.dag_state.read();
        for authority in authorities {
            let block = dag_state.get_last_block_for_authority(authority);

            debug!("Latest block for {authority}: {block:?} as requested from {peer}");

            // no reason to serve back the genesis block - it's equal as if it has not received any block
            if block.round() != GENESIS_ROUND {
                blocks.push(block);
            }
        }

        // Return the serialised blocks
        let result = blocks
            .into_iter()
            .map(|block| block.serialized().clone())
            .collect::<Vec<_>>();

        Ok(result)
    }

    async fn handle_get_latest_rounds(
        &self,
        _peer: AuthorityIndex,
    ) -> ConsensusResult<(Vec<Round>, Vec<Round>)> {
        fail_point_async!("consensus-rpc-response");

        let highest_received_rounds = self.round_tracker.read().local_highest_received_rounds();

        let blocks = self
            .dag_state
            .read()
            .get_last_cached_block_per_authority(Round::MAX);
        let highest_accepted_rounds = blocks
            .into_iter()
            .map(|(block, _)| block.round())
            .collect::<Vec<_>>();

        Ok((highest_received_rounds, highest_accepted_rounds))
    }
}

#[async_trait]
impl<C: CoreThreadDispatcher> ObserverNetworkService for AuthorityService<C> {
    async fn handle_stream_blocks(
        &self,
        _peer: NodeId,
        _highest_round_per_authority: Vec<u64>,
    ) -> ConsensusResult<ObserverBlockStream> {
        // TODO: Implement observer block streaming
        todo!("Observer block streaming not yet implemented")
    }

    async fn handle_fetch_blocks(
        &self,
        _peer: NodeId,
        _block_refs: Vec<BlockRef>,
    ) -> ConsensusResult<Vec<Bytes>> {
        // TODO: implement observer fetch blocks, similar to validator fetch_blocks but
        // without highest_accepted_rounds.
        Err(ConsensusError::NetworkRequest(
            "Observer fetch blocks not yet implemented".to_string(),
        ))
    }

    async fn handle_fetch_commits(
        &self,
        _peer: NodeId,
        _commit_range: CommitRange,
    ) -> ConsensusResult<(Vec<TrustedCommit>, Vec<VerifiedBlock>)> {
        // TODO: implement observer fetch commits, similar to validator fetch_commits.
        Err(ConsensusError::NetworkRequest(
            "Observer fetch commits not yet implemented".to_string(),
        ))
    }
}
struct Counter {
    count: usize,
    subscriptions_by_peer: BTreeMap<PeerId, usize>,
}

/// Atomically counts the number of active subscriptions to the block broadcast stream.
pub(crate) struct SubscriptionCounter {
    context: Arc<Context>,
    counter: parking_lot::Mutex<Counter>,
}

impl SubscriptionCounter {
    pub(crate) fn new(context: Arc<Context>) -> Self {
        // Set the subscribed peers by default to 0
        for (_, authority) in context.committee.authorities() {
            context
                .metrics
                .node_metrics
                .subscribed_by
                .with_label_values(&[authority.hostname.as_str()])
                .set(0);
        }

        Self {
            counter: parking_lot::Mutex::new(Counter {
                count: 0,
                subscriptions_by_peer: BTreeMap::new(),
            }),
            context,
        }
    }

    fn increment(&self, peer: &PeerId) -> Result<(), ConsensusError> {
        let mut counter = self.counter.lock();
        counter.count += 1;
        *counter
            .subscriptions_by_peer
            .entry(peer.clone())
            .or_insert(0) += 1;

        match peer {
            PeerId::Validator(authority) => {
                let peer_hostname = &self.context.committee.authority(*authority).hostname;
                self.context
                    .metrics
                    .node_metrics
                    .subscribed_by
                    .with_label_values(&[peer_hostname])
                    .set(1);
            }
            PeerId::Observer(_) => {
                self.context
                    .metrics
                    .node_metrics
                    .subscribed_by
                    .with_label_values(&["observer"])
                    .inc();
            }
        }

        Ok(())
    }

    fn decrement(&self, peer: &PeerId) -> Result<(), ConsensusError> {
        let mut counter = self.counter.lock();
        counter.count -= 1;
        *counter
            .subscriptions_by_peer
            .entry(peer.clone())
            .or_insert(0) -= 1;

        if counter.subscriptions_by_peer[peer] == 0 {
            match peer {
                PeerId::Validator(authority) => {
                    let peer_hostname = &self.context.committee.authority(*authority).hostname;
                    self.context
                        .metrics
                        .node_metrics
                        .subscribed_by
                        .with_label_values(&[peer_hostname])
                        .set(0);
                }
                PeerId::Observer(_) => {
                    self.context
                        .metrics
                        .node_metrics
                        .subscribed_by
                        .with_label_values(&["observer"])
                        .dec();
                }
            }
        }

        Ok(())
    }
}

/// Each broadcasted block stream wraps a broadcast receiver for blocks.
/// It yields blocks that are broadcasted after the stream is created.
type BroadcastedBlockStream = BroadcastStream<ExtendedBlock>;

/// Adapted from `tokio_stream::wrappers::BroadcastStream`. The main difference is that
/// this tolerates lags with only logging, without yielding errors.
pub(crate) struct BroadcastStream<T> {
    peer: PeerId,
    // Stores the receiver across poll_next() calls.
    inner: ReusableBoxFuture<
        'static,
        (
            Result<T, broadcast::error::RecvError>,
            broadcast::Receiver<T>,
        ),
    >,
    // Counts total subscriptions / active BroadcastStreams.
    subscription_counter: Arc<SubscriptionCounter>,
}

impl<T: 'static + Clone + Send> BroadcastStream<T> {
    pub fn new(
        peer: PeerId,
        rx: broadcast::Receiver<T>,
        subscription_counter: Arc<SubscriptionCounter>,
    ) -> Self {
        if let Err(err) = subscription_counter.increment(&peer) {
            match err {
                ConsensusError::Shutdown => {}
                _ => panic!("Unexpected error: {err}"),
            }
        }
        Self {
            peer,
            inner: ReusableBoxFuture::new(make_recv_future(rx)),
            subscription_counter,
        }
    }
}

impl<T: 'static + Clone + Send> Stream for BroadcastStream<T> {
    type Item = T;

    fn poll_next(
        mut self: Pin<&mut Self>,
        cx: &mut task::Context<'_>,
    ) -> task::Poll<Option<Self::Item>> {
        let peer = self.peer.clone();
        let maybe_item = loop {
            let (result, rx) = ready!(self.inner.poll(cx));
            self.inner.set(make_recv_future(rx));

            match result {
                Ok(item) => break Some(item),
                Err(broadcast::error::RecvError::Closed) => {
                    info!("Block BroadcastedBlockStream {} closed", peer);
                    break None;
                }
                Err(broadcast::error::RecvError::Lagged(n)) => {
                    warn!(
                        "Block BroadcastedBlockStream {} lagged by {} messages",
                        peer, n
                    );
                    continue;
                }
            }
        };
        task::Poll::Ready(maybe_item)
    }
}

impl<T> Drop for BroadcastStream<T> {
    fn drop(&mut self) {
        if let Err(err) = self.subscription_counter.decrement(&self.peer) {
            match err {
                ConsensusError::Shutdown => {}
                _ => panic!("Unexpected error: {err}"),
            }
        }
    }
}

async fn make_recv_future<T: Clone>(
    mut rx: broadcast::Receiver<T>,
) -> (
    Result<T, broadcast::error::RecvError>,
    broadcast::Receiver<T>,
) {
    let result = rx.recv().await;
    (result, rx)
}

// TODO: add a unit test for BroadcastStream.

#[cfg(test)]
mod tests {
    use std::{
        collections::{BTreeMap, BTreeSet},
        sync::Arc,
        time::Duration,
    };

    use async_trait::async_trait;
    use bytes::Bytes;
    use consensus_config::AuthorityIndex;
    use consensus_types::block::{BlockDigest, BlockRef, Round};
    use parking_lot::{Mutex, RwLock};
    use tokio::{sync::broadcast, time::sleep};

    use futures::StreamExt as _;

    use crate::{
        authority_service::AuthorityService,
        block::{BlockAPI, SignedBlock, TestBlock, VerifiedBlock},
        commit::{CertifiedCommits, CommitRange},
        commit_vote_monitor::CommitVoteMonitor,
        context::Context,
        core_thread::{CoreError, CoreThreadDispatcher},
        dag_state::DagState,
        error::ConsensusResult,
        network::{
            BlockStream, ExtendedSerializedBlock, ObserverNetworkClient, SynchronizerClient,
            ValidatorNetworkClient, ValidatorNetworkService,
        },
        round_tracker::RoundTracker,
        storage::mem_store::MemStore,
        synchronizer::Synchronizer,
        test_dag_builder::DagBuilder,
        transaction_certifier::TransactionCertifier,
    };
    struct FakeCoreThreadDispatcher {
        blocks: Mutex<Vec<VerifiedBlock>>,
    }

    impl FakeCoreThreadDispatcher {
        fn new() -> Self {
            Self {
                blocks: Mutex::new(vec![]),
            }
        }

        fn get_blocks(&self) -> Vec<VerifiedBlock> {
            self.blocks.lock().clone()
        }
    }

    #[async_trait]
    impl CoreThreadDispatcher for FakeCoreThreadDispatcher {
        async fn add_blocks(
            &self,
            blocks: Vec<VerifiedBlock>,
        ) -> Result<BTreeSet<BlockRef>, CoreError> {
            let block_refs = blocks.iter().map(|b| b.reference()).collect();
            self.blocks.lock().extend(blocks);
            Ok(block_refs)
        }

        async fn check_block_refs(
            &self,
            _block_refs: Vec<BlockRef>,
        ) -> Result<BTreeSet<BlockRef>, CoreError> {
            Ok(BTreeSet::new())
        }

        async fn add_certified_commits(
            &self,
            _commits: CertifiedCommits,
        ) -> Result<BTreeSet<BlockRef>, CoreError> {
            todo!()
        }

        async fn new_block(&self, _round: Round, _force: bool) -> Result<(), CoreError> {
            Ok(())
        }

        async fn get_missing_blocks(&self) -> Result<BTreeSet<BlockRef>, CoreError> {
            Ok(Default::default())
        }

        fn set_propagation_delay(&self, _propagation_delay: Round) -> Result<(), CoreError> {
            todo!()
        }

        fn set_last_known_proposed_round(&self, _round: Round) -> Result<(), CoreError> {
            todo!()
        }
    }

    #[derive(Default)]
    struct FakeNetworkClient {}

    #[async_trait]
    impl ValidatorNetworkClient for FakeNetworkClient {
        async fn send_block(
            &self,
            _peer: AuthorityIndex,
            _block: &VerifiedBlock,
            _timeout: Duration,
        ) -> ConsensusResult<()> {
            unimplemented!("Unimplemented")
        }

        async fn subscribe_blocks(
            &self,
            _peer: AuthorityIndex,
            _last_received: Round,
            _timeout: Duration,
        ) -> ConsensusResult<BlockStream> {
            unimplemented!("Unimplemented")
        }

        async fn fetch_blocks(
            &self,
            _peer: AuthorityIndex,
            _block_refs: Vec<BlockRef>,
            _fetch_after_rounds: Vec<Round>,
            _fetch_missing_ancestors: bool,
            _timeout: Duration,
        ) -> ConsensusResult<Vec<Bytes>> {
            unimplemented!("Unimplemented")
        }

        async fn fetch_commits(
            &self,
            _peer: AuthorityIndex,
            _commit_range: CommitRange,
            _timeout: Duration,
        ) -> ConsensusResult<(Vec<Bytes>, Vec<Bytes>)> {
            unimplemented!("Unimplemented")
        }

        async fn fetch_latest_blocks(
            &self,
            _peer: AuthorityIndex,
            _authorities: Vec<AuthorityIndex>,
            _timeout: Duration,
        ) -> ConsensusResult<Vec<Bytes>> {
            unimplemented!("Unimplemented")
        }

        async fn get_latest_rounds(
            &self,
            _peer: AuthorityIndex,
            _timeout: Duration,
        ) -> ConsensusResult<(Vec<Round>, Vec<Round>)> {
            unimplemented!("Unimplemented")
        }
    }

    #[async_trait]
    impl ObserverNetworkClient for FakeNetworkClient {
        async fn stream_blocks(
            &self,
            _peer: crate::network::PeerId,
            _highest_round_per_authority: Vec<u64>,
            _timeout: Duration,
        ) -> ConsensusResult<crate::network::ObserverBlockStream> {
            unimplemented!("Unimplemented")
        }

        async fn fetch_blocks(
            &self,
            _peer: crate::network::PeerId,
            _block_refs: Vec<BlockRef>,
            _timeout: Duration,
        ) -> ConsensusResult<Vec<Bytes>> {
            unimplemented!("Unimplemented")
        }

        async fn fetch_commits(
            &self,
            _peer: crate::network::PeerId,
            _commit_range: CommitRange,
            _timeout: Duration,
        ) -> ConsensusResult<(Vec<Bytes>, Vec<Bytes>)> {
            unimplemented!("Unimplemented")
        }
    }

    #[tokio::test(flavor = "current_thread", start_paused = true)]
    async fn test_handle_send_block() {
        let (context, _keys) = Context::new_for_test(4);
        let context = Arc::new(context);
        let block_verifier = Arc::new(crate::block_verifier::NoopBlockVerifier {});
        let commit_vote_monitor = Arc::new(CommitVoteMonitor::new(context.clone()));
        let core_dispatcher = Arc::new(FakeCoreThreadDispatcher::new());
        let (_tx_block_broadcast, rx_block_broadcast) = broadcast::channel(100);
        let fake_client = Arc::new(FakeNetworkClient::default());
        let network_client = Arc::new(SynchronizerClient::new(
            context.clone(),
            Some(fake_client.clone()),
            Some(fake_client.clone()),
        ));
        let store = Arc::new(MemStore::new());
        let dag_state = Arc::new(RwLock::new(DagState::new(context.clone(), store.clone())));
        let transaction_certifier =
            TransactionCertifier::new(context.clone(), block_verifier.clone(), dag_state.clone());
        let round_tracker = Arc::new(RwLock::new(RoundTracker::new(context.clone(), vec![])));
        let synchronizer = Synchronizer::start(
            network_client,
            context.clone(),
            core_dispatcher.clone(),
            commit_vote_monitor.clone(),
            block_verifier.clone(),
            transaction_certifier.clone(),
            round_tracker.clone(),
            dag_state.clone(),
            false,
        );
        let authority_service = Arc::new(AuthorityService::new(
            context.clone(),
            block_verifier,
            commit_vote_monitor,
            round_tracker,
            synchronizer,
            core_dispatcher.clone(),
            rx_block_broadcast,
            transaction_certifier,
            dag_state,
            store,
        ));

        // Test delaying blocks with time drift.
        let now = context.clock.timestamp_utc_ms();
        let max_drift = context.parameters.max_forward_time_drift;
        let input_block = VerifiedBlock::new_for_test(
            TestBlock::new(9, 0)
                .set_timestamp_ms(now + max_drift.as_millis() as u64)
                .build(),
        );

        let service = authority_service.clone();
        let serialized = ExtendedSerializedBlock {
            block: input_block.serialized().clone(),
            excluded_ancestors: vec![],
        };

        tokio::spawn({
            let service = service.clone();
            let context = context.clone();
            async move {
                service
                    .handle_send_block(context.committee.to_authority_index(0).unwrap(), serialized)
                    .await
                    .unwrap();
            }
        });

        sleep(max_drift / 2).await;

        let blocks = core_dispatcher.get_blocks();
        assert_eq!(blocks.len(), 1);
        assert_eq!(blocks[0], input_block);

        // Test invalid block.
        let invalid_block =
            VerifiedBlock::new_for_test(TestBlock::new(10, 1000).set_timestamp_ms(10).build());
        let extended_block = ExtendedSerializedBlock {
            block: invalid_block.serialized().clone(),
            excluded_ancestors: vec![],
        };
        service
            .handle_send_block(
                context.committee.to_authority_index(0).unwrap(),
                extended_block,
            )
            .await
            .unwrap_err();

        // Test invalid excluded ancestors.
        let invalid_excluded_ancestors = vec![
            bcs::to_bytes(&BlockRef::new(
                10,
                AuthorityIndex::new_for_test(1000),
                BlockDigest::MIN,
            ))
            .unwrap(),
            vec![3u8; 40],
            bcs::to_bytes(&invalid_block.reference()).unwrap(),
        ];
        let extended_block = ExtendedSerializedBlock {
            block: input_block.serialized().clone(),
            excluded_ancestors: invalid_excluded_ancestors,
        };
        service
            .handle_send_block(
                context.committee.to_authority_index(0).unwrap(),
                extended_block,
            )
            .await
            .unwrap_err();
    }

    #[tokio::test(flavor = "current_thread", start_paused = true)]
    async fn test_handle_fetch_blocks() {
        // GIVEN
        // Use NUM_AUTHORITIES and NUM_ROUNDS higher than max_blocks_per_sync to test limits.
        const NUM_AUTHORITIES: usize = 40;
        const NUM_ROUNDS: usize = 40;
        let (mut context, _keys) = Context::new_for_test(NUM_AUTHORITIES);
        context.parameters.max_blocks_per_fetch = 50;
        let context = Arc::new(context);
        let block_verifier = Arc::new(crate::block_verifier::NoopBlockVerifier {});
        let commit_vote_monitor = Arc::new(CommitVoteMonitor::new(context.clone()));
        let core_dispatcher = Arc::new(FakeCoreThreadDispatcher::new());
        let (_tx_block_broadcast, rx_block_broadcast) = broadcast::channel(100);
        let fake_client = Arc::new(FakeNetworkClient::default());
        let network_client = Arc::new(SynchronizerClient::new(
            context.clone(),
            Some(fake_client.clone()),
            Some(fake_client.clone()),
        ));
        let store = Arc::new(MemStore::new());
        let dag_state = Arc::new(RwLock::new(DagState::new(context.clone(), store.clone())));
        let transaction_certifier =
            TransactionCertifier::new(context.clone(), block_verifier.clone(), dag_state.clone());
        let round_tracker = Arc::new(RwLock::new(RoundTracker::new(context.clone(), vec![])));
        let synchronizer = Synchronizer::start(
            network_client,
            context.clone(),
            core_dispatcher.clone(),
            commit_vote_monitor.clone(),
            block_verifier.clone(),
            transaction_certifier.clone(),
            round_tracker.clone(),
            dag_state.clone(),
            false,
        );
        let authority_service = Arc::new(AuthorityService::new(
            context.clone(),
            block_verifier,
            commit_vote_monitor,
            round_tracker,
            synchronizer,
            core_dispatcher.clone(),
            rx_block_broadcast,
            transaction_certifier,
            dag_state.clone(),
            store,
        ));

        // GIVEN: 40 rounds of blocks in the dag state.
        let mut dag_builder = DagBuilder::new(context.clone());
        dag_builder
            .layers(1..=(NUM_ROUNDS as u32))
            .build()
            .persist_layers(dag_state.clone());
        dag_state.write().flush();
        let all_blocks = dag_builder.all_blocks();

        // WHEN: Request 2 blocks from round 40, fetch missing ancestors enabled.
        let missing_block_refs: Vec<BlockRef> = all_blocks
            .iter()
            .rev()
            .take(2)
            .map(|b| b.reference())
            .collect();
        let highest_accepted_rounds: Vec<Round> = vec![1; NUM_AUTHORITIES];
        let results = authority_service
            .handle_fetch_blocks(
                AuthorityIndex::new_for_test(0),
                missing_block_refs.clone(),
                highest_accepted_rounds,
                true,
            )
            .await
            .unwrap();

        // THEN: the expected number of unique blocks are returned.
        let blocks: BTreeMap<BlockRef, VerifiedBlock> = results
            .iter()
            .map(|b| {
                let signed = bcs::from_bytes(b).unwrap();
                let block = VerifiedBlock::new_verified(signed, b.clone());
                (block.reference(), block)
            })
            .collect();
        assert_eq!(blocks.len(), context.parameters.max_blocks_per_sync);
        // All missing blocks are returned.
        for b in &missing_block_refs {
            assert!(blocks.contains_key(b));
        }
        let num_missing_ancestors = blocks
            .keys()
            .filter(|b| b.round == NUM_ROUNDS as Round - 1)
            .count();
        assert_eq!(
            num_missing_ancestors,
            context.parameters.max_blocks_per_sync - missing_block_refs.len()
        );

        // WHEN: Request 2 blocks from round 37, fetch missing ancestors disabled.
        let missing_round = NUM_ROUNDS as Round - 3;
        let missing_block_refs: Vec<BlockRef> = all_blocks
            .iter()
            .filter(|b| b.reference().round == missing_round)
            .map(|b| b.reference())
            .take(2)
            .collect();
        let mut highest_accepted_rounds: Vec<Round> = vec![1; NUM_AUTHORITIES];
        // Try to fill up the blocks from the 1st authority in missing_block_refs.
        highest_accepted_rounds[missing_block_refs[0].author] = missing_round - 5;
        let results = authority_service
            .handle_fetch_blocks(
                AuthorityIndex::new_for_test(0),
                missing_block_refs.clone(),
                highest_accepted_rounds,
                false,
            )
            .await
            .unwrap();

        // THEN: the expected number of unique blocks are returned.
        let blocks: BTreeMap<BlockRef, VerifiedBlock> = results
            .iter()
            .map(|b| {
                let signed = bcs::from_bytes(b).unwrap();
                let block = VerifiedBlock::new_verified(signed, b.clone());
                (block.reference(), block)
            })
            .collect();
        assert_eq!(blocks.len(), context.parameters.max_blocks_per_sync);
        // All missing blocks are returned.
        for b in &missing_block_refs {
            assert!(blocks.contains_key(b));
        }
        // Ancestor blocks are from the expected rounds and authorities.
        let expected_authors = [missing_block_refs[0].author, missing_block_refs[1].author];
        for b in blocks.keys() {
            assert!(b.round <= missing_round);
            assert!(expected_authors.contains(&b.author));
        }

        // WHEN: Request with empty block_refs, fetch missing ancestors disabled.
        let mut highest_accepted_rounds: Vec<Round> = vec![1; NUM_AUTHORITIES];
        // Set a few authorities to higher accepted rounds.
        highest_accepted_rounds[0] = (NUM_ROUNDS as Round) - 5;
        highest_accepted_rounds[1] = (NUM_ROUNDS as Round) - 3;
        let results = authority_service
            .handle_fetch_blocks(
                AuthorityIndex::new_for_test(0),
                vec![],
                highest_accepted_rounds.clone(),
                false,
            )
            .await
            .unwrap();

        // THEN: the expected number of unique blocks are returned.
        let blocks: BTreeMap<BlockRef, VerifiedBlock> = results
            .iter()
            .map(|b| {
                let signed = bcs::from_bytes(b).unwrap();
                let block = VerifiedBlock::new_verified(signed, b.clone());
                (block.reference(), block)
            })
            .collect();
        assert_eq!(blocks.len(), context.parameters.max_blocks_per_fetch);
        // Blocks should be from all authorities, within the expected round range.
        for block_ref in blocks.keys() {
            let accepted = highest_accepted_rounds[block_ref.author];
            assert!(block_ref.round > accepted);
        }
        // Blocks should be fetched in ascending round order across authorities,
        // so blocks should have low rounds near the accepted rounds.
        let max_round_in_result = blocks.keys().map(|b| b.round).max().unwrap();
        // With 40 authorities mostly at accepted round 1 and max_blocks_per_fetch=50,
        // the min-heap fills ~1-2 rounds per authority.
        assert!(
            max_round_in_result <= 4,
            "Expected low rounds from fair round-order fetching, got max round {}",
            max_round_in_result
        );

        // WHEN: Request 5 block from round 40, not getting ancestors.
        let missing_block_refs: Vec<BlockRef> = all_blocks
            .iter()
            .filter(|b| b.reference().round == NUM_ROUNDS as Round - 10)
            .map(|b| b.reference())
            .take(5)
            .collect();
        let results = authority_service
            .handle_fetch_blocks(
                AuthorityIndex::new_for_test(0),
                missing_block_refs.clone(),
                vec![],
                false,
            )
            .await
            .unwrap();

        // THEN: the expected number of unique blocks are returned.
        let blocks: BTreeMap<BlockRef, VerifiedBlock> = results
            .iter()
            .map(|b| {
                let signed = bcs::from_bytes(b).unwrap();
                let block = VerifiedBlock::new_verified(signed, b.clone());
                (block.reference(), block)
            })
            .collect();
        assert_eq!(blocks.len(), 5);
        for b in &missing_block_refs {
            assert!(blocks.contains_key(b));
        }
    }

    #[tokio::test(flavor = "current_thread", start_paused = true)]
    async fn test_handle_fetch_latest_blocks() {
        // GIVEN
        let (context, _keys) = Context::new_for_test(4);
        let context = Arc::new(context);
        let block_verifier = Arc::new(crate::block_verifier::NoopBlockVerifier {});
        let commit_vote_monitor = Arc::new(CommitVoteMonitor::new(context.clone()));
        let core_dispatcher = Arc::new(FakeCoreThreadDispatcher::new());
        let (_tx_block_broadcast, rx_block_broadcast) = broadcast::channel(100);
        let fake_client = Arc::new(FakeNetworkClient::default());
        let network_client = Arc::new(SynchronizerClient::new(
            context.clone(),
            Some(fake_client.clone()),
            Some(fake_client.clone()),
        ));
        let store = Arc::new(MemStore::new());
        let dag_state = Arc::new(RwLock::new(DagState::new(context.clone(), store.clone())));
        let transaction_certifier =
            TransactionCertifier::new(context.clone(), block_verifier.clone(), dag_state.clone());
        let round_tracker = Arc::new(RwLock::new(RoundTracker::new(context.clone(), vec![])));
        let synchronizer = Synchronizer::start(
            network_client,
            context.clone(),
            core_dispatcher.clone(),
            commit_vote_monitor.clone(),
            block_verifier.clone(),
            transaction_certifier.clone(),
            round_tracker.clone(),
            dag_state.clone(),
            true,
        );
        let authority_service = Arc::new(AuthorityService::new(
            context.clone(),
            block_verifier,
            commit_vote_monitor,
            round_tracker,
            synchronizer,
            core_dispatcher.clone(),
            rx_block_broadcast,
            transaction_certifier,
            dag_state.clone(),
            store,
        ));

        // Create some blocks for a few authorities. Create some equivocations as well and store in dag state.
        let mut dag_builder = DagBuilder::new(context.clone());
        dag_builder
            .layers(1..=10)
            .authorities(vec![AuthorityIndex::new_for_test(2)])
            .equivocate(1)
            .build()
            .persist_layers(dag_state);

        // WHEN
        let authorities_to_request = vec![
            AuthorityIndex::new_for_test(1),
            AuthorityIndex::new_for_test(2),
        ];
        let results = authority_service
            .handle_fetch_latest_blocks(AuthorityIndex::new_for_test(1), authorities_to_request)
            .await;

        // THEN
        let serialised_blocks = results.unwrap();
        for serialised_block in serialised_blocks {
            let signed_block: SignedBlock =
                bcs::from_bytes(&serialised_block).expect("Error while deserialising block");
            let verified_block = VerifiedBlock::new_verified(signed_block, serialised_block);

            assert_eq!(verified_block.round(), 10);
        }
    }

    #[tokio::test(flavor = "current_thread", start_paused = true)]
    async fn test_handle_subscribe_blocks() {
        let (context, _keys) = Context::new_for_test(4);
        let context = Arc::new(context);
        let block_verifier = Arc::new(crate::block_verifier::NoopBlockVerifier {});
        let commit_vote_monitor = Arc::new(CommitVoteMonitor::new(context.clone()));
        let core_dispatcher = Arc::new(FakeCoreThreadDispatcher::new());
        let (_tx_block_broadcast, rx_block_broadcast) = broadcast::channel(100);
        let fake_client = Arc::new(FakeNetworkClient::default());
        let network_client = Arc::new(SynchronizerClient::new(
            context.clone(),
            Some(fake_client.clone()),
            Some(fake_client.clone()),
        ));
        let store = Arc::new(MemStore::new());
        let dag_state = Arc::new(RwLock::new(DagState::new(context.clone(), store.clone())));
        let transaction_certifier =
            TransactionCertifier::new(context.clone(), block_verifier.clone(), dag_state.clone());
        let round_tracker = Arc::new(RwLock::new(RoundTracker::new(context.clone(), vec![])));
        let synchronizer = Synchronizer::start(
            network_client,
            context.clone(),
            core_dispatcher.clone(),
            commit_vote_monitor.clone(),
            block_verifier.clone(),
            transaction_certifier.clone(),
            round_tracker.clone(),
            dag_state.clone(),
            false,
        );

        // Create 3 proposed blocks at rounds 5, 10, 15 for own authority (index 0)
        dag_state
            .write()
            .accept_block(VerifiedBlock::new_for_test(TestBlock::new(5, 0).build()));
        dag_state
            .write()
            .accept_block(VerifiedBlock::new_for_test(TestBlock::new(10, 0).build()));
        dag_state
            .write()
            .accept_block(VerifiedBlock::new_for_test(TestBlock::new(15, 0).build()));

        let authority_service = Arc::new(AuthorityService::new(
            context.clone(),
            block_verifier,
            commit_vote_monitor,
            round_tracker,
            synchronizer,
            core_dispatcher.clone(),
            rx_block_broadcast,
            transaction_certifier,
            dag_state.clone(),
            store,
        ));

        let peer = context.committee.to_authority_index(1).unwrap();

        // Case A: Subscribe with last_received = 100 (after all proposed blocks)
        // Should return last proposed block (round 15) as fallback
        {
            let mut stream = authority_service
                .handle_subscribe_blocks(peer, 100)
                .await
                .unwrap();
            let block: SignedBlock = bcs::from_bytes(&stream.next().await.unwrap().block).unwrap();
            assert_eq!(
                block.round(),
                15,
                "Should return last proposed block as fallback"
            );
            assert_eq!(block.author().value(), 0);
        }

        // Case B: Subscribe with last_received = 7 (includes rounds 10, 15)
        // Should return cached blocks from round 8+
        {
            let mut stream = authority_service
                .handle_subscribe_blocks(peer, 7)
                .await
                .unwrap();

            let block1: SignedBlock = bcs::from_bytes(&stream.next().await.unwrap().block).unwrap();
            assert_eq!(block1.round(), 10, "Should return block at round 10");

            let block2: SignedBlock = bcs::from_bytes(&stream.next().await.unwrap().block).unwrap();
            assert_eq!(block2.round(), 15, "Should return block at round 15");
        }
    }

    #[tokio::test(flavor = "current_thread", start_paused = true)]
    async fn test_handle_subscribe_blocks_not_proposed() {
        let (context, _keys) = Context::new_for_test(4);
        let context = Arc::new(context);
        let block_verifier = Arc::new(crate::block_verifier::NoopBlockVerifier {});
        let commit_vote_monitor = Arc::new(CommitVoteMonitor::new(context.clone()));
        let core_dispatcher = Arc::new(FakeCoreThreadDispatcher::new());
        let (_tx_block_broadcast, rx_block_broadcast) = broadcast::channel(100);
        let fake_client = Arc::new(FakeNetworkClient::default());
        let network_client = Arc::new(SynchronizerClient::new(
            context.clone(),
            Some(fake_client.clone()),
            Some(fake_client.clone()),
        ));
        let store = Arc::new(MemStore::new());
        let dag_state = Arc::new(RwLock::new(DagState::new(context.clone(), store.clone())));
        let transaction_certifier =
            TransactionCertifier::new(context.clone(), block_verifier.clone(), dag_state.clone());
        let round_tracker = Arc::new(RwLock::new(RoundTracker::new(context.clone(), vec![])));
        let synchronizer = Synchronizer::start(
            network_client,
            context.clone(),
            core_dispatcher.clone(),
            commit_vote_monitor.clone(),
            block_verifier.clone(),
            transaction_certifier.clone(),
            round_tracker.clone(),
            dag_state.clone(),
            false,
        );

        // No blocks added to DagState - only genesis exists

        let authority_service = Arc::new(AuthorityService::new(
            context.clone(),
            block_verifier,
            commit_vote_monitor,
            round_tracker,
            synchronizer,
            core_dispatcher.clone(),
            rx_block_broadcast,
            transaction_certifier,
            dag_state.clone(),
            store,
        ));

        let peer = context.committee.to_authority_index(1).unwrap();

        // Subscribe - no blocks have been proposed yet (only genesis exists)
        let mut stream = authority_service
            .handle_subscribe_blocks(peer, 0)
            .await
            .unwrap();

        // Should NOT receive any block (genesis must not be returned)
        use futures::poll;
        use std::task::Poll;
        let poll_result = poll!(stream.next());
        assert!(
            matches!(poll_result, Poll::Pending),
            "Should not receive genesis block on subscription stream"
        );
    }
}