consensus_core/

linearizer.rs

// Copyright (c) Mysten Labs, Inc.
// Modifications Copyright (c) 2024 IOTA Stiftung
// SPDX-License-Identifier: Apache-2.0

use std::{collections::HashSet, sync::Arc};

use consensus_config::AuthorityIndex;
use itertools::Itertools;
use parking_lot::RwLock;

use crate::{
    Round,
    block::{BlockAPI, BlockRef, VerifiedBlock},
    commit::{Commit, CommittedSubDag, TrustedCommit, sort_sub_dag_blocks},
    context::Context,
    dag_state::DagState,
    leader_schedule::LeaderSchedule,
};

/// The `StorageAPI` trait provides an interface for the block store and has
/// been mostly introduced for allowing to inject the test store in
/// `DagBuilder`.
pub(crate) trait BlockStoreAPI {
    fn get_blocks(&self, refs: &[BlockRef]) -> Vec<Option<VerifiedBlock>>;

    fn gc_round(&self) -> Round;

    fn gc_enabled(&self) -> bool;

    fn set_committed(&mut self, block_ref: &BlockRef) -> bool;

    fn is_committed(&self, block_ref: &BlockRef) -> bool;
}

impl BlockStoreAPI
    for parking_lot::lock_api::RwLockWriteGuard<'_, parking_lot::RawRwLock, DagState>
{
    fn get_blocks(&self, refs: &[BlockRef]) -> Vec<Option<VerifiedBlock>> {
        DagState::get_blocks(self, refs)
    }

    fn gc_round(&self) -> Round {
        DagState::gc_round(self)
    }

    fn gc_enabled(&self) -> bool {
        DagState::gc_enabled(self)
    }

    fn set_committed(&mut self, block_ref: &BlockRef) -> bool {
        DagState::set_committed(self, block_ref)
    }

    fn is_committed(&self, block_ref: &BlockRef) -> bool {
        DagState::is_committed(self, block_ref)
    }
}

/// Expand a committed sequence of leader into a sequence of sub-dags.
#[derive(Clone)]
pub(crate) struct Linearizer {
    /// In memory block store representing the dag state
    context: Arc<Context>,
    dag_state: Arc<RwLock<DagState>>,
    leader_schedule: Arc<LeaderSchedule>,
}

impl Linearizer {
    pub(crate) fn new(
        context: Arc<Context>,
        dag_state: Arc<RwLock<DagState>>,
        leader_schedule: Arc<LeaderSchedule>,
    ) -> Self {
        Self {
            dag_state,
            leader_schedule,
            context,
        }
    }

    /// Collect the sub-dag and the corresponding commit from a specific leader
    /// excluding any duplicates or blocks that have already been committed
    /// (within previous sub-dags).
    fn collect_sub_dag_and_commit(
        &mut self,
        leader_block: VerifiedBlock,
        reputation_scores_desc: Vec<(AuthorityIndex, u64)>,
    ) -> (CommittedSubDag, TrustedCommit) {
        let _s = self
            .context
            .metrics
            .node_metrics
            .scope_processing_time
            .with_label_values(&["Linearizer::collect_sub_dag_and_commit"])
            .start_timer();
        // Grab latest commit state from dag state
        let mut dag_state = self.dag_state.write();
        let last_commit_index = dag_state.last_commit_index();
        let last_commit_digest = dag_state.last_commit_digest();
        let last_commit_timestamp_ms = dag_state.last_commit_timestamp_ms();
        let last_committed_rounds = dag_state.last_committed_rounds();
        let timestamp_ms = leader_block.timestamp_ms().max(last_commit_timestamp_ms);

        // Now linearize the sub-dag starting from the leader block
        let to_commit = Self::linearize_sub_dag(
            &self.context,
            leader_block.clone(),
            last_committed_rounds,
            &mut dag_state,
        );

        drop(dag_state);

        // Create the Commit.
        let commit = Commit::new(
            last_commit_index + 1,
            last_commit_digest,
            timestamp_ms,
            leader_block.reference(),
            to_commit
                .iter()
                .map(|block| block.reference())
                .collect::<Vec<_>>(),
        );
        let serialized = commit
            .serialize()
            .unwrap_or_else(|e| panic!("Failed to serialize commit: {e}"));
        let commit = TrustedCommit::new_trusted(commit, serialized);

        // Create the corresponding committed sub dag
        let sub_dag = CommittedSubDag::new(
            leader_block.reference(),
            to_commit,
            timestamp_ms,
            commit.reference(),
            reputation_scores_desc,
        );

        (sub_dag, commit)
    }

    pub(crate) fn linearize_sub_dag(
        context: &Context,
        leader_block: VerifiedBlock,
        last_committed_rounds: Vec<u32>,
        dag_state: &mut impl BlockStoreAPI,
    ) -> Vec<VerifiedBlock> {
        let gc_enabled = dag_state.gc_enabled();
        // The GC round here is calculated based on the last committed round of the
        // leader block. The algorithm will attempt to commit blocks up to this
        // GC round. Once this commit has been processed and written to DagState, then
        // gc round will update and on the processing of the next commit we'll
        // have it already updated, so no need to do any gc_round recalculations here.
        // We just use whatever is currently in DagState.
        let gc_round: Round = dag_state.gc_round();
        let leader_block_ref = leader_block.reference();
        let mut buffer = vec![leader_block];

        let mut to_commit = Vec::new();

        // The new logic will perform the recursion without stopping at the highest
        // round round that has been committed per authority. Instead it will
        // allow to commit blocks that are lower than the highest committed round for an
        // authority but higher than gc_round.
        if context.protocol_config.consensus_linearize_subdag_v2() {
            assert!(
                dag_state.set_committed(&leader_block_ref),
                "Leader block with reference {leader_block_ref:?} attempted to be committed twice"
            );

            while let Some(x) = buffer.pop() {
                to_commit.push(x.clone());

                let ancestors: Vec<VerifiedBlock> = dag_state
                    .get_blocks(
                        &x.ancestors()
                            .iter()
                            .copied()
                            .filter(|ancestor| {
                                ancestor.round > gc_round && !dag_state.is_committed(ancestor)
                            })
                            .collect::<Vec<_>>(),
                    )
                    .into_iter()
                    .map(|ancestor_opt| {
                        ancestor_opt.expect("We should have all uncommitted blocks in dag state.")
                    })
                    .collect();

                for ancestor in ancestors {
                    buffer.push(ancestor.clone());
                    assert!(
                        dag_state.set_committed(&ancestor.reference()),
                        "Block with reference {:?} attempted to be committed twice",
                        ancestor.reference()
                    );
                }
            }
        } else {
            let mut committed = HashSet::new();
            assert!(committed.insert(leader_block_ref));

            while let Some(x) = buffer.pop() {
                to_commit.push(x.clone());

                let ancestors: Vec<VerifiedBlock> = dag_state
                    .get_blocks(
                        &x.ancestors()
                            .iter()
                            .copied()
                            .filter(|ancestor| {
                                // We skip the block if we already committed it or we reached a
                                // round that we already committed.
                                // TODO: for Fast Path we need to amend the recursion rule here and
                                // allow us to commit blocks all the way up to the `gc_round`.
                                // Some additional work will be needed to make sure that we keep the
                                // uncommitted blocks up to the `gc_round` across commits.
                                !committed.contains(ancestor)
                                    && last_committed_rounds[ancestor.author] < ancestor.round
                            })
                            .filter(|ancestor| {
                                // Keep the block if GC is not enabled or it is enabled and the
                                // block is above the gc_round. We do this
                                // to stop the recursion early and avoid going to deep when it's
                                // unnecessary.
                                !gc_enabled || ancestor.round > gc_round
                            })
                            .collect::<Vec<_>>(),
                    )
                    .into_iter()
                    .map(|ancestor_opt| {
                        ancestor_opt.expect("We should have all uncommitted blocks in dag state.")
                    })
                    .collect();

                for ancestor in ancestors {
                    buffer.push(ancestor.clone());
                    assert!(committed.insert(ancestor.reference()));
                }
            }
        }

        // The above code should have not yielded any blocks that are <= gc_round, but
        // just to make sure that we'll never commit anything that should be
        // garbage collected we attempt to prune here as well.
        if gc_enabled {
            assert!(
                to_commit.iter().all(|block| block.round() > gc_round),
                "No blocks <= {gc_round} should be committed. Leader round {leader_block_ref}, blocks {to_commit:?}."
            );
        }

        // Sort the blocks of the sub-dag blocks
        sort_sub_dag_blocks(&mut to_commit);

        to_commit
    }

    // This function should be called whenever a new commit is observed. This will
    // iterate over the sequence of committed leaders and produce a list of
    // committed sub-dags.
    pub(crate) fn handle_commit(
        &mut self,
        committed_leaders: Vec<VerifiedBlock>,
    ) -> Vec<CommittedSubDag> {
        if committed_leaders.is_empty() {
            return vec![];
        }

        // We check whether the leader schedule has been updated. If yes, then we'll
        // send the scores as part of the first sub dag.
        let schedule_updated = self
            .leader_schedule
            .leader_schedule_updated(&self.dag_state);

        let mut committed_sub_dags = vec![];
        for (i, leader_block) in committed_leaders.into_iter().enumerate() {
            let reputation_scores_desc = if schedule_updated && i == 0 {
                self.leader_schedule
                    .leader_swap_table
                    .read()
                    .reputation_scores_desc
                    .clone()
            } else {
                vec![]
            };

            // Collect the sub-dag generated using each of these leaders and the
            // corresponding commit.
            let (sub_dag, commit) =
                self.collect_sub_dag_and_commit(leader_block, reputation_scores_desc);

            self.update_blocks_pruned_metric(&sub_dag);

            // Buffer commit in dag state for persistence later.
            // This also updates the last committed rounds.
            self.dag_state.write().add_commit(commit.clone());

            committed_sub_dags.push(sub_dag);
        }

        // Committed blocks must be persisted to storage before sending them to IOTA and
        // executing their transactions.
        // Commit metadata can be persisted more lazily because they are recoverable.
        // Uncommitted blocks can wait to persist too.
        // But for simplicity, all unpersisted blocks and commits are flushed to
        // storage.
        self.dag_state.write().flush();

        committed_sub_dags
    }

    // Try to measure the number of blocks that get pruned due to GC. This is not
    // very accurate, but it can give us a good enough idea. We consider a block
    // as pruned when it is an ancestor of a block that has been committed as part
    // of the provided `sub_dag`, but it has not been committed as part of
    // previous commits. Right now we measure this via checking that highest
    // committed round for the authority as we don't an efficient look up
    // functionality to check if a block has been committed or not.
    fn update_blocks_pruned_metric(&self, sub_dag: &CommittedSubDag) {
        let (last_committed_rounds, gc_round) = {
            let dag_state = self.dag_state.read();
            (dag_state.last_committed_rounds(), dag_state.gc_round())
        };

        for block_ref in sub_dag
            .blocks
            .iter()
            .flat_map(|block| block.ancestors())
            .filter(
                |ancestor_ref| {
                    ancestor_ref.round <= gc_round
                        && last_committed_rounds[ancestor_ref.author] != ancestor_ref.round
                }, /* If the last committed round is the same as the pruned block's round, then
                    * we know for sure that it has been committed and it doesn't count here
                    * as pruned block. */
            )
            .unique()
        {
            let hostname = &self.context.committee.authority(block_ref.author).hostname;

            // If the last committed round from this authority is lower than the pruned
            // ancestor in question, then we know for sure that it has not been committed.
            let label_values = if last_committed_rounds[block_ref.author] < block_ref.round {
                &[hostname, "uncommitted"]
            } else {
                // If last committed round is higher for this authority, then we don't really
                // know it's status, but we know that there is a higher committed block from
                // this authority.
                &[hostname, "higher_committed"]
            };

            self.context
                .metrics
                .node_metrics
                .blocks_pruned_on_commit
                .with_label_values(label_values)
                .inc();
        }
    }
}

#[cfg(test)]
mod tests {
    use rstest::rstest;

    use super::*;
    use crate::{
        CommitIndex,
        commit::{CommitAPI as _, CommitDigest, DEFAULT_WAVE_LENGTH},
        context::Context,
        leader_schedule::{LeaderSchedule, LeaderSwapTable},
        storage::mem_store::MemStore,
        test_dag_builder::DagBuilder,
        test_dag_parser::parse_dag,
    };

    #[tokio::test]
    async fn test_handle_commit() {
        telemetry_subscribers::init_for_testing();
        let num_authorities = 4;
        let context = Arc::new(Context::new_for_test(num_authorities).0);
        let dag_state = Arc::new(RwLock::new(DagState::new(
            context.clone(),
            Arc::new(MemStore::new()),
        )));
        let leader_schedule = Arc::new(LeaderSchedule::new(
            context.clone(),
            LeaderSwapTable::default(),
        ));
        let mut linearizer = Linearizer::new(context.clone(), dag_state.clone(), leader_schedule);

        // Populate fully connected test blocks for round 0 ~ 10, authorities 0 ~ 3.
        let num_rounds: u32 = 10;
        let mut dag_builder = DagBuilder::new(context.clone());
        dag_builder
            .layers(1..=num_rounds)
            .build()
            .persist_layers(dag_state.clone());

        let leaders = dag_builder
            .leader_blocks(1..=num_rounds)
            .into_iter()
            .map(Option::unwrap)
            .collect::<Vec<_>>();

        let commits = linearizer.handle_commit(leaders.clone());
        for (idx, subdag) in commits.into_iter().enumerate() {
            tracing::info!("{subdag:?}");
            assert_eq!(subdag.leader, leaders[idx].reference());
            assert_eq!(subdag.timestamp_ms, leaders[idx].timestamp_ms());
            if idx == 0 {
                // First subdag includes the leader block only
                assert_eq!(subdag.blocks.len(), 1);
            } else {
                // Every subdag after will be missing the leader block from the previous
                // committed subdag
                assert_eq!(subdag.blocks.len(), num_authorities);
            }
            for block in subdag.blocks.iter() {
                assert!(block.round() <= leaders[idx].round());
            }
            assert_eq!(subdag.commit_ref.index, idx as CommitIndex + 1);
        }
    }

    #[tokio::test]
    async fn test_handle_commit_with_schedule_update() {
        telemetry_subscribers::init_for_testing();
        let num_authorities = 4;
        let context = Arc::new(Context::new_for_test(num_authorities).0);
        let dag_state = Arc::new(RwLock::new(DagState::new(
            context.clone(),
            Arc::new(MemStore::new()),
        )));
        const NUM_OF_COMMITS_PER_SCHEDULE: u64 = 10;
        let leader_schedule = Arc::new(
            LeaderSchedule::new(context.clone(), LeaderSwapTable::default())
                .with_num_commits_per_schedule(NUM_OF_COMMITS_PER_SCHEDULE),
        );
        let mut linearizer =
            Linearizer::new(context.clone(), dag_state.clone(), leader_schedule.clone());

        // Populate fully connected test blocks for round 0 ~ 20, authorities 0 ~ 3.
        let num_rounds: u32 = 20;
        let mut dag_builder = DagBuilder::new(context.clone());
        dag_builder
            .layers(1..=num_rounds)
            .build()
            .persist_layers(dag_state.clone());

        // Take the first 10 leaders
        let leaders = dag_builder
            .leader_blocks(1..=10)
            .into_iter()
            .map(Option::unwrap)
            .collect::<Vec<_>>();

        // Create some commits
        let commits = linearizer.handle_commit(leaders.clone());

        // Write them in DagState
        dag_state.write().add_scoring_subdags(commits);
        // Now update the leader schedule
        leader_schedule.update_leader_schedule_v2(&dag_state);
        assert!(
            leader_schedule.leader_schedule_updated(&dag_state),
            "Leader schedule should have been updated"
        );

        // Try to commit now the rest of the 10 leaders
        let leaders = dag_builder
            .leader_blocks(11..=20)
            .into_iter()
            .map(Option::unwrap)
            .collect::<Vec<_>>();

        // Now on the commits only the first one should contain the updated scores, the
        // other should be empty
        let commits = linearizer.handle_commit(leaders.clone());
        assert_eq!(commits.len(), 10);
        let scores = vec![
            (AuthorityIndex::new_for_test(1), 29),
            (AuthorityIndex::new_for_test(0), 29),
            (AuthorityIndex::new_for_test(3), 29),
            (AuthorityIndex::new_for_test(2), 29),
        ];
        assert_eq!(commits[0].reputation_scores_desc, scores);
        for commit in commits.into_iter().skip(1) {
            assert_eq!(commit.reputation_scores_desc, vec![]);
        }
    }

    // TODO: Remove when DistributedVoteScoring is enabled.
    #[tokio::test]
    async fn test_handle_commit_with_schedule_update_with_unscored_subdags() {
        telemetry_subscribers::init_for_testing();
        let num_authorities = 4;
        let context = Arc::new(Context::new_for_test(num_authorities).0);
        let dag_state = Arc::new(RwLock::new(DagState::new(
            context.clone(),
            Arc::new(MemStore::new()),
        )));
        const NUM_OF_COMMITS_PER_SCHEDULE: u64 = 10;
        let leader_schedule = Arc::new(
            LeaderSchedule::new(context.clone(), LeaderSwapTable::default())
                .with_num_commits_per_schedule(NUM_OF_COMMITS_PER_SCHEDULE),
        );
        let mut linearizer =
            Linearizer::new(context.clone(), dag_state.clone(), leader_schedule.clone());

        // Populate fully connected test blocks for round 0 ~ 20, authorities 0 ~ 3.
        let num_rounds: u32 = 20;
        let mut dag_builder = DagBuilder::new(context.clone());
        dag_builder
            .layers(1..=num_rounds)
            .build()
            .persist_layers(dag_state.clone());

        // Take the first 10 leaders
        let leaders = dag_builder
            .leader_blocks(1..=10)
            .into_iter()
            .map(Option::unwrap)
            .collect::<Vec<_>>();

        // Create some commits
        let commits = linearizer.handle_commit(leaders.clone());

        // Write them in DagState
        dag_state.write().add_unscored_committed_subdags(commits);

        // Now update the leader schedule
        leader_schedule.update_leader_schedule_v1(&dag_state);

        assert!(
            leader_schedule.leader_schedule_updated(&dag_state),
            "Leader schedule should have been updated"
        );

        // Try to commit now the rest of the 10 leaders
        let leaders = dag_builder
            .leader_blocks(11..=20)
            .into_iter()
            .map(Option::unwrap)
            .collect::<Vec<_>>();

        // Now on the commits only the first one should contain the updated scores, the
        // other should be empty
        let commits = linearizer.handle_commit(leaders.clone());
        assert_eq!(commits.len(), 10);
        let scores = vec![
            (AuthorityIndex::new_for_test(2), 9),
            (AuthorityIndex::new_for_test(1), 8),
            (AuthorityIndex::new_for_test(0), 8),
            (AuthorityIndex::new_for_test(3), 8),
        ];
        assert_eq!(commits[0].reputation_scores_desc, scores);

        for commit in commits.into_iter().skip(1) {
            assert_eq!(commit.reputation_scores_desc, vec![]);
        }
    }

    #[tokio::test]
    async fn test_handle_already_committed() {
        telemetry_subscribers::init_for_testing();
        let num_authorities = 4;
        let (mut context, _) = Context::new_for_test(num_authorities);
        context
            .protocol_config
            .set_consensus_gc_depth_for_testing(0);
        context
            .protocol_config
            .set_consensus_linearize_subdag_v2_for_testing(false);

        let context = Arc::new(context);

        let dag_state = Arc::new(RwLock::new(DagState::new(
            context.clone(),
            Arc::new(MemStore::new()),
        )));
        let leader_schedule = Arc::new(LeaderSchedule::new(
            context.clone(),
            LeaderSwapTable::default(),
        ));
        let mut linearizer =
            Linearizer::new(context.clone(), dag_state.clone(), leader_schedule.clone());
        let wave_length = DEFAULT_WAVE_LENGTH;

        let leader_round_wave_1 = 3;
        let leader_round_wave_2 = leader_round_wave_1 + wave_length;

        // Build a Dag from round 1..=6
        let mut dag_builder = DagBuilder::new(context.clone());
        dag_builder.layers(1..=leader_round_wave_2).build();

        // Now retrieve all the blocks up to round leader_round_wave_1 - 1
        // And then only the leader of round leader_round_wave_1
        // Also store those to DagState
        let mut blocks = dag_builder.blocks(0..=leader_round_wave_1 - 1);
        blocks.push(
            dag_builder
                .leader_block(leader_round_wave_1)
                .expect("Leader block should have been found"),
        );
        dag_state.write().accept_blocks(blocks.clone());

        let first_leader = dag_builder
            .leader_block(leader_round_wave_1)
            .expect("Wave 1 leader round block should exist");
        let mut last_commit_index = 1;
        let first_commit_data = TrustedCommit::new_for_test(
            last_commit_index,
            CommitDigest::MIN,
            0,
            first_leader.reference(),
            blocks.into_iter().map(|block| block.reference()).collect(),
        );
        dag_state.write().add_commit(first_commit_data);

        // Now take all the blocks from round `leader_round_wave_1` up to round
        // `leader_round_wave_2-1`
        let mut blocks = dag_builder.blocks(leader_round_wave_1..=leader_round_wave_2 - 1);
        // Filter out leader block of round `leader_round_wave_1`
        blocks.retain(|block| {
            !(block.round() == leader_round_wave_1
                && block.author() == leader_schedule.elect_leader(leader_round_wave_1, 0))
        });
        // Add the leader block of round `leader_round_wave_2`
        blocks.push(
            dag_builder
                .leader_block(leader_round_wave_2)
                .expect("Leader block should have been found"),
        );
        // Write them in dag state
        dag_state.write().accept_blocks(blocks.clone());

        let mut blocks: Vec<_> = blocks.into_iter().map(|block| block.reference()).collect();

        // Now get the latest leader which is the leader round of wave 2
        let leader = dag_builder
            .leader_block(leader_round_wave_2)
            .expect("Leader block should exist");

        last_commit_index += 1;
        let expected_second_commit = TrustedCommit::new_for_test(
            last_commit_index,
            CommitDigest::MIN,
            0,
            leader.reference(),
            blocks.clone(),
        );

        let commit = linearizer.handle_commit(vec![leader.clone()]);
        assert_eq!(commit.len(), 1);

        let subdag = &commit[0];
        tracing::info!("{subdag:?}");
        assert_eq!(subdag.leader, leader.reference());
        assert_eq!(subdag.timestamp_ms, leader.timestamp_ms());
        assert_eq!(subdag.commit_ref.index, expected_second_commit.index());

        // Using the same sorting as used in CommittedSubDag::sort
        blocks.sort_by(|a, b| a.round.cmp(&b.round).then_with(|| a.author.cmp(&b.author)));
        assert_eq!(
            subdag
                .blocks
                .clone()
                .into_iter()
                .map(|b| b.reference())
                .collect::<Vec<_>>(),
            blocks
        );
        for block in subdag.blocks.iter() {
            assert!(block.round() <= expected_second_commit.leader().round);
        }
    }

    /// This test will run the linearizer with GC disabled (gc_depth = 0) and gc
    /// enabled (gc_depth = 3) and make sure that for the exact same DAG the
    /// linearizer will commit different blocks according to the rules.
    #[rstest]
    #[tokio::test]
    async fn test_handle_commit_with_gc_simple(#[values(0, 3)] gc_depth: u32) {
        telemetry_subscribers::init_for_testing();

        let num_authorities = 4;
        let (mut context, _keys) = Context::new_for_test(num_authorities);
        context.protocol_config.set_gc_depth_for_testing(gc_depth);

        if gc_depth == 0 {
            context
                .protocol_config
                .set_consensus_linearize_subdag_v2_for_testing(false);
        }

        let context = Arc::new(context);
        let dag_state = Arc::new(RwLock::new(DagState::new(
            context.clone(),
            Arc::new(MemStore::new()),
        )));
        let leader_schedule = Arc::new(LeaderSchedule::new(
            context.clone(),
            LeaderSwapTable::default(),
        ));
        let mut linearizer = Linearizer::new(context.clone(), dag_state.clone(), leader_schedule);

        // Authorities of index 0->2 will always creates blocks that see each other, but
        // until round 5 they won't see the blocks of authority 3. For authority
        // 3 we create blocks that connect to all the other authorities.
        // On round 5 we finally make the other authorities see the blocks of authority
        // 3. Practically we "simulate" here a long chain created by authority 3
        // that is visible in round 5, but due to GC blocks of only round >=2 will
        // be committed, when GC is enabled. When GC is disabled all blocks will be
        // committed for rounds >= 1.
        let dag_str = "DAG {
                Round 0 : { 4 },
                Round 1 : { * },
                Round 2 : {
                    A -> [-D1],
                    B -> [-D1],
                    C -> [-D1],
                    D -> [*],
                },
                Round 3 : {
                    A -> [-D2],
                    B -> [-D2],
                    C -> [-D2],
                },
                Round 4 : {
                    A -> [-D3],
                    B -> [-D3],
                    C -> [-D3],
                    D -> [A3, B3, C3, D2],
                },
                Round 5 : { * },
            }";

        let (_, dag_builder) = parse_dag(dag_str).expect("Invalid dag");
        dag_builder.print();
        dag_builder.persist_all_blocks(dag_state.clone());

        let leaders = dag_builder
            .leader_blocks(1..=6)
            .into_iter()
            .flatten()
            .collect::<Vec<_>>();

        let commits = linearizer.handle_commit(leaders.clone());
        for (idx, subdag) in commits.into_iter().enumerate() {
            tracing::info!("{subdag:?}");
            assert_eq!(subdag.leader, leaders[idx].reference());
            assert_eq!(subdag.timestamp_ms, leaders[idx].timestamp_ms());
            if idx == 0 {
                // First subdag includes the leader block only
                assert_eq!(subdag.blocks.len(), 1);
            } else if idx == 1 {
                assert_eq!(subdag.blocks.len(), 3);
            } else if idx == 2 {
                // We commit:
                // * 1 block on round 4, the leader block
                // * 3 blocks on round 3, as no commit happened on round 3 since the leader was
                //   missing
                // * 2 blocks on round 2, again as no commit happened on round 3, we commit the
                //   "sub dag" of leader of round 3, which will be another 2 blocks
                assert_eq!(subdag.blocks.len(), 6);
            } else {
                // GC is enabled, so we expect to see only blocks of round >= 2
                if gc_depth > 0 {
                    // Now it's going to be the first time that a leader will see the blocks of
                    // authority 3 and will attempt to commit the long chain.
                    // However, due to GC it will only commit blocks of round > 1. That's because it
                    // will commit blocks up to previous leader's round (round =
                    // 4) minus the gc_depth = 3, so that will be gc_round = 4 - 3 = 1. So we expect
                    // to see on the sub dag committed blocks of round >= 2.
                    assert_eq!(subdag.blocks.len(), 5);

                    assert!(
                        subdag.blocks.iter().all(|block| block.round() >= 2),
                        "Found blocks that are of round < 2."
                    );

                    // Also ensure that gc_round has advanced with the latest committed leader
                    assert_eq!(dag_state.read().gc_round(), subdag.leader.round - gc_depth);
                } else {
                    // GC is disabled, so we expect to see all blocks of round >= 1
                    assert_eq!(subdag.blocks.len(), 6);
                    assert!(
                        subdag.blocks.iter().all(|block| block.round() >= 1),
                        "Found blocks that are of round < 1."
                    );

                    // GC round should never have moved
                    assert_eq!(dag_state.read().gc_round(), 0);
                }
            }
            for block in subdag.blocks.iter() {
                assert!(block.round() <= leaders[idx].round());
            }
            assert_eq!(subdag.commit_ref.index, idx as CommitIndex + 1);
        }
    }

    #[rstest]
    #[tokio::test]
    async fn test_handle_commit_below_highest_committed_round(#[values(3)] gc_depth: u32) {
        telemetry_subscribers::init_for_testing();

        let num_authorities = 4;
        let (mut context, _keys) = Context::new_for_test(num_authorities);
        context
            .protocol_config
            .set_consensus_gc_depth_for_testing(gc_depth);
        context
            .protocol_config
            .set_consensus_linearize_subdag_v2_for_testing(true);

        let context = Arc::new(context);
        let dag_state = Arc::new(RwLock::new(DagState::new(
            context.clone(),
            Arc::new(MemStore::new()),
        )));
        let leader_schedule = Arc::new(LeaderSchedule::new(
            context.clone(),
            LeaderSwapTable::default(),
        ));
        let mut linearizer = Linearizer::new(context.clone(), dag_state.clone(), leader_schedule);

        // Authority D will create an "orphaned" block on round 1 as it won't reference
        // to it on the block of round 2. Similar, no other authority will reference to
        // it on round 2. Then on round 3 the authorities A, B & C will link to
        // block D1. Once the DAG gets committed we should see the block D1 getting
        // committed as well. Normally ,as block D2 would have been committed
        // first block D1 should be omitted. With the new logic this is no longer true.
        let dag_str = "DAG {
                Round 0 : { 4 },
                Round 1 : { * },
                Round 2 : {
                    A -> [-D1],
                    B -> [-D1],
                    C -> [-D1],
                    D -> [-D1],
                },
                Round 3 : {
                    A -> [A2, B2, C2, D1],
                    B -> [A2, B2, C2, D1],
                    C -> [A2, B2, C2, D1],
                    D -> [A2, B2, C2, D2]
                },
                Round 4 : { * },
            }";

        let (_, dag_builder) = parse_dag(dag_str).expect("Invalid dag");
        dag_builder.print();
        dag_builder.persist_all_blocks(dag_state.clone());

        let leaders = dag_builder
            .leader_blocks(1..=4)
            .into_iter()
            .flatten()
            .collect::<Vec<_>>();

        let commits = linearizer.handle_commit(leaders.clone());
        for (idx, subdag) in commits.into_iter().enumerate() {
            tracing::info!("{subdag:?}");
            assert_eq!(subdag.leader, leaders[idx].reference());
            assert_eq!(subdag.timestamp_ms, leaders[idx].timestamp_ms());
            if idx == 0 {
                // First subdag includes the leader block only B1
                assert_eq!(subdag.blocks.len(), 1);
            } else if idx == 1 {
                // We commit:
                // * 1 block on round 2, the leader block C2
                // * 2 blocks on round 1, A1, C1
                assert_eq!(subdag.blocks.len(), 3);
            } else if idx == 2 {
                // We commit:
                // * 1 block on round 3, the leader block D3
                // * 3 blocks on round 2, A2, B2, D2
                assert_eq!(subdag.blocks.len(), 4);

                assert!(
                    subdag.blocks.iter().any(|block| block.round() == 2
                        && block.author() == AuthorityIndex::new_for_test(3)),
                    "Block D2 should have been committed."
                );
            } else if idx == 3 {
                // We commit:
                // * 1 block on round 4, the leader block A4
                // * 3 blocks on round 3, A3, B3, C3
                // * 1 block of round 1, D1
                assert_eq!(subdag.blocks.len(), 5);
                assert!(
                    subdag.blocks.iter().any(|block| block.round() == 1
                        && block.author() == AuthorityIndex::new_for_test(3)),
                    "Block D1 should have been committed."
                );
            } else {
                panic!("Unexpected subdag with index {idx:?}");
            }

            for block in subdag.blocks.iter() {
                assert!(block.round() <= leaders[idx].round());
            }
            assert_eq!(subdag.commit_ref.index, idx as CommitIndex + 1);
        }
    }
}