commonware_consensus/marshal/standard/

deferred.rs

//! Wrapper for consensus applications that handles epochs and block dissemination.
//!
//! # Overview
//!
//! [`Deferred`] is an adapter that wraps any [`VerifyingApplication`] implementation to handle
//! epoch transitions automatically. It intercepts consensus operations (propose, verify) and
//! ensures blocks are only produced within valid epoch boundaries.
//!
//! # Epoch Boundaries
//!
//! When the parent is the last block in an epoch (as determined by the [`Epocher`]), this wrapper
//! re-proposes that boundary block instead of building a new block. This avoids producing blocks
//! that would be pruned by the epoch transition.
//!
//! # Deferred Verification
//!
//! Before casting a notarize vote, [`Deferred`] waits for the block to become available and
//! then verifies that the block's embedded context matches the consensus context. However, it does not
//! wait for the application to finish verifying the block contents before voting. This enables verification
//! to run while we wait for a quorum of votes to form a certificate (hiding verification latency behind network
//! latency). Once a certificate is formed, we wait on the verification result in [`CertifiableAutomaton::certify`]
//! before voting to finalize (ensuring no invalid blocks are admitted to the canonical chain).
//!
//! # Usage
//!
//! Wrap your [`Application`] implementation with [`Deferred::new`] and provide it to your
//! consensus engine for the [`Automaton`] and [`Relay`]. The wrapper handles all epoch logic transparently.
//!
//! ```rust,ignore
//! let application = Deferred::new(
//!     context,
//!     my_application,
//!     marshal_mailbox,
//!     epocher,
//! );
//! ```
//!
//! # Implementation Notes
//!
//! - Genesis blocks are handled specially: epoch 0 returns the application's genesis block,
//!   while subsequent epochs use the last block of the previous epoch as genesis
//! - Blocks are automatically verified to be within the current epoch
//!
//! # Notarization and Data Availability
//!
//! In rare crash cases, it is possible for a notarization certificate to exist without a block being
//! available to the honest parties if [`CertifiableAutomaton::certify`] fails after a notarization is
//! formed.
//!
//! For this reason, it should not be expected that every notarized payload will be certifiable due
//! to the lack of an available block. However, if even one honest and online party has the block,
//! they will attempt to forward it to others via marshal's resolver.
//!
//! ```text
//!                                      ┌───────────────────────────────────────────────────┐
//!                                      ▼                                                   │
//! ┌─────────────────────┐   ┌─────────────────────┐   ┌─────────────────────┐   ┌─────────────────────┐
//! │          B1         │◀──│          B2         │◀──│          B3         │XXX│          B4         │
//! └─────────────────────┘   └─────────────────────┘   └──────────┬──────────┘   └─────────────────────┘
//!                                                                │
//!                                                          Failed Certify
//! ```
//!
//! # Future Work
//!
//! - To further reduce view latency, a participant could optimistically vote for a block prior to
//!   observing its availability during [`Automaton::verify`]. However, this would require updating
//!   other components (like [`crate::marshal`]) to handle backfill where notarization does not imply
//!   a block is fetchable (without modification, a malicious leader that withholds blocks during propose
//!   could get an honest node to exhaust their network rate limit fetching things that don't exist rather
//!   than blocks they need AND can fetch).

use crate::{
    marshal::{
        ancestry::AncestorStream,
        application::{
            validation::{is_inferred_reproposal_at_certify, LastBuilt},
            verification_tasks::VerificationTasks,
        },
        core::Mailbox,
        standard::{
            validation::{
                fetch_parent, precheck_epoch_and_reproposal, verify_with_parent, Decision,
            },
            Standard,
        },
        Update,
    },
    simplex::{types::Context, Plan},
    types::{Epoch, Epocher, Round},
    Application, Automaton, CertifiableAutomaton, CertifiableBlock, Epochable, Relay, Reporter,
    VerifyingApplication,
};
use commonware_cryptography::{certificate::Scheme, Digestible};
use commonware_macros::select;
use commonware_runtime::{
    telemetry::metrics::histogram::{Buckets, Timed},
    Clock, Metrics, Spawner,
};
use commonware_utils::{
    channel::{fallible::OneshotExt, oneshot},
    sync::Mutex,
};
use rand::Rng;
use std::sync::Arc;
use tracing::{debug, warn};

/// An [`Application`] adapter that handles epoch transitions and validates block ancestry.
///
/// This wrapper intercepts consensus operations to enforce epoch boundaries and validate
/// block ancestry. It prevents blocks from being produced outside their valid epoch,
/// handles the special case of re-proposing boundary blocks at epoch boundaries,
/// and ensures all blocks have valid parent linkage and contiguous heights.
///
/// # Ancestry Validation
///
/// Applications wrapped by [`Deferred`] can rely on the following ancestry checks being
/// performed automatically during verification:
/// - Parent digest matches the consensus context's expected parent
/// - Block height is exactly one greater than the parent's height
///
/// Verifying only the immediate parent is sufficient since the parent itself must have
/// been notarized by consensus, which guarantees it was verified and accepted by a quorum.
/// This means the entire ancestry chain back to genesis is transitively validated.
///
/// Applications do not need to re-implement these checks in their own verification logic.
///
/// # Context Recovery
///
/// With deferred verification, validators wait for data availability (DA) and verify the context
/// before voting. If a validator crashes after voting but before certification, they lose their in-memory
/// verification task. When recovering, validators extract context from a [`CertifiableBlock`].
///
/// _This embedded context is trustworthy because the notarizing quorum (which contains at least f+1 honest
/// validators) verified that the block's context matched the consensus context before voting._
#[derive(Clone)]
pub struct Deferred<E, S, A, B, ES>
where
    E: Rng + Spawner + Metrics + Clock,
    S: Scheme,
    A: Application<E>,
    B: CertifiableBlock,
    ES: Epocher,
{
    context: E,
    application: A,
    marshal: Mailbox<S, Standard<B>>,
    epocher: ES,
    last_built: LastBuilt<B>,
    verification_tasks: VerificationTasks<<B as Digestible>::Digest>,

    build_duration: Timed<E>,
}

impl<E, S, A, B, ES> Deferred<E, S, A, B, ES>
where
    E: Rng + Spawner + Metrics + Clock,
    S: Scheme,
    A: VerifyingApplication<
        E,
        Block = B,
        SigningScheme = S,
        Context = Context<B::Digest, S::PublicKey>,
    >,
    B: CertifiableBlock<Context = <A as Application<E>>::Context>,
    ES: Epocher,
{
    /// Creates a new [`Deferred`] wrapper.
    pub fn new(context: E, application: A, marshal: Mailbox<S, Standard<B>>, epocher: ES) -> Self {
        use prometheus_client::metrics::histogram::Histogram;

        let build_histogram = Histogram::new(Buckets::LOCAL);
        context.register(
            "build_duration",
            "Histogram of time taken for the application to build a new block, in seconds",
            build_histogram.clone(),
        );
        let build_duration = Timed::new(build_histogram, Arc::new(context.clone()));

        Self {
            context,
            application,
            marshal,
            epocher,
            last_built: Arc::new(Mutex::new(None)),
            verification_tasks: VerificationTasks::new(),

            build_duration,
        }
    }

    /// Verifies a proposed block's application-level validity.
    ///
    /// This method validates that:
    /// 1. The block's parent digest matches the expected parent
    /// 2. The block's height is exactly one greater than the parent's height
    /// 3. The underlying application's verification logic passes
    ///
    /// Verification is spawned in a background task and returns a receiver that will contain
    /// the verification result. Valid blocks are reported to the marshal as verified.
    #[inline]
    fn deferred_verify(
        &mut self,
        context: <Self as Automaton>::Context,
        block: B,
    ) -> oneshot::Receiver<bool> {
        let mut marshal = self.marshal.clone();
        let mut application = self.application.clone();
        let (mut tx, rx) = oneshot::channel();
        self.context
            .with_label("deferred_verify")
            .with_attribute("round", context.round)
            .spawn(move |runtime_context| async move {
                // Shared non-reproposal verification:
                // - fetch parent (using trusted round hint from consensus context)
                // - validate standard ancestry invariants
                // - run application verification over ancestry
                //
                // The helper preserves the prior early-exit behavior and returns
                // `None` when work should stop (for example receiver dropped or
                // parent unavailable).
                let application_valid = match verify_with_parent(
                    runtime_context,
                    context,
                    block,
                    &mut application,
                    &mut marshal,
                    &mut tx,
                )
                .await
                {
                    Some(valid) => valid,
                    None => return,
                };
                tx.send_lossy(application_valid);
            });

        rx
    }
}

impl<E, S, A, B, ES> Automaton for Deferred<E, S, A, B, ES>
where
    E: Rng + Spawner + Metrics + Clock,
    S: Scheme,
    A: VerifyingApplication<
        E,
        Block = B,
        SigningScheme = S,
        Context = Context<B::Digest, S::PublicKey>,
    >,
    B: CertifiableBlock<Context = <A as Application<E>>::Context>,
    ES: Epocher,
{
    type Digest = B::Digest;
    type Context = Context<Self::Digest, S::PublicKey>;

    /// Returns the genesis digest for a given epoch.
    ///
    /// For epoch 0, this returns the application's genesis block digest. For subsequent
    /// epochs, it returns the digest of the last block from the previous epoch, which
    /// serves as the genesis block for the new epoch.
    ///
    /// # Panics
    ///
    /// Panics if a non-zero epoch is requested but the previous epoch's final block is not
    /// available in storage. This indicates a critical error in the consensus engine startup
    /// sequence, as engines must always have the genesis block before starting.
    async fn genesis(&mut self, epoch: Epoch) -> Self::Digest {
        if epoch.is_zero() {
            return self.application.genesis().await.digest();
        }

        let prev = epoch.previous().expect("checked to be non-zero above");
        let last_height = self
            .epocher
            .last(prev)
            .expect("previous epoch should exist");
        let Some(block) = self.marshal.get_block(last_height).await else {
            // A new consensus engine will never be started without having the genesis block
            // of the new epoch (the last block of the previous epoch) already stored.
            unreachable!("missing starting epoch block at height {}", last_height);
        };
        block.digest()
    }

    /// Proposes a new block or re-proposes the epoch boundary block.
    ///
    /// This method builds a new block from the underlying application unless the parent block
    /// is the last block in the current epoch. When at an epoch boundary, it re-proposes the
    /// boundary block to avoid creating blocks that would be invalidated by the epoch transition.
    ///
    /// The proposal operation is spawned in a background task and returns a receiver that will
    /// contain the proposed block's digest when ready. The built block is cached for later
    /// broadcasting.
    async fn propose(
        &mut self,
        consensus_context: Context<Self::Digest, S::PublicKey>,
    ) -> oneshot::Receiver<Self::Digest> {
        let mut marshal = self.marshal.clone();
        let mut application = self.application.clone();
        let last_built = self.last_built.clone();
        let epocher = self.epocher.clone();

        // Metrics
        let build_duration = self.build_duration.clone();

        let (mut tx, rx) = oneshot::channel();
        self.context
            .with_label("propose")
            .with_attribute("round", consensus_context.round)
            .spawn(move |runtime_context| async move {
                let (parent_view, parent_digest) = consensus_context.parent;
                let parent_request = fetch_parent(
                    parent_digest,
                    // We are guaranteed that the parent round for any `consensus_context` is
                    // in the same epoch (recall, the boundary block of the previous epoch
                    // is the genesis block of the current epoch).
                    Some(Round::new(consensus_context.epoch(), parent_view)),
                    &mut application,
                    &mut marshal,
                )
                .await;

                let parent = select! {
                    _ = tx.closed() => {
                        debug!(reason = "consensus dropped receiver", "skipping proposal");
                        return;
                    },
                    result = parent_request => match result {
                        Ok(parent) => parent,
                        Err(_) => {
                            debug!(
                                ?parent_digest,
                                reason = "failed to fetch parent block",
                                "skipping proposal"
                            );
                            return;
                        }
                    },
                };

                // Special case: If the parent block is the last block in the epoch,
                // re-propose it as to not produce any blocks that will be cut out
                // by the epoch transition.
                let last_in_epoch = epocher
                    .last(consensus_context.epoch())
                    .expect("current epoch should exist");
                if parent.height() == last_in_epoch {
                    let digest = parent.digest();
                    {
                        let mut lock = last_built.lock();
                        *lock = Some((consensus_context.round, parent));
                    }

                    let success = tx.send_lossy(digest);
                    debug!(
                        round = ?consensus_context.round,
                        ?digest,
                        success,
                        "re-proposed parent block at epoch boundary"
                    );
                    return;
                }

                let ancestor_stream = AncestorStream::new(marshal.clone(), [parent]);
                let build_request = application.propose(
                    (
                        runtime_context.with_label("app_propose"),
                        consensus_context.clone(),
                    ),
                    ancestor_stream,
                );

                let mut build_timer = build_duration.timer();
                let built_block = select! {
                    _ = tx.closed() => {
                        debug!(reason = "consensus dropped receiver", "skipping proposal");
                        return;
                    },
                    result = build_request => match result {
                        Some(block) => block,
                        None => {
                            debug!(
                                ?parent_digest,
                                reason = "block building failed",
                                "skipping proposal"
                            );
                            return;
                        }
                    },
                };
                build_timer.observe();

                let digest = built_block.digest();
                {
                    let mut lock = last_built.lock();
                    *lock = Some((consensus_context.round, built_block));
                }

                let success = tx.send_lossy(digest);
                debug!(
                    round = ?consensus_context.round,
                    ?digest,
                    success,
                    "proposed new block"
                );
            });
        rx
    }

    async fn verify(
        &mut self,
        context: Context<Self::Digest, S::PublicKey>,
        digest: Self::Digest,
    ) -> oneshot::Receiver<bool> {
        let mut marshal = self.marshal.clone();
        let mut marshaled = self.clone();

        let (mut tx, rx) = oneshot::channel();
        self.context
            .with_label("optimistic_verify")
            .with_attribute("round", context.round)
            .spawn(move |_| async move {
                let block_request = marshal.subscribe_by_digest(Some(context.round), digest).await;
                let block = select! {
                    _ = tx.closed() => {
                        debug!(
                            reason = "consensus dropped receiver",
                            "skipping optimistic verification"
                        );
                        return;
                    },
                    result = block_request => match result {
                        Ok(block) => block,
                        Err(_) => {
                            debug!(
                                ?digest,
                                reason = "failed to fetch block for optimistic verification",
                                "skipping optimistic verification"
                            );
                            return;
                        }
                    },
                };

                // Shared pre-checks enforce:
                // - Block epoch membership.
                // - Re-proposal detection via `digest == context.parent.1`.
                //
                // Re-proposals return early and skip normal parent/height checks
                // because they were already verified when originally proposed and
                // parent-child checks would fail by construction when parent == block.
                let block = match precheck_epoch_and_reproposal(
                    &marshaled.epocher,
                    &mut marshal,
                    &context,
                    digest,
                    block,
                )
                .await
                {
                    Decision::Complete(valid) => {
                        if valid {
                            // Valid re-proposal. Create a completed verification task for `certify`.
                            let round = context.round;
                            let (task_tx, task_rx) = oneshot::channel();
                            task_tx.send_lossy(true);
                            marshaled.verification_tasks.insert(round, digest, task_rx);
                        }
                        // `Complete` means either immediate rejection or successful
                        // re-proposal handling with no further ancestry validation.
                        tx.send_lossy(valid);
                        return;
                    }
                    Decision::Continue(block) => block,
                };

                // Before casting a notarize vote, ensure the block's embedded context matches
                // the consensus context.
                //
                // This is a critical step - the notarize quorum is guaranteed to have at least
                // f+1 honest validators who will verify against this context, preventing a Byzantine
                // proposer from embedding a malicious context. The other f honest validators who did
                // not vote will later use the block-embedded context to help finalize if Byzantine
                // validators withhold their finalize votes.
                if block.context() != context {
                    debug!(
                        ?context,
                        block_context = ?block.context(),
                        "block-embedded context does not match consensus context during optimistic verification"
                    );
                    tx.send_lossy(false);
                    return;
                }

                // Begin the rest of the verification process asynchronously.
                let round = context.round;
                let task = marshaled.deferred_verify(context, block);
                marshaled.verification_tasks.insert(round, digest, task);

                tx.send_lossy(true);
            });
        rx
    }
}

impl<E, S, A, B, ES> CertifiableAutomaton for Deferred<E, S, A, B, ES>
where
    E: Rng + Spawner + Metrics + Clock,
    S: Scheme,
    A: VerifyingApplication<
        E,
        Block = B,
        SigningScheme = S,
        Context = Context<B::Digest, S::PublicKey>,
    >,
    B: CertifiableBlock<Context = <A as Application<E>>::Context>,
    ES: Epocher,
{
    async fn certify(&mut self, round: Round, digest: Self::Digest) -> oneshot::Receiver<bool> {
        // Attempt to retrieve the existing verification task for this (round, payload).
        let task = self.verification_tasks.take(round, digest);
        if let Some(task) = task {
            return task;
        }

        // No in-progress task means we never verified this proposal locally. We can use the
        // block's embedded context to help complete finalization when Byzantine validators
        // withhold their finalize votes. If a Byzantine proposer embedded a malicious context,
        // the f+1 honest validators from the notarizing quorum will verify against the proper
        // context and reject the mismatch, preventing a 2f+1 finalization quorum.
        //
        // Subscribe to the block and verify using its embedded context once available.
        debug!(
            ?round,
            ?digest,
            "subscribing to block for certification using embedded context"
        );
        let block_rx = self.marshal.subscribe_by_digest(Some(round), digest).await;
        let mut marshaled = self.clone();
        let epocher = self.epocher.clone();
        let (mut tx, rx) = oneshot::channel();
        self.context
            .with_label("certify")
            .with_attribute("round", round)
            .spawn(move |_| async move {
                let block = select! {
                    _ = tx.closed() => {
                        debug!(
                            reason = "consensus dropped receiver",
                            "skipping certification"
                        );
                        return;
                    },
                    result = block_rx => match result {
                        Ok(block) => block,
                        Err(_) => {
                            debug!(
                                ?digest,
                                reason = "failed to fetch block for certification",
                                "skipping certification"
                            );
                            return;
                        }
                    },
                };

                // Re-proposal detection for certify path: we don't have the consensus context,
                // only the block's embedded context from original proposal. Infer re-proposal from:
                // 1. Block is at epoch boundary (only boundary blocks can be re-proposed)
                // 2. Certification round's view > embedded context's view (re-proposals retain their
                //    original embedded context, so a later view indicates the block was re-proposed)
                // 3. Same epoch (re-proposals don't cross epoch boundaries)
                let embedded_context = block.context();
                let is_reproposal = is_inferred_reproposal_at_certify(
                    &epocher,
                    block.height(),
                    embedded_context.round,
                    round,
                );
                if is_reproposal {
                    // NOTE: It is possible that, during crash recovery, we call `marshal.verified`
                    // twice for the same block. That function is idempotent, so this is safe.
                    marshaled.marshal.verified(round, block).await;
                    tx.send_lossy(true);
                    return;
                }

                let verify_rx = marshaled.deferred_verify(embedded_context, block);
                if let Ok(result) = verify_rx.await {
                    tx.send_lossy(result);
                }
            });
        rx
    }
}

impl<E, S, A, B, ES> Relay for Deferred<E, S, A, B, ES>
where
    E: Rng + Spawner + Metrics + Clock,
    S: Scheme,
    A: Application<E, Block = B, Context = Context<B::Digest, S::PublicKey>>,
    B: CertifiableBlock<Context = <A as Application<E>>::Context>,
    ES: Epocher,
{
    type Digest = B::Digest;
    type PublicKey = S::PublicKey;
    type Plan = Plan<S::PublicKey>;

    async fn broadcast(&mut self, digest: Self::Digest, plan: Plan<S::PublicKey>) {
        match plan {
            Plan::Propose => {
                let Some((round, block)) = self.last_built.lock().take() else {
                    warn!("missing block to broadcast");
                    return;
                };
                if block.digest() != digest {
                    warn!(
                        round = %round,
                        digest = %block.digest(),
                        height = %block.height(),
                        "skipping requested broadcast of block with mismatched digest"
                    );
                    return;
                }
                debug!(
                    round = %round,
                    digest = %block.digest(),
                    height = %block.height(),
                    "requested broadcast of built block"
                );
                self.marshal.proposed(round, block).await;
            }
            Plan::Forward { round, peers } => {
                self.marshal.forward(round, digest, peers).await;
            }
        }
    }
}

impl<E, S, A, B, ES> Reporter for Deferred<E, S, A, B, ES>
where
    E: Rng + Spawner + Metrics + Clock,
    S: Scheme,
    A: Application<E, Block = B, Context = Context<B::Digest, S::PublicKey>>
        + Reporter<Activity = Update<B>>,
    B: CertifiableBlock<Context = <A as Application<E>>::Context>,
    ES: Epocher,
{
    type Activity = A::Activity;

    /// Relays a report to the underlying [`Application`] and cleans up old verification tasks.
    async fn report(&mut self, update: Self::Activity) {
        // Clean up verification tasks for rounds <= the finalized round.
        if let Update::Tip(round, _, _) = &update {
            self.verification_tasks.retain_after(round);
        }
        self.application.report(update).await
    }
}

#[cfg(test)]
mod tests {
    use super::Deferred;
    use crate::{
        marshal::mocks::{
            harness::{
                default_leader, make_raw_block, setup_network_with_participants, Ctx,
                StandardHarness, TestHarness, B, BLOCKS_PER_EPOCH, NAMESPACE, NUM_VALIDATORS, S, V,
            },
            verifying::MockVerifyingApp,
        },
        simplex::scheme::bls12381_threshold::vrf as bls12381_threshold_vrf,
        types::{Epoch, Epocher, FixedEpocher, Height, Round, View},
        Automaton, CertifiableAutomaton,
    };
    use commonware_cryptography::{
        certificate::{mocks::Fixture, ConstantProvider},
        sha256::Sha256,
        Digestible, Hasher as _,
    };
    use commonware_macros::{select, test_traced};
    use commonware_runtime::{deterministic, Clock, Metrics, Runner};
    use commonware_utils::NZUsize;
    use std::time::Duration;

    #[test_traced("INFO")]
    fn test_certify_lower_view_after_higher_view() {
        let runner = deterministic::Runner::timed(Duration::from_secs(60));
        runner.start(|mut context| async move {
            let Fixture {
                participants,
                schemes,
                ..
            } = bls12381_threshold_vrf::fixture::<V, _>(&mut context, NAMESPACE, NUM_VALIDATORS);
            let mut oracle =
                setup_network_with_participants(context.clone(), NZUsize!(1), participants.clone())
                    .await;

            let me = participants[0].clone();

            let setup = StandardHarness::setup_validator(
                context.with_label("validator_0"),
                &mut oracle,
                me.clone(),
                ConstantProvider::new(schemes[0].clone()),
            )
            .await;
            let marshal = setup.mailbox;

            let genesis = make_raw_block(Sha256::hash(b""), Height::zero(), 0);
            let mock_app: MockVerifyingApp<B, S> = MockVerifyingApp::new(genesis.clone());

            let mut marshaled = Deferred::new(
                context.clone(),
                mock_app,
                marshal.clone(),
                FixedEpocher::new(BLOCKS_PER_EPOCH),
            );

            // Create parent block at height 1
            let parent = make_raw_block(genesis.digest(), Height::new(1), 100);
            let parent_digest = parent.digest();
            marshal
                .clone()
                .proposed(Round::new(Epoch::new(0), View::new(1)), parent.clone())
                .await;

            // Block A at view 5 (height 2)
            let round_a = Round::new(Epoch::new(0), View::new(5));
            let context_a = Ctx {
                round: round_a,
                leader: me.clone(),
                parent: (View::new(1), parent_digest),
            };
            let block_a = B::new::<Sha256>(context_a.clone(), parent_digest, Height::new(2), 200);
            let commitment_a = block_a.digest();
            marshal.clone().proposed(round_a, block_a.clone()).await;

            // Block B at view 10 (height 2, different block same height)
            let round_b = Round::new(Epoch::new(0), View::new(10));
            let context_b = Ctx {
                round: round_b,
                leader: me.clone(),
                parent: (View::new(1), parent_digest),
            };
            let block_b = B::new::<Sha256>(context_b.clone(), parent_digest, Height::new(2), 300);
            let commitment_b = block_b.digest();
            marshal.clone().proposed(round_b, block_b.clone()).await;

            context.sleep(Duration::from_millis(10)).await;

            // Step 1: Verify block A at view 5
            let _ = marshaled.verify(context_a, commitment_a).await.await;

            // Step 2: Verify block B at view 10
            let _ = marshaled.verify(context_b, commitment_b).await.await;

            // Step 3: Certify block B at view 10 FIRST
            let certify_b = marshaled.certify(round_b, commitment_b).await;
            assert!(
                certify_b.await.unwrap(),
                "Block B certification should succeed"
            );

            // Step 4: Certify block A at view 5 - should succeed
            let certify_a = marshaled.certify(round_a, commitment_a).await;

            select! {
                result = certify_a => {
                    assert!(result.unwrap(), "Block A certification should succeed");
                },
                _ = context.sleep(Duration::from_secs(5)) => {
                    panic!("Block A certification timed out");
                },
            }
        })
    }

    #[test_traced("WARN")]
    fn test_marshaled_rejects_unsupported_epoch() {
        #[derive(Clone)]
        struct LimitedEpocher {
            inner: FixedEpocher,
            max_epoch: u64,
        }

        impl Epocher for LimitedEpocher {
            fn containing(&self, height: Height) -> Option<crate::types::EpochInfo> {
                let bounds = self.inner.containing(height)?;
                if bounds.epoch().get() > self.max_epoch {
                    None
                } else {
                    Some(bounds)
                }
            }

            fn first(&self, epoch: Epoch) -> Option<Height> {
                if epoch.get() > self.max_epoch {
                    None
                } else {
                    self.inner.first(epoch)
                }
            }

            fn last(&self, epoch: Epoch) -> Option<Height> {
                if epoch.get() > self.max_epoch {
                    None
                } else {
                    self.inner.last(epoch)
                }
            }
        }

        let runner = deterministic::Runner::timed(Duration::from_secs(60));
        runner.start(|mut context| async move {
            let Fixture {
                participants,
                schemes,
                ..
            } = bls12381_threshold_vrf::fixture::<V, _>(&mut context, NAMESPACE, NUM_VALIDATORS);
            let mut oracle =
                setup_network_with_participants(context.clone(), NZUsize!(1), participants.clone())
                    .await;

            let me = participants[0].clone();

            let setup = StandardHarness::setup_validator(
                context.with_label("validator_0"),
                &mut oracle,
                me.clone(),
                ConstantProvider::new(schemes[0].clone()),
            )
            .await;
            let marshal = setup.mailbox;

            let genesis = make_raw_block(Sha256::hash(b""), Height::zero(), 0);
            let mock_app: MockVerifyingApp<B, S> = MockVerifyingApp::new(genesis.clone());
            let limited_epocher = LimitedEpocher {
                inner: FixedEpocher::new(BLOCKS_PER_EPOCH),
                max_epoch: 0,
            };

            let mut marshaled =
                Deferred::new(context.clone(), mock_app, marshal.clone(), limited_epocher);

            // Create a parent block at height 19 (last block in epoch 0, which is supported)
            let parent_ctx = Ctx {
                round: Round::new(Epoch::zero(), View::new(19)),
                leader: default_leader(),
                parent: (View::zero(), genesis.digest()),
            };
            let parent =
                B::new::<Sha256>(parent_ctx.clone(), genesis.digest(), Height::new(19), 1000);
            let parent_digest = parent.digest();
            marshal
                .clone()
                .proposed(Round::new(Epoch::zero(), View::new(19)), parent.clone())
                .await;

            // Create a block at height 20 (first block in epoch 1, which is NOT supported)
            let unsupported_round = Round::new(Epoch::new(1), View::new(20));
            let unsupported_context = Ctx {
                round: unsupported_round,
                leader: me.clone(),
                parent: (View::new(19), parent_digest),
            };
            let block = B::new::<Sha256>(
                unsupported_context.clone(),
                parent_digest,
                Height::new(20),
                2000,
            );
            let block_commitment = block.digest();
            marshal
                .clone()
                .proposed(unsupported_round, block.clone())
                .await;

            context.sleep(Duration::from_millis(10)).await;

            // Call verify and wait for the result (verify returns optimistic result,
            // but also spawns deferred verification)
            let verify_result = marshaled
                .verify(unsupported_context, block_commitment)
                .await;
            // Wait for optimistic verify to complete so the verification task is registered
            let optimistic_result = verify_result.await;

            // The optimistic verify should return false because the block is in an unsupported epoch
            assert!(
                !optimistic_result.unwrap(),
                "Optimistic verify should reject block in unsupported epoch"
            );
        })
    }

    /// Test that marshaled rejects blocks when consensus context doesn't match block's embedded context.
    ///
    /// This tests that when verify() is called with a context that doesn't match what's embedded
    /// in the block, the verification should fail. A Byzantine proposer could broadcast a block
    /// with one embedded context but consensus could call verify() with a different context.
    #[test_traced("WARN")]
    fn test_marshaled_rejects_mismatched_context() {
        let runner = deterministic::Runner::timed(Duration::from_secs(30));
        runner.start(|mut context| async move {
            let Fixture {
                participants,
                schemes,
                ..
            } = bls12381_threshold_vrf::fixture::<V, _>(&mut context, NAMESPACE, NUM_VALIDATORS);
            let mut oracle =
                setup_network_with_participants(context.clone(), NZUsize!(1), participants.clone())
                    .await;

            let me = participants[0].clone();

            let setup = StandardHarness::setup_validator(
                context.with_label("validator_0"),
                &mut oracle,
                me.clone(),
                ConstantProvider::new(schemes[0].clone()),
            )
            .await;
            let marshal = setup.mailbox;

            let genesis = make_raw_block(Sha256::hash(b""), Height::zero(), 0);
            let mock_app: MockVerifyingApp<B, S> = MockVerifyingApp::new(genesis.clone());

            let mut marshaled = Deferred::new(
                context.clone(),
                mock_app,
                marshal.clone(),
                FixedEpocher::new(BLOCKS_PER_EPOCH),
            );

            // Create parent block at height 1 so the commitment is well-formed.
            let parent_ctx = Ctx {
                round: Round::new(Epoch::zero(), View::new(1)),
                leader: default_leader(),
                parent: (View::zero(), genesis.digest()),
            };
            let parent = B::new::<Sha256>(parent_ctx, genesis.digest(), Height::new(1), 100);
            let parent_commitment = parent.digest();
            marshal
                .clone()
                .proposed(Round::new(Epoch::zero(), View::new(1)), parent.clone())
                .await;

            // Build a block with context A (embedded in the block).
            let round_a = Round::new(Epoch::zero(), View::new(2));
            let context_a = Ctx {
                round: round_a,
                leader: me.clone(),
                parent: (View::new(1), parent_commitment),
            };
            let block_a = B::new::<Sha256>(context_a, parent.digest(), Height::new(2), 200);
            let commitment_a = block_a.digest();
            marshal.clone().proposed(round_a, block_a).await;

            context.sleep(Duration::from_millis(10)).await;

            // Verify using a different consensus context B (hash mismatch).
            let round_b = Round::new(Epoch::zero(), View::new(3));
            let context_b = Ctx {
                round: round_b,
                leader: participants[1].clone(),
                parent: (View::new(1), parent_commitment),
            };

            let verify_rx = marshaled.verify(context_b, commitment_a).await;
            select! {
                result = verify_rx => {
                    assert!(
                        !result.unwrap(),
                        "mismatched context hash should be rejected"
                    );
                },
                _ = context.sleep(Duration::from_secs(5)) => {
                    panic!("verify should reject mismatched context hash promptly");
                },
            }
        })
    }
}