Maintain attestations that reference unknown blocks (#2319)

## Issue Addressed

#635 

## Proposed Changes
- Keep attestations that reference a block we have not seen for 30secs before being re processed
- If we do import the block before that time elapses, it is reprocessed in that moment
- The first time it fails, do nothing wrt to gossipsub propagation or peer downscoring. If after being re processed it fails, downscore with a `LowToleranceError` and ignore the message.

beacon_node/beacon_chain/src/attestation_verification.rs

@@ -375,7 +375,7 @@ impl<T: BeaconChainTypes> VerifiedAggregatedAttestation<T> {
     pub fn verify(
         signed_aggregate: SignedAggregateAndProof<T::EthSpec>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, Error> {
+    ) -> Result<Self, (Error, SignedAggregateAndProof<T::EthSpec>)> {
         Self::verify_slashable(signed_aggregate, chain)
             .map(|verified_aggregate| {
                 if let Some(slasher) = chain.slasher.as_ref() {
@@ -383,7 +383,9 @@ impl<T: BeaconChainTypes> VerifiedAggregatedAttestation<T> {
                 }
                 verified_aggregate
             })
-            .map_err(|slash_info| process_slash_info(slash_info, chain))
+            .map_err(|(slash_info, original_aggregate)| {
+                (process_slash_info(slash_info, chain), original_aggregate)
+            })
     }
 
     /// Run the checks that happen before an indexed attestation is constructed.
@@ -509,17 +511,31 @@ impl<T: BeaconChainTypes> VerifiedAggregatedAttestation<T> {
     }
 
     /// Verify the attestation, producing extra information about whether it might be slashable.
+    // NOTE: Clippy considers the return too complex. This tuple is not used elsewhere so it is not
+    // worth creating an alias.
+    #[allow(clippy::type_complexity)]
     pub fn verify_slashable(
         signed_aggregate: SignedAggregateAndProof<T::EthSpec>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, AttestationSlashInfo<T, Error>> {
+    ) -> Result<
+        Self,
+        (
+            AttestationSlashInfo<T, Error>,
+            SignedAggregateAndProof<T::EthSpec>,
+        ),
+    > {
         use AttestationSlashInfo::*;
 
         let attestation = &signed_aggregate.message.aggregate;
         let aggregator_index = signed_aggregate.message.aggregator_index;
         let attestation_root = match Self::verify_early_checks(&signed_aggregate, chain) {
             Ok(root) => root,
-            Err(e) => return Err(SignatureNotChecked(signed_aggregate.message.aggregate, e)),
+            Err(e) => {
+                return Err((
+                    SignatureNotChecked(signed_aggregate.message.aggregate.clone(), e),
+                    signed_aggregate,
+                ))
+            }
         };
 
         let indexed_attestation =
@@ -546,7 +562,12 @@ impl<T: BeaconChainTypes> VerifiedAggregatedAttestation<T> {
                     .map_err(|e| BeaconChainError::from(e).into())
             }) {
                 Ok(indexed_attestation) => indexed_attestation,
-                Err(e) => return Err(SignatureNotChecked(signed_aggregate.message.aggregate, e)),
+                Err(e) => {
+                    return Err((
+                        SignatureNotChecked(signed_aggregate.message.aggregate.clone(), e),
+                        signed_aggregate,
+                    ))
+                }
             };
 
         // Ensure that all signatures are valid.
@@ -560,11 +581,11 @@ impl<T: BeaconChainTypes> VerifiedAggregatedAttestation<T> {
                     }
                 })
         {
-            return Err(SignatureInvalid(e));
+            return Err((SignatureInvalid(e), signed_aggregate));
         }
 
         if let Err(e) = Self::verify_late_checks(&signed_aggregate, attestation_root, chain) {
-            return Err(SignatureValid(indexed_attestation, e));
+            return Err((SignatureValid(indexed_attestation, e), signed_aggregate));
         }
 
         Ok(VerifiedAggregatedAttestation {
@@ -715,7 +736,7 @@ impl<T: BeaconChainTypes> VerifiedUnaggregatedAttestation<T> {
         attestation: Attestation<T::EthSpec>,
         subnet_id: Option<SubnetId>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, Error> {
+    ) -> Result<Self, (Error, Attestation<T::EthSpec>)> {
         Self::verify_slashable(attestation, subnet_id, chain)
             .map(|verified_unaggregated| {
                 if let Some(slasher) = chain.slasher.as_ref() {
@@ -723,26 +744,31 @@ impl<T: BeaconChainTypes> VerifiedUnaggregatedAttestation<T> {
                 }
                 verified_unaggregated
             })
-            .map_err(|slash_info| process_slash_info(slash_info, chain))
+            .map_err(|(slash_info, original_attestation)| {
+                (process_slash_info(slash_info, chain), original_attestation)
+            })
     }
 
     /// Verify the attestation, producing extra information about whether it might be slashable.
+    // NOTE: Clippy considers the return too complex. This tuple is not used elsewhere so it is not
+    // worth creating an alias.
+    #[allow(clippy::type_complexity)]
     pub fn verify_slashable(
         attestation: Attestation<T::EthSpec>,
         subnet_id: Option<SubnetId>,
         chain: &BeaconChain<T>,
-    ) -> Result<Self, AttestationSlashInfo<T, Error>> {
+    ) -> Result<Self, (AttestationSlashInfo<T, Error>, Attestation<T::EthSpec>)> {
         use AttestationSlashInfo::*;
 
         if let Err(e) = Self::verify_early_checks(&attestation, chain) {
-            return Err(SignatureNotChecked(attestation, e));
+            return Err((SignatureNotChecked(attestation.clone(), e), attestation));
         }
 
         let (indexed_attestation, committees_per_slot) =
             match obtain_indexed_attestation_and_committees_per_slot(chain, &attestation) {
                 Ok(x) => x,
                 Err(e) => {
-                    return Err(SignatureNotChecked(attestation, e));
+                    return Err((SignatureNotChecked(attestation.clone(), e), attestation));
                 }
             };
 
@@ -754,16 +780,21 @@ impl<T: BeaconChainTypes> VerifiedUnaggregatedAttestation<T> {
             chain,
         ) {
             Ok(t) => t,
-            Err(e) => return Err(SignatureNotCheckedIndexed(indexed_attestation, e)),
+            Err(e) => {
+                return Err((
+                    SignatureNotCheckedIndexed(indexed_attestation, e),
+                    attestation,
+                ))
+            }
         };
 
         // The aggregate signature of the attestation is valid.
         if let Err(e) = verify_attestation_signature(chain, &indexed_attestation) {
-            return Err(SignatureInvalid(e));
+            return Err((SignatureInvalid(e), attestation));
         }
 
         if let Err(e) = Self::verify_late_checks(&attestation, validator_index, chain) {
-            return Err(SignatureValid(indexed_attestation, e));
+            return Err((SignatureValid(indexed_attestation, e), attestation));
         }
 
         Ok(Self {

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1225,7 +1225,8 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         &self,
         unaggregated_attestation: Attestation<T::EthSpec>,
         subnet_id: Option<SubnetId>,
-    ) -> Result<VerifiedUnaggregatedAttestation<T>, AttestationError> {
+    ) -> Result<VerifiedUnaggregatedAttestation<T>, (AttestationError, Attestation<T::EthSpec>)>
+    {
         metrics::inc_counter(&metrics::UNAGGREGATED_ATTESTATION_PROCESSING_REQUESTS);
         let _timer =
             metrics::start_timer(&metrics::UNAGGREGATED_ATTESTATION_GOSSIP_VERIFICATION_TIMES);
@@ -1249,7 +1250,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     pub fn verify_aggregated_attestation_for_gossip(
         &self,
         signed_aggregate: SignedAggregateAndProof<T::EthSpec>,
-    ) -> Result<VerifiedAggregatedAttestation<T>, AttestationError> {
+    ) -> Result<
+        VerifiedAggregatedAttestation<T>,
+        (AttestationError, SignedAggregateAndProof<T::EthSpec>),
+    > {
         metrics::inc_counter(&metrics::AGGREGATED_ATTESTATION_PROCESSING_REQUESTS);
         let _timer =
             metrics::start_timer(&metrics::AGGREGATED_ATTESTATION_GOSSIP_VERIFICATION_TIMES);

beacon_node/beacon_chain/tests/attestation_verification.rs

@@ -223,7 +223,7 @@ fn aggregated_gossip_verification() {
                         .expect(&format!(
                             "{} should error during verify_aggregated_attestation_for_gossip",
                             $desc
-                        )),
+                        )).0,
                     $( $error ) |+ $( if $guard )?
                 ),
                 "case: {}",
@@ -606,7 +606,7 @@ fn unaggregated_gossip_verification() {
                         .expect(&format!(
                             "{} should error during verify_unaggregated_attestation_for_gossip",
                             $desc
-                        )),
+                        )).0,
                     $( $error ) |+ $( if $guard )?
                 ),
                 "case: {}",

beacon_node/beacon_chain/tests/store_tests.rs

@@ -332,7 +332,7 @@ fn epoch_boundary_state_attestation_processing() {
         {
             checked_pre_fin = true;
             assert!(matches!(
-                res.err().unwrap(),
+                res.err().unwrap().0,
                 AttnError::PastSlot {
                     attestation_slot,
                     earliest_permissible_slot,

beacon_node/beacon_chain/tests/tests.rs

@@ -537,7 +537,7 @@ fn attestations_with_increasing_slots() {
 
         if expected_attestation_slot < expected_earliest_permissible_slot {
             assert!(matches!(
-                res.err().unwrap(),
+                res.err().unwrap().0,
                 AttnError::PastSlot {
                     attestation_slot,
                     earliest_permissible_slot,

beacon_node/http_api/src/lib.rs

@@ -1790,8 +1790,8 @@ pub fn serve<T: BeaconChainTypes>(
                     let mut failures = Vec::new();
 
                     // Verify that all messages in the post are valid before processing further
-                    for (index, aggregate) in aggregates.as_slice().iter().enumerate() {
+                    for (index, aggregate) in aggregates.into_iter().enumerate() {
-                        match chain.verify_aggregated_attestation_for_gossip(aggregate.clone()) {
+                        match chain.verify_aggregated_attestation_for_gossip(aggregate) {
                             Ok(verified_aggregate) => {
                                 messages.push(PubsubMessage::AggregateAndProofAttestation(Box::new(
                                     verified_aggregate.aggregate().clone(),
@@ -1816,8 +1816,8 @@ pub fn serve<T: BeaconChainTypes>(
                             // It's reasonably likely that two different validators produce
                             // identical aggregates, especially if they're using the same beacon
                             // node.
-                            Err(AttnError::AttestationAlreadyKnown(_)) => continue,
+                            Err((AttnError::AttestationAlreadyKnown(_), _)) => continue,
-                            Err(e) => {
+                            Err((e, aggregate)) => {
                                 error!(log,
                                     "Failure verifying aggregate and proofs";
                                     "error" => format!("{:?}", e),

beacon_node/network/src/beacon_processor/block_delay_queue.rs

@@ -1,210 +0,0 @@
-//! Provides a mechanism which queues blocks for later processing when they arrive too early.
-//!
-//! When the `beacon_processor::Worker` imports a block that is acceptably early (i.e., within the
-//! gossip propagation tolerance) it will send it to this queue where it will be placed in a
-//! `DelayQueue` until the slot arrives. Once the block has been determined to be ready, it will be
-//! sent back out on a channel to be processed by the `BeaconProcessor` again.
-//!
-//! There is the edge-case where the slot arrives before this queue manages to process it. In that
-//! case, the block will be sent off for immediate processing (skipping the `DelayQueue`).
-use super::MAX_DELAYED_BLOCK_QUEUE_LEN;
-use beacon_chain::{BeaconChainTypes, GossipVerifiedBlock};
-use eth2_libp2p::PeerId;
-use futures::stream::{Stream, StreamExt};
-use futures::task::Poll;
-use slog::{crit, debug, error, Logger};
-use slot_clock::SlotClock;
-use std::collections::HashSet;
-use std::pin::Pin;
-use std::task::Context;
-use std::time::Duration;
-use task_executor::TaskExecutor;
-use tokio::sync::mpsc::{self, Receiver, Sender};
-use tokio::time::error::Error as TimeError;
-use tokio_util::time::DelayQueue;
-
-const TASK_NAME: &str = "beacon_processor_block_delay_queue";
-
-/// Queue blocks for re-processing with an `ADDITIONAL_DELAY` after the slot starts. This is to
-/// account for any slight drift in the system clock.
-const ADDITIONAL_DELAY: Duration = Duration::from_millis(5);
-
-/// Set an arbitrary upper-bound on the number of queued blocks to avoid DoS attacks. The fact that
-/// we signature-verify blocks before putting them in the queue *should* protect against this, but
-/// it's nice to have extra protection.
-const MAXIMUM_QUEUED_BLOCKS: usize = 16;
-
-/// A block that arrived early and has been queued for later import.
-pub struct QueuedBlock<T: BeaconChainTypes> {
-    pub peer_id: PeerId,
-    pub block: GossipVerifiedBlock<T>,
-    pub seen_timestamp: Duration,
-}
-
-/// Unifies the different messages processed by the block delay queue.
-enum InboundEvent<T: BeaconChainTypes> {
-    /// A block that has been received early that we should queue for later processing.
-    EarlyBlock(QueuedBlock<T>),
-    /// A block that was queued for later processing and is ready for import.
-    ReadyBlock(QueuedBlock<T>),
-    /// The `DelayQueue` returned an error.
-    DelayQueueError(TimeError),
-}
-
-/// Combines the `DelayQueue` and `Receiver` streams into a single stream.
-///
-/// This struct has a similar purpose to `tokio::select!`, however it allows for more fine-grained
-/// control (specifically in the ordering of event processing).
-struct InboundEvents<T: BeaconChainTypes> {
-    pub delay_queue: DelayQueue<QueuedBlock<T>>,
-    early_blocks_rx: Receiver<QueuedBlock<T>>,
-}
-
-impl<T: BeaconChainTypes> Stream for InboundEvents<T> {
-    type Item = InboundEvent<T>;
-
-    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
-        // Poll for expired blocks *before* we try to process new blocks.
-        //
-        // The sequential nature of blockchains means it is generally better to try and import all
-        // existing blocks before new ones.
-        match self.delay_queue.poll_expired(cx) {
-            Poll::Ready(Some(Ok(queued_block))) => {
-                return Poll::Ready(Some(InboundEvent::ReadyBlock(queued_block.into_inner())));
-            }
-            Poll::Ready(Some(Err(e))) => {
-                return Poll::Ready(Some(InboundEvent::DelayQueueError(e)));
-            }
-            // `Poll::Ready(None)` means that there are no more entries in the delay queue and we
-            // will continue to get this result until something else is added into the queue.
-            Poll::Ready(None) | Poll::Pending => (),
-        }
-
-        match self.early_blocks_rx.poll_recv(cx) {
-            Poll::Ready(Some(queued_block)) => {
-                return Poll::Ready(Some(InboundEvent::EarlyBlock(queued_block)));
-            }
-            Poll::Ready(None) => {
-                return Poll::Ready(None);
-            }
-            Poll::Pending => {}
-        }
-
-        Poll::Pending
-    }
-}
-
-/// Spawn a queue which will accept blocks via the returned `Sender`, potentially queue them until
-/// their slot arrives, then send them back out via `ready_blocks_tx`.
-pub fn spawn_block_delay_queue<T: BeaconChainTypes>(
-    ready_blocks_tx: Sender<QueuedBlock<T>>,
-    executor: &TaskExecutor,
-    slot_clock: T::SlotClock,
-    log: Logger,
-) -> Sender<QueuedBlock<T>> {
-    let (early_blocks_tx, early_blocks_rx): (_, Receiver<QueuedBlock<_>>) =
-        mpsc::channel(MAX_DELAYED_BLOCK_QUEUE_LEN);
-
-    let queue_future = async move {
-        let mut queued_block_roots = HashSet::new();
-
-        let mut inbound_events = InboundEvents {
-            early_blocks_rx,
-            delay_queue: DelayQueue::new(),
-        };
-
-        loop {
-            match inbound_events.next().await {
-                // Some block has been indicated as "early" and should be processed when the
-                // appropriate slot arrives.
-                Some(InboundEvent::EarlyBlock(early_block)) => {
-                    let block_slot = early_block.block.block.slot();
-                    let block_root = early_block.block.block_root;
-
-                    // Don't add the same block to the queue twice. This prevents DoS attacks.
-                    if queued_block_roots.contains(&block_root) {
-                        continue;
-                    }
-
-                    if let Some(duration_till_slot) = slot_clock.duration_to_slot(block_slot) {
-                        // Check to ensure this won't over-fill the queue.
-                        if queued_block_roots.len() >= MAXIMUM_QUEUED_BLOCKS {
-                            error!(
-                                log,
-                                "Early blocks queue is full";
-                                "queue_size" => MAXIMUM_QUEUED_BLOCKS,
-                                "msg" => "check system clock"
-                            );
-                            // Drop the block.
-                            continue;
-                        }
-
-                        queued_block_roots.insert(block_root);
-                        // Queue the block until the start of the appropriate slot, plus
-                        // `ADDITIONAL_DELAY`.
-                        inbound_events
-                            .delay_queue
-                            .insert(early_block, duration_till_slot + ADDITIONAL_DELAY);
-                    } else {
-                        // If there is no duration till the next slot, check to see if the slot
-                        // has already arrived. If it has already arrived, send it out for
-                        // immediate processing.
-                        //
-                        // If we can't read the slot or the slot hasn't arrived, simply drop the
-                        // block.
-                        //
-                        // This logic is slightly awkward since `SlotClock::duration_to_slot`
-                        // doesn't distinguish between a slot that has already arrived and an
-                        // error reading the slot clock.
-                        if let Some(now) = slot_clock.now() {
-                            if block_slot <= now && ready_blocks_tx.try_send(early_block).is_err() {
-                                error!(
-                                    log,
-                                    "Failed to send block";
-                                );
-                            }
-                        }
-                    }
-                }
-                // A block that was queued for later processing is now ready to be processed.
-                Some(InboundEvent::ReadyBlock(ready_block)) => {
-                    let block_root = ready_block.block.block_root;
-
-                    if !queued_block_roots.remove(&block_root) {
-                        // Log an error to alert that we've made a bad assumption about how this
-                        // program works, but still process the block anyway.
-                        error!(
-                            log,
-                            "Unknown block in delay queue";
-                            "block_root" => ?block_root
-                        );
-                    }
-
-                    if ready_blocks_tx.try_send(ready_block).is_err() {
-                        error!(
-                            log,
-                            "Failed to pop queued block";
-                        );
-                    }
-                }
-                Some(InboundEvent::DelayQueueError(e)) => crit!(
-                    log,
-                    "Failed to poll block delay queue";
-                    "e" => ?e
-                ),
-                None => {
-                    debug!(
-                        log,
-                        "Block delay queue stopped";
-                        "msg" => "shutting down"
-                    );
-                    break;
-                }
-            }
-        }
-    };
-
-    executor.spawn(queue_future, TASK_NAME);
-
-    early_blocks_tx
-}

beacon_node/network/src/beacon_processor/mod.rs

@@ -5,6 +5,7 @@
 //!
 //! - A "manager" task, which either spawns worker tasks or enqueues work.
 //! - One or more "worker" tasks which perform time-intensive work on the `BeaconChain`.
+//! - A task managing the scheduling of work that needs to be re-processed.
 //!
 //! ## Purpose
 //!
@@ -19,10 +20,12 @@
 //!
 //! ## Detail
 //!
-//! There is a single "manager" thread who listens to two event channels. These events are either:
+//! There is a single "manager" thread who listens to three event channels. These events are
+//! either:
 //!
 //! - A new parcel of work (work event).
 //! - Indication that a worker has finished a parcel of work (worker idle).
+//! - A work ready for reprocessing (work event).
 //!
 //! Then, there is a maximum of `n` "worker" blocking threads, where `n` is the CPU count.
 //!
@@ -37,7 +40,6 @@
 
 use crate::{metrics, service::NetworkMessage, sync::SyncMessage};
 use beacon_chain::{BeaconChain, BeaconChainTypes, BlockError, GossipVerifiedBlock};
-use block_delay_queue::{spawn_block_delay_queue, QueuedBlock};
 use eth2_libp2p::{
     rpc::{BlocksByRangeRequest, BlocksByRootRequest, StatusMessage},
     MessageId, NetworkGlobals, PeerId, PeerRequestId,
@@ -57,11 +59,14 @@ use types::{
     Attestation, AttesterSlashing, Hash256, ProposerSlashing, SignedAggregateAndProof,
     SignedBeaconBlock, SignedVoluntaryExit, SubnetId,
 };
+use work_reprocessing_queue::{
+    spawn_reprocess_scheduler, QueuedAggregate, QueuedBlock, QueuedUnaggregate, ReadyWork,
+};
 
 use worker::{Toolbox, Worker};
 
-mod block_delay_queue;
 mod tests;
+mod work_reprocessing_queue;
 mod worker;
 
 pub use worker::ProcessId;
@@ -77,14 +82,25 @@ pub const MAX_WORK_EVENT_QUEUE_LEN: usize = 16_384;
 /// set to the CPU count, but we set it high to be safe.
 const MAX_IDLE_QUEUE_LEN: usize = 16_384;
 
+/// The maximum size of the channel for re-processing work events.
+const MAX_SCHEDULED_WORK_QUEUE_LEN: usize = 16_384;
+
 /// The maximum number of queued `Attestation` objects that will be stored before we start dropping
 /// them.
 const MAX_UNAGGREGATED_ATTESTATION_QUEUE_LEN: usize = 16_384;
 
+/// The maximum number of queued `Attestation` objects that will be stored before we start dropping
+/// them.
+const MAX_UNAGGREGATED_ATTESTATION_REPROCESS_QUEUE_LEN: usize = 8_192;
+
 /// The maximum number of queued `SignedAggregateAndProof` objects that will be stored before we
 /// start dropping them.
 const MAX_AGGREGATED_ATTESTATION_QUEUE_LEN: usize = 1_024;
 
+/// The maximum number of queued `SignedAggregateAndProof` objects that will be stored before we
+/// start dropping them.
+const MAX_AGGREGATED_ATTESTATION_REPROCESS_QUEUE_LEN: usize = 1_024;
+
 /// The maximum number of queued `SignedBeaconBlock` objects received on gossip that will be stored
 /// before we start dropping them.
 const MAX_GOSSIP_BLOCK_QUEUE_LEN: usize = 1_024;
@@ -127,6 +143,7 @@ const MAX_BLOCKS_BY_ROOTS_QUEUE_LEN: usize = 1_024;
 
 /// The name of the manager tokio task.
 const MANAGER_TASK_NAME: &str = "beacon_processor_manager";
+
 /// The name of the worker tokio tasks.
 const WORKER_TASK_NAME: &str = "beacon_processor_worker";
 
@@ -148,6 +165,8 @@ pub const CHAIN_SEGMENT: &str = "chain_segment";
 pub const STATUS_PROCESSING: &str = "status_processing";
 pub const BLOCKS_BY_RANGE_REQUEST: &str = "blocks_by_range_request";
 pub const BLOCKS_BY_ROOTS_REQUEST: &str = "blocks_by_roots_request";
+pub const UNKNOWN_BLOCK_ATTESTATION: &str = "unknown_block_attestation";
+pub const UNKNOWN_BLOCK_AGGREGATE: &str = "unknown_block_aggregate";
 
 /// Used to send/receive results from a rpc block import in a blocking task.
 pub type BlockResultSender<E> = oneshot::Sender<Result<Hash256, BlockError<E>>>;
@@ -308,22 +327,6 @@ impl<T: BeaconChainTypes> WorkEvent<T> {
         }
     }
 
-    /// Create a new `Work` event for some block that was delayed for later processing.
-    pub fn delayed_import_beacon_block(
-        peer_id: PeerId,
-        block: Box<GossipVerifiedBlock<T>>,
-        seen_timestamp: Duration,
-    ) -> Self {
-        Self {
-            drop_during_sync: false,
-            work: Work::DelayedImportBlock {
-                peer_id,
-                block,
-                seen_timestamp,
-            },
-        }
-    }
-
     /// Create a new `Work` event for some exit.
     pub fn gossip_voluntary_exit(
         message_id: MessageId,
@@ -442,6 +445,57 @@ impl<T: BeaconChainTypes> WorkEvent<T> {
     }
 }
 
+impl<T: BeaconChainTypes> std::convert::From<ReadyWork<T>> for WorkEvent<T> {
+    fn from(ready_work: ReadyWork<T>) -> Self {
+        match ready_work {
+            ReadyWork::Block(QueuedBlock {
+                peer_id,
+                block,
+                seen_timestamp,
+            }) => Self {
+                drop_during_sync: false,
+                work: Work::DelayedImportBlock {
+                    peer_id,
+                    block: Box::new(block),
+                    seen_timestamp,
+                },
+            },
+            ReadyWork::Unaggregate(QueuedUnaggregate {
+                peer_id,
+                message_id,
+                attestation,
+                subnet_id,
+                should_import,
+                seen_timestamp,
+            }) => Self {
+                drop_during_sync: true,
+                work: Work::UnknownBlockAttestation {
+                    message_id,
+                    peer_id,
+                    attestation,
+                    subnet_id,
+                    should_import,
+                    seen_timestamp,
+                },
+            },
+            ReadyWork::Aggregate(QueuedAggregate {
+                peer_id,
+                message_id,
+                attestation,
+                seen_timestamp,
+            }) => Self {
+                drop_during_sync: true,
+                work: Work::UnknownBlockAggregate {
+                    message_id,
+                    peer_id,
+                    aggregate: attestation,
+                    seen_timestamp,
+                },
+            },
+        }
+    }
+}
+
 /// A consensus message (or multiple) from the network that requires processing.
 #[derive(Debug)]
 pub enum Work<T: BeaconChainTypes> {
@@ -453,12 +507,26 @@ pub enum Work<T: BeaconChainTypes> {
         should_import: bool,
         seen_timestamp: Duration,
     },
+    UnknownBlockAttestation {
+        message_id: MessageId,
+        peer_id: PeerId,
+        attestation: Box<Attestation<T::EthSpec>>,
+        subnet_id: SubnetId,
+        should_import: bool,
+        seen_timestamp: Duration,
+    },
     GossipAggregate {
         message_id: MessageId,
         peer_id: PeerId,
         aggregate: Box<SignedAggregateAndProof<T::EthSpec>>,
         seen_timestamp: Duration,
     },
+    UnknownBlockAggregate {
+        message_id: MessageId,
+        peer_id: PeerId,
+        aggregate: Box<SignedAggregateAndProof<T::EthSpec>>,
+        seen_timestamp: Duration,
+    },
     GossipBlock {
         message_id: MessageId,
         peer_id: PeerId,
@@ -525,6 +593,8 @@ impl<T: BeaconChainTypes> Work<T> {
             Work::Status { .. } => STATUS_PROCESSING,
             Work::BlocksByRangeRequest { .. } => BLOCKS_BY_RANGE_REQUEST,
             Work::BlocksByRootsRequest { .. } => BLOCKS_BY_ROOTS_REQUEST,
+            Work::UnknownBlockAttestation { .. } => UNKNOWN_BLOCK_ATTESTATION,
+            Work::UnknownBlockAggregate { .. } => UNKNOWN_BLOCK_AGGREGATE,
         }
     }
 }
@@ -554,8 +624,8 @@ enum InboundEvent<T: BeaconChainTypes> {
     WorkerIdle,
     /// There is new work to be done.
     WorkEvent(WorkEvent<T>),
-    /// A block that was delayed for import at a later slot has become ready.
+    /// A work event that was queued for re-processing has become ready.
-    QueuedBlock(Box<QueuedBlock<T>>),
+    ReprocessingWork(WorkEvent<T>),
 }
 
 /// Combines the various incoming event streams for the `BeaconProcessor` into a single stream.
@@ -567,8 +637,8 @@ struct InboundEvents<T: BeaconChainTypes> {
     idle_rx: mpsc::Receiver<()>,
     /// Used by upstream processes to send new work to the `BeaconProcessor`.
     event_rx: mpsc::Receiver<WorkEvent<T>>,
-    /// Used internally for queuing blocks for processing once their slot arrives.
+    /// Used internally for queuing work ready to be re-processed.
-    post_delay_block_queue_rx: mpsc::Receiver<QueuedBlock<T>>,
+    reprocess_work_rx: mpsc::Receiver<ReadyWork<T>>,
 }
 
 impl<T: BeaconChainTypes> Stream for InboundEvents<T> {
@@ -589,9 +659,9 @@ impl<T: BeaconChainTypes> Stream for InboundEvents<T> {
 
         // Poll for delayed blocks before polling for new work. It might be the case that a delayed
         // block is required to successfully process some new work.
-        match self.post_delay_block_queue_rx.poll_recv(cx) {
+        match self.reprocess_work_rx.poll_recv(cx) {
-            Poll::Ready(Some(queued_block)) => {
+            Poll::Ready(Some(ready_work)) => {
-                return Poll::Ready(Some(InboundEvent::QueuedBlock(Box::new(queued_block))));
+                return Poll::Ready(Some(InboundEvent::ReprocessingWork(ready_work.into())));
             }
             Poll::Ready(None) => {
                 return Poll::Ready(None);
@@ -643,7 +713,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
     pub fn spawn_manager(
         mut self,
         event_rx: mpsc::Receiver<WorkEvent<T>>,
-        work_journal_tx: Option<mpsc::Sender<String>>,
+        work_journal_tx: Option<mpsc::Sender<&'static str>>,
     ) {
         // Used by workers to communicate that they are finished a task.
         let (idle_tx, idle_rx) = mpsc::channel::<()>(MAX_IDLE_QUEUE_LEN);
@@ -655,6 +725,10 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
         let mut aggregate_debounce = TimeLatch::default();
         let mut attestation_queue = LifoQueue::new(MAX_UNAGGREGATED_ATTESTATION_QUEUE_LEN);
         let mut attestation_debounce = TimeLatch::default();
+        let mut unknown_block_aggregate_queue =
+            LifoQueue::new(MAX_AGGREGATED_ATTESTATION_REPROCESS_QUEUE_LEN);
+        let mut unknown_block_attestation_queue =
+            LifoQueue::new(MAX_UNAGGREGATED_ATTESTATION_REPROCESS_QUEUE_LEN);
 
         // Using a FIFO queue for voluntary exits since it prevents exit censoring. I don't have
         // a strong feeling about queue type for exits.
@@ -677,14 +751,13 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
         let mut bbrange_queue = FifoQueue::new(MAX_BLOCKS_BY_RANGE_QUEUE_LEN);
         let mut bbroots_queue = FifoQueue::new(MAX_BLOCKS_BY_ROOTS_QUEUE_LEN);
 
-        // The delayed block queues are used to re-queue blocks for processing at a later time if
+        // Channels for sending work to the re-process scheduler (`work_reprocessing_tx`) and to
-        // they're received early.
+        // receive them back once they are ready (`ready_work_rx`).
-        let (post_delay_block_queue_tx, post_delay_block_queue_rx) =
+        let (ready_work_tx, ready_work_rx) = mpsc::channel(MAX_SCHEDULED_WORK_QUEUE_LEN);
-            mpsc::channel(MAX_DELAYED_BLOCK_QUEUE_LEN);
+        let work_reprocessing_tx = {
-        let pre_delay_block_queue_tx = {
             if let Some(chain) = self.beacon_chain.upgrade() {
-                spawn_block_delay_queue(
+                spawn_reprocess_scheduler(
-                    post_delay_block_queue_tx,
+                    ready_work_tx,
                     &self.executor,
                     chain.slot_clock.clone(),
                     self.log.clone(),
@@ -704,7 +777,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
             let mut inbound_events = InboundEvents {
                 idle_rx,
                 event_rx,
-                post_delay_block_queue_rx,
+                reprocess_work_rx: ready_work_rx,
             };
 
             loop {
@@ -713,14 +786,8 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         self.current_workers = self.current_workers.saturating_sub(1);
                         None
                     }
-                    Some(InboundEvent::WorkEvent(event)) => Some(event),
+                    Some(InboundEvent::WorkEvent(event))
-                    Some(InboundEvent::QueuedBlock(queued_block)) => {
+                    | Some(InboundEvent::ReprocessingWork(event)) => Some(event),
-                        Some(WorkEvent::delayed_import_beacon_block(
-                            queued_block.peer_id,
-                            Box::new(queued_block.block),
-                            queued_block.seen_timestamp,
-                        ))
-                    }
                     None => {
                         debug!(
                             self.log,
@@ -750,7 +817,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
 
                     // We don't care if this message was successfully sent, we only use the journal
                     // during testing.
-                    let _ = work_journal_tx.try_send(id.to_string());
+                    let _ = work_journal_tx.try_send(id);
                 }
 
                 let can_spawn = self.current_workers < self.max_workers;
@@ -766,7 +833,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                     None if can_spawn => {
                         let toolbox = Toolbox {
                             idle_tx: idle_tx.clone(),
-                            delayed_block_tx: pre_delay_block_queue_tx.clone(),
+                            work_reprocessing_tx: work_reprocessing_tx.clone(),
                         };
 
                         // Check for chain segments first, they're the most efficient way to get
@@ -792,6 +859,12 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                             self.spawn_worker(item, toolbox);
                         } else if let Some(item) = attestation_queue.pop() {
                             self.spawn_worker(item, toolbox);
+                        // Aggregates and unaggregates queued for re-processing are older and we
+                        // care about fresher ones, so check those first.
+                        } else if let Some(item) = unknown_block_aggregate_queue.pop() {
+                            self.spawn_worker(item, toolbox);
+                        } else if let Some(item) = unknown_block_attestation_queue.pop() {
+                            self.spawn_worker(item, toolbox);
                         // Check RPC methods next. Status messages are needed for sync so
                         // prioritize them over syncing requests from other peers (BlocksByRange
                         // and BlocksByRoot)
@@ -820,7 +893,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                             if let Some(work_journal_tx) = &work_journal_tx {
                                 // We don't care if this message was successfully sent, we only use the journal
                                 // during testing.
-                                let _ = work_journal_tx.try_send(NOTHING_TO_DO.to_string());
+                                let _ = work_journal_tx.try_send(NOTHING_TO_DO);
                             }
                         }
                     }
@@ -857,7 +930,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         let work_id = work.str_id();
                         let toolbox = Toolbox {
                             idle_tx: idle_tx.clone(),
-                            delayed_block_tx: pre_delay_block_queue_tx.clone(),
+                            work_reprocessing_tx: work_reprocessing_tx.clone(),
                         };
 
                         match work {
@@ -890,6 +963,12 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                             Work::BlocksByRootsRequest { .. } => {
                                 bbroots_queue.push(work, work_id, &self.log)
                             }
+                            Work::UnknownBlockAttestation { .. } => {
+                                unknown_block_attestation_queue.push(work)
+                            }
+                            Work::UnknownBlockAggregate { .. } => {
+                                unknown_block_aggregate_queue.push(work)
+                            }
                         }
                     }
                 }
@@ -960,7 +1039,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
     /// Sends an message on `idle_tx` when the work is complete and the task is stopping.
     fn spawn_worker(&mut self, work: Work<T>, toolbox: Toolbox<T>) {
         let idle_tx = toolbox.idle_tx;
-        let delayed_block_tx = toolbox.delayed_block_tx;
+        let work_reprocessing_tx = toolbox.work_reprocessing_tx;
 
         // Wrap the `idle_tx` in a struct that will fire the idle message whenever it is dropped.
         //
@@ -1031,6 +1110,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         *attestation,
                         subnet_id,
                         should_import,
+                        Some(work_reprocessing_tx),
                         seen_timestamp,
                     ),
                     /*
@@ -1045,6 +1125,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         message_id,
                         peer_id,
                         *aggregate,
+                        Some(work_reprocessing_tx),
                         seen_timestamp,
                     ),
                     /*
@@ -1059,7 +1140,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         message_id,
                         peer_id,
                         *block,
-                        delayed_block_tx,
+                        work_reprocessing_tx,
                         seen_timestamp,
                     ),
                     /*
@@ -1069,7 +1150,12 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         peer_id,
                         block,
                         seen_timestamp,
-                    } => worker.process_gossip_verified_block(peer_id, *block, seen_timestamp),
+                    } => worker.process_gossip_verified_block(
+                        peer_id,
+                        *block,
+                        work_reprocessing_tx,
+                        seen_timestamp,
+                    ),
                     /*
                      * Voluntary exits received on gossip.
                      */
@@ -1106,7 +1192,7 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                      * Verification for beacon blocks received during syncing via RPC.
                      */
                     Work::RpcBlock { block, result_tx } => {
-                        worker.process_rpc_block(*block, result_tx)
+                        worker.process_rpc_block(*block, result_tx, work_reprocessing_tx)
                     }
                     /*
                      * Verification for a chain segment (multiple blocks).
@@ -1134,6 +1220,34 @@ impl<T: BeaconChainTypes> BeaconProcessor<T> {
                         request_id,
                         request,
                     } => worker.handle_blocks_by_root_request(peer_id, request_id, request),
+                    Work::UnknownBlockAttestation {
+                        message_id,
+                        peer_id,
+                        attestation,
+                        subnet_id,
+                        should_import,
+                        seen_timestamp,
+                    } => worker.process_gossip_attestation(
+                        message_id,
+                        peer_id,
+                        *attestation,
+                        subnet_id,
+                        should_import,
+                        None, // Do not allow this attestation to be re-processed beyond this point.
+                        seen_timestamp,
+                    ),
+                    Work::UnknownBlockAggregate {
+                        message_id,
+                        peer_id,
+                        aggregate,
+                        seen_timestamp,
+                    } => worker.process_gossip_aggregate(
+                        message_id,
+                        peer_id,
+                        *aggregate,
+                        None,
+                        seen_timestamp,
+                    ),
                 };
 
                 trace!(

beacon_node/network/src/beacon_processor/tests.rs

@@ -1,6 +1,7 @@
 #![cfg(not(debug_assertions))] // Tests are too slow in debug.
 #![cfg(test)]
 
+use crate::beacon_processor::work_reprocessing_queue::QUEUED_ATTESTATION_DELAY;
 use crate::beacon_processor::*;
 use crate::{service::NetworkMessage, sync::SyncMessage};
 use beacon_chain::test_utils::{
@@ -42,11 +43,13 @@ struct TestRig {
     chain: Arc<BeaconChain<T>>,
     next_block: SignedBeaconBlock<E>,
     attestations: Vec<(Attestation<E>, SubnetId)>,
+    next_block_attestations: Vec<(Attestation<E>, SubnetId)>,
+    next_block_aggregate_attestations: Vec<SignedAggregateAndProof<E>>,
     attester_slashing: AttesterSlashing<E>,
     proposer_slashing: ProposerSlashing,
     voluntary_exit: SignedVoluntaryExit,
     beacon_processor_tx: mpsc::Sender<WorkEvent<T>>,
-    work_journal_rx: mpsc::Receiver<String>,
+    work_journal_rx: mpsc::Receiver<&'static str>,
     _network_rx: mpsc::UnboundedReceiver<NetworkMessage<E>>,
     _sync_rx: mpsc::UnboundedReceiver<SyncMessage<E>>,
     environment: Option<Environment<E>>,
@@ -90,7 +93,7 @@ impl TestRig {
             "precondition: current slot is one after head"
         );
 
-        let (next_block, _next_state) =
+        let (next_block, next_state) =
             harness.make_block(head.beacon_state.clone(), harness.chain.slot().unwrap());
 
         let head_state_root = head.beacon_state_root();
@@ -111,6 +114,35 @@ impl TestRig {
             "precondition: attestations for testing"
         );
 
+        let next_block_attestations = harness
+            .get_unaggregated_attestations(
+                &AttestationStrategy::AllValidators,
+                &next_state,
+                next_block.state_root(),
+                next_block.canonical_root(),
+                next_block.slot(),
+            )
+            .into_iter()
+            .flatten()
+            .collect::<Vec<_>>();
+
+        let next_block_aggregate_attestations = harness
+            .make_attestations(
+                &harness.get_all_validators(),
+                &next_state,
+                next_block.state_root(),
+                next_block.canonical_root().into(),
+                next_block.slot(),
+            )
+            .into_iter()
+            .filter_map(|(_, aggregate_opt)| aggregate_opt)
+            .collect::<Vec<_>>();
+
+        assert!(
+            !next_block_attestations.is_empty(),
+            "precondition: attestation for next block are not empty"
+        );
+
         let attester_slashing = harness.make_attester_slashing(vec![0, 1]);
         let proposer_slashing = harness.make_proposer_slashing(2);
         let voluntary_exit = harness.make_voluntary_exit(3, harness.chain.epoch().unwrap());
@@ -174,6 +206,8 @@ impl TestRig {
             chain,
             next_block,
             attestations,
+            next_block_attestations,
+            next_block_aggregate_attestations,
             attester_slashing,
             proposer_slashing,
             voluntary_exit,
@@ -185,6 +219,10 @@ impl TestRig {
         }
     }
 
+    pub fn head_root(&self) -> Hash256 {
+        self.chain.head().unwrap().beacon_block_root
+    }
+
     pub fn enqueue_gossip_block(&self) {
         self.beacon_processor_tx
             .try_send(WorkEvent::gossip_beacon_block(
@@ -196,6 +234,11 @@ impl TestRig {
             .unwrap();
     }
 
+    pub fn enqueue_rpc_block(&self) {
+        let (event, _rx) = WorkEvent::rpc_beacon_block(Box::new(self.next_block.clone()));
+        self.beacon_processor_tx.try_send(event).unwrap();
+    }
+
     pub fn enqueue_unaggregated_attestation(&self) {
         let (attestation, subnet_id) = self.attestations.first().unwrap().clone();
         self.beacon_processor_tx
@@ -240,6 +283,36 @@ impl TestRig {
             .unwrap();
     }
 
+    pub fn enqueue_next_block_unaggregated_attestation(&self) {
+        let (attestation, subnet_id) = self.next_block_attestations.first().unwrap().clone();
+        self.beacon_processor_tx
+            .try_send(WorkEvent::unaggregated_attestation(
+                junk_message_id(),
+                junk_peer_id(),
+                attestation,
+                subnet_id,
+                true,
+                Duration::from_secs(0),
+            ))
+            .unwrap();
+    }
+
+    pub fn enqueue_next_block_aggregated_attestation(&self) {
+        let aggregate = self
+            .next_block_aggregate_attestations
+            .first()
+            .unwrap()
+            .clone();
+        self.beacon_processor_tx
+            .try_send(WorkEvent::aggregated_attestation(
+                junk_message_id(),
+                junk_peer_id(),
+                aggregate,
+                Duration::from_secs(0),
+            ))
+            .unwrap();
+    }
+
     fn runtime(&mut self) -> Arc<Runtime> {
         self.environment
             .as_mut()
@@ -265,27 +338,37 @@ impl TestRig {
         })
     }
 
-    /// Assert that the `BeaconProcessor` event journal is as `expected`.
+    /// Checks that the `BeaconProcessor` event journal contains the `expected` events in the given
+    /// order with a matching number of `WORKER_FREED` events in between. `NOTHING_TO_DO` events
+    /// are ignored.
     ///
-    /// ## Note
+    /// Given the described logic, `expected` must not contain `WORKER_FREED` or `NOTHING_TO_DO`
-    ///
+    /// events.
-    /// We won't attempt to listen for any more than `expected.len()` events. As such, it makes sense
+    pub fn assert_event_journal_contains_ordered(&mut self, expected: &[&str]) {
-    /// to use the `NOTHING_TO_DO` event to ensure that execution has completed.
+        assert!(expected
-    pub fn assert_event_journal(&mut self, expected: &[&str]) {
+            .iter()
-        let events = self.runtime().block_on(async {
+            .all(|ev| ev != &WORKER_FREED && ev != &NOTHING_TO_DO));
-            let mut events = vec![];
+
+        let (events, worker_freed_remaining) = self.runtime().block_on(async {
+            let mut events = Vec::with_capacity(expected.len());
+            let mut worker_freed_remaining = expected.len();
 
             let drain_future = async {
                 loop {
                     match self.work_journal_rx.recv().await {
-                        Some(event) => {
+                        Some(event) if event == WORKER_FREED => {
-                            events.push(event);
+                            worker_freed_remaining -= 1;
-
+                            if worker_freed_remaining == 0 {
-                            // Break as soon as we collect the desired number of events.
+                                // Break when all expected events are finished.
-                            if events.len() >= expected.len() {
                                 break;
                             }
                         }
+                        Some(event) if event == NOTHING_TO_DO => {
+                            // Ignore these.
+                        }
+                        Some(event) => {
+                            events.push(event);
+                        }
                         None => break,
                     }
                 }
@@ -294,9 +377,53 @@ impl TestRig {
             // Drain the expected number of events from the channel, or time out and give up.
             tokio::select! {
                 _ = tokio::time::sleep(STANDARD_TIMEOUT) => panic!(
-                    "timeout ({:?}) expired waiting for events. expected {:?} but got {:?}",
+                    "Timeout ({:?}) expired waiting for events. Expected {:?} but got {:?} waiting for {} `WORKER_FREED` events.",
                     STANDARD_TIMEOUT,
                     expected,
+                    events,
+                    worker_freed_remaining,
+                ),
+                _ = drain_future => {},
+            }
+
+            (events, worker_freed_remaining)
+        });
+
+        assert_eq!(events, expected);
+        assert_eq!(worker_freed_remaining, 0);
+    }
+
+    pub fn assert_event_journal(&mut self, expected: &[&str]) {
+        self.assert_event_journal_with_timeout(expected, STANDARD_TIMEOUT);
+    }
+
+    /// Assert that the `BeaconProcessor` event journal is as `expected`.
+    ///
+    /// ## Note
+    ///
+    /// We won't attempt to listen for any more than `expected.len()` events. As such, it makes sense
+    /// to use the `NOTHING_TO_DO` event to ensure that execution has completed.
+    pub fn assert_event_journal_with_timeout(&mut self, expected: &[&str], timeout: Duration) {
+        let events = self.runtime().block_on(async {
+            let mut events = Vec::with_capacity(expected.len());
+
+            let drain_future = async {
+                while let Some(event) = self.work_journal_rx.recv().await {
+                    events.push(event);
+
+                    // Break as soon as we collect the desired number of events.
+                    if events.len() >= expected.len() {
+                        break;
+                    }
+                }
+            };
+
+            // Drain the expected number of events from the channel, or time out and give up.
+            tokio::select! {
+                _ = tokio::time::sleep(timeout) => panic!(
+                    "Timeout ({:?}) expired waiting for events. Expected {:?} but got {:?}",
+                    timeout,
+                    expected,
                     events
                 ),
                 _ = drain_future => {},
@@ -305,13 +432,7 @@ impl TestRig {
             events
         });
 
-        assert_eq!(
+        assert_eq!(events, expected);
-            events,
-            expected
-                .into_iter()
-                .map(|s| s.to_string())
-                .collect::<Vec<_>>()
-        );
     }
 }
 
@@ -353,18 +474,18 @@ fn import_gossip_block_acceptably_early() {
     // processing.
     //
     // If this causes issues we might be able to make the block delay queue add a longer delay for
-    // processing, instead of just MAXIMUM_GOSSIP_CLOCK_DISPARITY. Speak to @paulhauner if this test
+    // processing, instead of just ADDITIONAL_QUEUED_BLOCK_DELAY. Speak to @paulhauner if this test
     // starts failing.
     rig.chain.slot_clock.set_slot(rig.next_block.slot().into());
     assert!(
-        rig.chain.head().unwrap().beacon_block_root != rig.next_block.canonical_root(),
+        rig.head_root() != rig.next_block.canonical_root(),
         "block not yet imported"
     );
 
     rig.assert_event_journal(&[DELAYED_IMPORT_BLOCK, WORKER_FREED, NOTHING_TO_DO]);
 
     assert_eq!(
-        rig.chain.head().unwrap().beacon_block_root,
+        rig.head_root(),
         rig.next_block.canonical_root(),
         "block should be imported and become head"
     );
@@ -395,12 +516,12 @@ fn import_gossip_block_unacceptably_early() {
 
     rig.assert_event_journal(&[GOSSIP_BLOCK, WORKER_FREED, NOTHING_TO_DO]);
 
-    // Waiting for 5 seconds is a bit arbtirary, however it *should* be long enough to ensure the
+    // Waiting for 5 seconds is a bit arbitrary, however it *should* be long enough to ensure the
     // block isn't imported.
     rig.assert_no_events_for(Duration::from_secs(5));
 
     assert!(
-        rig.chain.head().unwrap().beacon_block_root != rig.next_block.canonical_root(),
+        rig.head_root() != rig.next_block.canonical_root(),
         "block should not be imported"
     );
 }
@@ -421,7 +542,7 @@ fn import_gossip_block_at_current_slot() {
     rig.assert_event_journal(&[GOSSIP_BLOCK, WORKER_FREED, NOTHING_TO_DO]);
 
     assert_eq!(
-        rig.chain.head().unwrap().beacon_block_root,
+        rig.head_root(),
         rig.next_block.canonical_root(),
         "block should be imported and become head"
     );
@@ -445,6 +566,207 @@ fn import_gossip_attestation() {
     );
 }
 
+enum BlockImportMethod {
+    Gossip,
+    Rpc,
+}
+
+/// Ensure that attestations that reference an unknown block get properly re-queued and
+/// re-processed upon importing the block.
+fn attestation_to_unknown_block_processed(import_method: BlockImportMethod) {
+    let mut rig = TestRig::new(SMALL_CHAIN);
+
+    // Send the attestation but not the block, and check that it was not imported.
+
+    let initial_attns = rig.chain.naive_aggregation_pool.read().num_attestations();
+
+    rig.enqueue_next_block_unaggregated_attestation();
+
+    rig.assert_event_journal(&[GOSSIP_ATTESTATION, WORKER_FREED, NOTHING_TO_DO]);
+
+    assert_eq!(
+        rig.chain.naive_aggregation_pool.read().num_attestations(),
+        initial_attns,
+        "Attestation should not have been included."
+    );
+
+    // Send the block and ensure that the attestation is received back and imported.
+
+    let block_event = match import_method {
+        BlockImportMethod::Gossip => {
+            rig.enqueue_gossip_block();
+            GOSSIP_BLOCK
+        }
+        BlockImportMethod::Rpc => {
+            rig.enqueue_rpc_block();
+            RPC_BLOCK
+        }
+    };
+
+    rig.assert_event_journal_contains_ordered(&[block_event, UNKNOWN_BLOCK_ATTESTATION]);
+
+    // Run fork choice, since it isn't run when processing an RPC block. At runtime it is the
+    // responsibility of the sync manager to do this.
+    rig.chain.fork_choice().unwrap();
+
+    assert_eq!(
+        rig.head_root(),
+        rig.next_block.canonical_root(),
+        "Block should be imported and become head."
+    );
+
+    assert_eq!(
+        rig.chain.naive_aggregation_pool.read().num_attestations(),
+        initial_attns + 1,
+        "Attestation should have been included."
+    );
+}
+
+#[test]
+fn attestation_to_unknown_block_processed_after_gossip_block() {
+    attestation_to_unknown_block_processed(BlockImportMethod::Gossip)
+}
+
+#[test]
+fn attestation_to_unknown_block_processed_after_rpc_block() {
+    attestation_to_unknown_block_processed(BlockImportMethod::Rpc)
+}
+
+/// Ensure that attestations that reference an unknown block get properly re-queued and
+/// re-processed upon importing the block.
+fn aggregate_attestation_to_unknown_block(import_method: BlockImportMethod) {
+    let mut rig = TestRig::new(SMALL_CHAIN);
+
+    // Empty the op pool.
+    rig.chain
+        .op_pool
+        .prune_attestations(u64::max_value().into());
+    assert_eq!(rig.chain.op_pool.num_attestations(), 0);
+
+    // Send the attestation but not the block, and check that it was not imported.
+
+    let initial_attns = rig.chain.op_pool.num_attestations();
+
+    rig.enqueue_next_block_aggregated_attestation();
+
+    rig.assert_event_journal(&[GOSSIP_AGGREGATE, WORKER_FREED, NOTHING_TO_DO]);
+
+    assert_eq!(
+        rig.chain.op_pool.num_attestations(),
+        initial_attns,
+        "Attestation should not have been included."
+    );
+
+    // Send the block and ensure that the attestation is received back and imported.
+
+    let block_event = match import_method {
+        BlockImportMethod::Gossip => {
+            rig.enqueue_gossip_block();
+            GOSSIP_BLOCK
+        }
+        BlockImportMethod::Rpc => {
+            rig.enqueue_rpc_block();
+            RPC_BLOCK
+        }
+    };
+
+    rig.assert_event_journal_contains_ordered(&[block_event, UNKNOWN_BLOCK_AGGREGATE]);
+
+    // Run fork choice, since it isn't run when processing an RPC block. At runtime it is the
+    // responsibility of the sync manager to do this.
+    rig.chain.fork_choice().unwrap();
+
+    assert_eq!(
+        rig.head_root(),
+        rig.next_block.canonical_root(),
+        "Block should be imported and become head."
+    );
+
+    assert_eq!(
+        rig.chain.op_pool.num_attestations(),
+        initial_attns + 1,
+        "Attestation should have been included."
+    );
+}
+
+#[test]
+fn aggregate_attestation_to_unknown_block_processed_after_gossip_block() {
+    aggregate_attestation_to_unknown_block(BlockImportMethod::Gossip)
+}
+
+#[test]
+fn aggregate_attestation_to_unknown_block_processed_after_rpc_block() {
+    aggregate_attestation_to_unknown_block(BlockImportMethod::Rpc)
+}
+
+/// Ensure that attestations that reference an unknown block get properly re-queued and re-processed
+/// when the block is not seen.
+#[test]
+fn requeue_unknown_block_gossip_attestation_without_import() {
+    let mut rig = TestRig::new(SMALL_CHAIN);
+
+    // Send the attestation but not the block, and check that it was not imported.
+
+    let initial_attns = rig.chain.naive_aggregation_pool.read().num_attestations();
+
+    rig.enqueue_next_block_unaggregated_attestation();
+
+    rig.assert_event_journal(&[GOSSIP_ATTESTATION, WORKER_FREED, NOTHING_TO_DO]);
+
+    assert_eq!(
+        rig.chain.naive_aggregation_pool.read().num_attestations(),
+        initial_attns,
+        "Attestation should not have been included."
+    );
+
+    // Ensure that the attestation is received back but not imported.
+
+    rig.assert_event_journal_with_timeout(
+        &[UNKNOWN_BLOCK_ATTESTATION, WORKER_FREED, NOTHING_TO_DO],
+        Duration::from_secs(1) + QUEUED_ATTESTATION_DELAY,
+    );
+
+    assert_eq!(
+        rig.chain.naive_aggregation_pool.read().num_attestations(),
+        initial_attns,
+        "Attestation should not have been included."
+    );
+}
+
+/// Ensure that aggregate that reference an unknown block get properly re-queued and re-processed
+/// when the block is not seen.
+#[test]
+fn requeue_unknown_block_gossip_aggregated_attestation_without_import() {
+    let mut rig = TestRig::new(SMALL_CHAIN);
+
+    // Send the attestation but not the block, and check that it was not imported.
+
+    let initial_attns = rig.chain.op_pool.num_attestations();
+
+    rig.enqueue_next_block_aggregated_attestation();
+
+    rig.assert_event_journal(&[GOSSIP_AGGREGATE, WORKER_FREED, NOTHING_TO_DO]);
+
+    assert_eq!(
+        rig.chain.naive_aggregation_pool.read().num_attestations(),
+        initial_attns,
+        "Attestation should not have been included."
+    );
+
+    // Ensure that the attestation is received back but not imported.
+
+    rig.assert_event_journal_with_timeout(
+        &[UNKNOWN_BLOCK_AGGREGATE, WORKER_FREED, NOTHING_TO_DO],
+        Duration::from_secs(1) + QUEUED_ATTESTATION_DELAY,
+    );
+
+    assert_eq!(
+        rig.chain.op_pool.num_attestations(),
+        initial_attns,
+        "Attestation should not have been included."
+    );
+}
+
 /// Ensure a bunch of valid operations can be imported.
 #[test]
 fn import_misc_gossip_ops() {

beacon_node/network/src/beacon_processor/work_reprocessing_queue.rs

@@ -0,0 +1,500 @@
+//! Provides a mechanism which queues work for later processing.
+//!
+//! When the `beacon_processor::Worker` imports a block that is acceptably early (i.e., within the
+//! gossip propagation tolerance) it will send it to this queue where it will be placed in a
+//! `DelayQueue` until the slot arrives. Once the block has been determined to be ready, it will be
+//! sent back out on a channel to be processed by the `BeaconProcessor` again.
+//!
+//! There is the edge-case where the slot arrives before this queue manages to process it. In that
+//! case, the block will be sent off for immediate processing (skipping the `DelayQueue`).
+//!
+//! Aggregated and unaggregated attestations that failed verification due to referencing an unknown
+//! block will be re-queued until their block is imported, or until they expire.
+use super::MAX_SCHEDULED_WORK_QUEUE_LEN;
+use crate::metrics;
+use beacon_chain::{BeaconChainTypes, GossipVerifiedBlock, MAXIMUM_GOSSIP_CLOCK_DISPARITY};
+use eth2_libp2p::{MessageId, PeerId};
+use fnv::FnvHashMap;
+use futures::task::Poll;
+use futures::{Stream, StreamExt};
+use slog::{crit, debug, error, Logger};
+use slot_clock::SlotClock;
+use std::collections::{HashMap, HashSet};
+use std::pin::Pin;
+use std::task::Context;
+use std::time::Duration;
+use task_executor::TaskExecutor;
+use tokio::sync::mpsc::{self, Receiver, Sender};
+use tokio::time::error::Error as TimeError;
+use tokio_util::time::delay_queue::{DelayQueue, Key as DelayKey};
+use types::{Attestation, EthSpec, Hash256, SignedAggregateAndProof, SubnetId};
+
+const TASK_NAME: &str = "beacon_processor_reprocess_queue";
+const BLOCKS: &str = "blocks";
+const ATTESTATIONS: &str = "attestations";
+
+/// Queue blocks for re-processing with an `ADDITIONAL_QUEUED_BLOCK_DELAY` after the slot starts.
+/// This is to account for any slight drift in the system clock.
+const ADDITIONAL_QUEUED_BLOCK_DELAY: Duration = Duration::from_millis(5);
+
+/// For how long to queue aggregated and unaggregated attestations for re-processing.
+pub const QUEUED_ATTESTATION_DELAY: Duration = Duration::from_secs(12);
+
+/// Set an arbitrary upper-bound on the number of queued blocks to avoid DoS attacks. The fact that
+/// we signature-verify blocks before putting them in the queue *should* protect against this, but
+/// it's nice to have extra protection.
+const MAXIMUM_QUEUED_BLOCKS: usize = 16;
+
+/// How many attestations we keep before new ones get dropped.
+const MAXIMUM_QUEUED_ATTESTATIONS: usize = 1_024;
+
+/// Messages that the scheduler can receive.
+pub enum ReprocessQueueMessage<T: BeaconChainTypes> {
+    /// A block that has been received early and we should queue for later processing.
+    EarlyBlock(QueuedBlock<T>),
+    /// A block that was successfully processed. We use this to handle attestations for unknown
+    /// blocks.
+    BlockImported(Hash256),
+    /// An unaggregated attestation that references an unknown block.
+    UnknownBlockUnaggregate(QueuedUnaggregate<T::EthSpec>),
+    /// An aggregated attestation that references an unknown block.
+    UnknownBlockAggregate(QueuedAggregate<T::EthSpec>),
+}
+
+/// Events sent by the scheduler once they are ready for re-processing.
+pub enum ReadyWork<T: BeaconChainTypes> {
+    Block(QueuedBlock<T>),
+    Unaggregate(QueuedUnaggregate<T::EthSpec>),
+    Aggregate(QueuedAggregate<T::EthSpec>),
+}
+
+/// An Attestation for which the corresponding block was not seen while processing, queued for
+/// later.
+pub struct QueuedUnaggregate<T: EthSpec> {
+    pub peer_id: PeerId,
+    pub message_id: MessageId,
+    pub attestation: Box<Attestation<T>>,
+    pub subnet_id: SubnetId,
+    pub should_import: bool,
+    pub seen_timestamp: Duration,
+}
+
+/// An aggregated attestation for which the corresponding block was not seen while processing, queued for
+/// later.
+pub struct QueuedAggregate<T: EthSpec> {
+    pub peer_id: PeerId,
+    pub message_id: MessageId,
+    pub attestation: Box<SignedAggregateAndProof<T>>,
+    pub seen_timestamp: Duration,
+}
+
+/// A block that arrived early and has been queued for later import.
+pub struct QueuedBlock<T: BeaconChainTypes> {
+    pub peer_id: PeerId,
+    pub block: GossipVerifiedBlock<T>,
+    pub seen_timestamp: Duration,
+}
+
+/// Unifies the different messages processed by the block delay queue.
+enum InboundEvent<T: BeaconChainTypes> {
+    /// A block that was queued for later processing and is ready for import.
+    ReadyBlock(QueuedBlock<T>),
+    /// An aggregated or unaggregated attestation is ready for re-processing.
+    ReadyAttestation(QueuedAttestationId),
+    /// A `DelayQueue` returned an error.
+    DelayQueueError(TimeError, &'static str),
+    /// A message sent to the `ReprocessQueue`
+    Msg(ReprocessQueueMessage<T>),
+}
+
+/// Manages scheduling works that need to be later re-processed.
+struct ReprocessQueue<T: BeaconChainTypes> {
+    /// Receiver of messages relevant to schedule works for reprocessing.
+    work_reprocessing_rx: Receiver<ReprocessQueueMessage<T>>,
+    /// Sender of works once they become ready
+    ready_work_tx: Sender<ReadyWork<T>>,
+
+    /* Queues */
+    /// Queue to manage scheduled early blocks.
+    block_delay_queue: DelayQueue<QueuedBlock<T>>,
+    /// Queue to manage scheduled attestations.
+    attestations_delay_queue: DelayQueue<QueuedAttestationId>,
+
+    /* Queued items */
+    /// Queued blocks.
+    queued_block_roots: HashSet<Hash256>,
+    /// Queued aggregated attestations.
+    queued_aggregates: FnvHashMap<usize, (QueuedAggregate<T::EthSpec>, DelayKey)>,
+    /// Queued attestations.
+    queued_unaggregates: FnvHashMap<usize, (QueuedUnaggregate<T::EthSpec>, DelayKey)>,
+    /// Attestations (aggregated and unaggregated) per root.
+    awaiting_attestations_per_root: HashMap<Hash256, Vec<QueuedAttestationId>>,
+
+    /* Aux */
+    /// Next attestation id, used for both aggregated and unaggregated attestations
+    next_attestation: usize,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum QueuedAttestationId {
+    Aggregate(usize),
+    Unaggregate(usize),
+}
+
+impl<T: EthSpec> QueuedAggregate<T> {
+    pub fn beacon_block_root(&self) -> &Hash256 {
+        &self.attestation.message.aggregate.data.beacon_block_root
+    }
+}
+
+impl<T: EthSpec> QueuedUnaggregate<T> {
+    pub fn beacon_block_root(&self) -> &Hash256 {
+        &self.attestation.data.beacon_block_root
+    }
+}
+
+impl<T: BeaconChainTypes> Stream for ReprocessQueue<T> {
+    type Item = InboundEvent<T>;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        // NOTE: implementing `Stream` is not necessary but allows to maintain the future selection
+        // order fine-grained and separate from the logic of handling each message, which is nice.
+
+        // Poll for expired blocks *before* we try to process new blocks.
+        //
+        // The sequential nature of blockchains means it is generally better to try and import all
+        // existing blocks before new ones.
+        match self.block_delay_queue.poll_expired(cx) {
+            Poll::Ready(Some(Ok(queued_block))) => {
+                return Poll::Ready(Some(InboundEvent::ReadyBlock(queued_block.into_inner())));
+            }
+            Poll::Ready(Some(Err(e))) => {
+                return Poll::Ready(Some(InboundEvent::DelayQueueError(e, "block_queue")));
+            }
+            // `Poll::Ready(None)` means that there are no more entries in the delay queue and we
+            // will continue to get this result until something else is added into the queue.
+            Poll::Ready(None) | Poll::Pending => (),
+        }
+
+        match self.attestations_delay_queue.poll_expired(cx) {
+            Poll::Ready(Some(Ok(attestation_id))) => {
+                return Poll::Ready(Some(InboundEvent::ReadyAttestation(
+                    attestation_id.into_inner(),
+                )));
+            }
+            Poll::Ready(Some(Err(e))) => {
+                return Poll::Ready(Some(InboundEvent::DelayQueueError(e, "attestations_queue")));
+            }
+            // `Poll::Ready(None)` means that there are no more entries in the delay queue and we
+            // will continue to get this result until something else is added into the queue.
+            Poll::Ready(None) | Poll::Pending => (),
+        }
+
+        // Last empty the messages channel.
+        match self.work_reprocessing_rx.poll_recv(cx) {
+            Poll::Ready(Some(message)) => return Poll::Ready(Some(InboundEvent::Msg(message))),
+            Poll::Ready(None) | Poll::Pending => {}
+        }
+
+        Poll::Pending
+    }
+}
+
+/// Starts the job that manages scheduling works that need re-processing. The returned `Sender`
+/// gives the communicating channel to receive those works. Once a work is ready, it is sent back
+/// via `ready_work_tx`.
+pub fn spawn_reprocess_scheduler<T: BeaconChainTypes>(
+    ready_work_tx: Sender<ReadyWork<T>>,
+    executor: &TaskExecutor,
+    slot_clock: T::SlotClock,
+    log: Logger,
+) -> Sender<ReprocessQueueMessage<T>> {
+    let (work_reprocessing_tx, work_reprocessing_rx) = mpsc::channel(MAX_SCHEDULED_WORK_QUEUE_LEN);
+    // Basic sanity check.
+    assert!(ADDITIONAL_QUEUED_BLOCK_DELAY < MAXIMUM_GOSSIP_CLOCK_DISPARITY);
+
+    let mut queue = ReprocessQueue {
+        work_reprocessing_rx,
+        ready_work_tx,
+        block_delay_queue: DelayQueue::new(),
+        attestations_delay_queue: DelayQueue::new(),
+        queued_block_roots: HashSet::new(),
+        queued_aggregates: FnvHashMap::default(),
+        queued_unaggregates: FnvHashMap::default(),
+        awaiting_attestations_per_root: HashMap::new(),
+        next_attestation: 0,
+    };
+
+    executor.spawn(
+        async move {
+            while let Some(msg) = queue.next().await {
+                queue.handle_message(msg, &slot_clock, &log);
+            }
+
+            debug!(
+                log,
+                "Re-process queue stopped";
+                "msg" => "shutting down"
+            );
+        },
+        TASK_NAME,
+    );
+
+    work_reprocessing_tx
+}
+
+impl<T: BeaconChainTypes> ReprocessQueue<T> {
+    fn handle_message(&mut self, msg: InboundEvent<T>, slot_clock: &T::SlotClock, log: &Logger) {
+        use ReprocessQueueMessage::*;
+        match msg {
+            // Some block has been indicated as "early" and should be processed when the
+            // appropriate slot arrives.
+            InboundEvent::Msg(EarlyBlock(early_block)) => {
+                let block_slot = early_block.block.block.slot();
+                let block_root = early_block.block.block_root;
+
+                // Don't add the same block to the queue twice. This prevents DoS attacks.
+                if self.queued_block_roots.contains(&block_root) {
+                    return;
+                }
+
+                if let Some(duration_till_slot) = slot_clock.duration_to_slot(block_slot) {
+                    // Check to ensure this won't over-fill the queue.
+                    if self.queued_block_roots.len() >= MAXIMUM_QUEUED_BLOCKS {
+                        error!(
+                            log,
+                            "Early blocks queue is full";
+                            "queue_size" => MAXIMUM_QUEUED_BLOCKS,
+                            "msg" => "check system clock"
+                        );
+                        // Drop the block.
+                        return;
+                    }
+
+                    self.queued_block_roots.insert(block_root);
+                    // Queue the block until the start of the appropriate slot, plus
+                    // `ADDITIONAL_QUEUED_BLOCK_DELAY`.
+                    self.block_delay_queue.insert(
+                        early_block,
+                        duration_till_slot + ADDITIONAL_QUEUED_BLOCK_DELAY,
+                    );
+                } else {
+                    // If there is no duration till the next slot, check to see if the slot
+                    // has already arrived. If it has already arrived, send it out for
+                    // immediate processing.
+                    //
+                    // If we can't read the slot or the slot hasn't arrived, simply drop the
+                    // block.
+                    //
+                    // This logic is slightly awkward since `SlotClock::duration_to_slot`
+                    // doesn't distinguish between a slot that has already arrived and an
+                    // error reading the slot clock.
+                    if let Some(now) = slot_clock.now() {
+                        if block_slot <= now
+                            && self
+                                .ready_work_tx
+                                .try_send(ReadyWork::Block(early_block))
+                                .is_err()
+                        {
+                            error!(
+                                log,
+                                "Failed to send block";
+                            );
+                        }
+                    }
+                }
+            }
+            InboundEvent::Msg(UnknownBlockAggregate(queued_aggregate)) => {
+                if self.attestations_delay_queue.len() >= MAXIMUM_QUEUED_ATTESTATIONS {
+                    error!(
+                        log,
+                        "Aggregate attestation delay queue is full";
+                        "queue_size" => MAXIMUM_QUEUED_ATTESTATIONS,
+                        "msg" => "check system clock"
+                    );
+                    // Drop the attestation.
+                    return;
+                }
+
+                let att_id = QueuedAttestationId::Aggregate(self.next_attestation);
+
+                // Register the delay.
+                let delay_key = self
+                    .attestations_delay_queue
+                    .insert(att_id, QUEUED_ATTESTATION_DELAY);
+
+                // Register this attestation for the corresponding root.
+                self.awaiting_attestations_per_root
+                    .entry(*queued_aggregate.beacon_block_root())
+                    .or_default()
+                    .push(att_id);
+
+                // Store the attestation and its info.
+                self.queued_aggregates
+                    .insert(self.next_attestation, (queued_aggregate, delay_key));
+
+                self.next_attestation += 1;
+            }
+            InboundEvent::Msg(UnknownBlockUnaggregate(queued_unaggregate)) => {
+                if self.attestations_delay_queue.len() >= MAXIMUM_QUEUED_ATTESTATIONS {
+                    error!(
+                        log,
+                        "Attestation delay queue is full";
+                        "queue_size" => MAXIMUM_QUEUED_ATTESTATIONS,
+                        "msg" => "check system clock"
+                    );
+                    // Drop the attestation.
+                    return;
+                }
+
+                let att_id = QueuedAttestationId::Unaggregate(self.next_attestation);
+
+                // Register the delay.
+                let delay_key = self
+                    .attestations_delay_queue
+                    .insert(att_id, QUEUED_ATTESTATION_DELAY);
+
+                // Register this attestation for the corresponding root.
+                self.awaiting_attestations_per_root
+                    .entry(*queued_unaggregate.beacon_block_root())
+                    .or_default()
+                    .push(att_id);
+
+                // Store the attestation and its info.
+                self.queued_unaggregates
+                    .insert(self.next_attestation, (queued_unaggregate, delay_key));
+
+                self.next_attestation += 1;
+            }
+            InboundEvent::Msg(BlockImported(root)) => {
+                // Unqueue the attestations we have for this root, if any.
+                if let Some(queued_ids) = self.awaiting_attestations_per_root.remove(&root) {
+                    for id in queued_ids {
+                        metrics::inc_counter(
+                            &metrics::BEACON_PROCESSOR_REPROCESSING_QUEUE_MATCHED_ATTESTATIONS,
+                        );
+
+                        if let Some((work, delay_key)) = match id {
+                            QueuedAttestationId::Aggregate(id) => self
+                                .queued_aggregates
+                                .remove(&id)
+                                .map(|(aggregate, delay_key)| {
+                                    (ReadyWork::Aggregate(aggregate), delay_key)
+                                }),
+                            QueuedAttestationId::Unaggregate(id) => self
+                                .queued_unaggregates
+                                .remove(&id)
+                                .map(|(unaggregate, delay_key)| {
+                                    (ReadyWork::Unaggregate(unaggregate), delay_key)
+                                }),
+                        } {
+                            // Remove the delay.
+                            self.attestations_delay_queue.remove(&delay_key);
+
+                            // Send the work.
+                            if self.ready_work_tx.try_send(work).is_err() {
+                                error!(
+                                    log,
+                                    "Failed to send scheduled attestation";
+                                );
+                            }
+                        } else {
+                            // There is a mismatch between the attestation ids registered for this
+                            // root and the queued attestations. This should never happen.
+                            error!(
+                                log,
+                                "Unknown queued attestation for block root";
+                                "block_root" => ?root,
+                                "att_id" => ?id,
+                            );
+                        }
+                    }
+                }
+            }
+            // A block that was queued for later processing is now ready to be processed.
+            InboundEvent::ReadyBlock(ready_block) => {
+                let block_root = ready_block.block.block_root;
+
+                if !self.queued_block_roots.remove(&block_root) {
+                    // Log an error to alert that we've made a bad assumption about how this
+                    // program works, but still process the block anyway.
+                    error!(
+                        log,
+                        "Unknown block in delay queue";
+                        "block_root" => ?block_root
+                    );
+                }
+
+                if self
+                    .ready_work_tx
+                    .try_send(ReadyWork::Block(ready_block))
+                    .is_err()
+                {
+                    error!(
+                        log,
+                        "Failed to pop queued block";
+                    );
+                }
+            }
+            InboundEvent::DelayQueueError(e, queue_name) => {
+                crit!(
+                    log,
+                    "Failed to poll queue";
+                    "queue" => queue_name,
+                    "e" => ?e
+                )
+            }
+            InboundEvent::ReadyAttestation(queued_id) => {
+                metrics::inc_counter(
+                    &metrics::BEACON_PROCESSOR_REPROCESSING_QUEUE_EXPIRED_ATTESTATIONS,
+                );
+
+                if let Some((root, work)) = match queued_id {
+                    QueuedAttestationId::Aggregate(id) => {
+                        self.queued_aggregates
+                            .remove(&id)
+                            .map(|(aggregate, _delay_key)| {
+                                (
+                                    *aggregate.beacon_block_root(),
+                                    ReadyWork::Aggregate(aggregate),
+                                )
+                            })
+                    }
+                    QueuedAttestationId::Unaggregate(id) => self
+                        .queued_unaggregates
+                        .remove(&id)
+                        .map(|(unaggregate, _delay_key)| {
+                            (
+                                *unaggregate.beacon_block_root(),
+                                ReadyWork::Unaggregate(unaggregate),
+                            )
+                        }),
+                } {
+                    if self.ready_work_tx.try_send(work).is_err() {
+                        error!(
+                            log,
+                            "Failed to send scheduled attestation";
+                        );
+                    }
+
+                    if let Some(queued_atts) = self.awaiting_attestations_per_root.get_mut(&root) {
+                        if let Some(index) = queued_atts.iter().position(|&id| id == queued_id) {
+                            queued_atts.swap_remove(index);
+                        }
+                    }
+                }
+            }
+        }
+
+        metrics::set_gauge_vec(
+            &metrics::BEACON_PROCESSOR_REPROCESSING_QUEUE_TOTAL,
+            &[BLOCKS],
+            self.block_delay_queue.len() as i64,
+        );
+        metrics::set_gauge_vec(
+            &metrics::BEACON_PROCESSOR_REPROCESSING_QUEUE_TOTAL,
+            &[ATTESTATIONS],
+            self.attestations_delay_queue.len() as i64,
+        );
+    }
+}

beacon_node/network/src/beacon_processor/worker/gossip_methods.rs

@@ -13,11 +13,48 @@ use ssz::Encode;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 use tokio::sync::mpsc;
 use types::{
-    Attestation, AttesterSlashing, Hash256, ProposerSlashing, SignedAggregateAndProof,
+    Attestation, AttesterSlashing, EthSpec, Hash256, ProposerSlashing, SignedAggregateAndProof,
     SignedBeaconBlock, SignedVoluntaryExit, SubnetId,
 };
 
-use super::{super::block_delay_queue::QueuedBlock, Worker};
+use super::{
+    super::work_reprocessing_queue::{
+        QueuedAggregate, QueuedBlock, QueuedUnaggregate, ReprocessQueueMessage,
+    },
+    Worker,
+};
+
+/// Data for an aggregated or unaggregated attestation that failed verification.
+enum FailedAtt<T: EthSpec> {
+    Unaggregate {
+        attestation: Box<Attestation<T>>,
+        subnet_id: SubnetId,
+        should_import: bool,
+        seen_timestamp: Duration,
+    },
+    Aggregate {
+        attestation: Box<SignedAggregateAndProof<T>>,
+        seen_timestamp: Duration,
+    },
+}
+
+impl<T: EthSpec> FailedAtt<T> {
+    pub fn root(&self) -> &Hash256 {
+        match self {
+            FailedAtt::Unaggregate { attestation, .. } => &attestation.data.beacon_block_root,
+            FailedAtt::Aggregate { attestation, .. } => {
+                &attestation.message.aggregate.data.beacon_block_root
+            }
+        }
+    }
+
+    pub fn kind(&self) -> &'static str {
+        match self {
+            FailedAtt::Unaggregate { .. } => "unaggregated",
+            FailedAtt::Aggregate { .. } => "aggregated",
+        }
+    }
+}
 
 impl<T: BeaconChainTypes> Worker<T> {
     /* Auxiliary functions */
@@ -59,6 +96,7 @@ impl<T: BeaconChainTypes> Worker<T> {
     /// - Attempt to add it to the naive aggregation pool.
     ///
     /// Raises a log if there are errors.
+    #[allow(clippy::too_many_arguments)]
     pub fn process_gossip_attestation(
         self,
         message_id: MessageId,
@@ -66,6 +104,7 @@ impl<T: BeaconChainTypes> Worker<T> {
         attestation: Attestation<T::EthSpec>,
         subnet_id: SubnetId,
         should_import: bool,
+        reprocess_tx: Option<mpsc::Sender<ReprocessQueueMessage<T>>>,
         seen_timestamp: Duration,
     ) {
         let beacon_block_root = attestation.data.beacon_block_root;
@@ -75,12 +114,17 @@ impl<T: BeaconChainTypes> Worker<T> {
             .verify_unaggregated_attestation_for_gossip(attestation, Some(subnet_id))
         {
             Ok(attestation) => attestation,
-            Err(e) => {
+            Err((e, attestation)) => {
                 self.handle_attestation_verification_failure(
                     peer_id,
                     message_id,
-                    beacon_block_root,
+                    FailedAtt::Unaggregate {
-                    "unaggregated",
+                        attestation: Box::new(attestation),
+                        subnet_id,
+                        should_import,
+                        seen_timestamp,
+                    },
+                    reprocess_tx,
                     e,
                 );
                 return;
@@ -153,6 +197,7 @@ impl<T: BeaconChainTypes> Worker<T> {
         message_id: MessageId,
         peer_id: PeerId,
         aggregate: SignedAggregateAndProof<T::EthSpec>,
+        reprocess_tx: Option<mpsc::Sender<ReprocessQueueMessage<T>>>,
         seen_timestamp: Duration,
     ) {
         let beacon_block_root = aggregate.message.aggregate.data.beacon_block_root;
@@ -162,13 +207,16 @@ impl<T: BeaconChainTypes> Worker<T> {
             .verify_aggregated_attestation_for_gossip(aggregate)
         {
             Ok(aggregate) => aggregate,
-            Err(e) => {
+            Err((e, attestation)) => {
                 // Report the failure to gossipsub
                 self.handle_attestation_verification_failure(
                     peer_id,
                     message_id,
-                    beacon_block_root,
+                    FailedAtt::Aggregate {
-                    "aggregated",
+                        attestation: Box::new(attestation),
+                        seen_timestamp,
+                    },
+                    reprocess_tx,
                     e,
                 );
                 return;
@@ -238,7 +286,7 @@ impl<T: BeaconChainTypes> Worker<T> {
         message_id: MessageId,
         peer_id: PeerId,
         block: SignedBeaconBlock<T::EthSpec>,
-        delayed_import_tx: mpsc::Sender<QueuedBlock<T>>,
+        reprocess_tx: mpsc::Sender<ReprocessQueueMessage<T>>,
         seen_duration: Duration,
     ) {
         // Log metrics to track delay from other nodes on the network.
@@ -361,12 +409,12 @@ impl<T: BeaconChainTypes> Worker<T> {
 
                 metrics::inc_counter(&metrics::BEACON_PROCESSOR_GOSSIP_BLOCK_REQUEUED_TOTAL);
 
-                if delayed_import_tx
+                if reprocess_tx
-                    .try_send(QueuedBlock {
+                    .try_send(ReprocessQueueMessage::EarlyBlock(QueuedBlock {
                         peer_id,
                         block: verified_block,
                         seen_timestamp: seen_duration,
-                    })
+                    }))
                     .is_err()
                 {
                     error!(
@@ -378,7 +426,12 @@ impl<T: BeaconChainTypes> Worker<T> {
                     )
                 }
             }
-            Ok(_) => self.process_gossip_verified_block(peer_id, verified_block, seen_duration),
+            Ok(_) => self.process_gossip_verified_block(
+                peer_id,
+                verified_block,
+                reprocess_tx,
+                seen_duration,
+            ),
             Err(e) => {
                 error!(
                     self.log,
@@ -399,24 +452,34 @@ impl<T: BeaconChainTypes> Worker<T> {
         self,
         peer_id: PeerId,
         verified_block: GossipVerifiedBlock<T>,
+        reprocess_tx: mpsc::Sender<ReprocessQueueMessage<T>>,
         // This value is not used presently, but it might come in handy for debugging.
         _seen_duration: Duration,
     ) {
         let block = Box::new(verified_block.block.clone());
 
         match self.chain.process_block(verified_block) {
-            Ok(_block_root) => {
+            Ok(block_root) => {
                 metrics::inc_counter(&metrics::BEACON_PROCESSOR_GOSSIP_BLOCK_IMPORTED_TOTAL);
 
+                if reprocess_tx
+                    .try_send(ReprocessQueueMessage::BlockImported(block_root))
+                    .is_err()
+                {
+                    error!(
+                        self.log,
+                        "Failed to inform block import";
+                        "source" => "gossip",
+                        "block_root" => %block_root,
+                    )
+                };
+
                 trace!(
                     self.log,
                     "Gossipsub block processed";
                     "peer_id" => %peer_id
                 );
 
-                // The `MessageHandler` would be the place to put this, however it doesn't seem
-                // to have a reference to the `BeaconChain`. I will leave this for future
-                // works.
                 match self.chain.fork_choice() {
                     Ok(()) => trace!(
                         self.log,
@@ -627,14 +690,16 @@ impl<T: BeaconChainTypes> Worker<T> {
 
     /// Handle an error whilst verifying an `Attestation` or `SignedAggregateAndProof` from the
     /// network.
-    pub fn handle_attestation_verification_failure(
+    fn handle_attestation_verification_failure(
         &self,
         peer_id: PeerId,
         message_id: MessageId,
-        beacon_block_root: Hash256,
+        failed_att: FailedAtt<T::EthSpec>,
-        attestation_type: &str,
+        reprocess_tx: Option<mpsc::Sender<ReprocessQueueMessage<T>>>,
         error: AttnError,
     ) {
+        let beacon_block_root = failed_att.root();
+        let attestation_type = failed_att.kind();
         metrics::register_attestation_error(&error);
         match &error {
             AttnError::FutureEpoch { .. }
@@ -796,20 +861,15 @@ impl<T: BeaconChainTypes> Worker<T> {
                 self.gossip_penalize_peer(peer_id, PeerAction::LowToleranceError);
             }
             AttnError::UnknownHeadBlock { beacon_block_root } => {
-                // Note: its a little bit unclear as to whether or not this block is unknown or
-                // just old. See:
-                //
-                // https://github.com/sigp/lighthouse/issues/1039
-
-                // TODO: Maintain this attestation and re-process once sync completes
-                // TODO: We then score based on whether we can download the block and re-process.
                 trace!(
                     self.log,
                     "Attestation for unknown block";
                     "peer_id" => %peer_id,
                     "block" => %beacon_block_root
                 );
-                // we don't know the block, get the sync manager to handle the block lookup
+                if let Some(sender) = reprocess_tx {
+                    // We don't know the block, get the sync manager to handle the block lookup, and
+                    // send the attestation to be scheduled for re-processing.
                     self.sync_tx
                         .send(SyncMessage::UnknownBlockHash(peer_id, *beacon_block_root))
                         .unwrap_or_else(|_| {
@@ -819,7 +879,58 @@ impl<T: BeaconChainTypes> Worker<T> {
                                 "msg" => "UnknownBlockHash"
                             )
                         });
-                self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Ignore);
+                    let msg = match failed_att {
+                        FailedAtt::Aggregate {
+                            attestation,
+                            seen_timestamp,
+                        } => {
+                            metrics::inc_counter(
+                                &metrics::BEACON_PROCESSOR_AGGREGATED_ATTESTATION_REQUEUED_TOTAL,
+                            );
+                            ReprocessQueueMessage::UnknownBlockAggregate(QueuedAggregate {
+                                peer_id,
+                                message_id,
+                                attestation,
+                                seen_timestamp,
+                            })
+                        }
+                        FailedAtt::Unaggregate {
+                            attestation,
+                            subnet_id,
+                            should_import,
+                            seen_timestamp,
+                        } => {
+                            metrics::inc_counter(
+                                &metrics::BEACON_PROCESSOR_UNAGGREGATED_ATTESTATION_REQUEUED_TOTAL,
+                            );
+                            ReprocessQueueMessage::UnknownBlockUnaggregate(QueuedUnaggregate {
+                                peer_id,
+                                message_id,
+                                attestation,
+                                subnet_id,
+                                should_import,
+                                seen_timestamp,
+                            })
+                        }
+                    };
+
+                    if sender.try_send(msg).is_err() {
+                        error!(
+                            self.log,
+                            "Failed to send attestation for re-processing";
+                        )
+                    }
+                } else {
+                    // We shouldn't make any further attempts to process this attestation.
+                    // Downscore the peer.
+                    self.gossip_penalize_peer(peer_id, PeerAction::LowToleranceError);
+                    self.propagate_validation_result(
+                        message_id,
+                        peer_id,
+                        MessageAcceptance::Ignore,
+                    );
+                }
+
                 return;
             }
             AttnError::UnknownTargetRoot(_) => {
@@ -879,7 +990,6 @@ impl<T: BeaconChainTypes> Worker<T> {
                 self.propagate_validation_result(message_id, peer_id, MessageAcceptance::Reject);
                 self.gossip_penalize_peer(peer_id, PeerAction::LowToleranceError);
             }
-
             AttnError::InvalidSubnetId { received, expected } => {
                 /*
                  * The attestation was received on an incorrect subnet id.

beacon_node/network/src/beacon_processor/worker/mod.rs

@@ -1,4 +1,4 @@
-use super::QueuedBlock;
+use super::work_reprocessing_queue::ReprocessQueueMessage;
 use crate::{service::NetworkMessage, sync::SyncMessage};
 use beacon_chain::{BeaconChain, BeaconChainTypes};
 use slog::{error, Logger};
@@ -46,5 +46,5 @@ impl<T: BeaconChainTypes> Worker<T> {
 /// Contains the necessary items for a worker to do their job.
 pub struct Toolbox<T: BeaconChainTypes> {
     pub idle_tx: mpsc::Sender<()>,
-    pub delayed_block_tx: mpsc::Sender<QueuedBlock<T>>,
+    pub work_reprocessing_tx: mpsc::Sender<ReprocessQueueMessage<T>>,
 }

beacon_node/network/src/beacon_processor/worker/sync_methods.rs

@@ -1,4 +1,4 @@
-use super::Worker;
+use super::{super::work_reprocessing_queue::ReprocessQueueMessage, Worker};
 use crate::beacon_processor::worker::FUTURE_SLOT_TOLERANCE;
 use crate::beacon_processor::BlockResultSender;
 use crate::metrics;
@@ -7,6 +7,7 @@ use crate::sync::{BatchProcessResult, ChainId};
 use beacon_chain::{BeaconChainTypes, BlockError, ChainSegmentResult};
 use eth2_libp2p::PeerId;
 use slog::{crit, debug, error, info, trace, warn};
+use tokio::sync::mpsc;
 use types::{Epoch, Hash256, SignedBeaconBlock};
 
 /// Id associated to a block processing request, either a batch or a single block.
@@ -27,6 +28,7 @@ impl<T: BeaconChainTypes> Worker<T> {
         self,
         block: SignedBeaconBlock<T::EthSpec>,
         result_tx: BlockResultSender<T::EthSpec>,
+        reprocess_tx: mpsc::Sender<ReprocessQueueMessage<T>>,
     ) {
         let slot = block.slot();
         let block_result = self.chain.process_block(block);
@@ -40,6 +42,18 @@ impl<T: BeaconChainTypes> Worker<T> {
                 "slot" => slot,
                 "hash" => %root
             );
+
+            if reprocess_tx
+                .try_send(ReprocessQueueMessage::BlockImported(*root))
+                .is_err()
+            {
+                error!(
+                    self.log,
+                    "Failed to inform block import";
+                    "source" => "rpc",
+                    "block_root" => %root,
+                )
+            };
         }
 
         if result_tx.send(block_result).is_err() {

beacon_node/network/src/metrics.rs

@@ -301,6 +301,10 @@ lazy_static! {
         "beacon_processor_unaggregated_attestation_imported_total",
         "Total number of unaggregated attestations imported to fork choice, etc."
     );
+    pub static ref BEACON_PROCESSOR_UNAGGREGATED_ATTESTATION_REQUEUED_TOTAL: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_unaggregated_attestation_requeued_total",
+        "Total number of unaggregated attestations that referenced an unknown block and were re-queued."
+    );
     // Aggregated attestations.
     pub static ref BEACON_PROCESSOR_AGGREGATED_ATTESTATION_QUEUE_TOTAL: Result<IntGauge> = try_create_int_gauge(
         "beacon_processor_aggregated_attestation_queue_total",
@@ -314,6 +318,10 @@ lazy_static! {
         "beacon_processor_aggregated_attestation_imported_total",
         "Total number of aggregated attestations imported to fork choice, etc."
     );
+    pub static ref BEACON_PROCESSOR_AGGREGATED_ATTESTATION_REQUEUED_TOTAL: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_aggregated_attestation_requeued_total",
+        "Total number of aggregated attestations that referenced an unknown block and were re-queued."
+    );
 }
 
 lazy_static! {
@@ -370,6 +378,24 @@ lazy_static! {
         "beacon_block_gossip_slot_start_delay_time",
         "Duration between when the block is received and the start of the slot it belongs to.",
     );
+
+    /*
+     * Attestation reprocessing queue metrics.
+     */
+    pub static ref BEACON_PROCESSOR_REPROCESSING_QUEUE_TOTAL: Result<IntGaugeVec> =
+        try_create_int_gauge_vec(
+        "beacon_processor_reprocessing_queue_total",
+        "Count of items in a reprocessing queue.",
+        &["type"]
+    );
+    pub static ref BEACON_PROCESSOR_REPROCESSING_QUEUE_EXPIRED_ATTESTATIONS: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_reprocessing_queue_expired_attestations",
+        "Number of queued attestations which have expired before a matching block has been found"
+    );
+    pub static ref BEACON_PROCESSOR_REPROCESSING_QUEUE_MATCHED_ATTESTATIONS: Result<IntCounter> = try_create_int_counter(
+        "beacon_processor_reprocessing_queue_matched_attestations",
+        "Number of queued attestations where as matching block has been imported"
+    );
 }
 
 pub fn register_attestation_error(error: &AttnError) {

consensus/types/src/eth_spec.rs

@@ -52,7 +52,7 @@ pub trait EthSpec: 'static + Default + Sync + Send + Clone + Debug + PartialEq +
     /*
      * Misc
      */
-    type MaxValidatorsPerCommittee: Unsigned + Clone + Sync + Send + Debug + PartialEq + Eq + Unpin;
+    type MaxValidatorsPerCommittee: Unsigned + Clone + Sync + Send + Debug + PartialEq + Eq;
     /*
      * Time parameters
      */