Refuse to sign sync committee messages when head is optimistic (#3191)

## Issue Addressed

Resolves #3151 

## Proposed Changes

When fetching duties for sync committee contributions, check the value of `execution_optimistic` of the head block from the BN and refuse to sign any sync committee messages `if execution_optimistic == true`.

## Additional Info
- Is backwards compatible with older BNs
- Finding a way to add test coverage for this would be prudent. Open to suggestions.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1380,10 +1380,41 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     pub fn get_aggregated_sync_committee_contribution(
         &self,
         sync_contribution_data: &SyncContributionData,
-    ) -> Option<SyncCommitteeContribution<T::EthSpec>> {
-        self.naive_sync_aggregation_pool
+    ) -> Result<Option<SyncCommitteeContribution<T::EthSpec>>, Error> {
+        if let Some(contribution) = self
+            .naive_sync_aggregation_pool
             .read()
             .get(sync_contribution_data)
+        {
+            self.filter_optimistic_sync_committee_contribution(contribution)
+                .map(Option::Some)
+        } else {
+            Ok(None)
+        }
+    }
+
+    fn filter_optimistic_sync_committee_contribution(
+        &self,
+        contribution: SyncCommitteeContribution<T::EthSpec>,
+    ) -> Result<SyncCommitteeContribution<T::EthSpec>, Error> {
+        let beacon_block_root = contribution.beacon_block_root;
+        match self
+            .canonical_head
+            .fork_choice_read_lock()
+            .get_block_execution_status(&beacon_block_root)
+        {
+            // The contribution references a block that is not in fork choice, it must be
+            // pre-finalization.
+            None => Err(Error::SyncContributionDataReferencesFinalizedBlock { beacon_block_root }),
+            // The contribution references a fully valid `beacon_block_root`.
+            Some(execution_status) if execution_status.is_valid_or_irrelevant() => Ok(contribution),
+            // The contribution references a block that has not been verified by an EL (i.e. it
+            // is optimistic or invalid). Don't return the block, return an error instead.
+            Some(execution_status) => Err(Error::HeadBlockNotFullyVerified {
+                beacon_block_root,
+                execution_status,
+            }),
+        }
     }
 
     /// Produce an unaggregated `Attestation` that is valid for the given `slot` and `index`.

beacon_node/beacon_chain/src/errors.rs

@@ -184,6 +184,9 @@ pub enum BeaconChainError {
     CannotAttestToFinalizedBlock {
         beacon_block_root: Hash256,
     },
+    SyncContributionDataReferencesFinalizedBlock {
+        beacon_block_root: Hash256,
+    },
     RuntimeShutdown,
     TokioJoin(tokio::task::JoinError),
     ProcessInvalidExecutionPayload(JoinError),

beacon_node/http_api/src/lib.rs

@@ -2319,6 +2319,12 @@ pub fn serve<T: BeaconChainTypes>(
                 blocking_json_task(move || {
                     chain
                         .get_aggregated_sync_committee_contribution(&sync_committee_data)
+                        .map_err(|e| {
+                            warp_utils::reject::custom_bad_request(format!(
+                                "unable to fetch sync contribution: {:?}",
+                                e
+                            ))
+                        })?
                         .map(api_types::GenericResponse::from)
                         .ok_or_else(|| {
                             warp_utils::reject::custom_not_found(

validator_client/src/sync_committee_service.rs

@@ -4,7 +4,7 @@ use environment::RuntimeContext;
 use eth2::types::BlockId;
 use futures::future::join_all;
 use futures::future::FutureExt;
-use slog::{crit, debug, error, info, trace};
+use slog::{crit, debug, error, info, trace, warn};
 use slot_clock::SlotClock;
 use std::collections::HashMap;
 use std::ops::Deref;
@@ -174,17 +174,39 @@ impl<T: SlotClock + 'static, E: EthSpec> SyncCommitteeService<T, E> {
             return Ok(());
         }
 
-        // Fetch block root for `SyncCommitteeContribution`.
-        let block_root = self
+        // Fetch `block_root` and `execution_optimistic` for `SyncCommitteeContribution`.
+        let response = self
             .beacon_nodes
             .first_success(RequireSynced::Yes, |beacon_node| async move {
                 beacon_node.get_beacon_blocks_root(BlockId::Head).await
             })
             .await
             .map_err(|e| e.to_string())?
-            .ok_or_else(|| format!("No block root found for slot {}", slot))?
-            .data
-            .root;
+            .ok_or_else(|| format!("No block root found for slot {}", slot))?;
+
+        let block_root = response.data.root;
+        if let Some(execution_optimistic) = response.execution_optimistic {
+            if execution_optimistic {
+                warn!(
+                    log,
+                    "Refusing to sign sync committee messages for optimistic head block";
+                    "slot" => slot,
+                );
+                return Ok(());
+            }
+        } else if let Some(bellatrix_fork_epoch) = self.duties_service.spec.bellatrix_fork_epoch {
+            // If the slot is post Bellatrix, do not sign messages when we cannot verify the
+            // optimistic status of the head block.
+            if slot.epoch(E::slots_per_epoch()) > bellatrix_fork_epoch {
+                warn!(
+                    log,
+                    "Refusing to sign sync committee messages for a head block with an unknown \
+                    optimistic status";
+                    "slot" => slot,
+                );
+                return Ok(());
+            }
+        }
 
         // Spawn one task to publish all of the sync committee signatures.
         let validator_duties = slot_duties.duties;