Fix overly restrictive check in fork choice.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -60,7 +60,15 @@ pub enum BlockProcessingOutcome {
 #[derive(Debug, PartialEq)]
 pub enum AttestationProcessingOutcome {
     Processed,
-    UnknownHeadBlock { beacon_block_root: Hash256 },
+    UnknownHeadBlock {
+        beacon_block_root: Hash256,
+    },
+    /// The attestation is attesting to a state that is later than itself. (Viz., attesting to the
+    /// future).
+    AttestsToFutureState {
+        state: Slot,
+        attestation: Slot,
+    },
     Invalid(AttestationValidationError),
 }
 
@@ -582,12 +590,25 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                     per_slot_processing(&mut state, &self.spec)?;
                 }
 
+                let attestation_slot = state.get_attestation_data_slot(&attestation.data)?;
+
+                // Reject any attestation where the `state` loaded from `data.beacon_block_root`
+                // has a higher slot than the attestation.
+                //
+                // Permitting this would allow for attesters to vote on _future_ slots.
+                if attestation_slot > state.slot {
+                    Ok(AttestationProcessingOutcome::AttestsToFutureState {
+                        state: state.slot,
+                        attestation: attestation_slot,
+                    })
+                } else {
                     self.process_attestation_for_state_and_block(
                         attestation,
                         &state,
                         &attestation_head_block,
                     )
                 }
+            }
         } else {
             // Reject any block where we have not processed `attestation.data.beacon_block_root`.
             //

eth2/state_processing/src/per_block_processing/errors.rs

@@ -136,9 +136,6 @@ pub enum AttestationInvalid {
         delay: u64,
         attestation: Slot,
     },
-    /// The attestation is attesting to a state that is later than itself. (Viz., attesting to the
-    /// future).
-    AttestsToFutureState { state: Slot, attestation: Slot },
     /// Attestation slot is too far in the past to be included in a block.
     IncludedTooLate { state: Slot, attestation: Slot },
     /// Attestation target epoch does not match the current or previous epoch.

eth2/state_processing/src/per_block_processing/verify_attestation.rs

@@ -62,17 +62,6 @@ pub fn verify_attestation_for_state<T: EthSpec>(
         Invalid::BadShard
     );
 
-    let attestation_slot = state.get_attestation_data_slot(&data)?;
-
-    // An attestation cannot attest to a state that is later than itself.
-    verify!(
-        attestation_slot <= state.slot,
-        Invalid::AttestsToFutureState {
-            state: state.slot,
-            attestation: attestation_slot
-        }
-    );
-
     // Verify the Casper FFG vote and crosslink data.
     let parent_crosslink = verify_casper_ffg_vote(attestation, state)?;