avoid replaying long empty slots on old votes (#1304)

* avoid replaying long empty slots on old votes

* fixup!

* fixup!

* fixup!

beacon_chain/attestation_aggregation.nim

@@ -74,6 +74,31 @@ proc aggregate_attestations*(
 
   none(AggregateAndProof)
 
+proc isValidAttestationSlot(
+    pool: AttestationPool, attestationSlot: Slot, attestationBlck: BlockRef): bool =
+  # If we allow voting for very old blocks, the state transaction below will go
+  # nuts and keep processing empty slots
+  logScope:
+    attestationSlot
+    attestationBlck = shortLog(attestationBlck)
+
+  if not (attestationBlck.slot > pool.blockPool.finalizedHead.slot):
+    debug "voting for already-finalized block"
+    return false
+
+  # we'll also cap it at 4 epochs which is somewhat arbitrary, but puts an
+  # upper bound on the processing done to validate the attestation
+  # TODO revisit with less arbitrary approach
+  if not (attestationSlot >= attestationBlck.slot):
+    debug "voting for block that didn't exist at the time"
+    return false
+
+  if not ((attestationSlot - attestationBlck.slot) <= uint64(4 * SLOTS_PER_EPOCH)):
+    debug "voting for very old block"
+    return false
+
+  true
+
 # https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/p2p-interface.md#attestation-subnets
 proc isValidAttestation*(
     pool: var AttestationPool, attestation: Attestation, current_slot: Slot,
@@ -132,6 +157,10 @@ proc isValidAttestation*(
     pool.blockPool.addMissing(attestation.data.beacon_block_root)
     return false
 
+  if not isValidAttestationSlot(pool, attestation.data.slot, attestationBlck):
+    # Not in spec - check that rewinding to the state is sane
+    return false
+
   pool.blockPool.withState(
       pool.blockPool.tmpState,
       BlockSlot(blck: attestationBlck, slot: attestation.data.slot)):
@@ -222,6 +251,10 @@ proc isValidAggregatedAttestation*(
     debug "isValidAggregatedAttestation: attestation has no or invalid aggregation bits"
     return false
 
+  if not isValidAttestationSlot(pool, aggregate.data.slot, attestationBlck):
+    # Not in spec - check that rewinding to the state is sane
+    return false
+
   # [REJECT] aggregate_and_proof.selection_proof selects the validator as an
   # aggregator for the slot -- i.e. is_aggregator(state, aggregate.data.slot,
   # aggregate.data.index, aggregate_and_proof.selection_proof) returns True.

beacon_chain/beacon_node.nim

@@ -694,6 +694,7 @@ proc installBeaconApiHandlers(rpcServer: RpcServer, node: BeaconNode) =
                                       root: Option[Eth2Digest]) -> StringOfJson:
     requireOneOf(slot, root)
     if slot.isSome:
+      # TODO sanity check slot so that it doesn't cause excessive rewinding
       let blk = node.blockPool.head.blck.atSlot(slot.get)
       node.blockPool.withState(node.blockPool.tmpState, blk):
         return jsonResult(state)