Fix IncorrectAttestationSource error in attn. simulator (#5048)

* Don't produce attestations when syncing

* Handle `IncorrectAttestationSource`

beacon_node/beacon_chain/src/attestation_simulator.rs

@@ -4,7 +4,11 @@ use slot_clock::SlotClock;
 use std::sync::Arc;
 use task_executor::TaskExecutor;
 use tokio::time::sleep;
-use types::Slot;
+use types::{EthSpec, Slot};
+
+/// Don't run the attestation simulator if the head slot is this many epochs
+/// behind the wall-clock slot.
+const SYNCING_TOLERANCE_EPOCHS: u64 = 2;
 
 /// Spawns a routine which produces an unaggregated attestation at every slot.
 ///
@@ -58,33 +62,43 @@ async fn attestation_simulator_service<T: BeaconChainTypes>(
 }
 
 pub fn produce_unaggregated_attestation<T: BeaconChainTypes>(
-    inner_chain: Arc<BeaconChain<T>>,
+    chain: Arc<BeaconChain<T>>,
     current_slot: Slot,
 ) {
+    // Don't run the attestation simulator when the head slot is far behind the
+    // wall-clock slot.
+    //
+    // This helps prevent the simulator from becoming a burden by computing
+    // committees from old states.
+    let syncing_tolerance_slots = SYNCING_TOLERANCE_EPOCHS * T::EthSpec::slots_per_epoch();
+    if chain.best_slot() + syncing_tolerance_slots < current_slot {
+        return;
+    }
+
     // Since attestations for different committees are practically identical (apart from the committee index field)
     // Committee 0 is guaranteed to exist. That means there's no need to load the committee.
     let beacon_committee_index = 0;
 
     // Store the unaggregated attestation in the validator monitor for later processing
-    match inner_chain.produce_unaggregated_attestation(current_slot, beacon_committee_index) {
+    match chain.produce_unaggregated_attestation(current_slot, beacon_committee_index) {
         Ok(unaggregated_attestation) => {
             let data = &unaggregated_attestation.data;
 
             debug!(
-                inner_chain.log,
+                chain.log,
                 "Produce unagg. attestation";
                 "attestation_source" => data.source.root.to_string(),
                 "attestation_target" => data.target.root.to_string(),
             );
 
-            inner_chain
+            chain
                 .validator_monitor
                 .write()
                 .set_unaggregated_attestation(unaggregated_attestation);
         }
         Err(e) => {
             debug!(
-                inner_chain.log,
+                chain.log,
                 "Failed to simulate attestation";
                 "error" => ?e
             );