Fixes race condition at genesis (#5016)

* fixes race condition at genesis

beacon-chain/blockchain/receive_attestation.go

@@ -73,10 +73,11 @@ func (s *Service) IsValidAttestation(ctx context.Context, att *ethpb.Attestation
 }
 
 // This processes attestations from the attestation pool to account for validator votes and fork choice.
-func (s *Service) processAttestation() {
+func (s *Service) processAttestation(subscribedToStateEvents chan struct{}) {
 	// Wait for state to be initialized.
 	stateChannel := make(chan *feed.Event, 1)
 	stateSub := s.stateNotifier.StateFeed().Subscribe(stateChannel)
+	subscribedToStateEvents <- struct{}{}
 	<-stateChannel
 	stateSub.Unsubscribe()
 

beacon-chain/blockchain/service.go

@@ -137,6 +137,9 @@ func (s *Service) Start() {
 		}
 	}
 
+	// Make sure that attestation processor is subscribed and ready for state initializing event.
+	attestationProcessorSubscribed := make(chan struct{}, 1)
+
 	// If the chain has already been initialized, simply start the block processing routine.
 	if beaconState != nil {
 		log.Info("Blockchain data already exists in DB, initializing...")
@@ -183,6 +186,7 @@ func (s *Service) Start() {
 			stateChannel := make(chan *feed.Event, 1)
 			stateSub := s.stateNotifier.StateFeed().Subscribe(stateChannel)
 			defer stateSub.Unsubscribe()
+			<-attestationProcessorSubscribed
 			for {
 				select {
 				case event := <-stateChannel:
@@ -203,7 +207,7 @@ func (s *Service) Start() {
 		}()
 	}
 
-	go s.processAttestation()
+	go s.processAttestation(attestationProcessorSubscribed)
 }
 
 // processChainStartTime initializes a series of deposits from the ChainStart deposits in the eth1