ReceiveBlock: Only retrieve head block from DB if necessary (#4506)

* Only retrieve head block from DB if necessary

* remove redundant comment

Co-authored-by: prylabs-bulldozer[bot] <58059840+prylabs-bulldozer[bot]@users.noreply.github.com>

beacon-chain/blockchain/receive_block.go

@@ -85,16 +85,19 @@ func (s *Service) ReceiveBlockNoPubsub(ctx context.Context, block *ethpb.SignedB
 	if err != nil {
 		return errors.Wrap(err, "could not get head from fork choice service")
 	}
-	signedHeadBlock, err := s.beaconDB.Block(ctx, bytesutil.ToBytes32(headRoot))
-	if err != nil {
-		return errors.Wrap(err, "could not compute state from block head")
-	}
-	if signedHeadBlock == nil || signedHeadBlock.Block == nil {
-		return errors.New("nil head block")
-	}
 
 	// Only save head if it's different than the current head.
 	if !bytes.Equal(headRoot, s.HeadRoot()) {
+		signedHeadBlock, err := s.beaconDB.Block(ctx, bytesutil.ToBytes32(headRoot))
+		if err != nil {
+			return errors.Wrap(err, "could not compute state from block head")
+		}
+		if signedHeadBlock == nil || signedHeadBlock.Block == nil {
+			return errors.New("nil head block")
+		}
+
+		logCompetingBlock(root[:], blockCopy.Block.Slot, headRoot, signedHeadBlock.Block.Slot)
+
 		if err := s.saveHead(ctx, signedHeadBlock, bytesutil.ToBytes32(headRoot)); err != nil {
 			return errors.Wrap(err, "could not save head")
 		}
@@ -118,9 +121,6 @@ func (s *Service) ReceiveBlockNoPubsub(ctx context.Context, block *ethpb.SignedB
 	// Reports on block and fork choice metrics.
 	s.reportSlotMetrics(blockCopy.Block.Slot)
 
-	// Log if block is a competing block.
-	isCompetingBlock(root[:], blockCopy.Block.Slot, headRoot, signedHeadBlock.Block.Slot)
-
 	// Log state transition data.
 	logStateTransitionData(blockCopy.Block)
 
@@ -244,14 +244,12 @@ func (s *Service) ReceiveBlockNoVerify(ctx context.Context, block *ethpb.SignedB
 }
 
 // This checks if the block is from a competing chain, emits warning and updates metrics.
-func isCompetingBlock(root []byte, slot uint64, headRoot []byte, headSlot uint64) {
-	if !bytes.Equal(root[:], headRoot) {
-		log.WithFields(logrus.Fields{
-			"blkSlot":  slot,
-			"blkRoot":  hex.EncodeToString(root[:]),
-			"headSlot": headSlot,
-			"headRoot": hex.EncodeToString(headRoot),
-		}).Warn("Calculated head diffs from new block")
-		competingBlks.Inc()
-	}
+func logCompetingBlock(root []byte, slot uint64, headRoot []byte, headSlot uint64) {
+	log.WithFields(logrus.Fields{
+		"blkSlot":  slot,
+		"blkRoot":  hex.EncodeToString(root[:]),
+		"headSlot": headSlot,
+		"headRoot": hex.EncodeToString(headRoot),
+	}).Warn("Calculated head diffs from new block")
+	competingBlks.Inc()
 }