Author a sibling block in case best block's slot is same as current slot

lib/babe/babe.go

@@ -497,6 +497,25 @@ func (b *Service) handleSlot(epoch, slotNum uint64,
 		return err
 	}
 
+	bestBlockSlotNum, err := b.blockState.GetSlotForBlock(parentHeader.Hash())
+	if err != nil {
+		return fmt.Errorf("could not get slot for block %s: %w", parentHeader.Hash(), err)
+	}
+
+	if bestBlockSlotNum > slotNum {
+		return errLaggingSlot
+	}
+
+	if bestBlockSlotNum == slotNum {
+		// pick parent of best block instead to handle slot
+		newParentHeader, err := b.blockState.GetHeader(parentHeader.ParentHash)
+		if err != nil {
+			return fmt.Errorf("could not get header for hash %s: %w", parentHeader.ParentHash, err)
+		}
+
+		parentHeader = newParentHeader
+	}
+
 	if parentHeader == nil {
 		return errNilParentHeader
 	}

lib/babe/errors.go

@@ -78,6 +78,7 @@ var (
 	errInvalidSlotTechnique       = errors.New("invalid slot claiming technique")
 	errNoBABEAuthorityKeyProvided = errors.New("cannot create BABE service as authority; no keypair provided")
 	errLastDigestItemNotSeal      = errors.New("last digest item is not seal")
+	errLaggingSlot                = errors.New("cannot claim slot, current slot is smaller than slot of best block")
 
 	other         Other
 	invalidCustom InvalidCustom