Optional handlers (ChainSafe#643)

* initial if statements for optional handlers

* Squash commits

* clean up bulkiness of handler contract check at chain initialization

Author: vezenovm

chains/ethereum/chain.go

@@ -29,6 +29,7 @@ import (
 	erc721Handler "github.com/ChainSafe/ChainBridge/bindings/ERC721Handler"
 	"github.com/ChainSafe/ChainBridge/bindings/GenericHandler"
 	connection "github.com/ChainSafe/ChainBridge/connections/ethereum"
+	utils "github.com/ChainSafe/ChainBridge/shared/ethereum"
 	"github.com/ChainSafe/chainbridge-utils/blockstore"
 	"github.com/ChainSafe/chainbridge-utils/core"
 	"github.com/ChainSafe/chainbridge-utils/crypto/secp256k1"
@@ -116,13 +117,19 @@ func InitializeChain(chainCfg *core.ChainConfig, logger log15.Logger, sysErr cha
 	if err != nil {
 		return nil, err
 	}
-	err = conn.EnsureHasBytecode(cfg.erc20HandlerContract)
-	if err != nil {
-		return nil, err
+
+	if cfg.erc20HandlerContract != utils.ZeroAddress {
+		err = conn.EnsureHasBytecode(cfg.erc20HandlerContract)
+		if err != nil {
+			return nil, err
+		}
 	}
-	err = conn.EnsureHasBytecode(cfg.genericHandlerContract)
-	if err != nil {
-		return nil, err
+
+	if cfg.genericHandlerContract != utils.ZeroAddress {
+		err = conn.EnsureHasBytecode(cfg.genericHandlerContract)
+		if err != nil {
+			return nil, err
+		}
 	}
 
 	bridgeContract, err := bridge.NewBridge(cfg.bridgeContract, conn.Client())

chains/ethereum/config.go

@@ -91,14 +91,20 @@ func parseChainConfig(chainCfg *core.ChainConfig) (*Config, error) {
 		return nil, fmt.Errorf("must provide opts.bridge field for ethereum config")
 	}
 
-	config.erc20HandlerContract = common.HexToAddress(chainCfg.Opts[Erc20HandlerOpt])
-	delete(chainCfg.Opts, Erc20HandlerOpt)
+	if contract, ok := chainCfg.Opts[Erc20HandlerOpt]; ok {
+		config.erc20HandlerContract = common.HexToAddress(contract)
+		delete(chainCfg.Opts, Erc20HandlerOpt)
+	}
 
-	config.erc721HandlerContract = common.HexToAddress(chainCfg.Opts[Erc721HandlerOpt])
-	delete(chainCfg.Opts, Erc721HandlerOpt)
+	if contract, ok := chainCfg.Opts[Erc721HandlerOpt]; ok {
+		config.erc721HandlerContract = common.HexToAddress(contract)
+		delete(chainCfg.Opts, Erc721HandlerOpt)
+	}
 
-	config.genericHandlerContract = common.HexToAddress(chainCfg.Opts[GenericHandlerOpt])
-	delete(chainCfg.Opts, GenericHandlerOpt)
+	if contract, ok := chainCfg.Opts[GenericHandlerOpt]; ok {
+		config.genericHandlerContract = common.HexToAddress(contract)
+		delete(chainCfg.Opts, GenericHandlerOpt)
+	}
 
 	if gasPrice, ok := chainCfg.Opts[MaxGasPriceOpt]; ok {
 		price := big.NewInt(0)