core: minor cleanup in stateDB and Call (#96)

* core/vm/runtime: call OnTxStart before prepare state DB in Call

* core/state: simplify state selfdestruct

Author: sonhv0212

core/state/statedb.go

@@ -493,13 +493,13 @@ func (s *StateDB) SelfDestruct(addr common.Address) bool {
 	s.journal.append(selfDestructChange{
 		account:     &addr,
 		prev:        stateObject.selfDestructed,
-		prevbalance: new(big.Int).Set(stateObject.Balance()),
+		prevbalance: prev,
 	})
 	if s.logger != nil && s.logger.OnBalanceChange != nil && prev.Sign() > 0 {
 		s.logger.OnBalanceChange(addr, prev, n, tracing.BalanceDecreaseSelfdestruct)
 	}
 	stateObject.markSelfdestructed()
-	stateObject.data.Balance = new(big.Int)
+	stateObject.data.Balance = n
 
 	return true
 }

core/vm/runtime/runtime.go

@@ -194,13 +194,13 @@ func Call(address common.Address, input []byte, cfg *Config) ([]byte, uint64, er
 		statedb = cfg.State
 		rules   = cfg.ChainConfig.Rules(vmenv.Context.BlockNumber)
 	)
+	if cfg.EVMConfig.Tracer != nil && cfg.EVMConfig.Tracer.OnTxStart != nil {
+		cfg.EVMConfig.Tracer.OnTxStart(vmenv.GetVMContext(), types.NewTx(&types.LegacyTx{To: &address, Data: input, Value: cfg.Value, Gas: cfg.GasLimit}), cfg.Origin)
+	}
 	// Execute the preparatory steps for state transition which includes:
 	// - prepare accessList(post-berlin)
 	// - reset transient storage(eip 1153)
 	statedb.Prepare(rules, cfg.Origin, cfg.Coinbase, &address, vm.ActivePrecompiles(rules), nil)
-	if cfg.EVMConfig.Tracer != nil && cfg.EVMConfig.Tracer.OnTxStart != nil {
-		cfg.EVMConfig.Tracer.OnTxStart(vmenv.GetVMContext(), types.NewTx(&types.LegacyTx{To: &address, Data: input, Value: cfg.Value, Gas: cfg.GasLimit}), cfg.Origin)
-	}
 
 	// Call the code with the given configuration.
 	ret, leftOverGas, err := vmenv.Call(