core: transition db now also returns the required gas amount

Exposes some core methods to transition and compute new state
information and adds an additional return value to the transition db
method to fetch required gas for that particular message (excluding gas
refunds from any SSTORE[X] = 0 and SUICIDE.

Fixes #2395

core/state_transition.go

@@ -104,8 +104,9 @@ func IntrinsicGas(data []byte, contractCreation, homestead bool) *big.Int {
 	return igas
 }
 
-func ApplyMessage(env vm.Environment, msg Message, gp *GasPool) ([]byte, *big.Int, error) {
-	var st = StateTransition{
+// NewStateTransition initialises and returns a new state transition object.
+func NewStateTransition(env vm.Environment, msg Message, gp *GasPool) *StateTransition {
+	return &StateTransition{
 		gp:         gp,
 		env:        env,
 		msg:        msg,
@@ -116,7 +117,20 @@ func ApplyMessage(env vm.Environment, msg Message, gp *GasPool) ([]byte, *big.In
 		data:       msg.Data(),
 		state:      env.Db(),
 	}
-	return st.transitionDb()
+}
+
+// ApplyMessage computes the new state by applying the given message
+// against the old state within the environment.
+//
+// ApplyMessage returns the bytes returned by any EVM execution (if it took place),
+// the gas used (which includes gas refunds) and an error if it failed. An error always
+// indicates a core error meaning that the message would always fail for that particular
+// state and would never be accepted within a block.
+func ApplyMessage(env vm.Environment, msg Message, gp *GasPool) ([]byte, *big.Int, error) {
+	st := NewStateTransition(env, msg, gp)
+
+	ret, _, gasUsed, err := st.TransitionDb()
+	return ret, gasUsed, err
 }
 
 func (self *StateTransition) from() (vm.Account, error) {
@@ -209,7 +223,8 @@ func (self *StateTransition) preCheck() (err error) {
 	return nil
 }
 
-func (self *StateTransition) transitionDb() (ret []byte, usedGas *big.Int, err error) {
+// TransitionDb will move the state by applying the message against the given environment.
+func (self *StateTransition) TransitionDb() (ret []byte, requiredGas, usedGas *big.Int, err error) {
 	if err = self.preCheck(); err != nil {
 		return
 	}
@@ -220,7 +235,7 @@ func (self *StateTransition) transitionDb() (ret []byte, usedGas *big.Int, err e
 	contractCreation := MessageCreatesContract(msg)
 	// Pay intrinsic gas
 	if err = self.useGas(IntrinsicGas(self.data, contractCreation, homestead)); err != nil {
-		return nil, nil, InvalidTxError(err)
+		return nil, nil, nil, InvalidTxError(err)
 	}
 
 	vmenv := self.env
@@ -245,7 +260,7 @@ func (self *StateTransition) transitionDb() (ret []byte, usedGas *big.Int, err e
 	}
 
 	if err != nil && IsValueTransferErr(err) {
-		return nil, nil, InvalidTxError(err)
+		return nil, nil, nil, InvalidTxError(err)
 	}
 
 	// We aren't interested in errors here. Errors returned by the VM are non-consensus errors and therefor shouldn't bubble up
@@ -253,10 +268,12 @@ func (self *StateTransition) transitionDb() (ret []byte, usedGas *big.Int, err e
 		err = nil
 	}
 
+	requiredGas = new(big.Int).Set(self.gasUsed())
+
 	self.refundGas()
 	self.state.AddBalance(self.env.Coinbase(), new(big.Int).Mul(self.gasUsed(), self.gasPrice))
 
-	return ret, self.gasUsed(), err
+	return ret, requiredGas, self.gasUsed(), err
 }
 
 func (self *StateTransition) refundGas() {

eth/api.go

@@ -674,11 +674,11 @@ func (s *PublicBlockChainAPI) doCall(args CallArgs, blockNr rpc.BlockNumber) (st
 	vmenv := core.NewEnv(stateDb, s.config, s.bc, msg, block.Header(), s.config.VmConfig)
 	gp := new(core.GasPool).AddGas(common.MaxBig)
 
-	res, gas, err := core.ApplyMessage(vmenv, msg, gp)
+	res, requiredGas, _, err := core.NewStateTransition(vmenv, msg, gp).TransitionDb()
 	if len(res) == 0 { // backwards compatibility
-		return "0x", gas, err
+		return "0x", requiredGas, err
 	}
-	return common.ToHex(res), gas, err
+	return common.ToHex(res), requiredGas, err
 }
 
 // Call executes the given transaction on the state for the given block number.