rpc: add CalculateNetworkFee RPC method

pkg/core/blockchainer/policer.go

@@ -5,4 +5,5 @@ type Policer interface {
 	GetBaseExecFee() int64
 	GetMaxVerificationGAS() int64
 	GetStoragePrice() int64
+	FeePerByte() int64
 }

pkg/rpc/client/rpc.go

@@ -28,6 +28,20 @@ import (
 
 var errNetworkNotInitialized = errors.New("RPC client network is not initialized")
 
+// CalculateNetworkFee calculates network fee for transaction. The transaction may
+// have empty witnesses for contract signers and may have only verification scripts
+// filled for standard sig/multisig signers.
+func (c *Client) CalculateNetworkFee(tx *transaction.Transaction) (int64, error) {
+	var (
+		params = request.NewRawParams(tx.Bytes())
+		resp   int64
+	)
+	if err := c.performRequest("calculatenetworkfee", params, &resp); err != nil {
+		return 0, err
+	}
+	return resp, nil
+}
+
 // GetApplicationLog returns the contract log based on the specified txid.
 func (c *Client) GetApplicationLog(hash util.Uint256, trig *trigger.Type) (*result.ApplicationLog, error) {
 	var (

pkg/rpc/server/client_test.go

@@ -66,7 +66,7 @@ func TestClient_NEP17(t *testing.T) {
 	})
 }
 
-func TestAddNetworkFee(t *testing.T) {
+func TestAddNetworkFeeCalculateNetworkFee(t *testing.T) {
 	chain, rpcSrv, httpSrv := initServerWithInMemoryChain(t)
 	defer chain.Close()
 	defer rpcSrv.Shutdown()
@@ -110,6 +110,13 @@ func TestAddNetworkFee(t *testing.T) {
 			tx.Nonce = nonce
 			nonce++
 
+			tx.Scripts = []transaction.Witness{
+				{VerificationScript: acc0.GetVerificationScript()},
+			}
+			actualCalculatedNetFee, err := c.CalculateNetworkFee(tx)
+			require.NoError(t, err)
+
+			tx.Scripts = nil
 			require.NoError(t, c.AddNetworkFee(tx, extraFee, acc0))
 			actual := tx.NetworkFee
 
@@ -118,7 +125,8 @@ func TestAddNetworkFee(t *testing.T) {
 			expected := int64(io.GetVarSize(tx))*feePerByte + cFee + extraFee
 
 			require.Equal(t, expected, actual)
-			err := chain.VerifyTx(tx)
+			require.Equal(t, expected, actualCalculatedNetFee+extraFee)
+			err = chain.VerifyTx(tx)
 			if extraFee < 0 {
 				require.Error(t, err)
 			} else {
@@ -165,6 +173,15 @@ func TestAddNetworkFee(t *testing.T) {
 			tx.Nonce = nonce
 			nonce++
 
+			tx.Scripts = []transaction.Witness{
+				{VerificationScript: acc0.GetVerificationScript()},
+				{VerificationScript: acc1.GetVerificationScript()},
+			}
+			actualCalculatedNetFee, err := c.CalculateNetworkFee(tx)
+			require.NoError(t, err)
+
+			tx.Scripts = nil
+
 			require.NoError(t, c.AddNetworkFee(tx, extraFee, acc0, acc1))
 			actual := tx.NetworkFee
 
@@ -178,7 +195,8 @@ func TestAddNetworkFee(t *testing.T) {
 			expected := int64(io.GetVarSize(tx))*feePerByte + cFee + cFeeM + extraFee
 
 			require.Equal(t, expected, actual)
-			err := chain.VerifyTx(tx)
+			require.Equal(t, expected, actualCalculatedNetFee+extraFee)
+			err = chain.VerifyTx(tx)
 			if extraFee < 0 {
 				require.Error(t, err)
 			} else {
@@ -228,9 +246,19 @@ func TestAddNetworkFee(t *testing.T) {
 						Scopes:  transaction.Global,
 					},
 				}
+				// we need to fill standard verification scripts to use CalculateNetworkFee.
+				tx.Scripts = []transaction.Witness{
+					{VerificationScript: acc0.GetVerificationScript()},
+					{},
+				}
+				actual, err := c.CalculateNetworkFee(tx)
+				require.NoError(t, err)
+				tx.Scripts = nil
+
 				require.NoError(t, c.AddNetworkFee(tx, extraFee, acc0, acc1))
 				require.NoError(t, acc0.SignTx(testchain.Network(), tx))
 				tx.Scripts = append(tx.Scripts, transaction.Witness{})
+				require.Equal(t, tx.NetworkFee, actual+extraFee)
 				err = chain.VerifyTx(tx)
 				if extraFee < 0 {
 					require.Error(t, err)

pkg/rpc/server/server.go

@@ -20,6 +20,7 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/core"
 	"github.com/nspcc-dev/neo-go/pkg/core/block"
 	"github.com/nspcc-dev/neo-go/pkg/core/blockchainer"
+	"github.com/nspcc-dev/neo-go/pkg/core/fee"
 	"github.com/nspcc-dev/neo-go/pkg/core/mpt"
 	"github.com/nspcc-dev/neo-go/pkg/core/state"
 	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
@@ -93,6 +94,7 @@ const (
 )
 
 var rpcHandlers = map[string]func(*Server, request.Params) (interface{}, *response.Error){
+	"calculatenetworkfee":    (*Server).calculateNetworkFee,
 	"getapplicationlog":      (*Server).getApplicationLog,
 	"getbestblockhash":       (*Server).getBestBlockHash,
 	"getblock":               (*Server).getBlock,
@@ -549,6 +551,83 @@ func (s *Server) validateAddress(reqParams request.Params) (interface{}, *respon
 	return validateAddress(param.Value), nil
 }
 
+// calculateNetworkFee calculates network fee for the transaction.
+func (s *Server) calculateNetworkFee(reqParams request.Params) (interface{}, *response.Error) {
+	if len(reqParams) < 1 {
+		return 0, response.ErrInvalidParams
+	}
+	byteTx, err := reqParams[0].GetBytesBase64()
+	if err != nil {
+		return 0, response.WrapErrorWithData(response.ErrInvalidParams, err)
+	}
+	tx, err := transaction.NewTransactionFromBytes(byteTx)
+	if err != nil {
+		return 0, response.WrapErrorWithData(response.ErrInvalidParams, err)
+	}
+	hashablePart, err := tx.EncodeHashableFields()
+	if err != nil {
+		return 0, response.WrapErrorWithData(response.ErrInvalidParams, fmt.Errorf("failed to compute tx size: %w", err))
+	}
+	size := len(hashablePart) + io.GetVarSize(len(tx.Signers))
+	var (
+		ef     int64
+		netFee int64
+	)
+	for i, signer := range tx.Signers {
+		var verificationScript []byte
+		for _, w := range tx.Scripts {
+			if w.VerificationScript != nil && hash.Hash160(w.VerificationScript).Equals(signer.Account) {
+				// then it's a standard sig/multisig witness
+				verificationScript = w.VerificationScript
+				break
+			}
+		}
+		if verificationScript == nil { // then it still might be a contract-based verification
+			verificationErr := fmt.Sprintf("contract verification for signer #%d failed", i)
+			res, respErr := s.runScriptInVM(trigger.Verification, []byte{}, signer.Account, tx)
+			if respErr != nil && errors.Is(respErr.Cause, core.ErrUnknownVerificationContract) {
+				// it's neither a contract-based verification script nor a standard witness attached to
+				// the tx, so the user did not provide enough data to calculate fee for that witness =>
+				// it's a user error
+				return 0, response.NewRPCError(verificationErr, respErr.Cause.Error(), respErr.Cause)
+			}
+			if respErr != nil {
+				return 0, respErr
+			}
+			if res.State != "HALT" {
+				cause := fmt.Errorf("invalid VM state %s due to an error: %s", res.State, res.FaultException)
+				return 0, response.NewRPCError(verificationErr, cause.Error(), cause)
+			}
+			if l := len(res.Stack); l != 1 {
+				cause := fmt.Errorf("result stack length should be equal to 1, got %d", l)
+				return 0, response.NewRPCError(verificationErr, cause.Error(), cause)
+			}
+			isOK, err := res.Stack[0].TryBool()
+			if err != nil {
+				cause := fmt.Errorf("resulting stackitem cannot be converted to Boolean: %w", err)
+				return 0, response.NewRPCError(verificationErr, cause.Error(), cause)
+			}
+			if !isOK {
+				cause := errors.New("`verify` method returned `false` on stack")
+				return 0, response.NewRPCError(verificationErr, cause.Error(), cause)
+			}
+			netFee += res.GasConsumed
+			size += io.GetVarSize([]byte{}) * 2 // both scripts are empty
+			continue
+		}
+
+		if ef == 0 {
+			ef = s.chain.GetPolicer().GetBaseExecFee()
+		}
+		fee, sizeDelta := fee.Calculate(ef, verificationScript)
+		netFee += fee
+		size += sizeDelta
+	}
+	fee := s.chain.GetPolicer().FeePerByte()
+	netFee += int64(size) * fee
+	return netFee, nil
+}
+
 // getApplicationLog returns the contract log based on the specified txid or blockid.
 func (s *Server) getApplicationLog(reqParams request.Params) (interface{}, *response.Error) {
 	hash, err := reqParams.Value(0).GetUint256()