[rpc] Stub out estimate gas cost (#2162)

* [rpc] Stub out estimate gas cost

* Fix estimate gas

* Fix different block numbers

* Add to apiv2

Co-authored-by: flicker-harmony <52401354+flicker-harmony@users.noreply.github.com>

internal/hmyapi/apiv1/blockchain.go

@@ -18,6 +18,7 @@ import (
 	"github.com/harmony-one/harmony/core/vm"
 	internal_bls "github.com/harmony-one/harmony/crypto/bls"
 	internal_common "github.com/harmony-one/harmony/internal/common"
+	"github.com/harmony-one/harmony/internal/params"
 	"github.com/harmony-one/harmony/internal/utils"
 	"github.com/harmony-one/harmony/shard"
 	"github.com/harmony-one/harmony/staking/network"
@@ -635,6 +636,69 @@ func (s *PublicBlockChainAPI) GetDelegationByDelegatorAndValidator(ctx context.C
 	return nil, nil
 }
 
+// doEstimateGas ..
+func doEstimateGas(ctx context.Context, b Backend, args CallArgs, gasCap *big.Int) (hexutil.Uint64, error) {
+	// Binary search the gas requirement, as it may be higher than the amount used
+	var (
+		lo  uint64 = params.TxGas - 1
+		hi  uint64
+		cap uint64
+	)
+	blockNum := rpc.LatestBlockNumber
+	if args.Gas != nil && uint64(*args.Gas) >= params.TxGas {
+		hi = uint64(*args.Gas)
+	} else {
+		// Retrieve the block to act as the gas ceiling
+		block, err := b.BlockByNumber(ctx, blockNum)
+		if err != nil {
+			return 0, err
+		}
+		hi = block.GasLimit()
+	}
+	if gasCap != nil && hi > gasCap.Uint64() {
+		// log.Warn("Caller gas above allowance, capping", "requested", hi, "cap", gasCap)
+		hi = gasCap.Uint64()
+	}
+	cap = hi
+
+	// Use zero-address if none other is available
+	if args.From == nil {
+		args.From = &common.Address{}
+	}
+	// Create a helper to check if a gas allowance results in an executable transaction
+	executable := func(gas uint64) bool {
+		args.Gas = (*hexutil.Uint64)(&gas)
+
+		_, _, failed, err := doCall(ctx, b, args, blockNum, vm.Config{}, 0, big.NewInt(int64(cap)))
+		if err != nil || failed {
+			return false
+		}
+		return true
+	}
+	// Execute the binary search and hone in on an executable gas limit
+	for lo+1 < hi {
+		mid := (hi + lo) / 2
+		if !executable(mid) {
+			lo = mid
+		} else {
+			hi = mid
+		}
+	}
+	// Reject the transaction as invalid if it still fails at the highest allowance
+	if hi == cap {
+		if !executable(hi) {
+			return 0, fmt.Errorf("gas required exceeds allowance (%d) or always failing transaction", cap)
+		}
+	}
+	return hexutil.Uint64(hi), nil
+}
+
+// EstimateGas returns an estimate of the amount of gas needed to execute the
+// given transaction against the current pending block.
+func (s *PublicBlockChainAPI) EstimateGas(ctx context.Context, args CallArgs) (hexutil.Uint64, error) {
+	return doEstimateGas(ctx, s.b, args, nil)
+}
+
 // GetCurrentUtilityMetrics ..
 func (s *PublicBlockChainAPI) GetCurrentUtilityMetrics() (*network.UtilityMetric, error) {
 	if s.b.GetShardID() == shard.BeaconChainShardID {

internal/hmyapi/apiv2/blockchain.go

@@ -18,6 +18,7 @@ import (
 	"github.com/harmony-one/harmony/core/vm"
 	internal_bls "github.com/harmony-one/harmony/crypto/bls"
 	internal_common "github.com/harmony-one/harmony/internal/common"
+	"github.com/harmony-one/harmony/internal/params"
 	"github.com/harmony-one/harmony/internal/utils"
 	"github.com/harmony-one/harmony/shard"
 	"github.com/harmony-one/harmony/staking/network"
@@ -597,6 +598,69 @@ func (s *PublicBlockChainAPI) GetDelegationByDelegatorAndValidator(ctx context.C
 	return nil, nil
 }
 
+// doEstimateGas ..
+func doEstimateGas(ctx context.Context, b Backend, args CallArgs, gasCap *big.Int) (hexutil.Uint64, error) {
+	// Binary search the gas requirement, as it may be higher than the amount used
+	var (
+		lo  uint64 = params.TxGas - 1
+		hi  uint64
+		cap uint64
+	)
+	blockNum := rpc.LatestBlockNumber
+	if args.Gas != nil && uint64(*args.Gas) >= params.TxGas {
+		hi = uint64(*args.Gas)
+	} else {
+		// Retrieve the block to act as the gas ceiling
+		block, err := b.BlockByNumber(ctx, blockNum)
+		if err != nil {
+			return 0, err
+		}
+		hi = block.GasLimit()
+	}
+	if gasCap != nil && hi > gasCap.Uint64() {
+		// log.Warn("Caller gas above allowance, capping", "requested", hi, "cap", gasCap)
+		hi = gasCap.Uint64()
+	}
+	cap = hi
+
+	// Use zero-address if none other is available
+	if args.From == nil {
+		args.From = &common.Address{}
+	}
+	// Create a helper to check if a gas allowance results in an executable transaction
+	executable := func(gas uint64) bool {
+		args.Gas = (*hexutil.Uint64)(&gas)
+
+		_, _, failed, err := doCall(ctx, b, args, blockNum, vm.Config{}, 0, big.NewInt(int64(cap)))
+		if err != nil || failed {
+			return false
+		}
+		return true
+	}
+	// Execute the binary search and hone in on an executable gas limit
+	for lo+1 < hi {
+		mid := (hi + lo) / 2
+		if !executable(mid) {
+			lo = mid
+		} else {
+			hi = mid
+		}
+	}
+	// Reject the transaction as invalid if it still fails at the highest allowance
+	if hi == cap {
+		if !executable(hi) {
+			return 0, fmt.Errorf("gas required exceeds allowance (%d) or always failing transaction", cap)
+		}
+	}
+	return hexutil.Uint64(hi), nil
+}
+
+// EstimateGas returns an estimate of the amount of gas needed to execute the
+// given transaction against the current pending block.
+func (s *PublicBlockChainAPI) EstimateGas(ctx context.Context, args CallArgs) (hexutil.Uint64, error) {
+	return doEstimateGas(ctx, s.b, args, nil)
+}
+
 // GetCurrentUtilityMetrics ..
 func (s *PublicBlockChainAPI) GetCurrentUtilityMetrics() (*network.UtilityMetric, error) {
 	if s.b.GetShardID() == shard.BeaconChainShardID {