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internal/ethapi: optimize & clean up EstimateGas (#27710)
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Optimizations:

- Previously, if a transaction was reverting, EstimateGas would exhibit worst-case behavior and binary search up to the max gas limit (~40 state-clone + tx executions). This change allows EstimateGas to return after only a single unconstrained execution in this scenario.
- Uses the gas used from the unconstrained execution to bias the remaining binary search towards the likely solution in a simple way that doesn't impact the worst case. For a typical contract-invoking transaction, this reduces the median number of state-clone+executions from 25 to 18 (28% reduction).

Cleanup:

- added & improved function + code comments
- correct the EstimateGas documentation to clarify the gas limit determination is at latest block, not pending, if the blockNr is unspecified.
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@roberto-bayardo
roberto-bayardo committed Aug 18, 2023
1 parent 649deb6 commit 950ccdd
Showing 1 changed file with 65 additions and 57 deletions.
122 changes: 65 additions & 57 deletions internal/ethapi/api.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1162,12 +1162,30 @@ func (s *BlockChainAPI) Call(ctx context.Context, args TransactionArgs, blockNrO
return result.Return(), result.Err
}

// executeEstimate is a helper that executes the transaction under a given gas limit and returns
// true if the transaction fails for a reason that might be related to not enough gas. A non-nil
// error means execution failed due to reasons unrelated to the gas limit.
func executeEstimate(ctx context.Context, b Backend, args TransactionArgs, state *state.StateDB, header *types.Header, gasCap uint64, gasLimit uint64) (bool, *core.ExecutionResult, error) {
args.Gas = (*hexutil.Uint64)(&gasLimit)
result, err := doCall(ctx, b, args, state, header, nil, nil, 0, gasCap)
if err != nil {
if errors.Is(err, core.ErrIntrinsicGas) {
return true, nil, nil // Special case, raise gas limit
}
return true, nil, err // Bail out
}
return result.Failed(), result, nil
}

// DoEstimateGas returns the lowest possible gas limit that allows the transaction to run
// successfully at block `blockNrOrHash`. It returns error if the transaction would revert, or if
// there are unexpected failures. The gas limit is capped by both `args.Gas` (if non-nil &
// non-zero) and `gasCap` (if non-zero).
func DoEstimateGas(ctx context.Context, b Backend, args TransactionArgs, blockNrOrHash rpc.BlockNumberOrHash, overrides *StateOverride, gasCap uint64) (hexutil.Uint64, error) {
// Binary search the gas requirement, as it may be higher than the amount used
// Binary search the gas limit, as it may need to be higher than the amount used
var (
lo uint64 = params.TxGas - 1
hi uint64
cap uint64
lo uint64 // lowest-known gas limit where tx execution fails
hi uint64 // lowest-known gas limit where tx execution succeeds
)
// Use zero address if sender unspecified.
if args.From == nil {
Expand Down Expand Up @@ -1198,16 +1216,17 @@ func DoEstimateGas(ctx context.Context, b Backend, args TransactionArgs, blockNr
} else {
feeCap = common.Big0
}

state, header, err := b.StateAndHeaderByNumberOrHash(ctx, blockNrOrHash)
if state == nil || err != nil {
return 0, err
}
if err := overrides.Apply(state); err != nil {
return 0, err
}

// Recap the highest gas limit with account's available balance.
if feeCap.BitLen() != 0 {
state, _, err := b.StateAndHeaderByNumberOrHash(ctx, blockNrOrHash)
if err != nil {
return 0, err
}
err = overrides.Apply(state)
if err != nil {
return 0, err
}
balance := state.GetBalance(*args.From) // from can't be nil
available := new(big.Int).Set(balance)
if args.Value != nil {
Expand All @@ -1234,39 +1253,42 @@ func DoEstimateGas(ctx context.Context, b Backend, args TransactionArgs, blockNr
log.Warn("Caller gas above allowance, capping", "requested", hi, "cap", gasCap)
hi = gasCap
}
cap = hi

// Create a helper to check if a gas allowance results in an executable transaction
executable := func(gas uint64, state *state.StateDB, header *types.Header) (bool, *core.ExecutionResult, error) {
args.Gas = (*hexutil.Uint64)(&gas)

result, err := doCall(ctx, b, args, state, header, nil, nil, 0, gasCap)
if err != nil {
if errors.Is(err, core.ErrIntrinsicGas) {
return true, nil, nil // Special case, raise gas limit
}
return true, nil, err // Bail out
}
return result.Failed(), result, nil
}
state, header, err := b.StateAndHeaderByNumberOrHash(ctx, blockNrOrHash)
if state == nil || err != nil {
return 0, err
}
err = overrides.Apply(state)
// We first execute the transaction at the highest allowable gas limit, since if this fails we
// can return error immediately.
failed, result, err := executeEstimate(ctx, b, args, state.Copy(), header, gasCap, hi)
if err != nil {
return 0, err
}
// Execute the binary search and hone in on an executable gas limit
if failed {
if result != nil && result.Err != vm.ErrOutOfGas {
if len(result.Revert()) > 0 {
return 0, newRevertError(result)
}
return 0, result.Err
}
return 0, fmt.Errorf("gas required exceeds allowance (%d)", hi)
}
// For almost any transaction, the gas consumed by the unconstrained execution above
// lower-bounds the gas limit required for it to succeed. One exception is those txs that
// explicitly check gas remaining in order to successfully execute within a given limit, but we
// probably don't want to return a lowest possible gas limit for these cases anyway.
lo = result.UsedGas - 1

// Binary search for the smallest gas limit that allows the tx to execute successfully.
for lo+1 < hi {
s := state.Copy()
mid := (hi + lo) / 2
failed, _, err := executable(mid, s, header)

// If the error is not nil(consensus error), it means the provided message
// call or transaction will never be accepted no matter how much gas it is
// assigned. Return the error directly, don't struggle any more.
if mid > lo*2 {
// Most txs don't need much higher gas limit than their gas used, and most txs don't
// require near the full block limit of gas, so the selection of where to bisect the
// range here is skewed to favor the low side.
mid = lo * 2
}
failed, _, err = executeEstimate(ctx, b, args, state.Copy(), header, gasCap, mid)
if err != nil {
// This should not happen under normal conditions since if we make it this far the
// transaction had run without error at least once before.
log.Error("execution error in estimate gas", "err", err)
return 0, err
}
if failed {
Expand All @@ -1275,28 +1297,14 @@ func DoEstimateGas(ctx context.Context, b Backend, args TransactionArgs, blockNr
hi = mid
}
}
// Reject the transaction as invalid if it still fails at the highest allowance
if hi == cap {
failed, result, err := executable(hi, state, header)
if err != nil {
return 0, err
}
if failed {
if result != nil && result.Err != vm.ErrOutOfGas {
if len(result.Revert()) > 0 {
return 0, newRevertError(result)
}
return 0, result.Err
}
// Otherwise, the specified gas cap is too low
return 0, fmt.Errorf("gas required exceeds allowance (%d)", 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.
// EstimateGas returns the lowest possible gas limit that allows the transaction to run
// successfully at block `blockNrOrHash`, or the latest block if `blockNrOrHash` is unspecified. It
// returns error if the transaction would revert or if there are unexpected failures. The returned
// value is capped by both `args.Gas` (if non-nil & non-zero) and the backend's RPCGasCap
// configuration (if non-zero).
func (s *BlockChainAPI) EstimateGas(ctx context.Context, args TransactionArgs, blockNrOrHash *rpc.BlockNumberOrHash, overrides *StateOverride) (hexutil.Uint64, error) {
bNrOrHash := rpc.BlockNumberOrHashWithNumber(rpc.LatestBlockNumber)
if blockNrOrHash != nil {
Expand Down

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