// SPDX-License-Identifier: MIT pragma solidity 0.8.15; import { Initializable } from "@openzeppelin/contracts/proxy/utils/Initializable.sol"; import { Math } from "@openzeppelin/contracts/utils/math/Math.sol"; import { Burn } from "../libraries/Burn.sol"; import { Arithmetic } from "../libraries/Arithmetic.sol"; /** * @custom:upgradeable * @title ResourceMetering * @notice ResourceMetering implements an EIP-1559 style resource metering system where pricing * updates automatically based on current demand. */ abstract contract ResourceMetering is Initializable { /** * @notice Represents the various parameters that control the way in which resources are * metered. Corresponds to the EIP-1559 resource metering system. * * @custom:field prevBaseFee Base fee from the previous block(s). * @custom:field prevBoughtGas Amount of gas bought so far in the current block. * @custom:field prevBlockNum Last block number that the base fee was updated. */ struct ResourceParams { uint128 prevBaseFee; uint64 prevBoughtGas; uint64 prevBlockNum; } /** * @notice Represents the configuration for the EIP-1559 based curve for the deposit gas * market. These values should be set with care as it is possible to set them in * a way that breaks the deposit gas market. The target resource limit is defined as * maxResourceLimit / elasticityMultiplier. This struct was designed to fit within a * single word. There is additional space for additions in the future. * * @custom:field maxResourceLimit Represents the maximum amount of deposit gas that * can be purchased per block. * @custom:field elasticityMultiplier Determines the target resource limit along with * the resource limit. * @custom:field baseFeeMaxChangeDenominator Determines max change on fee per block. * @custom:field minimumBaseFee The min deposit base fee, it is clamped to this * value. * @custom:field systemTxMaxGas The amount of gas supplied to the system * transaction. This should be set to the same number * that the op-node sets as the gas limit for the * system transaction. * @custom:field maximumBaseFee The max deposit base fee, it is clamped to this * value. */ struct ResourceConfig { uint32 maxResourceLimit; uint8 elasticityMultiplier; uint8 baseFeeMaxChangeDenominator; uint32 minimumBaseFee; uint32 systemTxMaxGas; uint128 maximumBaseFee; } /** * @notice EIP-1559 style gas parameters. */ ResourceParams public params; /** * @notice Reserve extra slots (to a total of 50) in the storage layout for future upgrades. */ uint256[48] private __gap; /** * @notice Meters access to a function based an amount of a requested resource. * * @param _amount Amount of the resource requested. */ modifier metered(uint64 _amount) { // Record initial gas amount so we can refund for it later. uint256 initialGas = gasleft(); // Run the underlying function. _; // Run the metering function. _metered(_amount, initialGas); } /** * @notice An internal function that holds all of the logic for metering a resource. * * @param _amount Amount of the resource requested. * @param _initialGas The amount of gas before any modifier execution. */ function _metered(uint64 _amount, uint256 _initialGas) internal { // Update block number and base fee if necessary. uint256 blockDiff = block.number - params.prevBlockNum; ResourceConfig memory config = _resourceConfig(); int256 targetResourceLimit = int256(uint256(config.maxResourceLimit)) / int256(uint256(config.elasticityMultiplier)); if (blockDiff > 0) { // Handle updating EIP-1559 style gas parameters. We use EIP-1559 to restrict the rate // at which deposits can be created and therefore limit the potential for deposits to // spam the L2 system. Fee scheme is very similar to EIP-1559 with minor changes. int256 gasUsedDelta = int256(uint256(params.prevBoughtGas)) - targetResourceLimit; int256 baseFeeDelta = (int256(uint256(params.prevBaseFee)) * gasUsedDelta) / (targetResourceLimit * int256(uint256(config.baseFeeMaxChangeDenominator))); // Update base fee by adding the base fee delta and clamp the resulting value between // min and max. int256 newBaseFee = Arithmetic.clamp({ _value: int256(uint256(params.prevBaseFee)) + baseFeeDelta, _min: int256(uint256(config.minimumBaseFee)), _max: int256(uint256(config.maximumBaseFee)) }); // If we skipped more than one block, we also need to account for every empty block. // Empty block means there was no demand for deposits in that block, so we should // reflect this lack of demand in the fee. if (blockDiff > 1) { // Update the base fee by repeatedly applying the exponent 1-(1/change_denominator) // blockDiff - 1 times. Simulates multiple empty blocks. Clamp the resulting value // between min and max. newBaseFee = Arithmetic.clamp({ _value: Arithmetic.cdexp({ _coefficient: newBaseFee, _denominator: int256(uint256(config.baseFeeMaxChangeDenominator)), _exponent: int256(blockDiff - 1) }), _min: int256(uint256(config.minimumBaseFee)), _max: int256(uint256(config.maximumBaseFee)) }); } // Update new base fee, reset bought gas, and update block number. params.prevBaseFee = uint128(uint256(newBaseFee)); params.prevBoughtGas = 0; params.prevBlockNum = uint64(block.number); } // Make sure we can actually buy the resource amount requested by the user. params.prevBoughtGas += _amount; require( int256(uint256(params.prevBoughtGas)) <= int256(uint256(config.maxResourceLimit)), "ResourceMetering: cannot buy more gas than available gas limit" ); // Determine the amount of ETH to be paid. uint256 resourceCost = uint256(_amount) * uint256(params.prevBaseFee); // We currently charge for this ETH amount as an L1 gas burn, so we convert the ETH amount // into gas by dividing by the L1 base fee. We assume a minimum base fee of 1 gwei to avoid // division by zero for L1s that don't support 1559 or to avoid excessive gas burns during // periods of extremely low L1 demand. One-day average gas fee hasn't dipped below 1 gwei // during any 1 day period in the last 5 years, so should be fine. uint256 gasCost = resourceCost / Math.max(block.basefee, 1 gwei); // Give the user a refund based on the amount of gas they used to do all of the work up to // this point. Since we're at the end of the modifier, this should be pretty accurate. Acts // effectively like a dynamic stipend (with a minimum value). uint256 usedGas = _initialGas - gasleft(); if (gasCost > usedGas) { Burn.gas(gasCost - usedGas); } } /** * @notice Virtual function that returns the resource config. Contracts that inherit this * contract must implement this function. * * @return ResourceConfig */ function _resourceConfig() internal virtual returns (ResourceConfig memory); /** * @notice Sets initial resource parameter values. This function must either be called by the * initializer function of an upgradeable child contract. */ // solhint-disable-next-line func-name-mixedcase function __ResourceMetering_init() internal onlyInitializing { params = ResourceParams({ prevBaseFee: 1 gwei, prevBoughtGas: 0, prevBlockNum: uint64(block.number) }); } }