fix(evm): don't pay the beneficiary for the unused EIP-8037 reservoir

[Xowiek]

ArcEvmHandler::reward_beneficiary pays the beneficiary for:

let gas_used = exec_result.gas().used();

Under EIP-8037 that also counts gas the caller gets back. last_frame_result rebuilds the top-level gas over the whole tx gas limit, returns only the regular remaining, and keeps the unused state gas reservoir separately:

*gas = Gas::new_spent(evm.ctx().tx().gas_limit());
gas.erase_cost(remaining);
gas.set_reservoir(reservoir);

so used() — tx.gas_limit - remaining - refunded — still includes the reservoir. Arc doesn't override reimburse_caller, and revm's version hands remaining + reservoir + refunded back to the caller. The reservoir ends up both refunded and paid to the beneficiary.

revm's own post_execution::reward_beneficiary leaves it out for exactly this reason:

// Exclude reservoir gas (EIP-8037) from the used gas — reservoir is unused and reimbursed.
let effective_used = gas.used().saturating_sub(gas.reservoir());

EIP-8037 support landed in revm-handler 18.0.0. The Arc override was written before that and didn't pick the subtraction up when revm was bumped.

With a 20M gas limit, 1M regular gas left, a 3M reservoir and a price of 10, the caller is charged 200,000,000. On main, reimbursement plus reward come to:

  left: 230000000
 right: 200000000

The extra 30,000,000 is reservoir × price of native coin that was never debited from anyone.

This isn't reachable on any network today. The reservoir is only non-zero with EIP-8037 enabled, revm enables it at SpecId::AMSTERDAM, and no Arc chain spec schedules Amsterdam — mainnet and localdev stop at Osaka, testnet and devnet at Prague. So the change is a no-op for every existing block. create_evm already turns off revm's Amsterdam EIP-7708 logs ahead of the spec being enabled; this is the same kind of change for the fee path.

The added test runs revm's reimburse_caller and Arc's reward_beneficiary on the gas last_frame_result produces, and checks the two credits equal what the caller was charged. It fails on main with the output above. cargo test -p arc-evm passes (150 tests), as do fmt and clippy.

[ZhiyuCircle]

Hi @Xowiek,

Thank you for your interest in contributing to Arc Node, and apologies for the delay in getting back to this PR.

We're closing out the pull request backlog that predates our current contribution policy. This PR is being closed because it does not reference a GitHub issue. All PRs must reference an existing issue using the format Closes: #XXX, and the author must be assigned to that issue before the PR is opened.

This is not a judgement on the change itself. If you'd still like to land it:

1. Open an issue describing the problem, or find the existing one
2. Comment on the issue requesting assignment, and wait for maintainer approval
3. Open a fresh PR once you have been assigned

Please see CONTRIBUTING.md for details. Thanks again for taking the time to contribute.