Gas calculation for precompiles incorrectly varies with HAPI exchange rate

[lukelee-sl]

Description

Gas calculation appears to vary with hbar exchange rate, compromising Hedera's policy of fixed fee gas costs.

Steps to reproduce

Run the following unit test in the build:

private static final long GAS_COST_IN_TINYCENT = 852;

@Test
void exchangeRateChangesCausesUnstableGasCost() {
    final var mintFeeTinycents = 10000123L;
    final var fixedHbarEquiv = 30000;
    final var pre0900CentEquiv = 269100;
    final var post0900CentEquiv = 269232;
    System.out.println("Pre-09:00 gas cost: " + gasCostGiven(mintFeeTinycents, fixedHbarEquiv, pre0900CentEquiv));
    System.out.println("Post-09:00 gas cost: " + gasCostGiven(mintFeeTinycents, fixedHbarEquiv, post0900CentEquiv));
}

private long gasCostGiven(final long tinycentFee, final int hbarEquiv, final int centEquiv) {
    final var rate = ExchangeRate.newBuilder().setHbarEquiv(hbarEquiv).setCentEquiv(centEquiv).build();
    final var gasPriceInTinybar = getTinybarsFromTinyCents(rate, GAS_COST_IN_TINYCENT);
    final var feeInTinbyar = getTinybarsFromTinyCents(rate, tinycentFee);
    final var baseGasCost = (feeInTinbyar + gasPriceInTinybar - 1L) / gasPriceInTinybar;
    return baseGasCost + (baseGasCost / 5L);
}

with the following exchange rates:

At 08:59 CST
curl -X 'GET' \
  'https://mainnet-public.mirrornode.hedera.com/api/v1/network/exchangerate?timestamp=1703343540' \
  -H 'accept: application/json'
and

At 09:01 CST
curl -X 'GET' \
  'https://mainnet-public.mirrornode.hedera.com/api/v1/network/exchangerate?timestamp=1703343660' \
  -H 'accept: application/json'

Additional context

Proof on mainnet for same function call at two different times (with two different exchange rates): fail https://hashscan.io/mainnet/transaction/1703343586.545013838 success https://hashscan.io/mainnet/transaction/1703344296.632845003 fail gas: 14,232 success gas: 14,226

Impact

dApps cannot be certain the gas consumed by a smart contract function call when the gas changes with HAPI exchange rate. This compromises Hedera's stated policy that fees are denominated in USD.

Hedera network

mainnet

Version

v0.45.0

Operating system

None

[Nana-EC]

We should ensure documentation around this occurrence is clear for community

[MrValioBg]

The following table represents values from before and after the fix for Precision Loss.

Since before, the gas was changing with the exchange rate, the table represents a test with a fixed exchange rate. Additionally, after the changes, the "Gas Now" value should be constant, no matter what the exchange rate is.

Exchange rate tested with: hbarEquiv = "1"; centEquiv = "12"; 1hbar = ~0.12$

Keep in mind that since the current behavior is unstable and increasing the price, for example to hbarEquiv = 1; centEquiv = 39 can lead to getting 1–25000 more gas units.

The following gas includes the intrinsic gas + cost for the contractCall to the contract that interacts with the HTS system contract. However, it should be pretty similar for every operation, so we can easily track the delta.

System Contract	Previous Gas	Gas now	Diff %
approve() -> Redirect/ERC	733069	734267	0.16%
approve()	730664	731862	0.16%
LazyCreate via Constructor	97484	97484	0%
transferFrom() -> on the ERC20 Contract + lazy create	749543	750754	0.16%
createFungibleTokenWithCustomFees()	164204	164204	0%
createNonFungibleTokenWithCustomFees()	168742	168742	0%
createFungibleToken()	164965	164965	0%
createNonFungibleToken()	166112	166112	0%
cryptoTransfer() -> hBar only	32105	33304	3.7%
cryptoTransfer() -> Fungible	-	47085
cryptoTransfer() -> Fungible + Internal Auto-Associate	750112	752509	0.31%
cryptoTransfer() -> Non-Fungible	-	44666
cryptoTransfer() -> Non-Fungible + Internal Auto-Associate	747693	750090	0.32%
transferNFTs() -> With Internal Auto-associate	748151	750548	0.32%
transferNFTs()	43925	45124	2.72%
transferNFT()	41036	42235	2.92%
transferToken()	40970	42169	2.92%
transferToken() -> With Internal Auto-Associate	745196	747593	0.32%
transferFrom()	41065	42264	2.91%
transferFrom() -> ERC	43534	44733	2.75%
burn()	14085	15284	8.51%
mint()	14085	15284	8.51%
balanceOf()	27567	30074	9.09%
name()	27700	30207	9.05%
getTokenCustomFees()	28914	31421	8.67%
getTokenInfo()	76298	78805	3.28%
View Ops	100	2607 ~ $0.0001	2600%

Note: For view functions, we now calculate using the actual price from the canonical price, converted into gas, rather than just using a fixed 100 gas. All of the view functions currently use the BASE price for TokenGetInfo from canonical-prices.json.

This leads to an increase in the gas cost for the view operation from 100 to approximately 2600. ( In the table above we see higher values for view calls, because of the intrinsic/call gas cost.)