Incorrectly computing Fee APY in Curve 3pool

Backscratcher.apy(self, _: ApySamples) -> Apy at https://github.com/yearn/yearn-exporter/blob/master/yearn/special.py overestimates the APY from virtual price by looking back at APY from virtual price increases from the start of the pool instead of comparing the increase in virtual price from a year ago. This causes it to incorrectly measure APY from fees as ~2.3% instead of the APY from fees in the last year of ~.3%.

This is the function with the error

   def apy(self, _: ApySamples) -> Apy:
        curve_3_pool = contract("0xbEbc44782C7dB0a1A60Cb6fe97d0b483032FF1C7")
        curve_reward_distribution = contract("0xA464e6DCda8AC41e03616F95f4BC98a13b8922Dc")
        curve_voting_escrow = contract("0x5f3b5DfEb7B28CDbD7FAba78963EE202a494e2A2")
        voter = "0xF147b8125d2ef93FB6965Db97D6746952a133934"
        crv_price = magic.get_price("0xD533a949740bb3306d119CC777fa900bA034cd52")
        yvecrv_price = magic.get_price("0xc5bDdf9843308380375a611c18B50Fb9341f502A")

        total_vecrv = curve_voting_escrow.totalSupply()
        yearn_vecrv = curve_voting_escrow.balanceOf(voter)
        vault_supply = self.vault.totalSupply()

        week = 7 * 86400
        epoch = math.floor(time() / week) * week - week
        tokens_per_week = curve_reward_distribution.tokens_per_week(epoch) / 1e18
        virtual_price = curve_3_pool.get_virtual_price() / 1e18
        # although we call this APY, this is actually APR since there is no compounding
        apy = (tokens_per_week * virtual_price * 52) / ((total_vecrv / 1e18) * crv_price)
        vault_boost = (yearn_vecrv / vault_supply) * (crv_price / yvecrv_price)
        composite = {
            "currentBoost": vault_boost,
            "boostedApy": apy * vault_boost,
            "totalApy": apy * vault_boost,
            "poolApy": apy,
            "baseApy": apy,
        }
        return Apy("backscratcher", apy, apy, ApyFees(), composite=composite)

    def tvl(self, block=None) -> Tvl:
        total_assets = self.vault.totalSupply(block_identifier=block)
        try:
            price = magic.get_price(self.token, block=block)
        except PriceError:
            price = None
        tvl = total_assets * price / 10 ** self.vault.decimals(block_identifier=block) if price else None
        return Tvl(total_assets, price, tvl)

This section in particular is wrong.

        virtual_price = curve_3_pool.get_virtual_price() / 1e18
        # although we call this APY, this is actually APR since there is no compounding
        apy = (tokens_per_week * virtual_price * 52) / ((total_vecrv / 1e18) * crv_price)

curve_3_pool.get_virtual_price() tracks the lp token value increase from swap fees from the start of the pool more than 2 years ago. The 3pool contract was deployed on Sep 6, 2020

To fix this you can update the function like this.

block_a_year_ago = some_function_to_get_the_block_nearest_to_a_year_ago()
previous_virtual_price = curve_3_pool.get_virtual_price(block_identifier=block_a_year_ago) / 1e18   
current_virtual_price = curve_3_pool.get_virtual_price() / 1e18
virtual_price_increase = 1 + (current_virtual_price - previous_virtual_price)
  # although we call this APY, this is actually APR since there is no compounding
  apy = (tokens_per_week * virtual_price_increase * 52) / ((total_vecrv / 1e18) * crv_price)

The way it is written now overestimates the APY by about 2%.

three_pool = eth_client.toChecksumAddress('0xbebc44782c7db0a1a60cb6fe97d0b483032ff1c7')
from analytics.abis import CURVE_STABLE_SWAP_ABI
contract = eth_client.eth.contract(three_pool, abi=CURVE_STABLE_SWAP_ABI)
contract.functions.get_virtual_price().call()

block_a_year_ago = 14194418
now_block = 16616116

previous_virtual_price = contract.functions.get_virtual_price().call(block_identifier=block_a_year_ago) / 1e18
current_virtual_price = contract.functions.get_virtual_price().call(block_identifier=now_block) / 1e18
virtual_price_increase = 1 + (current_virtual_price - previous_virtual_price)
previous_virtual_price, current_virtual_price, virtual_price_increase
$ (1.0203125062827059, 1.0237601567870114, 1.0034476505043055)

The virtual price only increases by ~0.34% in the last year instead of 2.37% since the lifetime of the pool.

yearn / yearn-exporter

Incorrectly computing Fee APY in Curve 3pool #482