Inconsistency in Istanbul.LookbackWindow validation

[piersy]

Description

The celo-blockchain has the following validation

celo-blockchain/consensus/istanbul/config.go

Lines 123 to 125 in 43ae190

	if chainConfig.Istanbul.LookbackWindow >= chainConfig.Istanbul.Epoch-2 {
	return fmt.Errorf("istanbul.lookbackwindow must be less than istanbul.epoch-2")
	}

And celo-monorepo has the following.

    require(
      window <= getEpochSize().sub(2),
      "UptimeLookbackWindow must be smaller or equal to epochSize - 2"
    );

https://github.com/celo-org/celo-monorepo/blob/64e618b9b856073305dd5748fc04fc772ff72714/packages/protocol/contracts/governance/BlockchainParameters.sol#L110-L113

Proposed Solution

I'm not sure which is correct, but I'm thinking maybe the validation should be removed from the blockchain, since validation seems to be done at the contract level making the blockchain validation redundant and the error returned from the contract seems quite readable.

I noticed also that this parameter is set in 2 places in the config.

celo-blockchain/params/config.go

Line 235 in 43ae190

LookbackWindow uint64 `json:"lookbackwindow"` // The number of blocks to look back when calculating uptime

and

celo-blockchain/mycelo/genesis/config.go

Line 146 in 43ae190

UptimeLookbackWindow uint64 `json:"uptimeLookbackWindow"`

Revert: UptimeLookbackWindow must be smaller or equal to epochSize - 2