fix(contract): init Reentrancy Guard

[PierrickGT]

Issue: code-423n4/2021-06-pooltogether-findings#60

[aodhgan]

What is the default behaviour of the reentrancy guard when its not initialized? Just default to 0?

ACK

[PierrickGT]

What is the default behaviour of the reentrancy guard when its not initialized? Just default to 0?

ACK

Looking at the code, I feel like it would behave the same way than when initialized.

https://github.com/OpenZeppelin/openzeppelin-contracts-upgradeable/blob/master/contracts/security/ReentrancyGuardUpgradeable.sol#L44

By initializing it, _status is then equal to _NOT_ENTERED, so 1. But without initializing it, _status would be equal to 0, which wouldn't be a problem cause once the function called, it would still set _status to _ENTERED which is equal to 2.

https://github.com/OpenZeppelin/openzeppelin-contracts-upgradeable/blob/91e0b106804c3f2ca7379577c680e4233e2be09c/contracts/security/ReentrancyGuardUpgradeable.sol#L59

[asselstine]

@PierrickGT @aodhgan. Likely gas costs.

[kamescg]

To my knowledge reverting back to 0 would have been cheaper, since no storage is required for reverting to the default.

https://eips.ethereum.org/EIPS/eip-2200

If a contract with empty storage sets slot 0 to 1, then back to 0, it will be charged 20000 + 200 - 19800 = 400 gas.

When original value is 0, we want to prove that:
Case I: If the final value ends up still being 0, we want to charge 200 * N gases, because no disk write is needed.
Case II: If the final value ends up being a non-zero value, we want to charge 20000 + 200 * (N-1) gas, because it requires writing this slot to disk.

[kamescg]

@PierrickGT @asselstinesSimilar to pooltogether/pooltogether-yearnv2-yield-source#8 (review) may want to remove transferOwnership from initialize to adhere with other PT contracts, even thought contract doesn't include additional function for setting config params?