Document use of `increaseAllowance`

[emizzle]

Documents the use of increaseAllowance in OpenZeppelin's ERC20 implementation.

The increaseAllowance function of OpenZeppelin's ERC20 implementation is being used, which is not part of the standard ERC20 interface. If the Codex token must be an ERC20 token (eg DAI), then that token will need to be wrapped in OpenZeppelin's ERC20Wrapped to ensure that increaseAllowance is available.

The reason increaseAllowance is needed is because the sales state machine handles concurrent slot fills, which requires token approval. Because these approvals happen concurrently, simply calling approve is not sufficient as each approve amount will overwrite the previous. As an example, if a single node attempts to fill two slots simultaneously, it will send two approve transactions followed by two fillSlot transactions. The first two approve transactions will each approve the collateral amount for one fillSlot, the second overwriting the first, so the total approved will only ever be enough for one fillSlot. So after the first fillSlot is sent, the second one will fail as not enough tokens will have been approved. The solution is use increaseAllowance instead of approve which increases the allowance instead of overwriting it.

[emizzle]

May no longer be needed: https://github.com/codex-storage/nim-codex/issues/550#issuecomment-1754481085

[markspanbroek]

Closing. Will be handled in https://github.com/codex-storage/nim-codex/issues/598