Staking Rewards Single Token

Synthetix' StakingRewards rewritten for maximum efficiency when the staking token is the same as the reward token.

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Any EVM chain: 0xa8101F6Ec7080dE84233f1eE4fc1D6A2C1568327

Features

• Very low gas cost on all operations
• Role based permission control allowing non-admin funders
• compound() function for staking the rewards
• stake() function also compounds the rewards
• withdraw() function also claims the rewards
• addReward() function for transferring and funding the contract

Non-features

• No ERC20 token recovery function
• No funding using the account balance
• No pointless reenterancy guards
• No safeTransfer (use a proper token)

DO NOT DIRECTLY TRANSFER TOKENS TO THE CONTRACT! Only fund through the addReward() function. This model is similar to MiniChef, and incompatible with notifyRewardAmount() method of original StakingRewards. So, DO NOT DIRECTLY TRANSFER TOKENS TO THE CONTRACT!

Limitations

1. The sum of all tokens added through addReward() cannot exceed 2**96-1,
2. A user's staked balance cannot exceed 2**96-1.

Assumptions

1. block.timestamp < 2**64 - 2**32,
2. rewardToken returns false or reverts on failing transfers,
3. Number of users does not exceed (2**256-1)/(2**96-1).

Fixed Vulnerability

Original StakingRewards has a serious vulnerability when the StakingToken is the same as the RewardsToken. The contract owner can fund the rewards using the staked balance. This allows users to claim rewards from each others' staked balance, effectively ruining the state. To prevent this there are to options. The first options is keeping track of the reserves and updating a variable whenever tokens move to or from the contract. The second options is combining transferring tokens and funding the rewards into one function. I opted for the second for gas efficiency. Therefore, use the appropriate function for funding rewards and DO NOT DIRECTLY TRANSFER TOKENS TO THE CONTRACT!

Gas Comparison

My StakingRewards	Costs	Original StakingRewards	Costs
addReward	59k	transfer+notifyRewardAmount	~120k
stake	60k	getReward+stake	~175k
		stake	~100k
withdraw (full amount)	53k	exit	95k
withdraw	53k-57k	getReward+withdraw	150k-155k
		withdraw	75k-80k
compound	41k	getReward+stake	~175k
harvest	54k	getReward	75k

Disclaimer

Contract yet to be properly tested. Use at your own risk. I am also using unchecked math. That is considered a risky practice. However I plan to verify the logic.