The official implementation of the Peg Stability Module. There are two main components to the PSM:
This is an exact duplicate of the GemJoinX adapter for the given collateral type with two modifications.
First the method signature of join() is changed to include the original message sender at the end as well as adding the auth modifier. This should look like:
function join(address urn, uint256 wad) external note -> function join(address urn, uint256 wad, address _msgSender) external note auth
Second, all instances of msg.sender are replaced with _msgSender in the join() function.
In this repository I have added join-5-auth.sol for the PSM-friendly version of join-5.sol which is used for USDC. This can be applied to any other gem join adapter.
This is the actual PSM module which acts as a authed special vault sitting behind the AuthGemJoinX contract. DssPsm allows you to either call sellGem() or buyGem() to trade ERC20 DAI for the gem or vice versa. Upon calling one of these functions the PSM vault will either lock gems in the join adapter, take out a dai loan and issue ERC20 DAI to the specified user or do that process in reverse.
The PSM requires ERC20 approvals to pull in the tokens.
To use sellGem(usr, amt) you must first call gem.approve(<gemJoinAddress>, amt). Example:
// Trade 100 USDC for 100 DAI - fee
usdc.approve(0x0A59649758aa4d66E25f08Dd01271e891fe52199, 100 * (10 ** 6));
psm.sellGem(address(this), 100 * (10 ** 6));
To use buyGem(usr, amt) you must first call dai.approve(<psmAddress>, amt + fee). Example:
// Trade DAI + fee for 100 USDC
uint256 WAD = 10 ** 18;
dai.approve(0x89B78CfA322F6C5dE0aBcEecab66Aee45393cC5A, 100 * (psm.tout() + WAD));
psm.buyGem(address(this), 100 * (10 ** 6));
Please note the fee behaviour is not the same for both functions.
When calling sellGem(), DAI is minted at a 1:1 rate to which the fee is removed from that amount. So if you have 100 USDC with 1% fee and you call sellGem() then you will recieve 100 DAI - 1% * 100 DAI = 99 DAI. In this case the fee is subtracted from the result amount.
When calling buyGem(), you specify the amount of the gem you want to recieve. This fee is then added on top of this value to determine how much ERC20 DAI you require in your account. So if you call buyGem() for 100 USDC with a 1% fee then you require 100 DAI + 1% * 100 DAI = 101 DAI.
Please note this was a conscious decision to avoid dealing with decimal division and rounding leftovers.
USDC GemJoin: 0x0A59649758aa4d66E25f08Dd01271e891fe52199
USDC PSM: 0x89B78CfA322F6C5dE0aBcEecab66Aee45393cC5A
USDP GemJoin: 0x7bbd8cA5e413bCa521C2c80D8d1908616894Cf21
USDP PSM: 0x961Ae24a1Ceba861D1FDf723794f6024Dc5485Cf
USDC GemJoin: 0x4BA159Ad37FD80D235b4a948A8682747c74fDc0E
USDC PSM: 0xe4dC42e438879987e287A6d9519379936d7b065A
