Add _setCollateralPaused function to Comptroller

[nourharidy]

Add a _setCollateralPaused(CToken cToken, bool state) public returns (bool) function the Comptroller that disables future borrows for all users using a cToken as collateral

Similar to the other guardian roles, the function can be called by either the pauseGuardian or the admin (governance). However, only admin can pass state as false. Both can pass it as true.

If state is passed as true, the Comptroller should prevent future borrows across all Anchor markets for any user that uses the cToken as a collateral by checking checkMembership(address account, CToken cToken) function. If passed as false, the effect should be reversed.

The function should allow multiple cTokens to be paused as collateral at the same time.

Changes should be made to https://github.com/InverseFinance/anchor/blob/master/contracts/Comptroller.sol and its dependencies only.

The modified Comptroller will replace the existing Anchor Comptroller on Ethereum by executing a a proxy pattern upgrade on the Unitroller https://etherscan.io/address/0x4dcf7407ae5c07f8681e1659f626e114a7667339

Changes to the Comptroller must follow proxy pattern best practices in order to avoid an upgrade flaw

[beepidibop]

Saw your tweet, here's the PR

I have no idea if it'll work tbh. Just thought I'd give it a shot.

#21

[georgeroman]

Isn't this already covered by the below method and check?

anchor/contracts/Comptroller.sol

Lines 1012 to 1020 in 9c3239c

	function _setBorrowPaused(CToken cToken, bool state) public returns (bool) {
	require(markets[address(cToken)].isListed, "cannot pause a market that is not listed");
	require(msg.sender == pauseGuardian || msg.sender == admin, "only pause guardian and admin can pause");
	require(msg.sender == admin || state == true, "only admin can unpause");
	borrowGuardianPaused[address(cToken)] = state;
	emit ActionPaused(cToken, "Borrow", state);
	return state;
	}

anchor/contracts/Comptroller.sol

Lines 331 to 332 in 9c3239c

	// Pausing is a very serious situation - we revert to sound the alarms
	require(!borrowGuardianPaused[cToken], "borrow is paused");

[beepidibop]

Isn't this already covered by the below method and check?

Different scenarios. The above case is enough if you want to pause borrowing a token completely. The issue requested a way to stop said token from being a collateral for new loans.

E.g.
Pausing ETH borrowing would prevent people from borrowing more ETH, but doesn't stop them from borrowing WBTC with ETH deposits.