M-6 Vault _setProviders could revert

[0xdcota]

[M-6] For some assets, _setProviders will revert if new providers overlap with previous ones.

Description

The _setProviders function has two purposes: setting the providers array with a new set of providers and giving maximum approval for vault assets to all of the new providers.

function _setProviders(ILendingProvider[] memory providers) internal override {
    uint256 len = providers.length;
    for (uint256 i = 0; i < len;) {
     ----
      IERC20(asset()).approve(
        providers[i].approvedOperator(asset(), asset(), debtAsset()), type(uint256).max
      );
      IERC20(debtAsset()).approve(
        providers[i].approvedOperator(debtAsset(), asset(), debtAsset()), type(uint256).max
      );
    ----  
    }
    _providers = providers;
        -------
}

This code works for normal tokens, but for tokens that revert on approve if they already have an allowance, it will revert.

For example, USDT. If a new set of providers have even one provider from the previous, it will revert.

Remediation

Consider resetting allowance to 0 before setting it to the max. or Consider using forceApprove from the OZ SafeERC20 library.