[Draft] Improvement of Price Oracle Sentinel

[miguelmtzinf]

No description provided.

[miguelmtzinf]

Trafeoffs of this approach:

• The price check is only made in 1 single asset (the borrowed asset on borrow, the debt asset on liquidation)
• The price fetch is done twice: when calculating health factor, when validating the action
• There is only 1 asset price validated, but not the other assets part of the user's position.

Gas comparison:

|                         ·  Min        ·  Max        ·  Avg             │
| without oracle sentinel ·     236441  ·     362175  ·        286168    │
| original version        ·     252095  ·     377829  ·        301822    │
| modified version        ·     260518  ·     386252  ·        310245    │

[miguelmtzinf]

Alternative: create a keeper function at PriceOracleSentinel that checks price stability of all the reserves.

  function keeper() external {
    address[] memory reserveList = IPool(ADDRESSES_PROVIDER.getPool()).getReservesList();
    uint256 i = 0;
    uint256 reserveLength = reserveList.length;
    uint256 updatedAt = block.timestamp;
    for (i = 0; i < reserveLength; ) {
      (, , , updatedAt, ) = AggregatorV3Interface(_aaveOracle.getSourceOfAsset(reserveList[i]))
        .latestRoundData();
      _isPriceStale = (block.timestamp - updatedAt) > _priceExpirationTime;
      if (_isPriceStale) break;
      unchecked {
        ++i;
      }
    }
  }

  function isBorrowAllowed(address priceOracle, address asset) public view override returns (bool) {
  return _isUpAndGracePeriodPassed() && !_isPriceStale;
}

The flag _isPriceStale can be converted to a mapping of asset prices (similar to the userConfig map) so the whole user position can be taken into account when interacting