aave-protocol-v2/contracts/protocol/libraries/aave-upgradeability/VersionedInitializable.sol

// SPDX-License-Identifier: agpl-3.0
pragma solidity 0.6.12;

/**
 * @title VersionedInitializable
 *
 * @dev Helper contract to implement initializer functions. To use it, replace
 * the constructor with a function that has the `initializer` modifier.
 * WARNING: Unlike constructors, initializer functions must be manually
 * invoked. This applies both to deploying an Initializable contract, as well
 * as extending an Initializable contract via inheritance.
 * WARNING: When used with inheritance, manual care must be taken to not invoke
 * a parent initializer twice, or ensure that all initializers are idempotent,
 * because this is not dealt with automatically as with constructors.
 *
 * @author Aave, inspired by the OpenZeppelin Initializable contract
 */
abstract contract VersionedInitializable {
  /**
   * @dev Indicates that the contract has been initialized.
   */
  uint256 private lastInitializedRevision = 0;

  /**
   * @dev Indicates that the contract is in the process of being initialized.
   */
  bool private initializing;

  /**
   * @dev Modifier to use in the initializer function of a contract.
   */
  modifier initializer() {
    uint256 revision = getRevision();
    require(
      initializing || isConstructor() || revision > lastInitializedRevision,
      'Contract instance has already been initialized'
    );

    bool isTopLevelCall = !initializing;
    if (isTopLevelCall) {
      initializing = true;
      lastInitializedRevision = revision;
    }

    _;

    if (isTopLevelCall) {
      initializing = false;
    }
  }

  /**
   * @dev returns the revision number of the contract
   * Needs to be defined in the inherited class as a constant.
   **/
  function getRevision() internal pure virtual returns (uint256);

  /**
   * @dev Returns true if and only if the function is running in the constructor
   **/
  function isConstructor() private view returns (bool) {
    // extcodesize checks the size of the code stored in an address, and
    // address returns the current address. Since the code is still not
    // deployed when running a constructor, any checks on its code size will
    // yield zero, making it an effective way to detect if a contract is
    // under construction or not.
    uint256 cs;
    //solium-disable-next-line
    assembly {
      cs := extcodesize(address())
    }
    return cs == 0;
  }

  // Reserved storage space to allow for layout changes in the future.
  uint256[50] private ______gap;
}