ReentrancyGuard.sol

pragma solidity >=0.5.0 <0.6.0;

/**
 * @title Helps contracts guard against reentrancy attacks.
 * @author Remco Bloemen <remco@2π.com>, Eenae <alexey@mixbytes.io>
 * @dev If you mark a function `nonReentrant`, you should also
 * mark it `external`.
 */
contract ReentrancyGuard {
    /// @dev Constant for unlocked guard state - non-zero to prevent extra gas costs.
    /// See: https://github.com/OpenZeppelin/openzeppelin-solidity/issues/1056
    uint256 internal constant REENTRANCY_GUARD_FREE = 1;

    /// @dev Constant for locked guard state
    uint256 internal constant REENTRANCY_GUARD_LOCKED = 2;

    /**
     * @dev We use a single lock for the whole contract.
     */
    uint256 internal reentrancyLock = REENTRANCY_GUARD_FREE;

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * If you mark a function `nonReentrant`, you should also
     * mark it `external`. Calling one `nonReentrant` function from
     * another is not supported. Instead, you can implement a
     * `private` function doing the actual work, and an `external`
     * wrapper marked as `nonReentrant`.
     */
    modifier nonReentrant() {
        require(reentrancyLock == REENTRANCY_GUARD_FREE, "nonReentrant");
        reentrancyLock = REENTRANCY_GUARD_LOCKED;
        _;
        reentrancyLock = REENTRANCY_GUARD_FREE;
    }
}