openzeppelin-contracts/contracts/ReentrancyGuard.sol

pragma solidity ^0.4.8;

/**
 * @title Helps contracts guard agains rentrancy attacks.
 * @author Remco Bloemen <remco@2π.com>
 * @notice If you mark a function `nonReentrant`, you should also
 * mark it `external`.
 */
contract ReentrancyGuard {

  /**
   * @dev We use a single lock for the whole contract. 
   */
  bool private rentrancy_lock = false;

  /**
   * @dev Prevents a contract from calling itself, directly or indirectly.
   * @notice 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 a `external`
   * wrapper marked as `nonReentrant`.
   */
  modifier nonReentrant() {
    if(rentrancy_lock == false) {
      rentrancy_lock = true;
      _;
      rentrancy_lock = false;
    } else {
      throw;
    }
  }

}