这个修饰符nonReentrant是如何工作的？

Question

这是地址0x0d4535644fFeC908e39a711A01852Be40Bfe1C97的源代码，修饰符nonReentrant能防止重入漏洞吗？

contract ReentrancyGuard {
/// @dev counter to allow mutex lock with only one SSTORE operation
uint256 private _guardCounter;

constructor () internal {
    // The counter starts at one to prevent changing it from zero to a non-zero
    // value, which is a more expensive operation.
    _guardCounter = 1;
}

/**
 * @dev Prevents a contract from calling itself, directly or indirectly.
 * Calling a `nonReentrant` function from another `nonReentrant`
 * function is not supported. It is possible to prevent this from happening
 * by making the `nonReentrant` function external, and make it call a
 * `private` function that does the actual work.
 */
modifier nonReentrant() {
    _guardCounter += 1;
    uint256 localCounter = _guardCounter;
    _;
    require(localCounter == _guardCounter);
}

}

Answer 1

它基本上检查localCounter的值只增加了一个。这意味着函数只被调用一次(在事务中)。

实际的函数在_;上执行。在此之前，它将计数器增加1，并将值分配给局部变量(与全局变量_guardCounter相反)。如果函数自身调用，则代码再次进入此阶段，计数器再次增加一个(全局和本地)。

在函数完成执行后，它确保本地计数器与全局计数器相同。只有当函数只被调用一次时，本地值才是相同的，否则全局计数器已经增加了多次。如果它被多次调用，则执行将恢复。