ReEntry_reinitializationofreturnval.sol

// SPDX-License-Identifier: MIT
pragma solidity  0.8.15;

/// Q: Can I use declared return values to prevent reentrancy attacks?
/// A: No.
/// @notice return values of function are reinitialized on each unique entry and as such cannot prevent rerentry attacks
/// pattern might be useful if return value is global

contract Reentrance {

    uint public fliptest1;
    Attacker A;

    constructor() {
        A = new Attacker();
    }

    error cannotReenter(uint x, bool z);
    bool s;
    function notReentrant() public returns (bool) {
        if(s) revert cannotReenter(fliptest1, s);
        s= true;
        bytes memory b = abi.encode(msg.sig);
        unchecked { ++fliptest1; }
        if (fliptest1 > 10) return s;
        A.reEnter(b);
    }

    function getFlip2() public view returns (uint) {
        return A.fliptest2();
    }
}

contract Attacker {
    uint public fliptest2;

    function reEnter(bytes memory sig_) public {
        msg.sender.call(sig_);
        unchecked { ++fliptest2; }
    }
}