Understand the functionality of one contract

Question

I am trying to understand the following contract sample:

// Refund contract for trust DAO #16

contract DAO {
    function balanceOf(address addr) returns (uint);
    function transferFrom(address from, address to, uint balance) returns (bool);
    uint public totalSupply;
}

contract WithdrawDAO {
    DAO constant public mainDAO = DAO(0x200450f06520bdd6c527622a273333384d870efb);
    address public trustee = 0x7c81d252d9d1295058cd3620835f37e0eedd8840;

    function withdraw(){
        uint balance = mainDAO.balanceOf(msg.sender);

        if (!mainDAO.transferFrom(msg.sender, this, balance) || !msg.sender.send(balance))
            throw;
    }

    function trusteeWithdraw() {
        trustee.send((this.balance + mainDAO.balanceOf(this)) - mainDAO.totalSupply());
    }
}

I am a bit confused about the withdraw function. What if the first transfer transferFrom always failed due to certain reason? Would it somehow lock the fund and therefore the second fund transfer can never happen? Thanks a lot!

Answer 1

First of all, in Solidity x || y is evaluated in lazy way, so in case x is true, y is not evaluated at all. This means, that in case transferFrom will fail, send will not be called.

Also, throw "reverts" the transaction, i.e. rolls back all changes made be the transaction, including any ether or token transfers. So if transferFrom will fail, the transaction will be reverted, and no changes made by it will be persisted in blockchain.

Though, the fact that transaction was (unsuccessfully) executed will be persisted, and transaction publisher will pay fee for this, and this fee will be collected by miner as a reward for (unsuccessfully) executing the transaction. This is fair, because miner anyway spent resources on this transaction.