Add CanRefundGas contract

[medvedev1088]

🎉 Description

Something like this:

pragma solidity^0.4.11;

contract CanRefundGas {

    modifier refundGasCost() {
        uint remainingGasStart = msg.gas;

        _;

        uint remainingGasEnd = msg.gas;
        uint usedGas = remainingGasStart - remainingGasEnd;
        // Add intrinsic gas and transfer gas. Need to account for gas stipend as well.
        usedGas += 21000 + 9700;
        // Possibly need to check max gasprice and usedGas here to limit 
        // possibility for abuse.
        uint gasCost = usedGas * tx.gasprice;
        // Refund gas cost
        tx.origin.transfer(gasCost);
    }
}

contract SomeContract is CanRefundGas {

    event SomeEvent(address sender);

    // Need to allow depositing ether to the contract
    function() public payable {
    }

    function doSomething() external refundGasCost {
        SomeEvent(msg.sender);  
    }
}

• [ ] 🐛 This is a bug report.
• [x] 📈 This is a feature request.

Originally discussed here https://ethereum.stackexchange.com/questions/38479/how-to-make-someone-else-pay-for-gas/38517#38517

[nventuro]

Hm, this is rather interesting indeed! Closing due to perceived lack of interest, and the fact that we'll probably move forward with some sort of metatx solution, but we'll be keeping this in the back of our minds, thanks!