Gas refunds via “Self-destruct” prior to a revert()

Question

In a transaction that eventually calls revert(), if prior to the revert we have SELFDESTRUCT some contracts and thus received a gas refund, does the gas refund also get reverted?

Answer 1

An interesting question! I could not find anything in the docs about this, but I also couldn't find anything about selfdestruct's gas refund (until I noticed this How do gas refunds work? ).

I did some experimenting with the following contracts:

pragma solidity ^0.7.0;

contract A {
    function die() public {
        selfdestruct(msg.sender);
    }
}

contract B
{
    A aRef;
    function init() public {
        aRef = new A();
    }
    function killIt() public {
        uint i;
        for (i = 0; i < 10000; i++) { }
        aRef.die();
        revert();
    }
}

So what I did was:

1. Deploy B
2. Run init
3. Run killIt

I checked the gas costs in two cases:

1. Executing just like I pasted above. The call to killIt costs 588914
2. Executing with line aRef.die(); commented out. The call to killIt costs 581421

Obviously the selfdestruct's gas refund is not applied here so the refund is wasted.

Answer 2

A gas refund is only applied at the end of a transaction. Any state changes are reverted immediately when a transaction is reverted, so only the remaining gas for the transaction is refunded. That means that the gas refund is also reverted.