What if a dynamic call at the end of a function uses all the remaining gas of the caller

dynamic-callexternal-functiongas-limitout-of-gassolidity

Suppose we have a contact like this:

contract x {

   function y() external
   {
      // do some magic!
      if (some_condition) {
        another_contract_address.call(abi.encodeWithSignature("another_contract_func()"));
      }
   }

}

which does something in a function y and then dynamically calls another method of another contract as its last sentence under specific conditions. As far as I know, all the remaining gas of the caller contract will be used for executing the callee function. Regarding the fact that this dynamic call is the last expression in y, what will happen if this dynamic call exhausts all the remaining gas? Does the transaction finish successfully (due to the fact that the RETURN opcode uses 0 gas) or it will be failed with an out of gas error?

Best Answer

The states will get reverted back to initial state (the state when you called the function). But the gas fees and incentive which you'ld have provided to miner will not be refunded . Transaction block will be present there for eternity. any amount of ether if you have paid will be returned back. Dynamic call will function like if it was a function call of same contract. So on revertion process the state variable of dynamic contract calling and caller contract both will get reverted.

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[Ethereum] How to know, inside the contract function, the amount of gas that will be refunded in the end

There's no way to know the value of the EVM's refund counter from inside the EVM; if you want to account for refunds, you'll have to do them yourself, manually.

[Ethereum] passing an element of struct array as a function argument is pass by value or reference

The first argument of function StringsEqual is passed by reference and not by value because you are explicitly specifying state variable _a in storage.

To declare function StringsEqual as pure you need to pass an argument by value as following -

function stringsEqual(string memory _a, string memory _b) internal pure returns (bool) {
    bytes memory a = bytes(_a);
    bytes memory b = bytes(_b);
    if (a.length != b.length)
        return false;
    // @todo unroll this loop
    for (uint i = 0; i < a.length; i ++)
    {
        if (a[i] != b[i])
            return false;
    }
    return true;

}

To conclude: when you are explicitly defining storage type of an argument asstorage you are passing an argument to a function by reference.

If you don't specify storage type of a function argument then function arguments are always in memory and you are passing an argument to a function by value.

To add further clarification I tried following :

function stringsEqual(string storage  _a, string  _b) internal  returns (bool) {
    bytes memory a = bytes(_a);
    bytes memory b = bytes(_b);
    if (a.length != b.length)
        return false;
    // @todo unroll this loop
    for (uint i = 0; i < a.length; i ++)
    {
        if (a[i] != b[i])
            return false;
    }
    _a = "xyz";
    _b = "xyz";
    return true;

}

And I get the following error which confirms that argument is passed by reference :

browser/pass_by_reference_value.sol:198:14: TypeError: Type literal_string "xyz" is not implicitly convertible to expected type string storage pointer.

Best Answer

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[Ethereum] passing an element of struct array as a function argument is pass by value or reference

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