What is the gas limit for a fallback function called when the method does not exist?

Question

According to the docs a non matching function identifier will trigger the fallback function. So what happens if someone calls my contract with "thisMethodDoesntExist()", would the fallback have a limit of 2300 like when it is triggered with send / transfer, or will it have gas from by the calling method like .call.value?

Answer 1

Surprisingly no, and the official documentation is indeed not even clear about this.

So here is a short test for printing the gas remaining at the beginning of the fallback function:

Solidity Contract:

pragma solidity 0.6.12;

interface Interface0 {
    function thisMethodDoesntExist() external;
}

contract Contract1 {
    // note that prior to solidity 0.6.0, you need to replace `fallback` with `function`
    fallback() external payable {
        string memory message;
        uint256 x = gasleft();
        while (x > 0) {
            message = string(abi.encodePacked(uint8(x % 10 + 48), message));
            x /= 10;
        }
        revert(message);
    }
}

contract Contract2 {
    function test(Interface0 contract1) external {
        contract1.thisMethodDoesntExist();
    }
}

Truffle 5.x Test:

const Contract1 = artifacts.require("Contract1");
const Contract2 = artifacts.require("Contract2");

contract("test", (accounts) => {
    it("test", async () => {
        contract1 = await Contract1.new();
        contract2 = await Contract2.new();
        try {
            await contract2.test(contract1.address);
        }
        catch (error) {
            console.log(error.message);
        }
    });
});

The printout of this test is:

VM Exception while processing transaction: revert 9328773 -- Reason given: 9328773