WalletDelegator.sol

// WHATEVER YOU DO, DON'T USE THIS.
// This is a bit of demo code posted to illustrate a principle. It is unaudited,
// probably full of bugs, and definintely not production ready.
// https://twitter.com/nicksdjohnson/status/1041642345467404291

import "./DumbWallet.sol";

contract WalletDelegator {
    mapping(address=>uint) public nonces;

    function callFor(DumbWallet wallet, address target, uint value, bytes data, bytes32 r, bytes32 s, uint8 v) payable {
        bytes32 sigHash = keccak256(abi.encodePacked(wallet, target, value, data, nonces[wallet]));
        address signer = ecrecover(sigHash, v, r, s);
        require(wallet.authorised(signer));
        wallet.invoke.value(msg.value)(target, value, data);
        nonces[wallet]++;
    }
}

This is not reentrancy safe, right?

I would call nonces[wallet]++; right after generating sigHash