Idea of a modifier for functions that call externally to assert the gas consumed in between. Protects against re-entrancy.

gas.sol

contract ConservativeSender {
  ERC20 token;
   
  modifier max_gas(uint max_delta) {
    uint initialGas = msg.gas;
    _;
    assert(initialGas - msg.gas < max_delta);
  }
  
  function proxyTransfer(address _to, uint _value)
           max_gas(110000) {
           
    token.transfer(_to, _value);
  }
}