"With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody."
Gas costs have observable side effects, therefore someone will depend on them.
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bool isEntered = false;
function doSomething() { if (isEntered) { throw; } isEntered = true; ...; isEntered = false; }
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I don't really understand why they went with this gas-limit solution. I feel like there must be something I'm missing cause its too obvious.
The gas limit on send was more of a belt-and-suspenders thing, not intended as the sole protection; that may have been a bad idea in retrospect, but at the time people were pretty focused on adding every protection possible against another DAO situation.
For some context, gas (or operations expense) usage was never designed for re-entrency protection. After the DAO attack, in order to prevent re-entrency attacks the Solidity interpreter limited the gas of external calls.
It's clever way to solve the problem. However now when we're trying to change around the gas for certain operations, it's jeopardizing previous contracts that relied on this "clever" solution.
To be honest, this is really bad, cause I'm not sure how Ethereum can ever safely adjust gas.
Edit:
I'm not sure how Ethereum can ever safely adjust gas... until they go back and "monkey patch" all the smart contracts that used the external gas call limit. Since we only can see the compiled code, there's no real way to know if a deployer explicitly wanted that external gas call limit, or was just using it for re-entrency protection.