Smart contracts calling each other, costs and atomicity doubt

Question

According to what I know, If you invoke function F on Contract C, you pay the gas fees related to F's operations and if for some reason something goes wrong, all state changes performed by F until that point are revoked. F works in an atomic way and the human being account that invoked F pays for the gas fees (even if operations are rolled-back).

The doubt I have comes from a situation in which F invokes some other function F1 on Contract C1, which can invoke some other function F2 on Contract C2 and so on.... human -> (C) F -> (C1) F1 -> (C2) F2 -> ...

First of all, it is my understanding that the original account would only pay for the gas fees of operations performed by F, then it would by C who pays gas fees for operations performed by F1, C1 would pay for operations on F2 on so on....

But what if Fn fails? would all state modifications performed by F up to Fn-1 be rolled-back? would the original account as well as contracts from C up to Cn-1 waste their gas fees?

Answer 1

The user pays gas for (C) F -> (C1) F1 -> (C2) F2 -> ...

Those execute in the context of a single atomic transaction. It might help to consider that it is usually not possible to run F without knowing what F1 returned.

Generally, if F2 fails, then the error will bubble up to C and transaction T. If T reverts then they all revert. It will turn up as a failed transaction and a failed transaction can't change anything.

An exception to that complete failure can be built in to a contract. C1 can call C2 in a way that does not automatically revert if C2 reverts and instead press on, usually branching on success/failure of C2. You can sometimes see that on Etherscan when it notes that "part of the transaction failed" even though the overall transaction succeeded.

Hope it helps