> What happens if the loop executes some non-idempotent calls? I guess printf debug still has some value :)

Then they'll do the same thing when you replay.

Non-idempotent system calls are tricky because they interact with the outside world - but that's still OK.

In Time Travel Debug, the process you're debugging is essentially in the Matrix. When it's being recorded everything acts as normal (and it'll see the real results of those non-idempotent calls).

When it's being debugged, any interaction with the outside system is prevented and replaced with the behaviour we saw at record time. It'll still think it's doing the non-idempotent calls, they just won't change (or depend upon) the state of the rest of the system.