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Generalize the concept of flash loans by keeping track of how much is "owed by the current call", i.e. how much funds have to be paid back before the call ends. If the state changes this owed field is updated immediately to keep the balance and on token transfer the the payback is accounted for. The strong invariant for tokenBalance is now almost trivial, as every hook preserves it (the invariant checks that the hooks are correct). We then check that a call will never increase the owed value, i.e., if something is owed it is paid back before the function exits.
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QGarchery
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We show the solvency invariant only for good tokens (where tokenBalance behave like an ERC-20) and explicitly handle bad tokens that behave non-deterministically (HAVOC_ALL).
This had to solve more problems with non-atomic accounting: At the time the evil token runs, we may be out of sync between the token balance and the supposed token balance, which is similar to a flash loan, that needs to be paid back at the end of the function.
Instead of introducing more variables, this pull request rewrites the whole flash loan mechanism and introduces a field "owed" that basically represents the amount of flash loan taken during this call. We can then show the strong solvency invariant exactly (owed will be negative if tokens were donated to the contract). We also show that owed will be decreased by every call, proving that all flash loans will be paid back.
The accounting is now updated atomically: E.g., when increasing the collateral of a user's position, we make a flash loan by also increasing "owed". When the token transfer happens later, the owed is decreased again by the transferred tokens.
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