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test: fix uint128 overflow in IndexingMath round-trip fuzz - #57
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`_EXP_SCALED_ONE` is a uint56, so Solidity evaluated `_EXP_SCALED_ONE + index - 1` in uint128. Any index within 1e12 of `type(uint128).max` overflowed that addition. The test then reverted with Panic(0x11) before reaching its assertion. This failed the suite on roughly 1 in 20 fuzz seeds. Widen the headroom computation to uint256. Pin the top of the index range with a deterministic regression test.
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Closing since the issue was addressed in 10702a0 |
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Follow-up to #56.
testFuzz_roundTripfails on roughly 1 in 20 fuzz seeds, so it will break CI intermittently on this branch and on anything stacked on it.Cause
The test reserves worst-case round-trip inflation headroom with:
_EXP_SCALED_ONEis auint56andindexis auint128, so Solidity evaluates that expression inuint128. For anyindexwithin1e12oftype(uint128).maxthe addition overflows, and the test reverts withPanic(0x11)before it ever reaches the assertion it is meant to make. The library itself is fine — this is purely a defect in the test's own arithmetic.Reproduce on
d21c0ef:Counterexample:
principal = 1,index = 2**128 - 4.Approach
Widen the computation to
uint256before the addition. I kept the+ index - 1ceiling-division form rather than boundingindexaway from its maximum: bounding would paper over the overflow and silently drop the top of the index range from coverage, which is exactly where the round-trip inflation bound is tightest and most worth pinning.The added
test_roundTrip_atMaxIndexcalls the fuzz body directly atindex == type(uint128).maxfor both the smallest and largest principal, so the regression is pinned deterministically instead of depending on a seed to rediscover it.Verification
forge test— 373 passed, 0 failed.FOUNDRY_FUZZ_RUNS=20000 forge test --match-path test/IndexingMath.t.sol— 27 passed, to confirm no other seed-dependent flake is hiding in the file.src/libs/IndexingMath.solcoverage stays at 100.00% lines / 100.00% branches / 100.00% funcs.forge fmt --checkdoes not flag this file. It does report pre-existing diffs acrosssrc/andscript/, untouched here.