fix(chain): prevent overflow in KeychainTxOutIndex::lookahead_to_target - #2253
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CapThunder19 wants to merge 1 commit into
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fix(chain): prevent overflow in KeychainTxOutIndex::lookahead_to_target#2253CapThunder19 wants to merge 1 commit into
KeychainTxOutIndex::lookahead_to_target#2253CapThunder19 wants to merge 1 commit into
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evanlinjin
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Aug 17, 2026
evanlinjin
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Thanks for the PR. Concept ACK.
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| /// Computes how many additional lookahead scripts are needed to cover `target_index` | ||
| /// (inclusive), given the next index that would be derived (`next_index`). | ||
| /// | ||
| /// Returns `None` if `target_index` is already covered (i.e. `target_index < next_index`). | ||
| fn lookahead_delta(target_index: u32, next_index: u32) -> Option<u32> { | ||
| target_index | ||
| .saturating_add(1) | ||
| .checked_sub(next_index) | ||
| .filter(|&index| index > 0) | ||
| } | ||
|
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Please inline this. A separate method here is not helpful.
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| /// `lookahead_delta` must not panic (via overflow) when `target_index` is `u32::MAX`, and | ||
| /// must saturate rather than silently returning a wrong/empty result. | ||
| #[test] | ||
| fn lookahead_delta_does_not_overflow_at_u32_max() { | ||
| // Fresh keychain (next_index = 0): delta should saturate to u32::MAX, not overflow. | ||
| assert_eq!( | ||
| KeychainTxOutIndex::<i32>::lookahead_delta(u32::MAX, 0), | ||
| Some(u32::MAX) | ||
| ); | ||
| // Target already covered by next_index: no lookahead needed. | ||
| assert_eq!( | ||
| KeychainTxOutIndex::<i32>::lookahead_delta(u32::MAX, u32::MAX), | ||
| None | ||
| ); | ||
| // Normal, non-boundary case still behaves as before. | ||
| assert_eq!(KeychainTxOutIndex::<i32>::lookahead_delta(10, 5), Some(6)); | ||
| } | ||
|
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This unit test does not add any value. Please remove.
Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## master #2253 +/- ##
==========================================
+ Coverage 78.36% 78.40% +0.03%
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Files 30 30
Lines 5945 5955 +10
Branches 281 281
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+ Hits 4659 4669 +10
Misses 1210 1210
Partials 76 76
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Description
lookahead_to_targetcomputedtarget_index + 1with uncheckedaddition. With
target_index == u32::MAX, this panics in debug buildsand silently no-ops in release builds (wraps to 0, so the lookahead
request is quietly ignored).
Fixed by extracting the delta calculation into a
lookahead_deltahelper using
saturating_addinstead of+, matching the patternalready used elsewhere in this file.
Testing
Added
lookahead_delta_does_not_overflow_at_u32_max, a unit test on thehelper directly. (Not an integration test calling
lookahead_to_targetend-to-end with
u32::MAX, since that would try to derive ~2^31 realkeys on a fresh index.)
cargo test --features miniscript --libcargo test --features miniscript --test test_keychain_txout_indexcargo fmt --check -p bdk_chaincargo clippy -p bdk_chain --features miniscript --lib -- -D warnings#2251