fix: define Candidate::weight as segwit-serialized, fixing multi-input candidates - #61
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…oups In a segwit transaction every legacy input still serializes an empty witness. `input_weight` accounted for that byte by adding it once per non-segwit *candidate*, which is only correct when a candidate represents a single input. A candidate grouping N legacy inputs was undercounted by N-1 weight units, and one mixing legacy and segwit spends by its full legacy count -- `is_segwit` had to be true for the marker and flag, which suppressed the adjustment its legacy inputs still needed. Fix it at the definition instead of the use. `Candidate::weight` now means the input weight as serialized in a segwit transaction, so a legacy input counts its empty witness up front, where each input is still visible individually. `is_segwit` is then only the transaction-level marker and flag question, and grouping inputs of either script type into one candidate works. `input_weight` correspondingly stops adjusting per candidate: if nothing selected is segwit the transaction has no witness section at all, so every input takes its byte back. Existing tests only used single-input candidates, which is why this went unnoticed. The new test weighs one transaction under three groupings of the same three inputs -- one per candidate, the two legacy inputs shared, and all three shared -- and asserts all agree with the real serialized weight. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Bug:
CoinSelector::input_weightundercounts the transaction weight when aCandidaterepresents more than one input.In a segwit transaction, every input serializes a witness — a legacy input serializes an empty one, costing 1 WU. A segwit input already counts its witness in
Candidate::weight; a legacy input doesn't, soinput_weightcompensated:But the cost is 1 WU per legacy input, not per candidate:
is_segwitistrue, so the branch is skipped)Fix: put the byte into
weightwhen the candidate is built, while each input is still individually visible.Candidate::weightnow means the weight of the inputs as serialized in a segwit transaction: legacy inputs include their empty-witness byte.is_segwitonly decides whether the tx pays the 2 WU witness marker + flag.input_weightgives 1 WU per input back.⚠ Silent breaking change: hand-built candidates that use
txin.legacy_weight().to_wu()still compile but are now 1 WU short per legacy input — usetxin.legacy_weight().to_wu() + 1.Candidate::newalready does this for you.Test: one real tx (2 legacy + 1 segwit inputs) weighed under three groupings — one candidate per input, the legacy pair grouped, all three in one candidate. Each must equal
tx.weight(); all three fail on unfixedmaster.🤖 Generated with Claude Code