spend: deterministic, canonical input construction - #2192
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Coin selection (both the BnB algorithm and the descending-value-per-weight fallback) depends on the order in which candidate coins are supplied. Callers build that list from DatabaseConnection::coins, which returns a HashMap, so the order is randomized per run. When several coins are equivalent for selection (equal value per weight unit), which coins get selected and the resulting input order are non-deterministic. Sort candidates by outpoint before selecting, so ties break deterministically and a given set of coins always yields the same spend. This keeps spend and recovery reproducible and testable.
Building a coin's transaction input and PSBT input (its sequence, witness_utxo, and whether a non_witness_utxo is included for non-Taproot) is inlined in create_spend. A payjoin receiver being added separately reimplements the same construction and diverges from it: it hardcodes the sequence and always sets a non_witness_utxo, even for Taproot, making a payjoin input distinguishable from an ordinary spend input. Extract the construction into a shared coin_to_psbt_input helper so spend, recovery and payjoin build inputs identically. This is a pure refactor of the spend path here; the payjoin receiver will reuse the helper once rebased on top.
bc1cindy
marked this pull request as ready for review
July 9, 2026 13:54
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build spend/recovery inputs one canonical, deterministic way, so callers reusing this path can't be told apart from a normal spend.
coin_to_psbt_inputhelper: a single place builds a coin's input (sequence, witness / non-witness utxo), instead of each path diverging.divergent input construction is itself a wallet fingerprint.
ref: payjoin/rust-payjoin#1597 (comment)
closes #2191