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consensus/doc: explain unreachable bad-txns-fee-outofrange check

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After the previous conditions were validated, document why `bad-txns-fee-outofrange` is unreachable once `nValueIn` and `value_out` are `MoneyRange` and `nValueIn >= value_out`.

Although unreachable, keep the check itself in place (instead of removing) as it's part of consensus-critical code; the comment serves as a proof for future refactors.

Inspired by b-c-cov coverage reports:
* "bad-txns-fee-outofrange" - https://maflcko.github.io/b-c-cov/test_bitcoin.coverage/src/consensus/tx_verify.cpp.gcov.html#L200
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Lőrinc
2026-01-31 19:20:20 +01:00
parent 232a2bce90
commit 82ef92c8d0
1 changed files with 5 additions and 0 deletions
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5
src/consensus/tx_verify.cpp
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@@ -197,6 +197,11 @@ bool Consensus::CheckTxInputs(const CTransaction& tx, TxValidationState& state,
// Tally transaction fees
const CAmount txfee_aux = nValueIn - value_out;
if (!MoneyRange(txfee_aux)) {
// Unreachable, given the following preconditions:
// * `value_out` comes from `tx.GetValueOut()`, which throws unless `MoneyRange(value_out)` and asserts `MoneyRange(nValueOut)` on return.
// * `MoneyRange(nValueIn)` was enforced in the input loop.
// * `nValueIn < value_out` was handled above, so `nValueIn >= value_out` here (and `txfee_aux >= 0`).
// Therefore `0 <= txfee_aux = nValueIn - value_out <= nValueIn <= MAX_MONEY`.
return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-txns-fee-outofrange");
}