Repository: bitcoin/bitcoin
File: src/wallet/spend.cpp (C++)

Code before:
    coin_selection_params.m_cost_of_change = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size) + coin_selection_params.m_change_fee;

    coin_selection_params.m_min_change_target = GenerateChangeTarget(std::floor(recipients_sum / vecSend.size()), coin_selection_params.m_change_fee, rng_fast);

    // The smallest change amount should be:
    // 1. at least equal to dust threshold
    // 2. at least 1 sat greater than fees to spend it at m_discard_feerate
    const auto dust = GetDustThreshold(change_prototype_txout, coin_selection_params.m_discard_feerate);
    const auto change_spend_fee = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size);
    coin_selection_params.min_viable_change = std::max(change_spend_fee + 1, dust);

    // Static vsize overhead + outputs vsize. 4 version, 4 nLocktime, 1 input count, 1 witness overhead (dummy, flag, stack size)
    coin_selection_params.tx_noinputs_size = 10 + GetSizeOfCompactSize(vecSend.size()); // bytes for output count

    // vouts to the payees
    for (const auto& recipient : vecSend)
    {
        CTxOut txout(recipient.nAmount, GetScriptForDestination(recipient.dest));

        // Include the fee cost for outputs.
        coin_selection_params.tx_noinputs_size += ::GetSerializeSize(txout);

        if (IsDust(txout, wallet.chain().relayDustFee())) {
            return util::Error{_("Transaction amount too small")};
        }
        txNew.vout.push_back(txout);
    }

    // Include the fees for things that aren't inputs, excluding the change output
    const CAmount not_input_fees = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.m_subtract_fee_outputs ? 0 : coin_selection_params.tx_noinputs_size);
    CAmount selection_target = recipients_sum + not_input_fees;

    // This can only happen if feerate is 0, and requested destinations are value of 0 (e.g. OP_RETURN)
    // and no pre-selected inputs. This will result in 0-input transaction, which is consensus-invalid anyways
    if (selection_target == 0 && !coin_control.HasSelected()) {
        return util::Error{_("Transaction requires one destination of non-0 value, a non-0 feerate, or a pre-selected input")};
    }

    // Fetch manually selected coins
    PreSelectedInputs preset_inputs;
    if (coin_control.HasSelected()) {
        auto res_fetch_inputs = FetchSelectedInputs(wallet, coin_control, coin_selection_params);
        if (!res_fetch_inputs) return util::Error{util::ErrorString(res_fetch_inputs)};
        preset_inputs = *res_fetch_inputs;
    }

    // Fetch wallet available coins if "other inputs" are
    // allowed (coins automatically selected by the wallet)
    CoinsResult available_coins;
    if (coin_control.m_allow_other_inputs) {
        available_coins = AvailableCoins(wallet, &coin_control, coin_selection_params.m_effective_feerate);

Review comment (suggestion):
Although being a bit pedantic, but as [this commit](https://github.com/bitcoin/bitcoin/pull/30050/commits/a9c7300135f0188daa5bce5491e2daf2dd8da8ae) is supposed to be move-only, the removal of `CTxOut txout...` in line 1107 and the replacement of `push_back` with `emplace_back` in line 1115 could have been part of the previous commit to avoid any confusion for future readers.

Code after:
    FeeCalculation feeCalc;
    coin_selection_params.m_effective_feerate = GetMinimumFeeRate(wallet, coin_control, &feeCalc);
    // Do not, ever, assume that it's fine to change the fee rate if the user has explicitly
    // provided one
    if (coin_control.m_feerate && coin_selection_params.m_effective_feerate > *coin_control.m_feerate) {
        return util::Error{strprintf(_("Fee rate (%s) is lower than the minimum fee rate setting (%s)"), coin_control.m_feerate->ToString(FeeEstimateMode::SAT_VB), coin_selection_params.m_effective_feerate.ToString(FeeEstimateMode::SAT_VB))};
    }
    if (feeCalc.reason == FeeReason::FALLBACK && !wallet.m_allow_fallback_fee) {
        // eventually allow a fallback fee
        return util::Error{strprintf(_("Fee estimation failed. Fallbackfee is disabled. Wait a few blocks or enable %s."), "-fallbackfee")};
    }

    // Calculate the cost of change
    // Cost of change is the cost of creating the change output + cost of spending the change output in the future.
    // For creating the change output now, we use the effective feerate.
    // For spending the change output in the future, we use the discard feerate for now.
    // So cost of change = (change output size * effective feerate) + (size of spending change output * discard feerate)
    coin_selection_params.m_change_fee = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.change_output_size);
    coin_selection_params.m_cost_of_change = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size) + coin_selection_params.m_change_fee;

    coin_selection_params.m_min_change_target = GenerateChangeTarget(std::floor(recipients_sum / vecSend.size()), coin_selection_params.m_change_fee, rng_fast);

    // The smallest change amount should be:
    // 1. at least equal to dust threshold
    // 2. at least 1 sat greater than fees to spend it at m_discard_feerate
    const auto dust = GetDustThreshold(change_prototype_txout, coin_selection_params.m_discard_feerate);
    const auto change_spend_fee = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size);
    coin_selection_params.min_viable_change = std::max(change_spend_fee + 1, dust);

    // Include the fees for things that aren't inputs, excluding the change output
    const CAmount not_input_fees = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.m_subtract_fee_outputs ? 0 : coin_selection_params.tx_noinputs_size);
    CAmount selection_target = recipients_sum + not_input_fees;

    // This can only happen if feerate is 0, and requested destinations are value of 0 (e.g. OP_RETURN)
    // and no pre-selected inputs. This will result in 0-input transaction, which is consensus-invalid anyways
    if (selection_target == 0 && !coin_control.HasSelected()) {
        return util::Error{_("Transaction requires one destination of non-0 value, a non-0 feerate, or a pre-selected input")};
    }

    // Fetch manually selected coins
    PreSelectedInputs preset_inputs;
    if (coin_control.HasSelected()) {
        auto res_fetch_inputs = FetchSelectedInputs(wallet, coin_control, coin_selection_params);
        if (!res_fetch_inputs) return util::Error{util::ErrorString(res_fetch_inputs)};
        preset_inputs = *res_fetch_inputs;
    }

    // Fetch wallet available coins if "other inputs" are
    // allowed (coins automatically selected by the wallet)
    CoinsResult available_coins;
    if (coin_control.m_allow_other_inputs) {