spend.cpp

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// Copyright (c) 2021-2022 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#include <algorithm>
#include <common/args.h>
#include <common/messages.h>
#include <common/system.h>
#include <consensus/amount.h>
#include <consensus/validation.h>
#include <interfaces/chain.h>
#include <node/types.h>
#include <numeric>
#include <policy/policy.h>
#include <primitives/transaction.h>
#include <script/script.h>
#include <script/signingprovider.h>
#include <script/solver.h>
#include <util/check.h>
#include <util/moneystr.h>
#include <util/rbf.h>
#include <util/trace.h>
#include <util/translation.h>
#include <util/transaction_identifier.h>
#include <wallet/coincontrol.h>
#include <wallet/fees.h>
#include <wallet/receive.h>
#include <wallet/spend.h>
#include <wallet/transaction.h>
#include <wallet/wallet.h>
 
#include <cmath>
 
using common::StringForFeeReason;
using common::TransactionErrorString;
using interfaces::FoundBlock;
using node::TransactionError;
 
TRACEPOINT_SEMAPHORE(coin_selection, selected_coins);
TRACEPOINT_SEMAPHORE(coin_selection, normal_create_tx_internal);
TRACEPOINT_SEMAPHORE(coin_selection, attempting_aps_create_tx);
TRACEPOINT_SEMAPHORE(coin_selection, aps_create_tx_internal);
 
namespace wallet {
static constexpr size_t OUTPUT_GROUP_MAX_ENTRIES{100};
 
static bool IsSegwit(const Descriptor& desc) {
    if (const auto typ = desc.GetOutputType()) return *typ != OutputType::LEGACY;
    return false;
}
 
static bool UseMaxSig(const std::optional<CTxIn>& txin, const CCoinControl* coin_control) {
    // Use max sig if watch only inputs were used or if this particular input is an external input
    // to ensure a sufficient fee is attained for the requested feerate.
    return coin_control && (coin_control->fAllowWatchOnly || (txin && coin_control->IsExternalSelected(txin->prevout)));
}
 
static std::optional<int64_t> MaxInputWeight(const Descriptor& desc, const std::optional<CTxIn>& txin,
                                             const CCoinControl* coin_control, const bool tx_is_segwit,
                                             const bool can_grind_r) {
    if (const auto sat_weight = desc.MaxSatisfactionWeight(!can_grind_r || UseMaxSig(txin, coin_control))) {
        if (const auto elems_count = desc.MaxSatisfactionElems()) {
            const bool is_segwit = IsSegwit(desc);
            // Account for the size of the scriptsig and the number of elements on the witness stack. Note
            // that if any input in the transaction is spending a witness program, we need to specify the
            // witness stack size for every input regardless of whether it is segwit itself.
            // NOTE: this also works in case of mixed scriptsig-and-witness such as in p2sh-wrapped segwit v0
            // outputs. In this case the size of the scriptsig length will always be one (since the redeemScript
            // is always a push of the witness program in this case, which is smaller than 253 bytes).
            const int64_t scriptsig_len = is_segwit ? 1 : GetSizeOfCompactSize(*sat_weight / WITNESS_SCALE_FACTOR);
            const int64_t witstack_len = is_segwit ? GetSizeOfCompactSize(*elems_count) : (tx_is_segwit ? 1 : 0);
            // previous txid + previous vout + sequence + scriptsig len + witstack size + scriptsig or witness
            // NOTE: sat_weight already accounts for the witness discount accordingly.
            return (32 + 4 + 4 + scriptsig_len) * WITNESS_SCALE_FACTOR + witstack_len + *sat_weight;
        }
    }
 
    return {};
}
 
int CalculateMaximumSignedInputSize(const CTxOut& txout, const COutPoint outpoint, const SigningProvider* provider, bool can_grind_r, const CCoinControl* coin_control)
{
    if (!provider) return -1;
 
    if (const auto desc = InferDescriptor(txout.scriptPubKey, *provider)) {
        if (const auto weight = MaxInputWeight(*desc, {}, coin_control, true, can_grind_r)) {
            return static_cast<int>(GetVirtualTransactionSize(*weight, 0, 0));
        }
    }
 
    return -1;
}
 
int CalculateMaximumSignedInputSize(const CTxOut& txout, const CWallet* wallet, const CCoinControl* coin_control)
{
    const std::unique_ptr<SigningProvider> provider = wallet->GetSolvingProvider(txout.scriptPubKey);
    return CalculateMaximumSignedInputSize(txout, COutPoint(), provider.get(), wallet->CanGrindR(), coin_control);
}
 
static std::unique_ptr<Descriptor> GetDescriptor(const CWallet* wallet, const CCoinControl* coin_control,
                                                 const CScript script_pubkey)
{
    MultiSigningProvider providers;
    for (const auto spkman: wallet->GetScriptPubKeyMans(script_pubkey)) {
        providers.AddProvider(spkman->GetSolvingProvider(script_pubkey));
    }
    if (coin_control) {
        providers.AddProvider(std::make_unique<FlatSigningProvider>(coin_control->m_external_provider));
    }
    return InferDescriptor(script_pubkey, providers);
}
 
static std::optional<int64_t> GetSignedTxinWeight(const CWallet* wallet, const CCoinControl* coin_control,
                                                  const CTxIn& txin, const CTxOut& txo, const bool tx_is_segwit,
                                                  const bool can_grind_r)
{
    // If weight was provided, use that.
    std::optional<int64_t> weight;
    if (coin_control && (weight = coin_control->GetInputWeight(txin.prevout))) {
        return weight.value();
    }
 
    // Otherwise, use the maximum satisfaction size provided by the descriptor.
    std::unique_ptr<Descriptor> desc{GetDescriptor(wallet, coin_control, txo.scriptPubKey)};
    if (desc) return MaxInputWeight(*desc, {txin}, coin_control, tx_is_segwit, can_grind_r);
 
    return {};
}
 
// txouts needs to be in the order of tx.vin
TxSize CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, const std::vector<CTxOut>& txouts, const CCoinControl* coin_control)
{
    // version + nLockTime + input count + output count
    int64_t weight = (4 + 4 + GetSizeOfCompactSize(tx.vin.size()) + GetSizeOfCompactSize(tx.vout.size())) * WITNESS_SCALE_FACTOR;
    // Whether any input spends a witness program. Necessary to run before the next loop over the
    // inputs in order to accurately compute the compactSize length for the witness data per input.
    bool is_segwit = std::any_of(txouts.begin(), txouts.end(), [&](const CTxOut& txo) {
        std::unique_ptr<Descriptor> desc{GetDescriptor(wallet, coin_control, txo.scriptPubKey)};
        if (desc) return IsSegwit(*desc);
        return false;
    });
    // Segwit marker and flag
    if (is_segwit) weight += 2;
 
    // Add the size of the transaction outputs.
    for (const auto& txo : tx.vout) weight += GetSerializeSize(txo) * WITNESS_SCALE_FACTOR;
 
    // Add the size of the transaction inputs as if they were signed.
    for (uint32_t i = 0; i < txouts.size(); i++) {
        const auto txin_weight = GetSignedTxinWeight(wallet, coin_control, tx.vin[i], txouts[i], is_segwit, wallet->CanGrindR());
        if (!txin_weight) return TxSize{-1, -1};
        assert(*txin_weight > -1);
        weight += *txin_weight;
    }
 
    // It's ok to use 0 as the number of sigops since we never create any pathological transaction.
    return TxSize{GetVirtualTransactionSize(weight, 0, 0), weight};
}
 
TxSize CalculateMaximumSignedTxSize(const CTransaction &tx, const CWallet *wallet, const CCoinControl* coin_control)
{
    std::vector<CTxOut> txouts;
    // Look up the inputs. The inputs are either in the wallet, or in coin_control.
    for (const CTxIn& input : tx.vin) {
        const auto mi = wallet->mapWallet.find(input.prevout.hash);
        // Can not estimate size without knowing the input details
        if (mi != wallet->mapWallet.end()) {
            assert(input.prevout.n < mi->second.tx->vout.size());
            txouts.emplace_back(mi->second.tx->vout.at(input.prevout.n));
        } else if (coin_control) {
            const auto& txout{coin_control->GetExternalOutput(input.prevout)};
            if (!txout) return TxSize{-1, -1};
            txouts.emplace_back(*txout);
        } else {
            return TxSize{-1, -1};
        }
    }
    return CalculateMaximumSignedTxSize(tx, wallet, txouts, coin_control);
}
 
size_t CoinsResult::Size() const
{
    size_t size{0};
    for (const auto& it : coins) {
        size += it.second.size();
    }
    return size;
}
 
std::vector<COutput> CoinsResult::All() const
{
    std::vector<COutput> all;
    all.reserve(coins.size());
    for (const auto& it : coins) {
        all.insert(all.end(), it.second.begin(), it.second.end());
    }
    return all;
}
 
void CoinsResult::Clear() {
    coins.clear();
}
 
void CoinsResult::Erase(const std::unordered_set<COutPoint, SaltedOutpointHasher>& coins_to_remove)
{
    for (auto& [type, vec] : coins) {
        auto remove_it = std::remove_if(vec.begin(), vec.end(), [&](const COutput& coin) {
            // remove it if it's on the set
            if (coins_to_remove.count(coin.outpoint) == 0) return false;
 
            // update cached amounts
            total_amount -= coin.txout.nValue;
            if (coin.HasEffectiveValue()) total_effective_amount = *total_effective_amount - coin.GetEffectiveValue();
            return true;
        });
        vec.erase(remove_it, vec.end());
    }
}
 
void CoinsResult::Shuffle(FastRandomContext& rng_fast)
{
    for (auto& it : coins) {
        std::shuffle(it.second.begin(), it.second.end(), rng_fast);
    }
}
 
void CoinsResult::Add(OutputType type, const COutput& out)
{
    coins[type].emplace_back(out);
    total_amount += out.txout.nValue;
    if (out.HasEffectiveValue()) {
        total_effective_amount = total_effective_amount.has_value() ?
                *total_effective_amount + out.GetEffectiveValue() : out.GetEffectiveValue();
    }
}
 
static OutputType GetOutputType(TxoutType type, bool is_from_p2sh)
{
    switch (type) {
        case TxoutType::WITNESS_V1_TAPROOT:
            return OutputType::BECH32M;
        case TxoutType::WITNESS_V0_KEYHASH:
        case TxoutType::WITNESS_V0_SCRIPTHASH:
            if (is_from_p2sh) return OutputType::P2SH_SEGWIT;
            else return OutputType::BECH32;
        case TxoutType::SCRIPTHASH:
        case TxoutType::PUBKEYHASH:
            return OutputType::LEGACY;
        default:
            return OutputType::UNKNOWN;
    }
}
 
// Fetch and validate the coin control selected inputs.
// Coins could be internal (from the wallet) or external.
util::Result<PreSelectedInputs> FetchSelectedInputs(const CWallet& wallet, const CCoinControl& coin_control,
                                            const CoinSelectionParams& coin_selection_params)
{
    PreSelectedInputs result;
    const bool can_grind_r = wallet.CanGrindR();
    std::map<COutPoint, CAmount> map_of_bump_fees = wallet.chain().calculateIndividualBumpFees(coin_control.ListSelected(), coin_selection_params.m_effective_feerate);
    for (const COutPoint& outpoint : coin_control.ListSelected()) {
        int64_t input_bytes = coin_control.GetInputWeight(outpoint).value_or(-1);
        if (input_bytes != -1) {
            input_bytes = GetVirtualTransactionSize(input_bytes, 0, 0);
        }
        CTxOut txout;
        if (auto ptr_wtx = wallet.GetWalletTx(outpoint.hash)) {
            // Clearly invalid input, fail
            if (ptr_wtx->tx->vout.size() <= outpoint.n) {
                return util::Error{strprintf(_("Invalid pre-selected input %s"), outpoint.ToString())};
            }
            txout = ptr_wtx->tx->vout.at(outpoint.n);
            if (input_bytes == -1) {
                input_bytes = CalculateMaximumSignedInputSize(txout, &wallet, &coin_control);
            }
        } else {
            // The input is external. We did not find the tx in mapWallet.
            const auto out{coin_control.GetExternalOutput(outpoint)};
            if (!out) {
                return util::Error{strprintf(_("Not found pre-selected input %s"), outpoint.ToString())};
            }
 
            txout = *out;
        }
 
        if (input_bytes == -1) {
            input_bytes = CalculateMaximumSignedInputSize(txout, outpoint, &coin_control.m_external_provider, can_grind_r, &coin_control);
        }
 
        if (input_bytes == -1) {
            return util::Error{strprintf(_("Not solvable pre-selected input %s"), outpoint.ToString())}; // Not solvable, can't estimate size for fee
        }
 
        /* Set some defaults for depth, spendable, solvable, safe, time, and from_me as these don't matter for preset inputs since no selection is being done. */
        COutput output(outpoint, txout, /*depth=*/ 0, input_bytes, /*spendable=*/ true, /*solvable=*/ true, /*safe=*/ true, /*time=*/ 0, /*from_me=*/ false, coin_selection_params.m_effective_feerate);
        output.ApplyBumpFee(map_of_bump_fees.at(output.outpoint));
        result.Insert(output, coin_selection_params.m_subtract_fee_outputs);
    }
    return result;
}
 
CoinsResult AvailableCoins(const CWallet& wallet,
                           const CCoinControl* coinControl,
                           std::optional<CFeeRate> feerate,
                           const CoinFilterParams& params)
{
    AssertLockHeld(wallet.cs_wallet);
 
    CoinsResult result;
    // Either the WALLET_FLAG_AVOID_REUSE flag is not set (in which case we always allow), or we default to avoiding, and only in the case where
    // a coin control object is provided, and has the avoid address reuse flag set to false, do we allow already used addresses
    bool allow_used_addresses = !wallet.IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE) || (coinControl && !coinControl->m_avoid_address_reuse);
    const int min_depth = {coinControl ? coinControl->m_min_depth : DEFAULT_MIN_DEPTH};
    const int max_depth = {coinControl ? coinControl->m_max_depth : DEFAULT_MAX_DEPTH};
    const bool only_safe = {coinControl ? !coinControl->m_include_unsafe_inputs : true};
    const bool can_grind_r = wallet.CanGrindR();
    std::vector<COutPoint> outpoints;
 
    std::set<Txid> trusted_parents;
    for (const auto& entry : wallet.mapWallet)
    {
        const Txid& txid = entry.first;
        const CWalletTx& wtx = entry.second;
 
        if (wallet.IsTxImmatureCoinBase(wtx) && !params.include_immature_coinbase)
            continue;
 
        int nDepth = wallet.GetTxDepthInMainChain(wtx);
        if (nDepth < 0)
            continue;
 
        // We should not consider coins which aren't at least in our mempool
        // It's possible for these to be conflicted via ancestors which we may never be able to detect
        if (nDepth == 0 && !wtx.InMempool())
            continue;
 
        bool safeTx = CachedTxIsTrusted(wallet, wtx, trusted_parents);
 
        // We should not consider coins from transactions that are replacing
        // other transactions.
        //
        // Example: There is a transaction A which is replaced by bumpfee
        // transaction B. In this case, we want to prevent creation of
        // a transaction B' which spends an output of B.
        //
        // Reason: If transaction A were initially confirmed, transactions B
        // and B' would no longer be valid, so the user would have to create
        // a new transaction C to replace B'. However, in the case of a
        // one-block reorg, transactions B' and C might BOTH be accepted,
        // when the user only wanted one of them. Specifically, there could
        // be a 1-block reorg away from the chain where transactions A and C
        // were accepted to another chain where B, B', and C were all
        // accepted.
        if (nDepth == 0 && wtx.mapValue.count("replaces_txid")) {
            safeTx = false;
        }
 
        // Similarly, we should not consider coins from transactions that
        // have been replaced. In the example above, we would want to prevent
        // creation of a transaction A' spending an output of A, because if
        // transaction B were initially confirmed, conflicting with A and
        // A', we wouldn't want to the user to create a transaction D
        // intending to replace A', but potentially resulting in a scenario
        // where A, A', and D could all be accepted (instead of just B and
        // D, or just A and A' like the user would want).
        if (nDepth == 0 && wtx.mapValue.count("replaced_by_txid")) {
            safeTx = false;
        }
 
        if (only_safe && !safeTx) {
            continue;
        }
 
        if (nDepth < min_depth || nDepth > max_depth) {
            continue;
        }
 
        bool tx_from_me = CachedTxIsFromMe(wallet, wtx, ISMINE_ALL);
 
        for (unsigned int i = 0; i < wtx.tx->vout.size(); i++) {
            const CTxOut& output = wtx.tx->vout[i];
            const COutPoint outpoint(txid, i);
 
            if (output.nValue < params.min_amount || output.nValue > params.max_amount)
                continue;
 
            // Skip manually selected coins (the caller can fetch them directly)
            if (coinControl && coinControl->HasSelected() && coinControl->IsSelected(outpoint))
                continue;
 
            if (wallet.IsLockedCoin(outpoint) && params.skip_locked)
                continue;
 
            if (wallet.IsSpent(outpoint))
                continue;
 
            isminetype mine = wallet.IsMine(output);
 
            if (mine == ISMINE_NO) {
                continue;
            }
 
            if (!allow_used_addresses && wallet.IsSpentKey(output.scriptPubKey)) {
                continue;
            }
 
            std::unique_ptr<SigningProvider> provider = wallet.GetSolvingProvider(output.scriptPubKey);
 
            int input_bytes = CalculateMaximumSignedInputSize(output, COutPoint(), provider.get(), can_grind_r, coinControl);
            // Because CalculateMaximumSignedInputSize infers a solvable descriptor to get the satisfaction size,
            // it is safe to assume that this input is solvable if input_bytes is greater than -1.
            bool solvable = input_bytes > -1;
            bool spendable = ((mine & ISMINE_SPENDABLE) != ISMINE_NO) || (((mine & ISMINE_WATCH_ONLY) != ISMINE_NO) && (coinControl && coinControl->fAllowWatchOnly && solvable));
 
            // Filter by spendable outputs only
            if (!spendable && params.only_spendable) continue;
 
            // Obtain script type
            std::vector<std::vector<uint8_t>> script_solutions;
            TxoutType type = Solver(output.scriptPubKey, script_solutions);
 
            // If the output is P2SH and solvable, we want to know if it is
            // a P2SH (legacy) or one of P2SH-P2WPKH, P2SH-P2WSH (P2SH-Segwit). We can determine
            // this from the redeemScript. If the output is not solvable, it will be classified
            // as a P2SH (legacy), since we have no way of knowing otherwise without the redeemScript
            bool is_from_p2sh{false};
            if (type == TxoutType::SCRIPTHASH && solvable) {
                CScript script;
                if (!provider->GetCScript(CScriptID(uint160(script_solutions[0])), script)) continue;
                type = Solver(script, script_solutions);
                is_from_p2sh = true;
            }
 
            result.Add(GetOutputType(type, is_from_p2sh),
                       COutput(outpoint, output, nDepth, input_bytes, spendable, solvable, safeTx, wtx.GetTxTime(), tx_from_me, feerate));
 
            outpoints.push_back(outpoint);
 
            // Checks the sum amount of all UTXO's.
            if (params.min_sum_amount != MAX_MONEY) {
                if (result.GetTotalAmount() >= params.min_sum_amount) {
                    return result;
                }
            }
 
            // Checks the maximum number of UTXO's.
            if (params.max_count > 0 && result.Size() >= params.max_count) {
                return result;
            }
        }
    }
 
    if (feerate.has_value()) {
        std::map<COutPoint, CAmount> map_of_bump_fees = wallet.chain().calculateIndividualBumpFees(outpoints, feerate.value());
 
        for (auto& [_, outputs] : result.coins) {
            for (auto& output : outputs) {
                output.ApplyBumpFee(map_of_bump_fees.at(output.outpoint));
            }
        }
    }
 
    return result;
}
 
CoinsResult AvailableCoinsListUnspent(const CWallet& wallet, const CCoinControl* coinControl, CoinFilterParams params)
{
    params.only_spendable = false;
    return AvailableCoins(wallet, coinControl, /*feerate=*/ std::nullopt, params);
}
 
const CTxOut& FindNonChangeParentOutput(const CWallet& wallet, const COutPoint& outpoint)
{
    AssertLockHeld(wallet.cs_wallet);
    const CWalletTx* wtx{Assert(wallet.GetWalletTx(outpoint.hash))};
 
    const CTransaction* ptx = wtx->tx.get();
    int n = outpoint.n;
    while (OutputIsChange(wallet, ptx->vout[n]) && ptx->vin.size() > 0) {
        const COutPoint& prevout = ptx->vin[0].prevout;
        const CWalletTx* it = wallet.GetWalletTx(prevout.hash);
        if (!it || it->tx->vout.size() <= prevout.n ||
            !wallet.IsMine(it->tx->vout[prevout.n])) {
            break;
        }
        ptx = it->tx.get();
        n = prevout.n;
    }
    return ptx->vout[n];
}
 
std::map<CTxDestination, std::vector<COutput>> ListCoins(const CWallet& wallet)
{
    AssertLockHeld(wallet.cs_wallet);
 
    std::map<CTxDestination, std::vector<COutput>> result;
 
    CCoinControl coin_control;
    CoinFilterParams coins_params;
    coins_params.only_spendable = false;
    coins_params.skip_locked = false;
    for (const COutput& coin : AvailableCoins(wallet, &coin_control, /*feerate=*/std::nullopt, coins_params).All()) {
        CTxDestination address;
        if ((coin.spendable || (wallet.IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS) && coin.solvable))) {
            if (!ExtractDestination(FindNonChangeParentOutput(wallet, coin.outpoint).scriptPubKey, address)) {
                // For backwards compatibility, we convert P2PK output scripts into PKHash destinations
                if (auto pk_dest = std::get_if<PubKeyDestination>(&address)) {
                    address = PKHash(pk_dest->GetPubKey());
                } else {
                    continue;
                }
            }
            result[address].emplace_back(coin);
        }
    }
    return result;
}
 
FilteredOutputGroups GroupOutputs(const CWallet& wallet,
                          const CoinsResult& coins,
                          const CoinSelectionParams& coin_sel_params,
                          const std::vector<SelectionFilter>& filters,
                          std::vector<OutputGroup>& ret_discarded_groups)
{
    FilteredOutputGroups filtered_groups;
 
    if (!coin_sel_params.m_avoid_partial_spends) {
        // Allowing partial spends means no grouping. Each COutput gets its own OutputGroup
        for (const auto& [type, outputs] : coins.coins) {
            for (const COutput& output : outputs) {
                // Get mempool info
                size_t ancestors, descendants;
                wallet.chain().getTransactionAncestry(output.outpoint.hash, ancestors, descendants);
 
                // Create a new group per output and add it to the all groups vector
                OutputGroup group(coin_sel_params);
                group.Insert(std::make_shared<COutput>(output), ancestors, descendants);
 
                // Each filter maps to a different set of groups
                bool accepted = false;
                for (const auto& sel_filter : filters) {
                    const auto& filter = sel_filter.filter;
                    if (!group.EligibleForSpending(filter)) continue;
                    filtered_groups[filter].Push(group, type, /*insert_positive=*/true, /*insert_mixed=*/true);
                    accepted = true;
                }
                if (!accepted) ret_discarded_groups.emplace_back(group);
            }
        }
        return filtered_groups;
    }
 
    // We want to combine COutputs that have the same scriptPubKey into single OutputGroups
    // except when there are more than OUTPUT_GROUP_MAX_ENTRIES COutputs grouped in an OutputGroup.
    // To do this, we maintain a map where the key is the scriptPubKey and the value is a vector of OutputGroups.
    // For each COutput, we check if the scriptPubKey is in the map, and if it is, the COutput is added
    // to the last OutputGroup in the vector for the scriptPubKey. When the last OutputGroup has
    // OUTPUT_GROUP_MAX_ENTRIES COutputs, a new OutputGroup is added to the end of the vector.
    typedef std::map<std::pair<CScript, OutputType>, std::vector<OutputGroup>> ScriptPubKeyToOutgroup;
    const auto& insert_output = [&](
            const std::shared_ptr<COutput>& output, OutputType type, size_t ancestors, size_t descendants,
            ScriptPubKeyToOutgroup& groups_map) {
        std::vector<OutputGroup>& groups = groups_map[std::make_pair(output->txout.scriptPubKey,type)];
 
        if (groups.size() == 0) {
            // No OutputGroups for this scriptPubKey yet, add one
            groups.emplace_back(coin_sel_params);
        }
 
        // Get the last OutputGroup in the vector so that we can add the COutput to it
        // A pointer is used here so that group can be reassigned later if it is full.
        OutputGroup* group = &groups.back();
 
        // Check if this OutputGroup is full. We limit to OUTPUT_GROUP_MAX_ENTRIES when using -avoidpartialspends
        // to avoid surprising users with very high fees.
        if (group->m_outputs.size() >= OUTPUT_GROUP_MAX_ENTRIES) {
            // The last output group is full, add a new group to the vector and use that group for the insertion
            groups.emplace_back(coin_sel_params);
            group = &groups.back();
        }
 
        group->Insert(output, ancestors, descendants);
    };
 
    ScriptPubKeyToOutgroup spk_to_groups_map;
    ScriptPubKeyToOutgroup spk_to_positive_groups_map;
    for (const auto& [type, outs] : coins.coins) {
        for (const COutput& output : outs) {
            size_t ancestors, descendants;
            wallet.chain().getTransactionAncestry(output.outpoint.hash, ancestors, descendants);
 
            const auto& shared_output = std::make_shared<COutput>(output);
            // Filter for positive only before adding the output
            if (output.GetEffectiveValue() > 0) {
                insert_output(shared_output, type, ancestors, descendants, spk_to_positive_groups_map);
            }
 
            // 'All' groups
            insert_output(shared_output, type, ancestors, descendants, spk_to_groups_map);
        }
    }
 
    // Now we go through the entire maps and pull out the OutputGroups
    const auto& push_output_groups = [&](const ScriptPubKeyToOutgroup& groups_map, bool positive_only) {
        for (const auto& [script, groups] : groups_map) {
            // Go through the vector backwards. This allows for the first item we deal with being the partial group.
            for (auto group_it = groups.rbegin(); group_it != groups.rend(); group_it++) {
                const OutputGroup& group = *group_it;
 
                // Each filter maps to a different set of groups
                bool accepted = false;
                for (const auto& sel_filter : filters) {
                    const auto& filter = sel_filter.filter;
                    if (!group.EligibleForSpending(filter)) continue;
 
                    // Don't include partial groups if there are full groups too and we don't want partial groups
                    if (group_it == groups.rbegin() && groups.size() > 1 && !filter.m_include_partial_groups) {
                        continue;
                    }
 
                    OutputType type = script.second;
                    // Either insert the group into the positive-only groups or the mixed ones.
                    filtered_groups[filter].Push(group, type, positive_only, /*insert_mixed=*/!positive_only);
                    accepted = true;
                }
                if (!accepted) ret_discarded_groups.emplace_back(group);
            }
        }
    };
 
    push_output_groups(spk_to_groups_map, /*positive_only=*/ false);
    push_output_groups(spk_to_positive_groups_map, /*positive_only=*/ true);
 
    return filtered_groups;
}
 
FilteredOutputGroups GroupOutputs(const CWallet& wallet,
                                  const CoinsResult& coins,
                                  const CoinSelectionParams& params,
                                  const std::vector<SelectionFilter>& filters)
{
    std::vector<OutputGroup> unused;
    return GroupOutputs(wallet, coins, params, filters, unused);
}
 
// Returns true if the result contains an error and the message is not empty
static bool HasErrorMsg(const util::Result<SelectionResult>& res) { return !util::ErrorString(res).empty(); }
 
util::Result<SelectionResult> AttemptSelection(interfaces::Chain& chain, const CAmount& nTargetValue, OutputGroupTypeMap& groups,
                               const CoinSelectionParams& coin_selection_params, bool allow_mixed_output_types)
{
    // Run coin selection on each OutputType and compute the Waste Metric
    std::vector<SelectionResult> results;
    for (auto& [type, group] : groups.groups_by_type) {
        auto result{ChooseSelectionResult(chain, nTargetValue, group, coin_selection_params)};
        // If any specific error message appears here, then something particularly wrong happened.
        if (HasErrorMsg(result)) return result; // So let's return the specific error.
        // Append the favorable result.
        if (result) results.push_back(*result);
    }
    // If we have at least one solution for funding the transaction without mixing, choose the minimum one according to waste metric
    // and return the result
    if (results.size() > 0) return *std::min_element(results.begin(), results.end());
 
    // If we can't fund the transaction from any individual OutputType, run coin selection one last time
    // over all available coins, which would allow mixing.
    // If TypesCount() <= 1, there is nothing to mix.
    if (allow_mixed_output_types && groups.TypesCount() > 1) {
        return ChooseSelectionResult(chain, nTargetValue, groups.all_groups, coin_selection_params);
    }
    // Either mixing is not allowed and we couldn't find a solution from any single OutputType, or mixing was allowed and we still couldn't
    // find a solution using all available coins
    return util::Error();
};
 
util::Result<SelectionResult> ChooseSelectionResult(interfaces::Chain& chain, const CAmount& nTargetValue, Groups& groups, const CoinSelectionParams& coin_selection_params)
{
    // Vector of results. We will choose the best one based on waste.
    std::vector<SelectionResult> results;
    std::vector<util::Result<SelectionResult>> errors;
    auto append_error = [&] (util::Result<SelectionResult>&& result) {
        // If any specific error message appears here, then something different from a simple "no selection found" happened.
        // Let's save it, so it can be retrieved to the user if no other selection algorithm succeeded.
        if (HasErrorMsg(result)) {
            errors.emplace_back(std::move(result));
        }
    };
 
    // Maximum allowed weight for selected coins.
    int max_transaction_weight = coin_selection_params.m_max_tx_weight.value_or(MAX_STANDARD_TX_WEIGHT);
    int tx_weight_no_input = coin_selection_params.tx_noinputs_size * WITNESS_SCALE_FACTOR;
    int max_selection_weight = max_transaction_weight - tx_weight_no_input;
    if (max_selection_weight <= 0) {
        return util::Error{_("Maximum transaction weight is less than transaction weight without inputs")};
    }
 
    // SFFO frequently causes issues in the context of changeless input sets: skip BnB when SFFO is active
    if (!coin_selection_params.m_subtract_fee_outputs) {
        if (auto bnb_result{SelectCoinsBnB(groups.positive_group, nTargetValue, coin_selection_params.m_cost_of_change, max_selection_weight)}) {
            results.push_back(*bnb_result);
        } else append_error(std::move(bnb_result));
    }
 
    // Deduct change weight because remaining Coin Selection algorithms can create change output
    int change_outputs_weight = coin_selection_params.change_output_size * WITNESS_SCALE_FACTOR;
    max_selection_weight -= change_outputs_weight;
    if (max_selection_weight < 0 && results.empty()) {
        return util::Error{_("Maximum transaction weight is too low, can not accommodate change output")};
    }
 
    // The knapsack solver has some legacy behavior where it will spend dust outputs. We retain this behavior, so don't filter for positive only here.
    if (auto knapsack_result{KnapsackSolver(groups.mixed_group, nTargetValue, coin_selection_params.m_min_change_target, coin_selection_params.rng_fast, max_selection_weight)}) {
        results.push_back(*knapsack_result);
    } else append_error(std::move(knapsack_result));
 
    if (coin_selection_params.m_effective_feerate > CFeeRate{3 * coin_selection_params.m_long_term_feerate}) { // Minimize input set for feerates of at least 3×LTFRE (default: 30 ṩ/vB+)
        if (auto cg_result{CoinGrinder(groups.positive_group, nTargetValue, coin_selection_params.m_min_change_target, max_selection_weight)}) {
            cg_result->RecalculateWaste(coin_selection_params.min_viable_change, coin_selection_params.m_cost_of_change, coin_selection_params.m_change_fee);
            results.push_back(*cg_result);
        } else {
            append_error(std::move(cg_result));
        }
    }
 
    if (auto srd_result{SelectCoinsSRD(groups.positive_group, nTargetValue, coin_selection_params.m_change_fee, coin_selection_params.rng_fast, max_selection_weight)}) {
        results.push_back(*srd_result);
    } else append_error(std::move(srd_result));
 
    if (results.empty()) {
        // No solution found, retrieve the first explicit error (if any).
        // future: add 'severity level' to errors so the worst one can be retrieved instead of the first one.
        return errors.empty() ? util::Error() : std::move(errors.front());
    }
 
    // If the chosen input set has unconfirmed inputs, check for synergies from overlapping ancestry
    for (auto& result : results) {
        std::vector<COutPoint> outpoints;
        std::set<std::shared_ptr<COutput>> coins = result.GetInputSet();
        CAmount summed_bump_fees = 0;
        for (auto& coin : coins) {
            if (coin->depth > 0) continue; // Bump fees only exist for unconfirmed inputs
            outpoints.push_back(coin->outpoint);
            summed_bump_fees += coin->ancestor_bump_fees;
        }
        std::optional<CAmount> combined_bump_fee = chain.calculateCombinedBumpFee(outpoints, coin_selection_params.m_effective_feerate);
        if (!combined_bump_fee.has_value()) {
            return util::Error{_("Failed to calculate bump fees, because unconfirmed UTXOs depend on enormous cluster of unconfirmed transactions.")};
        }
        CAmount bump_fee_overestimate = summed_bump_fees - combined_bump_fee.value();
        if (bump_fee_overestimate) {
            result.SetBumpFeeDiscount(bump_fee_overestimate);
        }
        result.RecalculateWaste(coin_selection_params.min_viable_change, coin_selection_params.m_cost_of_change, coin_selection_params.m_change_fee);
    }
 
    // Choose the result with the least waste
    // If the waste is the same, choose the one which spends more inputs.
    return *std::min_element(results.begin(), results.end());
}
 
util::Result<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& available_coins, const PreSelectedInputs& pre_set_inputs,
                                          const CAmount& nTargetValue, const CCoinControl& coin_control,
                                          const CoinSelectionParams& coin_selection_params)
{
    // Deduct preset inputs amount from the search target
    CAmount selection_target = nTargetValue - pre_set_inputs.total_amount;
 
    // Return if automatic coin selection is disabled, and we don't cover the selection target
    if (!coin_control.m_allow_other_inputs && selection_target > 0) {
        return util::Error{_("The preselected coins total amount does not cover the transaction target. "
                             "Please allow other inputs to be automatically selected or include more coins manually")};
    }
 
    // Return if we can cover the target only with the preset inputs
    if (selection_target <= 0) {
        SelectionResult result(nTargetValue, SelectionAlgorithm::MANUAL);
        result.AddInputs(pre_set_inputs.coins, coin_selection_params.m_subtract_fee_outputs);
        result.RecalculateWaste(coin_selection_params.min_viable_change, coin_selection_params.m_cost_of_change, coin_selection_params.m_change_fee);
        return result;
    }
 
    // Return early if we cannot cover the target with the wallet's UTXO.
    // We use the total effective value if we are not subtracting fee from outputs and 'available_coins' contains the data.
    CAmount available_coins_total_amount = coin_selection_params.m_subtract_fee_outputs ? available_coins.GetTotalAmount() :
            (available_coins.GetEffectiveTotalAmount().has_value() ? *available_coins.GetEffectiveTotalAmount() : 0);
    if (selection_target > available_coins_total_amount) {
        return util::Error(); // Insufficient funds
    }
 
    // Start wallet Coin Selection procedure
    auto op_selection_result = AutomaticCoinSelection(wallet, available_coins, selection_target, coin_selection_params);
    if (!op_selection_result) return op_selection_result;
 
    // If needed, add preset inputs to the automatic coin selection result
    if (!pre_set_inputs.coins.empty()) {
        SelectionResult preselected(pre_set_inputs.total_amount, SelectionAlgorithm::MANUAL);
        preselected.AddInputs(pre_set_inputs.coins, coin_selection_params.m_subtract_fee_outputs);
        op_selection_result->Merge(preselected);
        op_selection_result->RecalculateWaste(coin_selection_params.min_viable_change,
                                                coin_selection_params.m_cost_of_change,
                                                coin_selection_params.m_change_fee);
 
        // Verify we haven't exceeded the maximum allowed weight
        int max_inputs_weight = coin_selection_params.m_max_tx_weight.value_or(MAX_STANDARD_TX_WEIGHT) - (coin_selection_params.tx_noinputs_size * WITNESS_SCALE_FACTOR);
        if (op_selection_result->GetWeight() > max_inputs_weight) {
            return util::Error{_("The combination of the pre-selected inputs and the wallet automatic inputs selection exceeds the transaction maximum weight. "
                                 "Please try sending a smaller amount or manually consolidating your wallet's UTXOs")};
        }
    }
    return op_selection_result;
}
 
util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, CoinsResult& available_coins, const CAmount& value_to_select, const CoinSelectionParams& coin_selection_params)
{
    unsigned int limit_ancestor_count = 0;
    unsigned int limit_descendant_count = 0;
    wallet.chain().getPackageLimits(limit_ancestor_count, limit_descendant_count);
    const size_t max_ancestors = (size_t)std::max<int64_t>(1, limit_ancestor_count);
    const size_t max_descendants = (size_t)std::max<int64_t>(1, limit_descendant_count);
    const bool fRejectLongChains = gArgs.GetBoolArg("-walletrejectlongchains", DEFAULT_WALLET_REJECT_LONG_CHAINS);
 
    // Cases where we have 101+ outputs all pointing to the same destination may result in
    // privacy leaks as they will potentially be deterministically sorted. We solve that by
    // explicitly shuffling the outputs before processing
    if (coin_selection_params.m_avoid_partial_spends && available_coins.Size() > OUTPUT_GROUP_MAX_ENTRIES) {
        available_coins.Shuffle(coin_selection_params.rng_fast);
    }
 
    // Coin Selection attempts to select inputs from a pool of eligible UTXOs to fund the
    // transaction at a target feerate. If an attempt fails, more attempts may be made using a more
    // permissive CoinEligibilityFilter.
    {
        // Place coins eligibility filters on a scope increasing order.
        std::vector<SelectionFilter> ordered_filters{
                // If possible, fund the transaction with confirmed UTXOs only. Prefer at least six
                // confirmations on outputs received from other wallets and only spend confirmed change.
                {CoinEligibilityFilter(1, 6, 0), /*allow_mixed_output_types=*/false},
                {CoinEligibilityFilter(1, 1, 0)},
        };
        // Fall back to using zero confirmation change (but with as few ancestors in the mempool as
        // possible) if we cannot fund the transaction otherwise.
        if (wallet.m_spend_zero_conf_change) {
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, 2)});
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, std::min(size_t{4}, max_ancestors/3), std::min(size_t{4}, max_descendants/3))});
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, max_ancestors/2, max_descendants/2)});
            // If partial groups are allowed, relax the requirement of spending OutputGroups (groups
            // of UTXOs sent to the same address, which are obviously controlled by a single wallet)
            // in their entirety.
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, max_ancestors-1, max_descendants-1, /*include_partial=*/true)});
            // Try with unsafe inputs if they are allowed. This may spend unconfirmed outputs
            // received from other wallets.
            if (coin_selection_params.m_include_unsafe_inputs) {
                ordered_filters.push_back({CoinEligibilityFilter(/*conf_mine=*/0, /*conf_theirs*/0, max_ancestors-1, max_descendants-1, /*include_partial=*/true)});
            }
            // Try with unlimited ancestors/descendants. The transaction will still need to meet
            // mempool ancestor/descendant policy to be accepted to mempool and broadcasted, but
            // OutputGroups use heuristics that may overestimate ancestor/descendant counts.
            if (!fRejectLongChains) {
                ordered_filters.push_back({CoinEligibilityFilter(0, 1, std::numeric_limits<uint64_t>::max(),
                                                                   std::numeric_limits<uint64_t>::max(),
                                                                   /*include_partial=*/true)});
            }
        }
 
        // Group outputs and map them by coin eligibility filter
        std::vector<OutputGroup> discarded_groups;
        FilteredOutputGroups filtered_groups = GroupOutputs(wallet, available_coins, coin_selection_params, ordered_filters, discarded_groups);
 
        // Check if we still have enough balance after applying filters (some coins might be discarded)
        CAmount total_discarded = 0;
        CAmount total_unconf_long_chain = 0;
        for (const auto& group : discarded_groups) {
            total_discarded += group.GetSelectionAmount();
            if (group.m_ancestors >= max_ancestors || group.m_descendants >= max_descendants) total_unconf_long_chain += group.GetSelectionAmount();
        }
 
        if (CAmount total_amount = available_coins.GetTotalAmount() - total_discarded < value_to_select) {
            // Special case, too-long-mempool cluster.
            if (total_amount + total_unconf_long_chain > value_to_select) {
                return util::Error{_("Unconfirmed UTXOs are available, but spending them creates a chain of transactions that will be rejected by the mempool")};
            }
            return util::Error{}; // General "Insufficient Funds"
        }
 
        // Walk-through the filters until the solution gets found.
        // If no solution is found, return the first detailed error (if any).
        // future: add "error level" so the worst one can be picked instead.
        std::vector<util::Result<SelectionResult>> res_detailed_errors;
        for (const auto& select_filter : ordered_filters) {
            auto it = filtered_groups.find(select_filter.filter);
            if (it == filtered_groups.end()) continue;
            if (auto res{AttemptSelection(wallet.chain(), value_to_select, it->second,
                                          coin_selection_params, select_filter.allow_mixed_output_types)}) {
                return res; // result found
            } else {
                // If any specific error message appears here, then something particularly wrong might have happened.
                // Save the error and continue the selection process. So if no solutions gets found, we can return
                // the detailed error to the upper layers.
                if (HasErrorMsg(res)) res_detailed_errors.emplace_back(std::move(res));
            }
        }
 
        // Return right away if we have a detailed error
        if (!res_detailed_errors.empty()) return std::move(res_detailed_errors.front());
 
 
        // General "Insufficient Funds"
        return util::Error{};
    }
}
 
static bool IsCurrentForAntiFeeSniping(interfaces::Chain& chain, const uint256& block_hash)
{
    if (chain.isInitialBlockDownload()) {
        return false;
    }
    constexpr int64_t MAX_ANTI_FEE_SNIPING_TIP_AGE = 8 * 60 * 60; // in seconds
    int64_t block_time;
    CHECK_NONFATAL(chain.findBlock(block_hash, FoundBlock().time(block_time)));
    if (block_time < (GetTime() - MAX_ANTI_FEE_SNIPING_TIP_AGE)) {
        return false;
    }
    return true;
}
 
static void DiscourageFeeSniping(CMutableTransaction& tx, FastRandomContext& rng_fast,
                                 interfaces::Chain& chain, const uint256& block_hash, int block_height)
{
    // All inputs must be added by now
    assert(!tx.vin.empty());
    // Discourage fee sniping.
    //
    // For a large miner the value of the transactions in the best block and
    // the mempool can exceed the cost of deliberately attempting to mine two
    // blocks to orphan the current best block. By setting nLockTime such that
    // only the next block can include the transaction, we discourage this
    // practice as the height restricted and limited blocksize gives miners
    // considering fee sniping fewer options for pulling off this attack.
    //
    // A simple way to think about this is from the wallet's point of view we
    // always want the blockchain to move forward. By setting nLockTime this
    // way we're basically making the statement that we only want this
    // transaction to appear in the next block; we don't want to potentially
    // encourage reorgs by allowing transactions to appear at lower heights
    // than the next block in forks of the best chain.
    //
    // Of course, the subsidy is high enough, and transaction volume low
    // enough, that fee sniping isn't a problem yet, but by implementing a fix
    // now we ensure code won't be written that makes assumptions about
    // nLockTime that preclude a fix later.
    if (IsCurrentForAntiFeeSniping(chain, block_hash)) {
        tx.nLockTime = block_height;
 
        // Secondly occasionally randomly pick a nLockTime even further back, so
        // that transactions that are delayed after signing for whatever reason,
        // e.g. high-latency mix networks and some CoinJoin implementations, have
        // better privacy.
        if (rng_fast.randrange(10) == 0) {
            tx.nLockTime = std::max(0, int(tx.nLockTime) - int(rng_fast.randrange(100)));
        }
    } else {
        // If our chain is lagging behind, we can't discourage fee sniping nor help
        // the privacy of high-latency transactions. To avoid leaking a potentially
        // unique "nLockTime fingerprint", set nLockTime to a constant.
        tx.nLockTime = 0;
    }
    // Sanity check all values
    assert(tx.nLockTime < LOCKTIME_THRESHOLD); // Type must be block height
    assert(tx.nLockTime <= uint64_t(block_height));
    for (const auto& in : tx.vin) {
        // Can not be FINAL for locktime to work
        assert(in.nSequence != CTxIn::SEQUENCE_FINAL);
        // May be MAX NONFINAL to disable both BIP68 and BIP125
        if (in.nSequence == CTxIn::MAX_SEQUENCE_NONFINAL) continue;
        // May be MAX BIP125 to disable BIP68 and enable BIP125
        if (in.nSequence == MAX_BIP125_RBF_SEQUENCE) continue;
        // The wallet does not support any other sequence-use right now.
        assert(false);
    }
}
 
size_t GetSerializeSizeForRecipient(const CRecipient& recipient)
{
    return ::GetSerializeSize(CTxOut(recipient.nAmount, GetScriptForDestination(recipient.dest)));
}
 
bool IsDust(const CRecipient& recipient, const CFeeRate& dustRelayFee)
{
    return ::IsDust(CTxOut(recipient.nAmount, GetScriptForDestination(recipient.dest)), dustRelayFee);
}
 
static util::Result<CreatedTransactionResult> CreateTransactionInternal(
        CWallet& wallet,
        const std::vector<CRecipient>& vecSend,
        std::optional<unsigned int> change_pos,
        const CCoinControl& coin_control,
        bool sign) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet)
{
    AssertLockHeld(wallet.cs_wallet);
 
    FastRandomContext rng_fast;
    CMutableTransaction txNew; // The resulting transaction that we make
 
    if (coin_control.m_version) {
        txNew.version = coin_control.m_version.value();
    }
 
    CoinSelectionParams coin_selection_params{rng_fast}; // Parameters for coin selection, init with dummy
    coin_selection_params.m_avoid_partial_spends = coin_control.m_avoid_partial_spends;
    coin_selection_params.m_include_unsafe_inputs = coin_control.m_include_unsafe_inputs;
    coin_selection_params.m_max_tx_weight = coin_control.m_max_tx_weight.value_or(MAX_STANDARD_TX_WEIGHT);
    int minimum_tx_weight = MIN_STANDARD_TX_NONWITNESS_SIZE * WITNESS_SCALE_FACTOR;
    if (coin_selection_params.m_max_tx_weight.value() < minimum_tx_weight || coin_selection_params.m_max_tx_weight.value() > MAX_STANDARD_TX_WEIGHT) {
        return util::Error{strprintf(_("Maximum transaction weight must be between %d and %d"), minimum_tx_weight, MAX_STANDARD_TX_WEIGHT)};
    }
    // Set the long term feerate estimate to the wallet's consolidate feerate
    coin_selection_params.m_long_term_feerate = wallet.m_consolidate_feerate;
    // Static vsize overhead + outputs vsize. 4 nVersion, 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
 
    CAmount recipients_sum = 0;
    const OutputType change_type = wallet.TransactionChangeType(coin_control.m_change_type ? *coin_control.m_change_type : wallet.m_default_change_type, vecSend);
    ReserveDestination reservedest(&wallet, change_type);
    unsigned int outputs_to_subtract_fee_from = 0; // The number of outputs which we are subtracting the fee from
    for (const auto& recipient : vecSend) {
        if (IsDust(recipient, wallet.chain().relayDustFee())) {
            return util::Error{_("Transaction amount too small")};
        }
 
        // Include the fee cost for outputs.
        coin_selection_params.tx_noinputs_size += GetSerializeSizeForRecipient(recipient);
        recipients_sum += recipient.nAmount;
 
        if (recipient.fSubtractFeeFromAmount) {
            outputs_to_subtract_fee_from++;
            coin_selection_params.m_subtract_fee_outputs = true;
        }
    }
 
    // Create change script that will be used if we need change
    CScript scriptChange;
    bilingual_str error; // possible error str
 
    // coin control: send change to custom address
    if (!std::get_if<CNoDestination>(&coin_control.destChange)) {
        scriptChange = GetScriptForDestination(coin_control.destChange);
    } else { // no coin control: send change to newly generated address
        // Note: We use a new key here to keep it from being obvious which side is the change.
        //  The drawback is that by not reusing a previous key, the change may be lost if a
        //  backup is restored, if the backup doesn't have the new private key for the change.
        //  If we reused the old key, it would be possible to add code to look for and
        //  rediscover unknown transactions that were written with keys of ours to recover
        //  post-backup change.
 
        // Reserve a new key pair from key pool. If it fails, provide a dummy
        // destination in case we don't need change.
        CTxDestination dest;
        auto op_dest = reservedest.GetReservedDestination(true);
        if (!op_dest) {
            error = _("Transaction needs a change address, but we can't generate it.") + Untranslated(" ") + util::ErrorString(op_dest);
        } else {
            dest = *op_dest;
            scriptChange = GetScriptForDestination(dest);
        }
        // A valid destination implies a change script (and
        // vice-versa). An empty change script will abort later, if the
        // change keypool ran out, but change is required.
        CHECK_NONFATAL(IsValidDestination(dest) != scriptChange.empty());
    }
    CTxOut change_prototype_txout(0, scriptChange);
    coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
 
    // Get size of spending the change output
    int change_spend_size = CalculateMaximumSignedInputSize(change_prototype_txout, &wallet, /*coin_control=*/nullptr);
    // If the wallet doesn't know how to sign change output, assume p2sh-p2wpkh
    // as lower-bound to allow BnB to do it's thing
    if (change_spend_size == -1) {
        coin_selection_params.change_spend_size = DUMMY_NESTED_P2WPKH_INPUT_SIZE;
    } else {
        coin_selection_params.change_spend_size = change_spend_size;
    }
 
    // Set discard feerate
    coin_selection_params.m_discard_feerate = GetDiscardRate(wallet);
 
    // Get the fee rate to use effective values in coin selection
    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-zero value, a non-zero 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);
    }
 
    // Choose coins to use
    auto select_coins_res = SelectCoins(wallet, available_coins, preset_inputs, /*nTargetValue=*/selection_target, coin_control, coin_selection_params);
    if (!select_coins_res) {
        // 'SelectCoins' either returns a specific error message or, if empty, means a general "Insufficient funds".
        const bilingual_str& err = util::ErrorString(select_coins_res);
        return util::Error{err.empty() ?_("Insufficient funds") : err};
    }
    const SelectionResult& result = *select_coins_res;
    TRACEPOINT(coin_selection, selected_coins,
           wallet.GetName().c_str(),
           GetAlgorithmName(result.GetAlgo()).c_str(),
           result.GetTarget(),
           result.GetWaste(),
           result.GetSelectedValue());
 
    // vouts to the payees
    txNew.vout.reserve(vecSend.size() + 1); // + 1 because of possible later insert
    for (const auto& recipient : vecSend)
    {
        txNew.vout.emplace_back(recipient.nAmount, GetScriptForDestination(recipient.dest));
    }
    const CAmount change_amount = result.GetChange(coin_selection_params.min_viable_change, coin_selection_params.m_change_fee);
    if (change_amount > 0) {
        CTxOut newTxOut(change_amount, scriptChange);
        if (!change_pos) {
            // Insert change txn at random position:
            change_pos = rng_fast.randrange(txNew.vout.size() + 1);
        } else if ((unsigned int)*change_pos > txNew.vout.size()) {
            return util::Error{_("Transaction change output index out of range")};
        }
        txNew.vout.insert(txNew.vout.begin() + *change_pos, newTxOut);
    } else {
        change_pos = std::nullopt;
    }
 
    // Shuffle selected coins and fill in final vin
    std::vector<std::shared_ptr<COutput>> selected_coins = result.GetShuffledInputVector();
 
    if (coin_control.HasSelected() && coin_control.HasSelectedOrder()) {
        // When there are preselected inputs, we need to move them to be the first UTXOs
        // and have them be in the order selected. We can use stable_sort for this, where we
        // compare with the positions stored in coin_control. The COutputs that have positions
        // will be placed before those that don't, and those positions will be in order.
        std::stable_sort(selected_coins.begin(), selected_coins.end(),
            [&coin_control](const std::shared_ptr<COutput>& a, const std::shared_ptr<COutput>& b) {
                auto a_pos = coin_control.GetSelectionPos(a->outpoint);
                auto b_pos = coin_control.GetSelectionPos(b->outpoint);
                if (a_pos.has_value() && b_pos.has_value()) {
                    return a_pos.value() < b_pos.value();
                } else if (a_pos.has_value() && !b_pos.has_value()) {
                    return true;
                } else {
                    return false;
                }
            });
    }
 
    // The sequence number is set to non-maxint so that DiscourageFeeSniping
    // works.
    //
    // BIP125 defines opt-in RBF as any nSequence < maxint-1, so
    // we use the highest possible value in that range (maxint-2)
    // to avoid conflicting with other possible uses of nSequence,
    // and in the spirit of "smallest possible change from prior
    // behavior."
    bool use_anti_fee_sniping = true;
    const uint32_t default_sequence{coin_control.m_signal_bip125_rbf.value_or(wallet.m_signal_rbf) ? MAX_BIP125_RBF_SEQUENCE : CTxIn::MAX_SEQUENCE_NONFINAL};
    txNew.vin.reserve(selected_coins.size());
    for (const auto& coin : selected_coins) {
        std::optional<uint32_t> sequence = coin_control.GetSequence(coin->outpoint);
        if (sequence) {
            // If an input has a preset sequence, we can't do anti-fee-sniping
            use_anti_fee_sniping = false;
        }
        txNew.vin.emplace_back(coin->outpoint, CScript{}, sequence.value_or(default_sequence));
 
        auto scripts = coin_control.GetScripts(coin->outpoint);
        if (scripts.first) {
            txNew.vin.back().scriptSig = *scripts.first;
        }
        if (scripts.second) {
            txNew.vin.back().scriptWitness = *scripts.second;
        }
    }
    if (coin_control.m_locktime) {
        txNew.nLockTime = coin_control.m_locktime.value();
        // If we have a locktime set, we can't use anti-fee-sniping
        use_anti_fee_sniping = false;
    }
    if (use_anti_fee_sniping) {
        DiscourageFeeSniping(txNew, rng_fast, wallet.chain(), wallet.GetLastBlockHash(), wallet.GetLastBlockHeight());
    }
 
    // Calculate the transaction fee
    TxSize tx_sizes = CalculateMaximumSignedTxSize(CTransaction(txNew), &wallet, &coin_control);
    int nBytes = tx_sizes.vsize;
    if (nBytes == -1) {
        return util::Error{_("Missing solving data for estimating transaction size")};
    }
    CAmount fee_needed = coin_selection_params.m_effective_feerate.GetFee(nBytes) + result.GetTotalBumpFees();
    const CAmount output_value = CalculateOutputValue(txNew);
    Assume(recipients_sum + change_amount == output_value);
    CAmount current_fee = result.GetSelectedValue() - output_value;
 
    // Sanity check that the fee cannot be negative as that means we have more output value than input value
    if (current_fee < 0) {
        return util::Error{Untranslated(STR_INTERNAL_BUG("Fee paid < 0"))};
    }
 
    // If there is a change output and we overpay the fees then increase the change to match the fee needed
    if (change_pos && fee_needed < current_fee) {
        auto& change = txNew.vout.at(*change_pos);
        change.nValue += current_fee - fee_needed;
        current_fee = result.GetSelectedValue() - CalculateOutputValue(txNew);
        if (fee_needed != current_fee) {
            return util::Error{Untranslated(STR_INTERNAL_BUG("Change adjustment: Fee needed != fee paid"))};
        }
    }
 
    // Reduce output values for subtractFeeFromAmount
    if (coin_selection_params.m_subtract_fee_outputs) {
        CAmount to_reduce = fee_needed - current_fee;
        unsigned int i = 0;
        bool fFirst = true;
        for (const auto& recipient : vecSend)
        {
            if (change_pos && i == *change_pos) {
                ++i;
            }
            CTxOut& txout = txNew.vout[i];
 
            if (recipient.fSubtractFeeFromAmount)
            {
                txout.nValue -= to_reduce / outputs_to_subtract_fee_from; // Subtract fee equally from each selected recipient
 
                if (fFirst) // first receiver pays the remainder not divisible by output count
                {
                    fFirst = false;
                    txout.nValue -= to_reduce % outputs_to_subtract_fee_from;
                }
 
                // Error if this output is reduced to be below dust
                if (IsDust(txout, wallet.chain().relayDustFee())) {
                    if (txout.nValue < 0) {
                        return util::Error{_("The transaction amount is too small to pay the fee")};
                    } else {
                        return util::Error{_("The transaction amount is too small to send after the fee has been deducted")};
                    }
                }
            }
            ++i;
        }
        current_fee = result.GetSelectedValue() - CalculateOutputValue(txNew);
        if (fee_needed != current_fee) {
            return util::Error{Untranslated(STR_INTERNAL_BUG("SFFO: Fee needed != fee paid"))};
        }
    }
 
    // fee_needed should now always be less than or equal to the current fees that we pay.
    // If it is not, it is a bug.
    if (fee_needed > current_fee) {
        return util::Error{Untranslated(STR_INTERNAL_BUG("Fee needed > fee paid"))};
    }
 
    // Give up if change keypool ran out and change is required
    if (scriptChange.empty() && change_pos) {
        return util::Error{error};
    }
 
    if (sign && !wallet.SignTransaction(txNew)) {
        return util::Error{_("Signing transaction failed")};
    }
 
    // Return the constructed transaction data.
    CTransactionRef tx = MakeTransactionRef(std::move(txNew));
 
    // Limit size
    if ((sign && GetTransactionWeight(*tx) > MAX_STANDARD_TX_WEIGHT) ||
        (!sign && tx_sizes.weight > MAX_STANDARD_TX_WEIGHT))
    {
        return util::Error{_("Transaction too large")};
    }
 
    if (current_fee > wallet.m_default_max_tx_fee) {
        return util::Error{TransactionErrorString(TransactionError::MAX_FEE_EXCEEDED)};
    }
 
    if (gArgs.GetBoolArg("-walletrejectlongchains", DEFAULT_WALLET_REJECT_LONG_CHAINS)) {
        // Lastly, ensure this tx will pass the mempool's chain limits
        auto result = wallet.chain().checkChainLimits(tx);
        if (!result) {
            return util::Error{util::ErrorString(result)};
        }
    }
 
    // Before we return success, we assume any change key will be used to prevent
    // accidental reuse.
    reservedest.KeepDestination();
 
    wallet.WalletLogPrintf("Coin Selection: Algorithm:%s, Waste Metric Score:%d\n", GetAlgorithmName(result.GetAlgo()), result.GetWaste());
    wallet.WalletLogPrintf("Fee Calculation: Fee:%d Bytes:%u Tgt:%d (requested %d) Reason:\"%s\" Decay %.5f: Estimation: (%g - %g) %.2f%% %.1f/(%.1f %d mem %.1f out) Fail: (%g - %g) %.2f%% %.1f/(%.1f %d mem %.1f out)\n",
              current_fee, nBytes, feeCalc.returnedTarget, feeCalc.desiredTarget, StringForFeeReason(feeCalc.reason), feeCalc.est.decay,
              feeCalc.est.pass.start, feeCalc.est.pass.end,
              (feeCalc.est.pass.totalConfirmed + feeCalc.est.pass.inMempool + feeCalc.est.pass.leftMempool) > 0.0 ? 100 * feeCalc.est.pass.withinTarget / (feeCalc.est.pass.totalConfirmed + feeCalc.est.pass.inMempool + feeCalc.est.pass.leftMempool) : 0.0,
              feeCalc.est.pass.withinTarget, feeCalc.est.pass.totalConfirmed, feeCalc.est.pass.inMempool, feeCalc.est.pass.leftMempool,
              feeCalc.est.fail.start, feeCalc.est.fail.end,
              (feeCalc.est.fail.totalConfirmed + feeCalc.est.fail.inMempool + feeCalc.est.fail.leftMempool) > 0.0 ? 100 * feeCalc.est.fail.withinTarget / (feeCalc.est.fail.totalConfirmed + feeCalc.est.fail.inMempool + feeCalc.est.fail.leftMempool) : 0.0,
              feeCalc.est.fail.withinTarget, feeCalc.est.fail.totalConfirmed, feeCalc.est.fail.inMempool, feeCalc.est.fail.leftMempool);
    return CreatedTransactionResult(tx, current_fee, change_pos, feeCalc);
}
 
util::Result<CreatedTransactionResult> CreateTransaction(
        CWallet& wallet,
        const std::vector<CRecipient>& vecSend,
        std::optional<unsigned int> change_pos,
        const CCoinControl& coin_control,
        bool sign)
{
    if (vecSend.empty()) {
        return util::Error{_("Transaction must have at least one recipient")};
    }
 
    if (std::any_of(vecSend.cbegin(), vecSend.cend(), [](const auto& recipient){ return recipient.nAmount < 0; })) {
        return util::Error{_("Transaction amounts must not be negative")};
    }
 
    LOCK(wallet.cs_wallet);
 
    auto res = CreateTransactionInternal(wallet, vecSend, change_pos, coin_control, sign);
    TRACEPOINT(coin_selection, normal_create_tx_internal,
           wallet.GetName().c_str(),
           bool(res),
           res ? res->fee : 0,
           res && res->change_pos.has_value() ? int32_t(*res->change_pos) : -1);
    if (!res) return res;
    const auto& txr_ungrouped = *res;
    // try with avoidpartialspends unless it's enabled already
    if (txr_ungrouped.fee > 0 /* 0 means non-functional fee rate estimation */ && wallet.m_max_aps_fee > -1 && !coin_control.m_avoid_partial_spends) {
        TRACEPOINT(coin_selection, attempting_aps_create_tx, wallet.GetName().c_str());
        CCoinControl tmp_cc = coin_control;
        tmp_cc.m_avoid_partial_spends = true;
 
        // Reuse the change destination from the first creation attempt to avoid skipping BIP44 indexes
        if (txr_ungrouped.change_pos) {
            ExtractDestination(txr_ungrouped.tx->vout[*txr_ungrouped.change_pos].scriptPubKey, tmp_cc.destChange);
        }
 
        auto txr_grouped = CreateTransactionInternal(wallet, vecSend, change_pos, tmp_cc, sign);
        // if fee of this alternative one is within the range of the max fee, we use this one
        const bool use_aps{txr_grouped.has_value() ? (txr_grouped->fee <= txr_ungrouped.fee + wallet.m_max_aps_fee) : false};
        TRACEPOINT(coin_selection, aps_create_tx_internal,
               wallet.GetName().c_str(),
               use_aps,
               txr_grouped.has_value(),
               txr_grouped.has_value() ? txr_grouped->fee : 0,
               txr_grouped.has_value() && txr_grouped->change_pos.has_value() ? int32_t(*txr_grouped->change_pos) : -1);
        if (txr_grouped) {
            wallet.WalletLogPrintf("Fee non-grouped = %lld, grouped = %lld, using %s\n",
                txr_ungrouped.fee, txr_grouped->fee, use_aps ? "grouped" : "non-grouped");
            if (use_aps) return txr_grouped;
        }
    }
    return res;
}
 
util::Result<CreatedTransactionResult> FundTransaction(CWallet& wallet, const CMutableTransaction& tx, const std::vector<CRecipient>& vecSend, std::optional<unsigned int> change_pos, bool lockUnspents, CCoinControl coinControl)
{
    // We want to make sure tx.vout is not used now that we are passing outputs as a vector of recipients.
    // This sets us up to remove tx completely in a future PR in favor of passing the inputs directly.
    assert(tx.vout.empty());
 
    // Set the user desired locktime
    coinControl.m_locktime = tx.nLockTime;
 
    // Set the user desired version
    coinControl.m_version = tx.version;
 
    // Acquire the locks to prevent races to the new locked unspents between the
    // CreateTransaction call and LockCoin calls (when lockUnspents is true).
    LOCK(wallet.cs_wallet);
 
    // Fetch specified UTXOs from the UTXO set to get the scriptPubKeys and values of the outputs being selected
    // and to match with the given solving_data. Only used for non-wallet outputs.
    std::map<COutPoint, Coin> coins;
    for (const CTxIn& txin : tx.vin) {
        coins[txin.prevout]; // Create empty map entry keyed by prevout.
    }
    wallet.chain().findCoins(coins);
 
    for (const CTxIn& txin : tx.vin) {
        const auto& outPoint = txin.prevout;
        PreselectedInput& preset_txin = coinControl.Select(outPoint);
        if (!wallet.IsMine(outPoint)) {
            if (coins[outPoint].out.IsNull()) {
                return util::Error{_("Unable to find UTXO for external input")};
            }
 
            // The input was not in the wallet, but is in the UTXO set, so select as external
            preset_txin.SetTxOut(coins[outPoint].out);
        }
        preset_txin.SetSequence(txin.nSequence);
        preset_txin.SetScriptSig(txin.scriptSig);
        preset_txin.SetScriptWitness(txin.scriptWitness);
    }
 
    auto res = CreateTransaction(wallet, vecSend, change_pos, coinControl, false);
    if (!res) {
        return res;
    }
 
    if (lockUnspents) {
        for (const CTxIn& txin : res->tx->vin) {
            wallet.LockCoin(txin.prevout);
        }
    }
 
    return res;
}
} // namespace wallet