Choosing a wallet transaction fee

1// Copyright (c) 2009-2010 Satoshi Nakamoto
2// Copyright (c) 2009-present The Bitcoin Core developers
3// Distributed under the MIT software license, see the accompanying
4// file COPYING or http://www.opensource.org/licenses/mit-license.php.
5
6#include <wallet/fees.h>
7
8#include <policy/feerate.h>
9#include <util/fees.h>
10#include <wallet/coincontrol.h>
11#include <wallet/wallet.h>
12
13#include <optional>
14
15namespace wallet {
16CAmount GetRequiredFee(const CWallet& wallet, unsigned int nTxBytes)
17{
18    return GetRequiredFeeRate(wallet).GetFee(static_cast<int32_t>(nTxBytes));
19}
20
21
22CAmount GetMinimumFee(const MinimumFeeRateResult& min_fee_rate, unsigned int nTxBytes)
23{
24    return min_fee_rate.fee_rate.GetFee(static_cast<int32_t>(nTxBytes));
25}
26
27CFeeRate GetRequiredFeeRate(const CWallet& wallet)
28{
29    return std::max(wallet.m_min_fee, wallet.chain().relayMinFee());
30}
31
32MinimumFeeRateResult GetMinimumFeeRate(const CWallet& wallet, const CCoinControl& coin_control)
33{
34    /* User control of how to calculate fee uses the following parameter precedence:
35       1. coin_control.m_feerate
36       2. coin_control.m_confirm_target
37       3. m_confirm_target (user-set member variable of wallet)
38       The first parameter that is set is used.
39    */
40    if (coin_control.m_feerate) { // 1.
41        CFeeRate fee_rate{*coin_control.m_feerate};
42        // Allow to override automatic min/max check over coin control instance
43        if (coin_control.fOverrideFeeRate) return {fee_rate, FeeReason::USER_SPECIFIED, std::nullopt};
44
45        CFeeRate required_feerate = GetRequiredFeeRate(wallet);
46        if (required_feerate > fee_rate) return {required_feerate, FeeReason::REQUIRED, std::nullopt};
47
48        return {fee_rate, FeeReason::USER_SPECIFIED, std::nullopt};
49    }
50
51    // We will use smart fee estimation
52    unsigned int target = coin_control.m_confirm_target ? *coin_control.m_confirm_target : wallet.m_confirm_target;
53    // By default estimates are economical iff we are signaling opt-in-RBF
54    bool conservative_estimate = !coin_control.m_signal_bip125_rbf.value_or(wallet.m_signal_rbf);
55    // Allow to override the default fee estimate mode over the CoinControl instance
56    if (coin_control.m_fee_mode == FeeEstimateMode::CONSERVATIVE)
57        conservative_estimate = true;
58    else if (coin_control.m_fee_mode == FeeEstimateMode::ECONOMICAL)
59        conservative_estimate = false;
60
61    const auto fee_estimation_res = wallet.chain().getFeeRateEstimate(target, conservative_estimate);
62    const FeeRateEstimation& estimation{FeeRateEstimationRef(fee_estimation_res)};
63    CFeeRate fee_rate{estimation.feerate};
64    FeeReason fee_reason{FeeReason::FEE_RATE_ESTIMATOR};
65    // Only fee rate estimator results have a returned target.
66    std::optional<int> returned_target{estimation.returned_target};
67    if (fee_rate == CFeeRate(0)) {
68        // if we don't have enough data for getFeeRateEstimate, then use fallback fee
69        fee_rate = wallet.m_fallback_fee;
70        fee_reason = FeeReason::FALLBACK;
71        returned_target = std::nullopt;
72        // directly return if fallback fee is disabled (feerate 0 == disabled)
73        if (wallet.m_fallback_fee == CFeeRate(0)) return {fee_rate, FeeReason::FALLBACK, std::nullopt};
74    }
75
76    // Obey mempool min fee when using smart fee estimation or fallback fee
77    CFeeRate min_mempool_feerate = wallet.chain().mempoolMinFee();
78    if (fee_rate < min_mempool_feerate) {
79        fee_rate = min_mempool_feerate;
80        fee_reason = FeeReason::MEMPOOL_MIN;
81        returned_target = std::nullopt;
82    }
83
84    CFeeRate required_feerate = GetRequiredFeeRate(wallet);
85    if (required_feerate > fee_rate) return {required_feerate, FeeReason::REQUIRED, std::nullopt};
86
87    return {fee_rate, fee_reason, returned_target};
88}
89
90CFeeRate GetDiscardRate(const CWallet& wallet)
91{
92    unsigned int highest_target = wallet.chain().maximumFeeEstimationTargetBlocks();
93    const auto res = wallet.chain().getFeeRateEstimate(highest_target, /*conservative=*/false);
94    auto discard_rate = res ? CFeeRate(res->feerate) : CFeeRate(0);
95    // Don't let discard_rate be greater than longest possible fee estimate if we get a valid fee estimate
96    discard_rate = (discard_rate == CFeeRate(0)) ? wallet.m_discard_rate : std::min(discard_rate, wallet.m_discard_rate);
97    // Discard rate must be at least dust relay feerate
98    discard_rate = std::max(discard_rate, wallet.chain().relayDustFee());
99    return discard_rate;
100}
101} // namespace wallet
102