wallet: Move discard feerate fetching to CreateTransaction

Instead of fetching the discard feerate for each SelectCoinsMinConf
iteration, fetch and cache it once during CreateTransaction so that it
is shared for each SelectCoinsMinConf through
coin_selection_params.m_discard_feerate.

Does not change behavior.

src/bench/coin_selection.cpp

@@ -51,7 +51,7 @@ static void CoinSelection(benchmark::Bench& bench)
     const CoinEligibilityFilter filter_standard(1, 6, 0);
     const CoinSelectionParams coin_selection_params(/* use_bnb= */ true, /* change_output_size= */ 34,
                                                     /* change_spend_size= */ 148, /* effective_fee= */ CFeeRate(0),
-                                                    /* long_term_feerate= */ CFeeRate(0),
+                                                    /* long_term_feerate= */ CFeeRate(0), /* discard_feerate= */ CFeeRate(0),
                                                     /* tx_no_inputs_size= */ 0, /* avoid_partial= */ false);
     bench.run([&] {
         std::set<CInputCoin> setCoinsRet;

src/wallet/test/coinselector_tests.cpp

@@ -37,7 +37,7 @@ CoinEligibilityFilter filter_confirmed(1, 1, 0);
 CoinEligibilityFilter filter_standard_extra(6, 6, 0);
 CoinSelectionParams coin_selection_params(/* use_bnb= */ false, /* change_output_size= */ 0,
                                           /* change_spend_size= */ 0, /* effective_fee= */ CFeeRate(0),
-                                          /* long_term_feerate= */ CFeeRate(0),
+                                          /* long_term_feerate= */ CFeeRate(0), /* discard_feerate= */ CFeeRate(0),
                                           /* tx_no_inputs_size= */ 0, /* avoid_partial= */ false);
 
 static void add_coin(const CAmount& nValue, int nInput, std::vector<CInputCoin>& set)
@@ -274,7 +274,7 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
     // Make sure that effective value is working in SelectCoinsMinConf when BnB is used
     CoinSelectionParams coin_selection_params_bnb(/* use_bnb= */ true, /* change_output_size= */ 0,
                                                   /* change_spend_size= */ 0, /* effective_fee= */ CFeeRate(3000),
-                                                  /* long_term_feerate= */ CFeeRate(1000),
+                                                  /* long_term_feerate= */ CFeeRate(1000), /* discard_feerate= */ CFeeRate(1000),
                                                   /* tx_no_inputs_size= */ 0, /* avoid_partial= */ false);
     CoinSet setCoinsRet;
     CAmount nValueRet;
@@ -647,11 +647,11 @@ BOOST_AUTO_TEST_CASE(SelectCoins_test)
         // Perform selection
         CoinSelectionParams coin_selection_params_knapsack(/* use_bnb= */ false, /* change_output_size= */ 34,
                                                            /* change_spend_size= */ 148, /* effective_fee= */ CFeeRate(0),
-                                                           /* long_term_feerate= */ CFeeRate(0),
+                                                           /* long_term_feerate= */ CFeeRate(0), /* discard_feerate= */ CFeeRate(0),
                                                            /* tx_no_inputs_size= */ 0, /* avoid_partial= */ false);
         CoinSelectionParams coin_selection_params_bnb(/* use_bnb= */ true, /* change_output_size= */ 34,
                                                       /* change_spend_size= */ 148, /* effective_fee= */ CFeeRate(0),
-                                                      /* long_term_feerate= */ CFeeRate(0),
+                                                      /* long_term_feerate= */ CFeeRate(0), /* discard_feerate= */ CFeeRate(0),
                                                       /* tx_no_inputs_size= */ 0, /* avoid_partial= */ false);
         CoinSet out_set;
         CAmount out_value = 0;

src/wallet/wallet.cpp

@@ -2374,7 +2374,7 @@ bool CWallet::SelectCoinsMinConf(const CAmount& nTargetValue, const CoinEligibil
         std::vector<OutputGroup> groups = GroupOutputs(coins, !coin_selection_params.m_avoid_partial_spends, effective_feerate, coin_selection_params.m_long_term_feerate, eligibility_filter, true /* positive_only */);
 
         // Calculate cost of change
-        CAmount cost_of_change = GetDiscardRate(*this).GetFee(coin_selection_params.change_spend_size) + coin_selection_params.effective_fee.GetFee(coin_selection_params.change_output_size);
+        CAmount cost_of_change = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size) + coin_selection_params.effective_fee.GetFee(coin_selection_params.change_output_size);
 
         // Calculate the fees for things that aren't inputs
         CAmount not_input_fees = coin_selection_params.effective_fee.GetFee(coin_selection_params.tx_noinputs_size);
@@ -2798,7 +2798,8 @@ bool CWallet::CreateTransactionInternal(
             CTxOut change_prototype_txout(0, scriptChange);
             coin_selection_params.change_output_size = GetSerializeSize(change_prototype_txout);
 
-            CFeeRate discard_rate = GetDiscardRate(*this);
+            // Set discard feerate
+            coin_selection_params.m_discard_feerate = GetDiscardRate(*this);
 
             // Get the fee rate to use effective values in coin selection
             coin_selection_params.effective_fee = GetMinimumFeeRate(*this, coin_control, &feeCalc);
@@ -2917,7 +2918,7 @@ bool CWallet::CreateTransactionInternal(
                     // Never create dust outputs; if we would, just
                     // add the dust to the fee.
                     // The nChange when BnB is used is always going to go to fees.
-                    if (IsDust(newTxOut, discard_rate) || bnb_used)
+                    if (IsDust(newTxOut, coin_selection_params.m_discard_feerate) || bnb_used)
                     {
                         nChangePosInOut = -1;
                         nFeeRet += nChange;
@@ -2969,7 +2970,7 @@ bool CWallet::CreateTransactionInternal(
                     if (nChangePosInOut == -1 && nSubtractFeeFromAmount == 0 && pick_new_inputs) {
                         unsigned int tx_size_with_change = nBytes + coin_selection_params.change_output_size + 2; // Add 2 as a buffer in case increasing # of outputs changes compact size
                         CAmount fee_needed_with_change = coin_selection_params.effective_fee.GetFee(tx_size_with_change);
-                        CAmount minimum_value_for_change = GetDustThreshold(change_prototype_txout, discard_rate);
+                        CAmount minimum_value_for_change = GetDustThreshold(change_prototype_txout, coin_selection_params.m_discard_feerate);
                         if (nFeeRet >= fee_needed_with_change + minimum_value_for_change) {
                             pick_new_inputs = false;
                             nFeeRet = fee_needed_with_change;

src/wallet/wallet.h

@@ -608,18 +608,20 @@ struct CoinSelectionParams
     size_t change_spend_size = 0;
     CFeeRate effective_fee = CFeeRate(0);
     CFeeRate m_long_term_feerate;
+    CFeeRate m_discard_feerate;
     size_t tx_noinputs_size = 0;
     //! Indicate that we are subtracting the fee from outputs
     bool m_subtract_fee_outputs = false;
     bool m_avoid_partial_spends = false;
 
     CoinSelectionParams(bool use_bnb, size_t change_output_size, size_t change_spend_size, CFeeRate effective_fee,
-                        CFeeRate long_term_feerate, size_t tx_noinputs_size, bool avoid_partial) :
+                        CFeeRate long_term_feerate, CFeeRate discard_feerate, size_t tx_noinputs_size, bool avoid_partial) :
         use_bnb(use_bnb),
         change_output_size(change_output_size),
         change_spend_size(change_spend_size),
         effective_fee(effective_fee),
         m_long_term_feerate(long_term_feerate),
+        m_discard_feerate(discard_feerate),
         tx_noinputs_size(tx_noinputs_size),
         m_avoid_partial_spends(avoid_partial)
     {}