test: add coverage for BnB-SFFO restriction

Verify the transaction creation process does not produce
a BnB solution when SFFO is enabled.
This is currently problematic because it could require a
change output. And BnB is specialized on changeless solutions.

src/wallet/test/coinselector_tests.cpp

@@ -7,10 +7,12 @@
 #include <policy/policy.h>
 #include <primitives/transaction.h>
 #include <random.h>
+#include <test/util/logging.h>
 #include <test/util/setup_common.h>
 #include <util/translation.h>
 #include <wallet/coincontrol.h>
 #include <wallet/coinselection.h>
+#include <wallet/fees.h>
 #include <wallet/spend.h>
 #include <wallet/test/util.h>
 #include <wallet/test/wallet_test_fixture.h>
@@ -83,7 +85,8 @@ static void add_coin(CoinsResult& available_coins, CWallet& wallet, const CAmoun
     uint256 txid = tx.GetHash();
 
     LOCK(wallet.cs_wallet);
-    auto ret = wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(txid), std::forward_as_tuple(MakeTransactionRef(std::move(tx)), TxStateInactive{}));
+    TxState state = (nAge > 0) ? TxStateConfirmed{uint256{}, nAge, 0} : TxState{TxStateInactive{}};
+    auto ret = wallet.mapWallet.emplace(std::piecewise_construct, std::forward_as_tuple(txid), std::forward_as_tuple(MakeTransactionRef(std::move(tx)), state));
     assert(ret.second);
     CWalletTx& wtx = (*ret.first).second;
     const auto& txout = wtx.tx->vout.at(nInput);
@@ -449,6 +452,37 @@ BOOST_AUTO_TEST_CASE(bnb_search_test)
     }
 }
 
+BOOST_AUTO_TEST_CASE(tx_creation_bnb_sffo_restriction)
+{
+    // Verify the transaction creation process does not produce a BnB solution when SFFO is enabled.
+    // This is currently problematic because it could require a change output. And BnB is specialized on changeless solutions.
+    std::unique_ptr<CWallet> wallet = NewWallet(m_node);
+    WITH_LOCK(wallet->cs_wallet, wallet->SetLastBlockProcessed(300, uint256{})); // set a high block so internal UTXOs are selectable
+
+    CTxDestination change_dest = *Assert(wallet->GetNewChangeDestination(OutputType::BECH32));
+    CTxOut change_out(0, GetScriptForDestination(change_dest));
+    size_t change_output_size = GetSerializeSize(change_out);
+
+    CFeeRate effective_feerate(10000);
+    CFeeRate discard_feerate = GetDiscardRate(*wallet);
+
+    CAmount change_fee = effective_feerate.GetFee(change_output_size);
+    size_t change_spend_size = CalculateMaximumSignedInputSize(change_out, wallet.get(), nullptr);
+    CAmount cost_of_change = discard_feerate.GetFee(change_spend_size) + change_fee;
+
+    // Add spendable coin at the BnB selection upper bound
+    CoinsResult available_coins;
+    add_coin(available_coins, *wallet, COIN + cost_of_change, /*feerate=*/CFeeRate(0), /*nAge=*/6*24, /*fIsFromMe=*/true, /*nInput=*/0, /*spendable=*/true);
+
+    // Now verify coin selection does not produce BnB result
+    ASSERT_DEBUG_LOG_NOT_FOUND("Coin Selection: Algorithm:bnb");
+    CCoinControl coin_control;
+    coin_control.m_feerate = effective_feerate;
+    coin_control.destChange = change_dest;
+    CRecipient recipient{GetScriptForRawPubKey({}), /*target=*/COIN, /*subtract_fee=*/true};
+    BOOST_CHECK(CreateTransaction(*wallet, {recipient}, /*change_pos=*/-1, coin_control));
+}
+
 BOOST_AUTO_TEST_CASE(knapsack_solver_test)
 {
     FastRandomContext rand{};