wallet test: Add test for subtract fee from recipient behavior

Behavior might have recently changed in bitcoin#17331 (it is not clear) but not
noticed because there is no test coverage.

This adds test coverage for current subtract from recipient behavior
without changing it.

src/Makefile.test.include

@@ -151,6 +151,7 @@ BITCOIN_TESTS =\
 if ENABLE_WALLET
 BITCOIN_TESTS += \
   wallet/test/psbt_wallet_tests.cpp \
+  wallet/test/spend_tests.cpp \
   wallet/test/wallet_tests.cpp \
   wallet/test/walletdb_tests.cpp \
   wallet/test/wallet_crypto_tests.cpp \

src/wallet/test/spend_tests.cpp

@@ -0,0 +1,48 @@
+// Copyright (c) 2021 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 <policy/fees.h>
+#include <validation.h>
+#include <wallet/coincontrol.h>
+#include <wallet/test/util.h>
+#include <wallet/test/wallet_test_fixture.h>
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(spend_tests, WalletTestingSetup)
+
+BOOST_FIXTURE_TEST_CASE(SubtractFee, TestChain100Setup)
+{
+    CChain& cchain = m_node.chainman->ActiveChain();
+    CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));
+    auto wallet = CreateSyncedWallet(*m_node.chain, cchain, coinbaseKey);
+
+    // Check subtract from recipient transaction equal to coinbase amount does
+    // not create change output.
+    CRecipient recipient{GetScriptForRawPubKey({}), 50 * COIN, true /* subtract fee */};
+    CTransactionRef tx;
+    CAmount fee;
+    int change_pos = -1;
+    bilingual_str error;
+    CCoinControl coin_control;
+    FeeCalculation fee_calc;
+    BOOST_CHECK(wallet->CreateTransaction({recipient}, tx, fee, change_pos, error, coin_control, fee_calc));
+    BOOST_CHECK_EQUAL(tx->vout.size(), 1);
+    BOOST_CHECK_EQUAL(tx->vout[0].nValue, recipient.nAmount - fee);
+    tx.reset();
+
+    // Check subtract from recipient transaction slightly less than coinbase
+    // amount also does not create change output and pays extra dust amount to
+    // recipient instead of miner
+    const CAmount dust_amount = 123;
+    const CAmount expected_fee = fee;
+    recipient.nAmount -= dust_amount;
+    BOOST_CHECK(wallet->CreateTransaction({recipient}, tx, fee, change_pos, error, coin_control, fee_calc));
+    BOOST_CHECK_EQUAL(tx->vout.size(), 1);
+    BOOST_CHECK_EQUAL(tx->vout[0].nValue, recipient.nAmount - fee + dust_amount);
+    BOOST_CHECK_EQUAL(fee, expected_fee);
+    tx.reset();
+}
+
+BOOST_AUTO_TEST_SUITE_END()