wallet test: Add test for subtract fee from recipient behavior

Behavior might have recently changed in #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.

Co-authored-by: Andrew Chow <achow101-github@achow101.com>

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 +1,61 @@
+// 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)
+{
+    CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));
+    auto wallet = CreateSyncedWallet(*m_node.chain, m_node.chainman->ActiveChain(), coinbaseKey);
+
+    // Check that a subtract-from-recipient transaction slightly less than the
+    // coinbase input amount does not create a change output (because it would
+    // be uneconomical to add and spend the output), and make sure it pays the
+    // leftover input amount which would have been change to the recipient
+    // instead of the miner.
+    auto check_tx = [&wallet](CAmount leftover_input_amount) {
+        CRecipient recipient{GetScriptForRawPubKey({}), 50 * COIN - leftover_input_amount, true /* subtract fee */};
+        CTransactionRef tx;
+        CAmount fee;
+        int change_pos = -1;
+        bilingual_str error;
+        CCoinControl coin_control;
+        coin_control.m_feerate.emplace(10000);
+        coin_control.fOverrideFeeRate = true;
+        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 + leftover_input_amount - fee);
+        BOOST_CHECK_GT(fee, 0);
+        return fee;
+    };
+
+    // Send full input amount to recipient, check that only nonzero fee is
+    // subtracted (to_reduce == fee).
+    const CAmount fee{check_tx(0)};
+
+    // Send slightly less than full input amount to recipient, check leftover
+    // input amount is paid to recipient not the miner (to_reduce == fee - 123)
+    BOOST_CHECK_EQUAL(fee, check_tx(123));
+
+    // Send full input minus fee amount to recipient, check leftover input
+    // amount is paid to recipient not the miner (to_reduce == 0)
+    BOOST_CHECK_EQUAL(fee, check_tx(fee));
+
+    // Send full input minus more than the fee amount to recipient, check
+    // leftover input amount is paid to recipient not the miner (to_reduce ==
+    // -123). This overpays the recipient instead of overpaying the miner more
+    // than double the neccesary fee.
+    BOOST_CHECK_EQUAL(fee, check_tx(fee + 123));
+}
+
+BOOST_AUTO_TEST_SUITE_END()