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The subtract fee from outputs assumes that the leftover input amount will be dropped to fees. However this only happens if that amount is less than the cost of change. In the event that it is higher than the cost of change, the leftover amount will actually become a change output. To avoid this scenario, force a change type which has a high cost of change.
65 lines
2.9 KiB
C++
65 lines
2.9 KiB
C++
// Copyright (c) 2021 The Bitcoin Core developers
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// Distributed under the MIT software license, see the accompanying
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// file COPYING or http://www.opensource.org/licenses/mit-license.php.
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#include <consensus/amount.h>
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#include <policy/fees.h>
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#include <validation.h>
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#include <wallet/coincontrol.h>
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#include <wallet/spend.h>
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#include <wallet/test/util.h>
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#include <wallet/test/wallet_test_fixture.h>
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#include <boost/test/unit_test.hpp>
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BOOST_FIXTURE_TEST_SUITE(spend_tests, WalletTestingSetup)
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BOOST_FIXTURE_TEST_CASE(SubtractFee, TestChain100Setup)
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{
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CreateAndProcessBlock({}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));
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auto wallet = CreateSyncedWallet(*m_node.chain, m_node.chainman->ActiveChain(), coinbaseKey);
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// Check that a subtract-from-recipient transaction slightly less than the
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// coinbase input amount does not create a change output (because it would
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// be uneconomical to add and spend the output), and make sure it pays the
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// leftover input amount which would have been change to the recipient
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// instead of the miner.
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auto check_tx = [&wallet](CAmount leftover_input_amount) {
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CRecipient recipient{GetScriptForRawPubKey({}), 50 * COIN - leftover_input_amount, true /* subtract fee */};
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CTransactionRef tx;
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CAmount fee;
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int change_pos = -1;
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bilingual_str error;
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CCoinControl coin_control;
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coin_control.m_feerate.emplace(10000);
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coin_control.fOverrideFeeRate = true;
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// We need to use a change type with high cost of change so that the leftover amount will be dropped to fee instead of added as a change output
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coin_control.m_change_type = OutputType::LEGACY;
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FeeCalculation fee_calc;
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BOOST_CHECK(CreateTransaction(*wallet, {recipient}, tx, fee, change_pos, error, coin_control, fee_calc));
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BOOST_CHECK_EQUAL(tx->vout.size(), 1);
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BOOST_CHECK_EQUAL(tx->vout[0].nValue, recipient.nAmount + leftover_input_amount - fee);
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BOOST_CHECK_GT(fee, 0);
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return fee;
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};
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// Send full input amount to recipient, check that only nonzero fee is
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// subtracted (to_reduce == fee).
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const CAmount fee{check_tx(0)};
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// Send slightly less than full input amount to recipient, check leftover
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// input amount is paid to recipient not the miner (to_reduce == fee - 123)
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BOOST_CHECK_EQUAL(fee, check_tx(123));
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// Send full input minus fee amount to recipient, check leftover input
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// amount is paid to recipient not the miner (to_reduce == 0)
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BOOST_CHECK_EQUAL(fee, check_tx(fee));
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// Send full input minus more than the fee amount to recipient, check
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// leftover input amount is paid to recipient not the miner (to_reduce ==
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// -123). This overpays the recipient instead of overpaying the miner more
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// than double the necessary fee.
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BOOST_CHECK_EQUAL(fee, check_tx(fee + 123));
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}
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BOOST_AUTO_TEST_SUITE_END()
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