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fees: Always round up fee calculated from a feerate

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When calculating the fee for a given tx size from a fee rate, we should
always round up to the next satoshi. Otherwise, if we round down (via
truncation), the calculated fee may result in a fee with a feerate
slightly less than targeted.

This is particularly important for coin selection as a slightly lower
feerate than expected can result in a variety of issues.
This commit is contained in:
Andrew Chow
2021-09-10 20:24:44 -04:00
parent 927586990e
commit 0fbaef9676
5 changed files with 14 additions and 9 deletions
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10
src/test/amount_tests.cpp
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@@ -48,13 +48,13 @@ BOOST_AUTO_TEST_CASE(GetFeeTest)
BOOST_CHECK_EQUAL(feeRate.GetFee(9e3), CAmount(-9e3));
feeRate = CFeeRate(123);
// Truncates the result, if not integer
// Rounds up the result, if not integer
BOOST_CHECK_EQUAL(feeRate.GetFee(0), CAmount(0));
BOOST_CHECK_EQUAL(feeRate.GetFee(8), CAmount(1)); // Special case: returns 1 instead of 0
BOOST_CHECK_EQUAL(feeRate.GetFee(9), CAmount(1));
BOOST_CHECK_EQUAL(feeRate.GetFee(121), CAmount(14));
BOOST_CHECK_EQUAL(feeRate.GetFee(122), CAmount(15));
BOOST_CHECK_EQUAL(feeRate.GetFee(999), CAmount(122));
BOOST_CHECK_EQUAL(feeRate.GetFee(9), CAmount(2));
BOOST_CHECK_EQUAL(feeRate.GetFee(121), CAmount(15));
BOOST_CHECK_EQUAL(feeRate.GetFee(122), CAmount(16));
BOOST_CHECK_EQUAL(feeRate.GetFee(999), CAmount(123));
BOOST_CHECK_EQUAL(feeRate.GetFee(1e3), CAmount(123));
BOOST_CHECK_EQUAL(feeRate.GetFee(9e3), CAmount(1107));