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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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6
src/policy/feerate.cpp
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@@ -7,6 +7,8 @@
#include <tinyformat.h>
#include <cmath>
CFeeRate::CFeeRate(const CAmount& nFeePaid, uint32_t num_bytes)
{
const int64_t nSize{num_bytes};
@@ -22,7 +24,9 @@ CAmount CFeeRate::GetFee(uint32_t num_bytes) const
{
const int64_t nSize{num_bytes};
CAmount nFee = nSatoshisPerK * nSize / 1000;
// Be explicit that we're converting from a double to int64_t (CAmount) here.
// We've previously had issues with the silent double->int64_t conversion.
CAmount nFee{static_cast<CAmount>(std::ceil(nSatoshisPerK * nSize / 1000.0))};
if (nFee == 0 && nSize != 0) {
if (nSatoshisPerK > 0) nFee = CAmount(1);