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mempool: introduce GetDustThreshold to export dust limit calculation #1742

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Sep 16, 2021
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mempool: introduce GetDustThreshold to export dust limit calculation #1742

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31 changes: 19 additions & 12 deletions mempool/policy.go
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Original file line number Diff line number Diff line change
Expand Up @@ -172,17 +172,10 @@ func checkPkScriptStandard(pkScript []byte, scriptClass txscript.ScriptClass) er
return nil
}

// IsDust returns whether or not the passed transaction output amount is
// considered dust or not based on the passed minimum transaction relay fee.
// Dust is defined in terms of the minimum transaction relay fee. In
// particular, if the cost to the network to spend coins is more than 1/3 of the
// minimum transaction relay fee, it is considered dust.
func IsDust(txOut *wire.TxOut, minRelayTxFee btcutil.Amount) bool {
// Unspendable outputs are considered dust.
if txscript.IsUnspendable(txOut.PkScript) {
return true
}

// GetDustThreshold calculates the dust limit for a *wire.TxOut by taking the
// size of a typical spending transaction and multiplying it by 3 to account
// for the minimum dust relay fee of 3000sat/kvb.
func GetDustThreshold(txOut *wire.TxOut) int64 {
// The total serialized size consists of the output and the associated
// input script to redeem it. Since there is no input script
// to redeem it yet, use the minimum size of a typical input script.
Expand Down Expand Up @@ -253,6 +246,20 @@ func IsDust(txOut *wire.TxOut, minRelayTxFee btcutil.Amount) bool {
totalSize += 107
}

return 3 * int64(totalSize)
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Why is the factor of 3 here retained? When in the original dust calculation, this value is used to ensure that the amount spent on fees to spend an output isn't greater than 1/3 of the relay fee.

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Ah ok, we chatted offline and derived why this is in place.

}

// IsDust returns whether or not the passed transaction output amount is
// considered dust or not based on the passed minimum transaction relay fee.
// Dust is defined in terms of the minimum transaction relay fee. In
// particular, if the cost to the network to spend coins is more than 1/3 of the
// minimum transaction relay fee, it is considered dust.
func IsDust(txOut *wire.TxOut, minRelayTxFee btcutil.Amount) bool {
// Unspendable outputs are considered dust.
if txscript.IsUnspendable(txOut.PkScript) {
return true
}

// The output is considered dust if the cost to the network to spend the
// coins is more than 1/3 of the minimum free transaction relay fee.
// minFreeTxRelayFee is in Satoshi/KB, so multiply by 1000 to
Expand All @@ -265,7 +272,7 @@ func IsDust(txOut *wire.TxOut, minRelayTxFee btcutil.Amount) bool {
//
// The following is equivalent to (value/totalSize) * (1/3) * 1000
// without needing to do floating point math.
return txOut.Value*1000/(3*int64(totalSize)) < int64(minRelayTxFee)
return txOut.Value*1000/GetDustThreshold(txOut) < int64(minRelayTxFee)
}

// checkTransactionStandard performs a series of checks on a transaction to
Expand Down