// IsDustOutput determines whether a transaction output is considered dust.
// Transactions with dust outputs are not standard and are rejected by mempools
// with default policies.
func IsDustOutput(output *wire.TxOut, relayFeePerKb btcutil.Amount) bool {
// Unspendable outputs which solely carry data are not checked for dust.
if txscript.GetScriptClass(output.PkScript) == txscript.NullDataTy {
return false
}
// All other unspendable outputs are considered dust.
if txscript.IsUnspendable(output.PkScript) {
return true
}
return IsDustAmount(btcutil.Amount(output.Value), len(output.PkScript),
relayFeePerKb)
}
// AddTxOuts adds all outputs in the passed transaction which are not provably
// unspendable to the view. When the view already has entries for any of the
// outputs, they are simply marked unspent. All fields will be updated for
// existing entries since it's possible it has changed during a reorg.
func (view *UtxoViewpoint) AddTxOuts(tx *btcutil.Tx, blockHeight int32) {
// When there are not already any utxos associated with the transaction,
// add a new entry for it to the view.
entry := view.LookupEntry(tx.Hash())
if entry == nil {
entry = newUtxoEntry(tx.MsgTx().Version, IsCoinBase(tx),
blockHeight)
view.entries[*tx.Hash()] = entry
} else {
entry.blockHeight = blockHeight
}
entry.modified = true
// Loop all of the transaction outputs and add those which are not
// provably unspendable.
for txOutIdx, txOut := range tx.MsgTx().TxOut {
if txscript.IsUnspendable(txOut.PkScript) {
continue
}
// Update existing entries. All fields are updated because it's
// possible (although extremely unlikely) that the existing
// entry is being replaced by a different transaction with the
// same hash. This is allowed so long as the previous
// transaction is fully spent.
if output, ok := entry.sparseOutputs[uint32(txOutIdx)]; ok {
output.spent = false
output.compressed = false
output.amount = txOut.Value
output.pkScript = txOut.PkScript
continue
}
// Add the unspent transaction output.
entry.sparseOutputs[uint32(txOutIdx)] = &utxoOutput{
spent: false,
compressed: false,
amount: txOut.Value,
pkScript: txOut.PkScript,
}
}
return
}
// 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 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.
//
// Pay-to-pubkey-hash bytes breakdown:
//
// Output to hash (34 bytes):
// 8 value, 1 script len, 25 script [1 OP_DUP, 1 OP_HASH_160,
// 1 OP_DATA_20, 20 hash, 1 OP_EQUALVERIFY, 1 OP_CHECKSIG]
//
// Input with compressed pubkey (148 bytes):
// 36 prev outpoint, 1 script len, 107 script [1 OP_DATA_72, 72 sig,
// 1 OP_DATA_33, 33 compressed pubkey], 4 sequence
//
// Input with uncompressed pubkey (180 bytes):
// 36 prev outpoint, 1 script len, 139 script [1 OP_DATA_72, 72 sig,
// 1 OP_DATA_65, 65 compressed pubkey], 4 sequence
//
// Pay-to-pubkey bytes breakdown:
//
// Output to compressed pubkey (44 bytes):
// 8 value, 1 script len, 35 script [1 OP_DATA_33,
// 33 compressed pubkey, 1 OP_CHECKSIG]
//
// Output to uncompressed pubkey (76 bytes):
// 8 value, 1 script len, 67 script [1 OP_DATA_65, 65 pubkey,
// 1 OP_CHECKSIG]
//
// Input (114 bytes):
// 36 prev outpoint, 1 script len, 73 script [1 OP_DATA_72,
// 72 sig], 4 sequence
//
// Theoretically this could examine the script type of the output script
// and use a different size for the typical input script size for
// pay-to-pubkey vs pay-to-pubkey-hash inputs per the above breakdowns,
// but the only combinination which is less than the value chosen is
// a pay-to-pubkey script with a compressed pubkey, which is not very
// common.
//
// The most common scripts are pay-to-pubkey-hash, and as per the above
// breakdown, the minimum size of a p2pkh input script is 148 bytes. So
// that figure is used. If the output being spent is a witness program,
// then we apply the witness discount to the size of the signature.
var totalSize int
if txscript.IsWitnessProgram(txOut.PkScript) {
witnessSig := (32 + 4 + 1 + (107 / blockchain.WitnessScaleFactor) + 4)
totalSize = txOut.SerializeSize() + witnessSig
} else {
totalSize = txOut.SerializeSize() + 148
}
// 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
// convert to bytes.
//
// Using the typical values for a pay-to-pubkey-hash transaction from
// the breakdown above and the default minimum free transaction relay
// fee of 1000, this equates to values less than 546 satoshi being
// considered dust.
//
// 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)
}