// fundTx attempts to fund a transaction sending amt bitcoin. The coins are
// selected such that the final amount spent pays enough fees as dictated by
// the passed fee rate. The passed fee rate should be expressed in
// satoshis-per-byte.
//
// NOTE: The memWallet's mutex must be held when this function is called.
func (m *memWallet) fundTx(tx *wire.MsgTx, amt btcutil.Amount, feeRate btcutil.Amount) error {
const (
// spendSize is the largest number of bytes of a sigScript
// which spends a p2pkh output: OP_DATA_73 <sig> OP_DATA_33 <pubkey>
spendSize = 1 + 73 + 1 + 33
)
var (
amtSelected btcutil.Amount
txSize int
)
for outPoint, utxo := range m.utxos {
// Skip any outputs that are still currently immature or are
// currently locked.
if !utxo.isMature(m.currentHeight) || utxo.isLocked {
continue
}
amtSelected += utxo.value
// Add the selected output to the transaction, updating the
// current tx size while accounting for the size of the future
// sigScript.
tx.AddTxIn(wire.NewTxIn(&outPoint, nil, nil))
txSize = tx.SerializeSize() + spendSize*len(tx.TxIn)
// Calculate the fee required for the txn at this point
// observing the specified fee rate. If we don't have enough
// coins from he current amount selected to pay the fee, then
// continue to grab more coins.
reqFee := btcutil.Amount(txSize * int(feeRate))
if amtSelected-reqFee < amt {
continue
}
// If we have any change left over, then add an additional
// output to the transaction reserved for change.
changeVal := amtSelected - amt - reqFee
if changeVal > 0 {
addr, err := m.newAddress()
if err != nil {
return err
}
pkScript, err := txscript.PayToAddrScript(addr)
if err != nil {
return err
}
changeOutput := &wire.TxOut{
Value: int64(changeVal),
PkScript: pkScript,
}
tx.AddTxOut(changeOutput)
}
return nil
}
// If we've reached this point, then coin selection failed due to an
// insufficient amount of coins.
return fmt.Errorf("not enough funds for coin selection")
}
