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wallet: ensure VSP payment doesn't create unmixable change #2595

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8839841
wallet: ensure VSP payent doesn't create unmixable change
matthawkins90 Dec 6, 2025
20317e4
Merge branch 'decred:master' into unmixable_change
matthawkins90 Jan 28, 2026
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19 changes: 16 additions & 3 deletions wallet/createtx.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1657,9 +1657,11 @@ func (w *Wallet) purchaseTickets(ctx context.Context, op errors.Op,
return purchaseTicketsResponse, err
}

// ReserveOutputsForAmount returns locked spendable outpoints from the given
// reserveOutputsForAmount returns locked spendable outpoints from the given
// account. It is the responsibility of the caller to unlock the outpoints.
func (w *Wallet) ReserveOutputsForAmount(ctx context.Context, account uint32, amount dcrutil.Amount, minconf int32) ([]Input, error) {
// This unexported func supports specifying a minimum non-zero change amount,
// so that the selected outputs will total at least "amount + minChange".
func (w *Wallet) reserveOutputsForAmount(ctx context.Context, account uint32, amount dcrutil.Amount, minconf int32, minChange dcrutil.Amount) ([]Input, error) {
defer w.lockedOutpointMu.Unlock()
w.lockedOutpointMu.Lock()

Expand All @@ -1668,10 +1670,15 @@ func (w *Wallet) ReserveOutputsForAmount(ctx context.Context, account uint32, am
// Get current block's height
_, tipHeight := w.txStore.MainChainTip(dbtx)

targetAmount := amount
if minChange > 0 {
targetAmount = amount + minChange
}

var err error
const minAmount = 0
const maxResults = 0
outputs, err = w.findEligibleOutputsAmount(dbtx, account, minconf, amount, tipHeight,
outputs, err = w.findEligibleOutputsAmount(dbtx, account, minconf, targetAmount, tipHeight,
minAmount, maxResults)
if err != nil {
return err
Expand All @@ -1690,6 +1697,12 @@ func (w *Wallet) ReserveOutputsForAmount(ctx context.Context, account uint32, am
return outputs, nil
}

// ReserveOutputsForAmount returns locked spendable outpoints from the given
// account. It is the responsibility of the caller to unlock the outpoints.
func (w *Wallet) ReserveOutputsForAmount(ctx context.Context, account uint32, amount dcrutil.Amount, minconf int32) ([]Input, error) {
return w.reserveOutputsForAmount(ctx, account, amount, minconf, 0)
}

func (w *Wallet) reserveOutputs(ctx context.Context, account uint32, minconf int32) ([]Input, error) {
defer w.lockedOutpointMu.Unlock()
w.lockedOutpointMu.Lock()
Expand Down
26 changes: 20 additions & 6 deletions wallet/wallet.go
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Original file line number Diff line number Diff line change
Expand Up @@ -4663,13 +4663,31 @@ func (s sigDataSource) GetScript(a stdaddr.Address) ([]byte, error) { return s.s
func (w *Wallet) CreateVspPayment(ctx context.Context, tx *wire.MsgTx, fee dcrutil.Amount,
feeAddr stdaddr.Address, feeAcct uint32, changeAcct uint32) error {

feeRate := w.RelayFee()

// Reserve new outputs to pay the fee if outputs have not already been
// reserved. This will be the case for fee payments that were begun on
// already purchased tickets, where the caller did not ensure that fee
// outputs would already be reserved.
if len(tx.TxIn) == 0 {
const minconf = 1
inputs, err := w.ReserveOutputsForAmount(ctx, feeAcct, fee, minconf)

minMixableChange := smallestMixChange(feeRate)

// Estimate tx fee for a typical VSP payment (1 input, 2 outputs)
// to ensure enough is reserved for mixable change after all fees.
estScriptSizes := []int{txsizes.RedeemP2PKHSigScriptSize}
estOutputs := []*wire.TxOut{
{PkScript: make([]byte, txsizes.P2PKHPkScriptSize)}, // fee output
{PkScript: make([]byte, txsizes.P2PKHPkScriptSize)}, // change output
}
estSize := txsizes.EstimateSerializeSize(estScriptSizes, estOutputs, 0)
estTxFee := txrules.FeeForSerializeSize(feeRate, estSize)

// Request inputs totaling at least (fee + minMixableChange + estTxFee)
// so change after paying VSP fee and tx fee will be mixable.
minChange := minMixableChange + dcrutil.Amount(estTxFee)
inputs, err := w.reserveOutputsForAmount(ctx, feeAcct, fee, minconf, minChange)
if err != nil {
return fmt.Errorf("unable to reserve outputs: %w", err)
}
Expand Down Expand Up @@ -4716,11 +4734,7 @@ func (w *Wallet) CreateVspPayment(ctx context.Context, tx *wire.MsgTx, fee dcrut
Version: vers,
PkScript: feeScript,
})
feeRate := w.RelayFee()
scriptSizes := make([]int, len(tx.TxIn))
for i := range scriptSizes {
scriptSizes[i] = txsizes.RedeemP2PKHSigScriptSize
}
scriptSizes := slices.Repeat([]int{txsizes.RedeemP2PKHSigScriptSize}, len(tx.TxIn))
est := txsizes.EstimateSerializeSize(scriptSizes, tx.TxOut, txsizes.P2PKHPkScriptSize)
change := input
change -= tx.TxOut[0].Value
Expand Down
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