staticaddr: fractional loop-in amount

interface.go

@@ -338,6 +338,8 @@ type StaticAddressLoopInRequest struct {
 	// swap payment. If the timeout is reached the swap will be aborted and
 	// the client can retry the swap if desired with different parameters.
 	PaymentTimeoutSeconds uint32
+
+	SelectedAmount btcutil.Amount
 }
 
 // LoopInTerms are the server terms on which it executes loop in swaps.

staticaddr/loopin/actions.go

@@ -68,9 +68,14 @@ func (f *FSM) InitHtlcAction(ctx context.Context,
 	}
 
 	// Calculate the swap invoice amount. The server needs to pay us the
-	// sum of all deposits minus the fees that the server charges for the
-	// swap.
-	swapInvoiceAmt := f.loopIn.TotalDepositAmount() - f.loopIn.QuotedSwapFee
+	// swap amount minus the fees that the server charges for the swap. The
+	// swap amount is either the total value of the selected deposits, or
+	// the selected amount if a specific amount was requested.
+	swapAmount := f.loopIn.TotalDepositAmount()
+	if f.loopIn.SelectedAmount > 0 {
+		swapAmount = f.loopIn.SelectedAmount
+	}
+	swapInvoiceAmt := swapAmount - f.loopIn.QuotedSwapFee
 
 	// Generate random preimage.
 	var swapPreimage lntypes.Preimage

staticaddr/loopin/loopin.go

@@ -93,6 +93,8 @@ type StaticAddressLoopIn struct {
 	// swap.
 	DepositOutpoints []string
 
+	SelectedAmount btcutil.Amount
+
 	// state is the current state of the swap.
 	state fsm.StateType
 
@@ -287,10 +289,18 @@ func (l *StaticAddressLoopIn) createHtlcTx(chainParams *chaincfg.Params,
 	weight := l.htlcWeight()
 	fee := feeRate.FeeForWeight(weight)
 
-	// Check if the server breaches our fee limits.
-	amt := float64(l.TotalDepositAmount())
-	feeLimit := btcutil.Amount(amt * maxFeePercentage)
+	// Determine the swap amount. If the user selected a specific amount, we
+	// use that and use the difference to the total deposit amount as the
+	// change.
+	swapAmt := float64(l.TotalDepositAmount())
+	var changeAmount btcutil.Amount
+	if l.SelectedAmount > 0 {
+		swapAmt = float64(l.SelectedAmount)
+		changeAmount = l.TotalDepositAmount() - l.SelectedAmount
+	}
 
+	// Check if the server breaches our fee limits.
+	feeLimit := btcutil.Amount(swapAmt * maxFeePercentage)
 	if fee > feeLimit {
 		return nil, fmt.Errorf("htlc tx fee %v exceeds max fee %v",
 			fee, feeLimit)
@@ -314,6 +324,14 @@ func (l *StaticAddressLoopIn) createHtlcTx(chainParams *chaincfg.Params,
 
 	msgTx.AddTxOut(sweepOutput)
 
+	// We expect change to be sent back to our static address output script.
+	if changeAmount > 0 {
+		msgTx.AddTxOut(&wire.TxOut{
+			Value:    int64(changeAmount),
+			PkScript: l.AddressParams.PkScript,
+		})
+	}
+
 	return msgTx, nil
 }
 

staticaddr/loopin/manager.go

@@ -20,7 +20,9 @@ import (
 	"github.com/lightninglabs/loop/staticaddr/deposit"
 	"github.com/lightninglabs/loop/swapserverrpc"
 	looprpc "github.com/lightninglabs/loop/swapserverrpc"
+	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lntypes"
+	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/routing/route"
 )
 
@@ -205,8 +207,8 @@ func (m *Manager) Run(ctx context.Context, currentHeight uint32) error {
 			case request.respChan <- resp:
 
 			case <-ctx.Done():
-				// Noify subroutines that the main loop has been
-				// canceled.
+				// Notify subroutines that the main loop has
+				// been canceled.
 				close(m.exitChan)
 
 				return ctx.Err()
@@ -549,6 +551,15 @@ func (m *Manager) initiateLoopIn(ctx context.Context,
 	}
 	totalDepositAmount := tmp.TotalDepositAmount()
 
+	// If the selected amount would leave a dust change output or exceeds
+	// the total deposits value, we return an error.
+	dustLimit := lnwallet.DustLimitForSize(input.P2TRSize)
+	if totalDepositAmount-req.SelectedAmount < dustLimit {
+		return nil, fmt.Errorf("selected amount %v leaves "+
+			"dust or exceeds total deposit value %v",
+			req.SelectedAmount, totalDepositAmount)
+	}
+
 	// Check that the label is valid.
 	err := labels.Validate(req.Label)
 	if err != nil {
@@ -616,6 +627,7 @@ func (m *Manager) initiateLoopIn(ctx context.Context,
 	}
 
 	swap := &StaticAddressLoopIn{
+		SelectedAmount:        req.SelectedAmount,
 		DepositOutpoints:      req.DepositOutpoints,
 		Deposits:              deposits,
 		Label:                 req.Label,