Merge pull request #303 from ElementsProject/pre-swapcheck-fee

swap: calculate required balance including fee

lnd/client.go

@@ -131,7 +131,7 @@ func (l *Client) SpendableMsat(scid string) (uint64, error) {
 				return 0, err
 			}
 			spendable := (uint64(ch.GetLocalBalance()) -
-				ch.GetLocalConstraints().GetChanReserveSat()*1000)
+				ch.GetLocalConstraints().GetChanReserveSat()) * 1000
 			// since the max htlc limit is not always set reliably,
 			// the check is skipped if it is not set.
 			if maxHtlcAmtMsat == 0 {
@@ -168,7 +168,7 @@ func (l *Client) ReceivableMsat(scid string) (uint64, error) {
 				return 0, err
 			}
 			receivable := (uint64(ch.GetRemoteBalance()) -
-				ch.GetRemoteConstraints().GetChanReserveSat()*1000)
+				ch.GetRemoteConstraints().GetChanReserveSat()) * 1000
 			// since the max htlc limit is not always set reliably,
 			// the check is skipped if it is not set.
 			if maxHtlcAmtMsat == 0 {

swap/actions.go

@@ -589,20 +589,29 @@ func (r *PayFeeInvoiceAction) Execute(services *SwapServices, swap *SwapData) Ev
 	}
 
 	ll := services.lightning
-	// policy := services.policy
+
 	_, msatAmt, _, err := ll.DecodePayreq(swap.SwapOutAgreement.Payreq)
 	if err != nil {
 		return swap.HandleError(err)
 	}
+
 	sp, err := ll.SpendableMsat(swap.SwapOutRequest.Scid)
 	if err != nil {
 		return swap.HandleError(err)
 	}
 
-	if sp <= swap.SwapOutRequest.Amount*1000 {
-		return swap.HandleError(err)
+	// Calculate the total required balance
+	// This includes the swap amount (multiplied by 1000 to convert from sat to msat)
+	// plus the fee in millisatoshis
+	requiredBalance := swap.SwapOutRequest.Amount*1000 + msatAmt
+
+	// Check if the spendable balance (sp) is sufficient
+	if sp <= requiredBalance {
+		return swap.HandleError(fmt.Errorf("not enough spendable msat: %d, expected: %d", sp, requiredBalance))
 	}
-	success, failureReason, err := ll.ProbePayment(swap.SwapOutRequest.Scid, swap.SwapOutRequest.Amount*1000)
+
+	// Probe the payment to check if it's possible
+	success, failureReason, err := ll.ProbePayment(swap.SwapOutRequest.Scid, requiredBalance)
 	if err != nil {
 		return swap.HandleError(err)
 	}

swap/service.go

@@ -391,14 +391,6 @@ func (s *SwapService) SwapOut(peer string, chain string, channelId string, initi
 		return nil, fmt.Errorf("exceeding spendable amount_msat: %d", sp)
 	}
 
-	success, failureReason, err := s.swapServices.lightning.ProbePayment(channelId, amtSat*1000)
-	if err != nil {
-		return nil, err
-	}
-	if !success {
-		return nil, fmt.Errorf("the prepayment probe was unsuccessful: %s", failureReason)
-	}
-
 	swap := newSwapOutSenderFSM(s.swapServices, initiator, peer)
 	err = s.lockSwap(swap.SwapId.String(), channelId, swap)
 	if err != nil {