channeldb/invoice: fail extra HTLC sets immeidately

invoices/invoiceregistry.go

@@ -1063,6 +1063,23 @@ func (i *InvoiceRegistry) notifyExitHopHtlcLocked(
 			i.notifyHodlSubscribers(htlcSettleResolution)
 		}
 
+		// If concurrent payments were attempted to this invoice before
+		// the current one was ultimately settled, cancel back any of
+		// the HTLCs immediately. As a result of the settle, the HTLCs
+		// in other HTLC sets are automatically converted to a canceled
+		// state when updating the invoice.
+		canceledHtlcSet := invoice.HTLCSetCompliment(
+			setID, channeldb.HtlcStateCanceled,
+		)
+		for key, htlc := range canceledHtlcSet {
+			htlcFailResolution := NewFailResolution(
+				key, int32(htlc.AcceptHeight),
+				ResultInvoiceAlreadySettled,
+			)
+
+			i.notifyHodlSubscribers(htlcFailResolution)
+		}
+
 	// If we accepted the htlc, subscribe to the hodl invoice and return
 	// an accept resolution with the htlc's accept time on it.
 	case *htlcAcceptResolution:

invoices/resolution_result.go

@@ -61,6 +61,10 @@ const (
 	// invoice that is already canceled.
 	ResultInvoiceAlreadyCanceled
 
+	// ResultInvoiceAlreadySettled is returned when trying to pay an invoice
+	// that is already settled.
+	ResultInvoiceAlreadySettled
+
 	// ResultAmountTooLow is returned when an invoice is underpaid.
 	ResultAmountTooLow
 
@@ -133,6 +137,9 @@ func (f FailResolutionResult) FailureString() string {
 	case ResultInvoiceAlreadyCanceled:
 		return "invoice already canceled"
 
+	case ResultInvoiceAlreadySettled:
+		return "invoice alread settled"
+
 	case ResultAmountTooLow:
 		return "amount too low"